Blasting-Media.com

Iron Grits Blasting Media

Owing to our rich industrial experience, we are offering a quality range of Iron Grit to our clients at industry leading prices. Our offered Iron Grits are white structures that are manufactured with the best grade iron shots. The Iron Grits offered by us are perfect to be used for cutting or filling actions. These grits are extensively utilized in the air air blast applications for removing old paints, blasting & cutting stones and cleaning of castings & metallurgical material.

Features:

  • Hard
  • Sharp edge
  • Highly abrasive
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Grit Blasting for Superalloy Aerospace Components

Question:

We are currently nickel plating our Rene base materials prior to brazing. But this is a very expensive and tedious operation. After reading (Nicrobraz News, Summer ’92) about the use of NicroBlast Grit as a means of preparing base metals for brazing, we have several questions. What air pressure should we use? Do we need to dust-off or degrease the braze joint before or after blasting? What happens if grit gets caught in the joint and is not removed prior to brazing, and how long can NicroBlast Grit be used before it must be replaced?

Answer:

Grit blasting base metals prior to brazing improves the wet-ability of the surface for the brazing filler metal (BFM). The grit “works” the surface, essentially “peening” it, which helps the BFM wet the surface better (spreads out further) than the original untreated surface. Nickel plating also improves surface wet-ability; one of its main advantages is that it covers, and thus, prevents oxidation of, metallic elements such as aluminum and titanium in the base metal.

Figure 1 shows qualitative comparisons of how a super-alloy base metal surface was wetted by a nickel BFM after the super-alloy surface was prepared by several different techniques. The wetting angle (0) shows how well a surface is wetted by a BFM. The smaller the angle, the better the wetting.

We normally recommend that a 100 psig air pressure be used in your blasting operation. We also suggest that you use a pressure-type blaster instead of a suction-type. In a pressure-type blaster, air pressure powers the gun as well as forces the blasting particles through it; versus the suction-type unit, where the exhaust fan pulls the air through the blaster.

Caution: When blasting thin sheet metal parts, it may be necessary to lower the air pressure to prevent distortion of the parts. In some cases, the thin material should not be blasted at all (such as a material that may distort excessively even under light pressure). For example, when blasting a 1″ diameter tube with a 0 032’1 wall thickness, the O.D. of the tube will increase in size as the blasting time is increased. This is due to the compressive stress in the surface which results from continuous blasting (peening). See Figure 2.

We strongly recommend that any parts which are to be blasted be thoroughly degreased before blasting. Be sure the degreasing fluids are clean, and that all oil and grease have been completely removed from the parts prior to blasting.

After the parts have been degreased and blasted, we recommend that they be dusted with a clean cloth prior to BFM application. If the parts have been tack-welded prior to being blasted, there will probably be some particles of grit lodged in the joint area. If the grit particles cannot be easily removed, and the blasting media used was NicroBlast Grit, it is acceptable to leave them in place because the nickel BFMs used in your brazing operation will dissolve (or bond to) any NicroBlast particles. Since they are non-contaminating, they will not interfere with the brazing operation or joint properties. However, if the grit particles are non-metallic (aluminum oxide, glass bead, etc.), voids may result in the area of entrapment.

The blasting cabinet should have an adequate air sweep to ensure that all fine particles are continuously removed from the cabinet during the blasting operation. While blasting is taking place, the NicroBlast Grit will be broken up into finer particles. As long as the parts being blasted are free of all oxides, oil, grease, carbon and other contaminants, then it is not necessary to replace the NicroBlast Grit. It is only necessary to remove the dust created by the continual wearing away of the blasting grit, and to add grit to the cabinet, as required. An interesting experience occurred some years ago, which illustrates the importance of air sweep. Although some parts, which were free of all contaminants, had been blasted, the blasting cabinet did not have adequate air sweep. Consequently, the fine particles, which were being created from the breakdown of the NicroBlast Grit, continued to buildup until they resembled dark talcum-like powder, which felt like pasty carbon to the touch. This film coated the parts being blasted and prevented any further effective blasting operations. Even though we had presumed that the parts were oily and greasy, the contamination was merely due to our failure to remove the fine dust that was being created by continual breakdown of the grit over time.

Other considerations. Some Rene alloys contain large amounts of titanium and aluminum (about 8% total). This raises the following items:

NicroBlast Grit is very effective if the brazing temperature is maintained at 1950° F (1066° C) after blasting. If the parts are brazed at 2150° F (1177° C) instead, the aluminum and titanium diffuse to the surface much faster, forming an oxide layer which may inhibit the wetting action of the BFM. Thus, the brazing temperature will have a large effect on the results of the blasting operation (the higher the brazing temperature, the higher the risk that the blasting operation may not remain effective).

  1. The atmosphere quality will also effect the results of the blasting operation. When the brazing furnace atmosphere quality is exceedingly good (maximum leak-up rate of 1-to-2 microns per hour, and an extremely clean furnace with no visible surface contamination on any inside surfaces), these base metals can be brazed immediately after the degreasing operation, with no other surface preparation required. However, there are very few furnaces that have this capability.
  2. Where the furnace atmosphere quality is less than perfect, the presence of any contamination in the furnace and/ or small leak in the furnace can result in oxidation of both the titanium and aluminum content in the base metal. This oxidation may occur so rapidly that the blasting operation could be rendered totally ineffective (particularly with base metals in which titanium and aluminum levels are high).

Hopefully this information provides you with valuable insights into both the strengths and limitations of NicroBlast Grit, and how to maximize its effectiveness in your operations.

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Build Your Own Grit Blasting Cabinet

A Little Background

Like many people, when I first began my restoration project (my 1956 Ford F250) I discovered that a grit blast cabinet, also called a sand blast cabinet, is a great tool to have around. It cleans many parts faster and better than any other method. If you do a little research on the subject of grit blasting cabinets you will find that there are a number of top-quality units available from various vendors. If you want to blast larger parts, though, you get into the industrial-strength units that are usually beyond the budget of typical restoration or rebuild project. Costs for larger cabinets usually start at over $2,000 and go up to well over $5,000. I found I needed a larger cabinet to handle many parts, but I just couldn’t afford to shell out the kind of money that a commercial unit costs. The only option was to begin experimenting with various designs of my own. The idea was not to redesign the available commercial units with all their capabilities but to provide a simple, workable cabinet with enough interior room to handle bigger parts.

Since this was my first time building a grit blasting cabinet it was quite a learning experience. Hopefully I can pass on to you some of the lessons that I learned the hard way building the first few cabinet versions. I think it’s important to let you know the reasoning behind certain features since some of them are not obvious but came into use through hard experience and tinkering. Here are a few of the biggest issues I dealt with.


From my experience, the single biggest issue with blasting cabinets is visibility. If you can’t see, you can’t clean. Keeping the dust level low and providing sufficient lighting and a good-size viewing port make any blasting job a whole lot easier. The visibility battle began with my very first cabinet design. That first version was nothing more than an 18″ x 18″ x 28″ wooden box with a door, a small viewing window, armholes, and an exhaust fan. Boy did I learn a few things from that experience I could blast for about two minutes tops before I couldn’t see anything at all. I tried a number of dust retention methods including a larger exhaust fan and a variety of filter types including a homemade water scrubber stack. That water scrubber was actually pretty neat and it did work but it was too cumbersome and messy to use in the long run. I also looked into electrostatic filtration but that was prohibitively expensive.

I finally reached the conclusion that the plain sand I was using was a major part of the problem. It just created too big a mess inside the cabinet for proper visibility. All grit breaks down with use but the plain sand tends to break into small, rounded particles. Those little rounded particles are no good for blasting but are great for making a big cloud inside the cabinet if you want to obscure your vision. In addition, inhaling this fine silica dust can cause serious health problems such as silicosis. After a lot of reading and research, I found a material called CrystalGrit from Virginia Materials, Inc.. CrystalGrit is specular hematite, which has a number of advantages over plain sand including:

  1. It doesn’t break down quickly,
  2. It’s just about as cheap as plain sand (except the shipping),
  3. It is very dense so when it does break down, it tends to settle and not form a dust cloud, and
  4. When it does break down, it shears leaving sharp, angular surfaces that still cut rust very well.

The difference between the grit types was like night and day. With the CrystalGrit I found I could actually see what I was doing and I didn’t need an elaborate dust retention setup. I still made a few changes to improve the visibility such as better light placement, inlet and exhaust baffles, and a better viewing window system. I’ll describe these in detail later.

Another big problem is that grit bounces off the part being blasted and strikes the viewing window creating small pits. The process is the same one used to etch glass. I was using a polycarbonate window with a scratch resistant surface but after a few hours it would still need replacement. After wearing out a number of windows on my box, I decided that there had to be a better way. After a little thought my brother suggested I use the same system they use for racing helmet visors and, I found out later, for commercial grit blast cabinets – oops. I need to read more closely next time I do research I guess. I now use tempered glass for the window and attach a replaceable polycarbonate film on the inside with masking tape. When the film gets pitted, it’s easily replaced and the glass window stays like new.

I have been using my grit blaster in its current form for a couple of years now and I have yet to find any major problems with it. There are things that could be improved, to be sure, but I am happy with the current design. Now let’s look at the design details and how to build it.

Designing A Large Grit Blasting Cabinet

Before starting the design, I needed to determine what features are important and what I could afford to leave out. If I tried to include every option the cabinet will end up costing more than an industrial unit. Here are some of the main design considerations.

First, many people prefer a funnel shaped bottom on their grit blaster to automatically recycle the grit to the grit bucket. I’m willing to load up with a bigger amount of grit and stop every couple hours to manually recycle the grit back to the grit bucket. Since automatic grit recycling isn’t critical to me I’ve simplified the design and used a flat bottom on the cabinet. This flat bottom also lets me easily put different height and shape tables inside to support the parts while I work on them so I can handle both small and large parts. Second, I’m willing to run an exhaust pipe outside so I don’t need a 100% efficient dust retention system although it has to keep the dust down as I discussed earlier. Finally, I dislike is standing for hours on end while blasting, so the design has to accommodate a seated operator in reasonable comfort.

The simplest cabinet design is a big, square box and the least expensive and easiest material to build it out of is either oriented strand board (OSB), particle board, or plywood. I chose particle board for my project but the other materials should work just fine too. All of the materials come in 4′ x 8′ sheets so the obvious cabinet size to build would be a 4′ x 4′ x 4′ cube  big enough to handle all my parts, small enough to fit in my garage, and the right size to minimize construction effort. I chose ” thick board to provide the strength needed at the joints without reinforcing. When I first designed the box, I hadn’t considered it, but the thicker material also allowed me to cut side ports for longer parts without worrying too much about the sidewall strength. This was a great bonus when I needed to clean a differential during the cold winter months. The photo in Figure 1 shows the side port with a cover in place since I’m not using it right now. You can probably use ” board to save money and decrease the weight but you will have to provide joint reinforcement (2″ x 2″ or similar) and you might not be able to cut side ports without sidewall reinforcement.

The following paragraphs contain a description of the cabinet I built along with some of the reasons I did things the way I did. My approach is far from the only way to do this project and I encourage readers to experiment, substitute components, or make other improvements to meet their individual needs. As far as construction goes, a friend or two to help with the main cabinet assembly is strongly advised. You need to be comfortable with either a hand or power saw and with other common hand tools to do the assembly. You will also need to do some minor electrical wiring for the lights and fan. If you don’t feel you are qualified to do this wiring please get a qualified professional or knowledgeable friend to help out.

Please read through the ENTIRE process carefully before starting.  I’ve tried to make sure there aren’t any gotcha’s but a little study on your part could save you a lot of aggravation in case I’ve missed something. A parts list for the cabinet including vendors and estimated costs is included at the end of the article.

Building The Cabinet

The 4′ x 8′ sheets get cut to the sizes shown in Figure 2. Note that the front door is just a section cut from the 4′ x 4′ panel for the cabinet front  DO NOT cut out the front door panel yet! We want the front panel in one piece during assembly to make sure we get things square and true. I’ll describe how to cut out the front door later.

It’s easier if you cut the round ports in the back and bottom panels after you cut the panels to size but before you assemble the box. It’s also a good idea to attach the inlet port baffle to the back panel before you assemble the cabinet. Center the baffle over the 10″ diameter inlet vent hole and attach it and the spacers with four screws in the corners of the baffle as shown in Figure 3. The baffle keeps flying grit from going out the inlet port and into your workspace but allows air to enter since the spacers hold it off the back panel just a little. We’ll deal with the cutouts in the front door after assembly of the main cabinet.

We needed two people to assemble the box, but I suppose you could figure out a way to do it yourself if you needed to. If you are assembling it outdoors make sure you have a door big enough to get it back indoors when you are finished putting the box together No point in providing comic relief for the neighbors, eh? Stand the two sides, the top, and the bottom panels on their front edges on a flat, hard surface (garage floor, driveway pad, etc.).  Clamp them in place with a strap clamp or several long pipe clamps. Place the back on top of the rear edges of the four panels and make sure everything fits square and true. Make any required adjustments (trimming, etc.) now. Pay attention to which side is the outside of the back and bottom panels or the holes won’t be in the right places when you’re done.

With the back, two side, top, and bottom panels still clamped in place pre-drill holes for the screws that will hold the cabinet panels together. I used six or eight #8 x 2-1/2″ particle board screws in each edge. Now let’s put these five panels together.

1.      Remove the back panel.

2.      Unclamp the top and one side panel and remove them as well.

3.      Keep the other side and the bottom panels standing on their front edges and apply construction adhesive to the joint between them.

4.      Install the screws in this joint while holding the panels at a right angle.

5.      Place the second side panel on its front edge, apply adhesive to the joint between it and the bottom panel and install the screws in this joint.

6.      Place the top panel on its front edge, apply adhesive to the joints between it and the two side panels, and install the screws in these two joints.

7.      Finally, apply construction adhesive to the back edges of the two side, top, and bottom panels, replace the back panel, and install screws in all four joints.

Note that the construction adhesive is both to provide strength as well as to seal the edges so fine grit doesn’t work it’s way through. Don’t be afraid to use a little extra and wipe the excess off the outside after you’re done assembling things. If you’re really a neat freak you can wipe off any excess on the inside as well.

After waiting for the adhesive to cure, carefully turn the cabinet over so it’s sitting flat on the back panel. Place the front panel on the edges of the sides, top, and bottom panels and check for a good fit. We don’t want to attach the section of the front panel that will be our door so carefully mark the top and bottom edges of the front panel door cutout on the two side panels. Avoid putting adhesive or screws between the marks and you’ll be able to remove the door after we attach the front panel and make the door cuts. Pre-drill holes for screws in the front panel-to-box joints, apply adhesive, and drive in the screws.

At this point you should have a big cube with some round holes in it. I strongly recommend letting the adhesive cure according to the manufacturer’s instructions. You’ll need to move the cabinet around a little and there’s no point in tearing a joint apart at this point because you’re too impatient to wait a little for the adhesive to cure.

With the cabinet sitting on it’s back we’re ready to cut out the door and take care of a couple other details.

1.      Cut two, three-inch-long pieces of the 6″ diameter PVC drain pipe and smooth all the edges with a file or sandpaper.

2.      Rough up the outside surfaces of the pipe with coarse sandpaper so the adhesive will bond properly when we apply it later.

3.      Use one of the pieces of pipe to trace the armhole cutouts on the front panel. You want the holes just big enough to accept the pipe so trace the OD of the pipe closely.

4.      Mark the square cutout for the viewing window.

5.      Cut out the viewing window and armholes with a saber saw.

6.      Test fit the armholes with the pipes and, if necessary, enlarge the holes a little until they fit.

On to the front door cutout and hinge installation.

1.      Set your circular saw so it cuts just a hair over ” deep and run it across the front panel along the upper and lower door cutout lines. If you’ve been careful with the adhesive and screws the door panel should come right off.

2.      Decide which side you want the door hinges on right-handed folks will probably want them on the left side and vice-versa.

3.      Cut a 47″ long piece of 2×4 and attach it with adhesive and screws to the front edge of the side where the hinges are going. See Figure 4.

4.      With the door panel off, place four Popsicle sticks on the front edges of the side panels and replace the door so it is sitting about 1/8″ off the sides. This gap is for the weather strip seal we’ll install later.

5.      Place two of the three hinges about 1″ from the top and bottom of the door opening and center the third one. Use the long arm of the hinge flat against the side of the cabinet and the tab flat against the door. The hinge arms have to be rotated backwards so the pin is on the outside of the corner. If the hinges don’t rotate quite far enough use a file to remove a little material where they bind.

6.      Attach the hinges with #8 flat head particle board screws.

7.      Remove the Popsicle sticks, attach the door handle opposite the hinges, and test the door to make sure it moves freely.

8.      Cut two 45″ long pieces of 2×4 and, with screws and adhesive, install them horizontally at the top and bottom edges of the door opening as shown in Figure 4. The 2x4s should extend about ” past the door cutout to provide a place for the door weather strip to seal against. Choose 2x4s with smooth and straight surfaces where the seal will contact.

9.      Install the weather stripping around the inside edge of the door so it contacts the 2x4s on the top, bottom, and hinge side and the front edge of the side panel on the door handle side.

10.  Insert two or three flat washers behind the turn buttons and attach two turn buttons along both the top and bottom edges of the door opening so the door can be held closed. The washers should hold the turn buttons off the front panel just enough to allow the turn buttons to pass over the raised door surface. Adjust the number of washers to get a good, snug fit all around the door.

11.  If necessary, fill any gaps in the 2x4s or side panel edge with construction adhesive and flatten the surface toward the door with a putty knife. Wait until the adhesive has cured to try the door fit again.

Time to cut the grit strainer hole, so tip the cabinet onto its top panel. I found swimming pool strainer baskets work just fine as grit strainers. They have holes about the right size for the grit but small enough to trap any parts you drop. They also catch any crud that might plug up the siphon tube or grit gun. They’re made of durable plastic and are cheap to replace if you break it or it eventually wears out. In the parts list I’ve included the part number of the one I use with my abrasive but you may have to try a couple different ones to find the right size strainer holes for different abrasives. Cut the hole in the bottom panel for the grit sieve as shown in Figure 1. The hole should be just a little smaller than the inlet opening of the strainer you choose. With the hole cut, place the strainer over it and drive #10×3/4 pan-head sheet metal screws into the bottom panel so their heads catch the lip of the strainer and hold it in place. I prefer round strainers and I put four screws evenly spaced around 190 degrees of the circle as shown in Figure 5. That holds the strainer firmly but allows me to squeeze it and slide it out without removing the screws. Remove the grit sieve for now to avoid damaging it.

The Cabinet Base

The base should provide solid support since the cabinet, parts, and grit can add up to substantial weight. The base design shown in Figure 6 has worked well for me. It provides knee space for the operator, support on three sides and across the middle of the cabinet, and is stiffened to avoid rocking. Other designs are possible  just make sure the base can fully support the cabinet and handle the load, which can reach 1000 lbs or more with a large batch of dense grit and some heavy parts.

Start by cutting all the 2x4s and the particle board gussets. There should be sufficient particle board left from the last sheet to make the 8 gussets. Assemble the five 2x4s that make up the top of the base exactly as shown in Figure 6. Use adhesive and the #10×3-3/4″ wood screws. With the top frame assembled, attach the eight gussets using adhesive and #10×1-1/2″ wood screws. Be careful to stagger the joints exactly as shown in the figure. Use 4 or 5 screws into each 2×4 to ensure a strong joint. Finally, attach the legs to the gussets and the top frame using adhesive and wood screws. Use the #10×1-1/2″ wood screws to attach the legs to the gussets and the #10×3-3/4″ wood screws to attach the legs to the top frame through the gussets. Let the adhesive in the base cure according to the manufacturer’s instructions before performing any more work on the cabinet.

Once the adhesive in the base is cured set it up in the location you intend to put the cabinet. Keep in mind that you need access to the cabinet front door and some space at the rear for the air inlet and exhaust ports and exhaust hose. The corrugated exhaust pipe needs at least 18 inches to turn 90 degrees unless you use an elbow to attach the hose to the cabinet back panel. With the base set up get a friend or two to help lift the cabinet onto the base. Line up the rear and sides of the base and cabinet but do not fasten them yet. Sit down at the operator’s position, put your arms through the armholes, and determine if the cabinet is at the correct height for you. It’s still relatively easy to put blocks under the legs or cut them off at this point. Make height adjustments as necessary. Once you are happy with the setup drive three #10×1-1/2″ wood screws through the bottom of the cabinet and into each rail of the top frame of the base. The cabinet should now be solidly in place on the base. If there is any tendency for the legs to rock use a small wood or metal spacer under the short leg to make the base solid on the floor.

 

Exhaust System

The exhaust system consists of a baffle, exhaust pipe, fan, and dust retainer. The exhaust must be sent outdoors since the dust retainer is not 100% efficient. If you exhaust the dust into your garage or workshop everything in the space will become coated with a combination of fine grit and rust. Don’t say I didn’t warn you. Plan where you will route your exhaust pipe and where you want the exhaust fan, dust retainer, and their housing to be when operating the cabinet. I run mine out the garage door to the back yard. Once you’ve got your plan, attach one end of the corrugated plastic exhaust pipe to the exhaust opening on the rear panel of the cabinet. I inserted the pipe so it extended about ” inside the cabinet and then drove six of the #8×3/4″ particle board screws through the pipe and into the cabinet back panel. I then used the construction adhesive to seal both the inside and outside seams to produce an airtight and strong connection as shown in Figure 7. If desired, you could use an elbow to attach the pipe to reduce the rear clearance required for the cabinet.

The other end of the exhaust pipe gets connected to the fan housing. I used the simple box shown in Figure 8 for my housing. I connected the pipe to the housing rear panel using the same method I used for the cabinet end. The fan gets mounted inside the housing front panel as shown in the figure. There are markings on the fan to indicate the airflow direction. Make sure to install the fan so the air blows in the direction indicated in the figure or you’ll have a mess in your garage the first time you turn it on, since the grit will be blown back through the inlet and grit sieve ports instead of out through the fan.

 

I did not use any adhesive when assembling the housing since I want to be able to service the fan if necessary. The housing parts are small and it’s not too difficult to cut them accurately enough to minimize air leaks. A little duck tape around the outside takes care of any small leaks that remain. I realized that having the fan in the dust path would probably shorten its life but I wasn’t sure how bad it would be damaged. I’ve been using the same fan now for over three years and it’s still working. I figure if I have to replace it every few years it’s not too big a sacrifice for a simple and effective dust exhaust system.

 

The wiring to the fan is straightforward.

1.      Drill a 1-1/2″ diameter hole through the center of one side of the exhaust housing.

2.      Install one of the cable connectors in the center rear of one of the electrical connection boxes.

3.      Install a second cable connector in one of the sides of the connection box.

(Make sure the screw clamps for both connectors are on the outside of the box.)

4.      Cut off the female end of one of the 25′ extension cords.

5.      Feed the cord through the connector on the side of the box and then through the connector on the rear of the box.

6.      Tighten the screws on both connectors to clamp the cord in place leaving about 12″ of cord extending out the rear connector.

7.      Position the connector box with the rear connector in the 1-1/2″ hole you drilled earlier in the housing.

8.      Fasten the box to the side of the housing using #10×3/4″ pan head screws.

9.      Strip the three wires inside the housing and connect the black and white wires to the two fan terminals. You can solder them or buy the special crimp-on connectors that fit the small tabs on the fan. The green wire gets connected to the metal fan housing with a small machine screw and nut.

10.  Put the cover on the electrical connection box.

11.  Plug the cord in to turn on the fan and test your installation.

At this point you’re probably wondering about this “dust retainer” thingy I’ve mentioned a few times. Well, it’s a large pillowcase placed over the exhaust housing. I hold it in place by tying a loose knot in the open end of the fabric behind the housing. When the fan starts up the pillowcase balloons up and is forced up against the back of the housing. It can’t get free because the hole I’ve left with the knot is big enough to fit around the exhaust pipe but not big enough to fit over the housing.

A couple tips are in order here. NEVER try to wash the dust retainer in your everyday washing machine. You’ll be very sorry for several months afterward. Even sorrier if you’re married and your spouse finds out. The best way to clean the retainer is to turn it inside out and hand wash it in a bucket of warm water with some detergent. Let it air dry and you’re ready to go.

Next, insert the armhole PVC pipes so that about 2″ extends into the cabinet. This leaves a lip on the outside for adhesive and a short length on the inside to attach the gloves. Apply adhesive to both the inside and outside of the joint between the pipe and the door so provide a strong attachment as well as an airtight seal. Don’t get adhesive all over the inside pipe extensions since that’s where we’ll be attaching the gloves. Just a solid bead that leaves an inch or so of clean pipe for glove attachment is fine.

While that adhesive is curing, lay a bead of adhesive around the window opening and drive two small screws to the left and right on a line where the lower edge of the window will be. Install the glass centered on the opening and resting on the small screws to support it. Use duck tape on the window edges to hold it against the door while the adhesive cures.

Completing The Cabinet

The last phase of construction is to connect and attach the remaining auxiliary items. First, install the exhaust port baffle as shown in Figure 9. Use adhesive to seal the top and side seams and use three #10×1-1/2″ wood screws along each side seam to hold the baffle to the cabinet back and side panels. The baffle allows air and dust to be drawn up from the bottom of the cabinet to the exhaust port but the heavier grit falls back down and stays in the cabinet.

Prepare for the lighting installation by cutting off the female end of the second extension cord. Cut off approximately 10 feet of the cord on the opposite end from the male plug. We’ll use this spare cord to connect the lights inside the cabinet and the remaining 15 feet with the male plug will run to an outlet to provide the power. Connect one end of the spare cord to one of the light fixtures. The ground wire is not used here since the fixture has no exposed metal parts. Just trim the green wire off and connect the black wire to the gold (brass) contact and the white wire to the silver contact. Install this light fixture on the inside of the cabinet centered directly above the door opening. Snug the mounting screws down but don’t over-tighten them or you’ll end up with a broken fixture.

 

Drill a 1-1/2″ diameter hole through the cabinet where you want the power cord to exit. It should be near the door hinges but in a location where it won’t get caught in the door or otherwise damaged. There should be sufficient room around the hole to mount the second electrical box the same way the first box was mounted on the exhaust housing.

Install one of the cable connectors in the center rear of the second electrical connection box. Install a second cable connector in one of the sides of the connection box. Make sure the screw clamps for both connectors are on the outside of the box. Feed the cut end of the 15-foot cord through the connector on the side of the box and leave about 6 inches of cord inside the box. Tighten the screws on the side connector to hold the cord firmly in place. Strip the outer insulation off the wires inside the box.

 

Route the wire from the light fixture you’ve installed across the top of the cabinet and to the 1-1/2″ hole you drilled. Hold the wire firmly in place every 4 to 6 inches with the wire mounting clips. Run the wire through the hole and then through the rear cable connector on the electrical box. Hold the box in place over the 1-1/2″ hole and cut the cable leaving about 6 inches inside the box.

Mount the two remaining light fixtures on the inside of the front door about 6 inches to either side of the viewing window. Use the remaining length of the spare cord to connect these two fixtures in parallel and route their supply cord to the 1-1/2″ hole in the same way as the top light fixture. Read the paragraph below before routing the cord to avoid getting it in the way of the light shields. Run the cord through the hole and then through the rear connector on the electrical box. Now, carefully position the electrical box over the 1-1/2″ hole and leave a loop of wire for the door lights so that the wire will not be damaged as the door is opened and closed. Too much loose cord invites damage from parts and too little will cause the cord to be stretched or pinched with door motion. When you have the cord adjusted correctly, pull the box away from the hole just enough to tighten the rear connector screws and hold the two cords running through it firmly in place. Now attach the connection box to the cabinet, strip and connect the three black wires together, the three white wires together, and the three green wires together. Use wire nuts on the black and white wires but attach all three green wires to the metal connection box using a small machine screw and nut. Put the cover on the electrical connection box and install three 100W bulbs in the fixtures. Plug the cord in to test your installation.

Next, install the light shields on the door to avoid having the door lights shine back in your face through the viewing window. You can see some older versions of the shields in Figure 10. Cut the aluminum sheet in half so you have two pieces each 6″ x 12″. Bend a ” strip along the 12″ side of each piece to a right angle and drill three evenly spaced 3/16″ holes through it for mounting screws. Mount each piece vertically between one of the door light fixtures and the viewing window so that you can’t see the light bulbs as you look left or right through the glass. Use the #8 x ” particle board screws to mount the shields.

With the adhesive cured on the armhole pipes take each glove and pull the cuff over the inside of the armhole pipe. Slide a cable clamp over the glove and tighten it down to connect the glove cuff to the pipe. I use a strip of felt between the clamp and the glove cuff to avoid cutting or tearing the glove with the clamp. Make sure to get the correct glove on the correct side and make sure the gloves are rotated with the thumbs up and in for a comfortable fit.

Time to install the air and grit pickup hoses and the gun. Note the two holes shown in the front of the cabinet for tubing feedthrough in Figure 1. Choose one to be the grit pickup and one to be the air supply. Drill the grit pickup hole so that the plastic grit tube that came with the gun will just fit through. Insert the tube so that about 24″ of tube remain on the inside of the cabinet with the gun attached.

I wanted to keep my compressor air hose intact and use the gun both inside and outside the cabinet. The easiest way to do this was to make a short air line with a male quick-connect on one end and a female on the other. Connect the female quick-connect-to-hose-barb fitting to one end of the 6′ piece of ” polyethylene tubing. Drill a ” hole through the cabinet front panel for the air supply. Feed the cut end of the polyethylene tubing through the hole from the inside until there is about 24″ of tubing left inside the cabinet. Connect the female fitting to the male quick-connect on the grit gun. Finally, connect the male quick-connect-to-hose-barb fitting to the cut end of the tubing outside the cabinet. You are now all set to connect your air line and test your installation.

The last step is to modify the grit bucket so it catches the grit that falls through the grit sieve. I’ve included a grit blast kit in the parts list that has a plastic bucket with the appropriate funnel shape and suction tube at the bottom. Unfortunately, the manufacturers and vendors tend to change these molded plastic units fairly often. I don’t know why they do this, maybe to keep up with the fast-paced world of grit bucket fashion? Anyway, I cut off the top of the grit bucket that comes with the kit in the parts list and it worked fine catching the grit. You may have to make some other modifications to your bucket if the style has changed since then. The bottom line is that you need a bucket with reasonable capacity, a funnel-shaped lower section, and a suction tube at the bottom. The only other requirements are that the opening at the top is big enough to catch the grit from the grit sieve and the unit is short enough to fit under the cabinet.

 

Figure 10 is an interior and Figure 11 is a front view of the cabinet after much use. This particular cabinet is a little older and has some minor variations from the description above but it is pretty much what you should see when you are done with your project.

Using the Cabinet

 

As I mentioned at the beginning of the article, I wanted to be able to put various tables, part holders, etc. inside the cabinet to make the work easier. The flat cabinet bottom doesn’t allow for automatic grit recycling but it does make it easy to handle items and put tables or fixtures inside. The easiest way to recycle the grit in this cabinet is to remove any items inside and use a short end of 2 x 4 and your hands (with work gloves on!) to push the grit to the front corner and into the grit sieve hole.

The heavy rubber gloves are designed to keep the skin on your hands when you’re blasting. Unfortunately, they also make holding small parts difficult. I’ve found that clamping small parts in a vise grip before putting them in the cabinet gives you have something to hold on while blasting. You’ll have to stop and change the clamp position to get the entire surface blasted but it’s better than searching around in the grit pile trying to find that little bugger after you drop it. Another tip is to put a nylon wire tie on the gun trigger to keep your trigger finger from permanently freezing in the “squeeze” position. I’m too cheap to buy a foot-operated air valve so I just disconnect the quick-connect air supply fitting to the gun to stop the airflow.

Even with the exhaust system the fine dust in the cabinet gradually builds up on everything inside. I periodically pull the grit suction tube off the gun port and use the straight compressed air from the gun to kick up all the dust on the cabinet sides, top, and especially the lights. After a minute or two with the exhaust fan on, the inside is clear again and I reinstall the grit tube and continue blasting.

I’d strongly recommend that you use a compressor of sufficient size to avoid the frustration of low air pressure and incomplete surface coverage. There is also the possibility of burning out either the motor or pump from overwork, I learned that lesson the hard way. The minimum I’ve been able to get away with is about 20 cfm at 80 psi for heavy-duty work. I do use lower pressure for more delicate items but I wish I had something like 25 cfm at 100 psi to really make the hard jobs easier.

As I mentioned before, I’ve used the large cabinet to blast the rear axle from my F-250 by cutting a port in the right side and putting each end of the axle inside the cabinet in turn. I supported the outboard end of the axle with an engine hoist although the ” thick cabinet side panel took a fair amount of the weight as well. Some duck tape and a plastic garbage bag sealed the opening between the axle and the cabinet side panel when the axle was in place. An oversize scrap piece of particle board with weather stripping attached to the edges closes off the port when I’m not blasting longer parts that don’t fit in the interior.

One change I would make to the current design is to use a larger armhole diameter for better comfort. I’m sure there is 8″ diameter PVC pipe available somewhere and the glove cuffs would probably stretch to go around it. If anyone would like to try this you might try looking at sewage pipe supply houses, since they tend to have larger diameter plastic pipe.

Optional Parts And Tips

The polycarbonate film in the optional parts list comes in 24″ x 24″ squares. I use a utility knife to cut each 24″ square into four smaller squares that fit in the viewing window cutout in the door.  Four pieces of masking tape seal the edges of the film to the window. As I mentioned before, the film takes the abuse from the flying grit rather than having it pit the glass. When the film gets too hazy I just replace it with a new sheet. That’s a lot better than having to replace the glass.

Again, as far as grit material, I highly recommend the specular hematite product called CrystalGrit from Virginia Materials, Inc. I’ve included information on it in the optional parts list. It does tend to wear out gun nozzles faster than normal, but the advantages make it worthwhile, in my opinion. To handle the extra wear, I’ve included some alumina tube in the optional parts list that is exactly the same ID and OD as the nozzles for my blasting gun.  I cut the tube to length with a carbide hack saw blade and make my own nozzles for about $0.75 each instead of $12 retail. McMaster-Carr has a good range of alumina tube sizes so if your gun has a different nozzle diameter you may still be able to save some money using this method.

Another option to strongly consider is an oil/water removal unit right at the air supply entry to the cabinet. I have one on my compressor but the air is still warm there and it holds quite a bit of water vapor. By the time the air reaches the cabinet it’s cooled down and an oil/water removal unit takes a lot of water out so it doesn’t go on your parts. I’ve included a reasonable unit in the optional parts list.

The most important issue when using the cabinet is safety. ALWAYS use a particle mask to avoid breathing any residual dust that might leak from the cabinet and ALWAYS wear safety glasses in case the viewing window gets broken while you have you’re peering through it.

That pretty much covers the procedure for building the large grit blast box. I hope yours works as well as mine has. Remember, if you think of a better idea or don’t like something about the design, change it! This is your grit blaster and you have to use it. That’s what building your own tools is all about.

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Grit Blasting Instead Of Scabbling

The task

Constructing concrete structures.

The problem

A construction company was building one of the stations for the London Underground Jubilee Line extension. The main structure of the station is concrete which was being cast in situ in stages. The meeting surfaces of each section have to be prepared before the adjacent section is cast to ensure an effective bond. In some parts of the structure, this was done using a special expanded metal material which forms the bond itself. However, this material is not suitable for use on thin sections of concrete which meant the joints in these sections had to be prepared in some other way. They could have been prepared using impulsive scabbling tools such as needle guns, but workers would have been exposed to vibration magnitudes of about 18 m/s2 for up to 2 hours per shift. These tools also produce high noise levels.

The solution

The surfaces were prepared by grit blasting. A sub-contract gang was able to blast about 300 m 2 of the surfaces per day per worker. Before the work began, screens were erected around the area to be blasted to prevent dust from blowing around the site. The grit blasting method compares favourably with the use of impulsive scabblers which may prepare as little as 8 m 2 of the surface per day per worker.

The result

  • The operators are not exposed to any vibration with grit blasting.
  • Grit blasting is much faster than scabbling but may increase exposure to dust and noise which will require further assessment and control measures.
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Grit Blasting Painting

We undertake grit blasting of metal components up to almost any size using Olivine, a naturally occurring silica free expendable material. Grit blasting, a mild abrasive is suited to the removal of surface contaminants, light rust, edge burrs and coatings from most metals including aluminium, cast iron and mild steel.

Degreasing

All metalwork undergoes either vapour degreasing, manual degreasing or grit blasting prior to coating in order to remove oil, grease and chemical contaminants which can hinder the coating process.

Blasting

Metalwork is blasted with an angular Olivine grit which produces an excellent key for subsequent powder coating or wet spray. Ideal applications include:

  • Large & heavy items such as plant enclosures & structures
  • New metal prior to powder coating or wet spray
  • Castings
  • Antiques
  • Vehicles
  • Stainless steel & aluminium components
  • Old radiators.
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Steel Grit Sizing

Steel grit is crushed steel shot. They have sharp edges that helps cleaning the surfaces easly.They are used for different aplication (ship building, pipe building, steel construction, granite cutting, tank manufacturing etc.). Our products are very successful in abrasive blasting operations. Because we apply a special heat treatment. After this heat treatment, steel grits have unifom martensitic microstructure. 

Normally, grits are manufactured 8 different sizes and 4 different hardnesses  Our products are named according to SAE standarts. 

Our standart hardness grades are  

  • G Grade 45-50 HRC
  • GL Grade 53-58 HRC
  • GP Grade 58-63 HRC
  • GH Grade 64 HRC Min  

Our standart sizes are 12, 14, 16, 18, 25,40, 50, 80  But we can manufacture with special hardness and different sizes depending our cutomers demand.

Casted steel shots are hardened in a furnace and crushed in a crusher. Crushed grits has a very high hardness grades. At these grades steel can be easly broken. To prevent this brittelness, applied a tempering process to the grits. And its hardness decreased to the target hardness values.  Aftere these operations grits are ready for sieving and packaging 

Our all steel grit sizes are G12, G14, G16, G18, G25, G40, G50, G80, GL12, GL14, GL16, GL18, GL25, GL40, GL50, GL80, GP12, GP14, GP16, GP18, GP25, GP40, GP50, GP80, GH12, GH14, GH16, GH18, GH25, GH40, GH50, GH80

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Steel Abrasive Grit Blasting

Steel abrasives are steel particles that are used as abrasive or peening media. They are usually available in two different shapes (shot and grit) that address different industrial applications.

Steel shot refers to spherical grains made of molten steel through an atomization (“granulation”) process, available in different sizes and hardnesses.

Steel grit characterizes grains with a predominantly angular shape. These grains are obtained by crushing steel shot, therefore they exhibit sharp edges and broken sections. Harder than steel shot, it is also available in different sizes and hardnesses.

Properties

Most steel abrasives are made of a high-carbon steel composition, the best compromise between mechanical properties, efficiency and durability. The most important properties for steel abrasives are hardness, grain size and shape, toughness and cleanliness (lack of oxides, contaminants, etc.).

Recyclability and environmental impact

The recyclability of steel shot and grit ranges between 2000 to 3000 cycles. Due to its high recyclability level, steel shot and grit tend to generate less waste when compared to other expendable abrasives.

Hardness

Steel shot or grit is usually available at different hardness levels, ranging between 40 and 65 on the Rockwell scale (400 to 850 on the Vickers hardness scale).

Industrial applications

Cleaning

Steel shot and grit are used in cleaning applications for removal of loose material on metal surfaces. This type of cleaning is common in automotive industry (motor blocks, cylinder heads, etc.).

Surface preparation

Surface preparation is as a series of operations including cleaning and physical modification of a surface. Steel shot and grit are used in surface preparation process for cleaning metal surfaces which are covered with mill scale, dirt, rust, or paint coatings and for physically modifying the metal surface such as creating roughness for better application of paint and coating.

Stone cutting

Steel grit is used in cutting hard stones, such as granite. The grit is used in large multi-blade frames which cut the blocks of granite into thin slices.

Shot peening

Shot peening is the repeated striking of a metal surface by hard shot particles. These multiple impacts produce a deformation on the metal surface but also improve the durability of the metal part. The media used in this application is spherical rather than angular. The reason is that spherical shots are more resistant to the fracture which happens due to the striking impact.

Industrial uses

Steel shot and grit address numerous sectors since cleaning, surface preparation or shot peening applications are used by many industries as a part of their construction, renovation or repair processes. The main industrial sectors employing steel abrasives are:

  • Automotive industry
  • Construction
  • Metallurgy
  • Petrochemical industry

Production

The annual steel abrasive production in the world is estimated to be above 1 million tonnes, the world’s largest producer being Winoa Group (previously known as Wheelabrator Allevard) by production and capacity.

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Standard CNC Abrasive Grit Blasting System

Our standard CNC grit blaster has a capacity to process parts up to 48″ (1.2m) in diameter by 48″ (1.2m) tall while occupying less than 100 ft (9 m) of floor space. This machine is the most efficient, cost effective robotic grit blasting machine in the industry. Typical applications include surface preparation of complex geometries prior to thermal spray coating.

Standard

This system features a rugged Progressive 4 axis Gantry Robot, Fanuc or Siemens CNC, and a coordinated 5th axis shuttle spindle for precise positioning and easy part loading. Our integral dust collector, Sweco classifier, grit media delivery and reclaim allows for a very small footprint.

Optional

  • PRIMS process reporting for collection of key process parameters and PM monitoring
  • Grit blast media adder plus media storage for processing with up to 4 different sizes of abrasive blast media
  • Sweco access platform and stairs
  • Closed loop air pressure
  • Closed loop non-ferrous media flow control
  • Manual blast station in door
  • Machine mounted jib hoist
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General Automatic Abrasive Grit Blasting Machine

This grit blasting system can process a wide variety of parts and geometries and is designed for flexibility and simplicity. The spindle assembly and table top unit can swing out through the front door for easy part loading or unloading by an overhead crane. Parts up to 54″ (1370 mm) diameter can be loaded inside the machine. It utilizes up to 2 venturi suction or direct pressure nozzles and a gun mover which can be programmed for a range of motions. The front door is also equipped with hand ports to enable fully manual sandblasting and is fully interlocked for safe operation.

Standard

  • Integral design incorporates dust house, abrasive blast media reclaim and delivery
  • Complete system occupies only 100 ft� (9.3 m�)
  • 2-axis gun mover
  • Variable speed spindle with swingout design
  • Touchscreen operator interface
  • Simple programming and operation
  • Ergonomic – easy part load/unload
  • Process flexibility – manual and automatic modes of operation

Optional

  • Closed loop air pressure
  • Closed loop abrasive blast media flow
  • Automatic sandblast media adder
  • Sweco vibratory screening system
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Rotary Indexing System for Abrasive Grit Blasting

Rotary indexing table machines are available with both suction and pressure media grit blast delivery. The machines are used in high volume production applications, and can be manually or automatically loaded.

Standard

This system has a standard 4 position index table which features a rugged abrasive blasting indexer. The base sandblasting machine has 2 spindles per station and can be configured with one to four spindles per station.

Optional

  • Automatic media adder for continuous abrasive grit blasting
  • Grit media classifier for precise control of media size
  • Light curtain for enhanced ergonomics
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