Wed Sep 05, 2007 3:43 pm
Shade Tree Sand Blasting Booth
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- Blasting Intro
Sand blasting, more properly called Abrasive Blasting, is one of the tools I find indispensable for cleaning up parts. Like most things, there are costs and benefits associated with abrasive blasting. I have a love/hate relationship with blasting. I love the way it cleans out crevices and the texture it leaves on the part surface. I'm also fascinated with blasting glass and mirrors. With some creative masking you can make multiple level frosted glass for use in furniture or other home projects. I hate the mess, the dust, the hassle getting and storing abrasive, and that the grit gets into every nook and cranny. (Including eyes, ears, nose, mouth, and hair.)
But there are times when it is the only game in town for cleaning up parts. A 4" wire brush on a grinder will take care of a lot of things, but any area with a radius less than 1 1/2" is usually out of reach. Using a wire brush cup will allow some additional access, but these still leave areas that blasting does best. Just like it will etch glass, it will remove mineral deposits on the inside of the radiator inlet and outlet pipes. On every Cub casting there are areas that you just can't reach with a typical powered wire brush.
Abrasive blasting requires a compressed air source, a container of abrasive grit, and a gun. How much air? It depends on the gun and the abrasive chosen. But plan on at least 5 cfm or more to get enough flow to do any good. What kind of abrasive grit? It depends on what you are blasting and the finish desired. This is one of those areas in which there is no substitute for commercial grit. Material removal is due to the cutting is the edge of the grit. Playbox sand is typically tumbled or river sand meaning that all the sharp edges have been knocked off. Surface preparation requires different media. You'll hear about shot blasting, or glass bead blasting but I suspect this is more in the vein of high-power automotive rather than basic Cub work. Your media choices include sand, slag, glass beads, walnut shells, silicon carbide, aluminum oxide, and others. The grit will range in size from fairly coarse to dust-like and is determined by the use. Guns come with different nozzle sizes, and made of different materials. Cheap guns will have a hardened steel nozzle, ceramic nozzles will be more expensive but last longer. Assuming you already a compressor, you can be in the "Blasting Business" for the price of dinner for 2 at a nice restaurant. I get my slag at Grainger's for about $25/80 pounds and have a cheap gun also bought at Grainger's about 30 years ago.
- Blasting Protection
If you decide to start blasting, you will need a place to do and protection while doing it. There are two approaches to blasting protection. Either wrap you in a protective environment or wrap the work in a protective environment. YOU MUST DO ONE OR THE OTHER. If you stand out in the side yard and start blasting, you will need, at a minimum, tight fitting goggles, a dust mask to cover your nose and mouth, ear plugs, a long sleeve shirt, long pants, gloves, and a roll of masking tape. The masking tape is used to seal the shirt cuffs to the gloves, the pant cuffs to your shoes, and whatever else will collect dust. As you can imagine this is a real PITA and you may be tempted to short corners. The first time you blow your nose after sand blasting, you want to re-think your position.
The other alternative, putting the work in a protective environment, is a much more attractive option. A sand blasting cabinet is a sealed box with rubber/cloth gloves, a gun hookup, and a viewing window. All the action takes place inside the cabinet so you can work in "shirt-sleeve" comfort. You can find blasting cabinets ranging in size from 15"x25"x12" up through 22"x24"x47" and beyond. You will need a cabinet large enough to handle the largest piece you will need to blast. But the luxury of a cabinet comes at a price. Expect to pay about $90 for the bench top size to $700 for the 2'x2'x4' cabinet. And larger cabinets are much more expensive, figure as much as you paid for your Cub to 2 or 3 times what your Cub cost. But there is an alternative to buying a blast cabinet.
- Basic Blasting Booth Requirements
A minimum blasting cabinet is
- a sealed box with one or more doors
- latching and sealed door
- a replaceable window to see what you are blasting
- gloves/sleeves to protect your hands
- air and grit hookups for the gun
- sump for the used grit
Additional options could include
- lights to help see the work better
- a system to keep the dust down
- material handling setups like a "lazy susan" or racks to hold parts
- a filter in the sump to separate dust from blast abrasive
- a reclamation system to allow reuse of blasting abrasive
- Blasting Booth Construction
When I started working on the Cub, I decided that I would need to do some blasting and figured that the hood was the largest piece the cabinet would need to hold. The idea of paying somebody $3000 for a metal box just rubbed me the wrong way. I started looking at the pieces in my piles "good stuff", and decided that I could build a blasting cabinet that had an interior volume of 24"x24"x60". I wanted to make this booth rust-proof since we live in the Sunny Soggy South. As a result you will notice that almost everything is either glass or aluminum. The few steel pieces are all galvanized, plated, or covered with caulk. What follows is my solution to building a blasting booth.
- Building the Box
I had a couple of frosted glass shower doors that came out of a bathroom remodel. (The wife decided they were too hard to keep clean.) Since I had a pair, they became the sides of the box. These shower doors have an aluminum frame that allowed them to be attached to outer storm door pieces including the hinge piece. The back wall is the glass panel from a storm door that was removed when I installed new doors in the house. This rear panel is held in place with acrylic latex caulking and a few screws just in case. Here is a sketch of the components of the booth.
The vertical corners are storm door frame pieces, and the horizontals are aluminum extrusions. Everything is either screwed together with self-tapping screws or glued together with adhesive caulk. Here is a shot of the completed booth from the left rear corner showing the side and rear panels, the sump bucket, and interior shelf supports. You can see the rear panel extends from the ground up to the top of the box.
The shower door and rear panel glass is tempered so they must be left full size. I had another storm door panel that I tried to cut. Trust me on this, unless you know the secrets of cutting tempered glass, use them in the "as found" condition. I ended up with a million little-bitty pieces of glass. The front door was cut down to be the same height as the shower doors and the same width as the rear panel. Here is a shot of the door showing the window insert, glove insert, and the 2 blank inserts. In addition, you can see the gravity fed grit hopper and latch location. Notice the pop rivets in the door corners holding the frame pieces together.
With the door open, you can see the interior workspace, the aluminum sump funnel floor, the wire shelf to put parts on, the latch location and in the door, the left sleeve/glove. The floor is made out of old aluminum siding pieced together and pop riveted. The shelf supports are pieces of aluminum angle screwed into the shower door aluminum frame.
All the extrusions used for sills and on the base came from some old jalousie window units I replaced in the house. Storm door frames could also be used for this purpose. The abrasive blasting gun I use is designed to siphon sand up from a bucket, but siphon feed is not as reliable as gravity feed. The hopper is a piece of 6" PVC pipe with an end cap glued on. The end cap is tapped for 1/2" NPT thread and a plastic hose adapter screwed into place. The hopper is connected to the gun via a 5/8" ID plastic hose. When it seems like I'm not removing material, I can squeeze the hose to know if there is grit in the system. If not, the hopper is empty. If the hose feels "solid" then I know I have a blockage in the gun. To fill the hopper, I use a plastic butter tub. About 4 tubs worth will fill the hopper and give me about 20 minutes of blasting time. Parts to be blasted are placed on the wire rack which is supported on both sides by aluminum angle screwed into the shower door frame. The wire rack is from the refrigerator we replaced a few years ago. Once the grit has done its job, it falls to the floor and funnels down into a 5 gallon plastic bucket. The next photo shows how the floor was constructed on pieces of aluminum siding left over from a remodel.
The siding was bent to rough shape, drilled for pop-rivets, and held in place by Clecos. Once all the drilling was done, the Clecos were removed one by one and replaced with pop-rivets. (Look for Clecos on ebay. They are used in aircraft sheet metal work to hold the skin on the ribs until the rivets can be set. Depending on your job, you may only need a dozen Clecos. If you do any sheetmetal work that are very handy to have.) Once the floor was riveted, all the seams were caulked and smoothed to promote free flow of the grit into the bucket.
We now have a vertical box with 3 sides and a floor. The door is cut down from the original storm door size to fit the booth. The frame has a flange against which the inserts seat. These inserts are designed so that they can be moved up and down in the door frame.
Arrangement A is for most items that fit on the wire rack, B is working in the top part of a long item, C is working on the bottom part of a long item, and D shows that both the window and the blanks can be changed in size if there is a need. Using changable inserts makes the most of the large volume in the booth. The inserts themselves happened to be what I had on hand, but they seem to work. The bottom blank insert is a piece of 1/2" foil-backed foam insulation board. The upper blank insert is a piece of aluminum siding cut to fit. All the blanks do is prevent the grit from leaving the box. They do not have to withstand any pressure and they are not in the path of the blasting gun. My window insert is a piece of lexan but a lexan/glass combination is preferable. The lexan will become etched from bouncing grit. After a while, it will be like looking through a dust cloud. The Mk.I Mod2 version of the window has a piece of window glass taped to the inside. The glass is easy to clean, and when it becomes etched, it can be replaced. The lexan will prevent any damage to you if a part were to fall against the glass and break it. The sleeve insert holds the sleeve/glove assembly so your hands and arms are protected against flying grit. (Trust me on this... You need to protect your skin from flying grit. Think of the flying grit as infinitely flexible 60 grit sandpaper. Now think of the sandpaper being vigorously applied to your skin. Kinda' like roadrash at zero miles per hour.) The sleeve insert is made out of a pair of old circuit boards. These boards are strong, light, and made of fiberglass. Arm holes were cut in the boards and the sleeve/glove assembly is sandwiched in between the boards and secured with stainless steel 4-40 hardware.
The sleeve glove assembly is composed of 2 shirt sleeves and an unlined leather glove. You will need 2 of these assemblies, one for each arm. I went to the Mission/Salvation Army/Charity Store got 2 shirts. Color is not important, but weight and a tight weave are. The sleeves were cut off the shirts about an inch above the shoulder seam. One of the sleeves is the inner and the other is the outer. The glove will be attached to the inner sleeve and be covered by the outer sleeve. In my booth, the outer sleeve is orange (shown above) and the inner sleeve is yellow (shown below). Here is a photo of the attachment of the glove to the inner sleeve.
I used pop rivets with washers on both sides to secure the glove to the inner sleeve. The sleeve material is on both sides of the glove cuff so the rivet has to be long enough for 2 washers, 2 layers of sleeve, and a layer of glove. I used about 8 pop rivets equally spaced around the glove cuff. The purpose is to secure the glove to the sleeve and prevent grit getting in between the glove and inner sleeve. Notice also that the location of the sleeve/glove attachment is not at the sleeve cuff. By having the inner sleeve extend almost to the glove fingers, the sleeve material tends to slow down and flying grit moving up the sleeve. The outer sleeve covers the inner sleeve/glove attachment and further reduces grit infiltration. The sleeves have to be long enough for your reach into the cabinet. An alternative for shirt sleeves might be a pair of Levi pant legs. Although the sleeves are tailored for arms and fit much better than pant legs.
- Sealing the Box
So far we have a box with a functioning door and a funnel-shaped hole in the bottom. We could leave it at that, but blasting grit is not smart enough to say within the box. And the blasting media is sensitive to moisture. When it gets damp/wet, it tends to clump. Needless to say, these clumps do not feed through the gun very well. We need to seal the box both to keep the grit in and moisture out. A roof will go a long way towards keeping out mother nature, this includes bugs, insects, bees, wasps, etc. as well as rain and other precipitation. I don't want to open the door and come face to face with a flight of angry wasps. (You may be more tolerant)
My roof was built with material on hand and your structure may be different. But the roof should be water-proof, dust-proof and tightly attached to the box. It may not windy today, but it will be windy some day. The covering I used is some more aluminum siding and the trusses are constructed from drip-edge roof trim leftover from the re-roof. The drip-edge cuts easily with a pair of tin snips and was folded into shape. The formed drip-edge was pop riveted to the horizontal drip-edge. With the truss assembled, the gable panel was pop riveted onto the truss. After both trusses were assembled, they were screwed to the top of the shower doors. Then, more siding was pieced together using folded lap joints and placed on the trusses. Finally, the roof covering and trusses were drilled and pop riveted. The photo below shows one of the gable ends of the roof.
This construction method is amazingly strong. We've had some serious storms since the booth was put into operation and the inside remained dry in the middle of some Texas "frog stranglers". Once the roof is attached, the last step is to every joint/seam/possible opening. Not to keep the water out, but to keep the grit in. After all, this is the reason we built the booth.
There you have the basics for building your own abrasive blasting booth. With the money you have saved you can buy another Cub!
- Further Modifications
Since nothing is ever perfect, I've been making modifications to my booth as I've noticed things that could be improved. The first thing is to use a lexan/glass combination for the viewing window. Then I noticed that there was a lot of reflection on the window making it hard to see the blasting. There are a couple of ways to attack this issue. We can put lights inside the booth and/or we can incline the viewing window about 15 degrees from vertical. (The reason that airfield control towers have tilted glass is to reduce reflections so the controllers don't "miss" anybody.) So far, I've put lights in, but I think that inclining the window will have greater effect.
One of the problems with abrasive blasting is that the liberated material is usually ground to dust before it flys free. A small amount of blasting has minimal effect, but a large amount of blasting or blasting rust frees a bunch of dust. Since the booth has been sealed to keep the grit in, this dust has nowhere to go until it settles. As long as the gun is blasting, the air inside the booth keeps getting stirred up and the dust will not settle until after you quit blasting. I possible solution is to put a fan in the overhead space. The inlet would be outside the booth and the overpressure should drive the dust down through the floor funnel. Using a fan to suck the air out of the booth leads to complications like filtering the grit from the dust. I'm still debating both approaches. If I come up with a solution, I'll update this page.
The wire refrigerator rack seems to work just fine for all the pieces I've done so far. Another possibility is to install adjustable shelf brackets. To reduce handling I considered putting in some kind of lazy susan arrangement, but these require bearing surfaces of some sort. And the grit is hell on moving parts. If the part were suspended using a piece of nylon rope, it would still be able to rotate. When the moving parts of the rope break down, just replace it. A little more constrained would be a piece of nylon webbing since it has a "natural" or neutral orientation but can still be rotated. So many possibilities.
Reclaiming the grit for reuse is turning out to be much more difficult than expected. I've built a cyclone separator, tried using fabric filters, used a fan and a long chamber to grade the grit. I've even tried water separation techniques. So far no joy. Perhaps using a fan to suck the dust out before it recombines with the grit is the best approach. And just about the only thing I have not tried yet. But the chance to do science is driving me on.
The Bottom Line: Total cost for the booth was less than $15. This includes 2 shirts, 2 tubes of caulk, a box of screws, 200 pop rivets. Your mileage may vary.
- TOP SECRET -- BURN BEFORE READING
Emergency Sand Blasting Enclosure
When you absolutely must do some sand blasting and the item is too large for your cabinet, or you don't have a cabinet yet, there is a field expedient Sand Blasting Enclosure. Get a clear/natural/see through 55 gallon plastic garbage bag. Put the part inside the bag. Put a glove on your gun hand, then put your gun hand and gun inside the bag. Secure the bag around your wrist. Taping the bag is usually the best way. (Cable ties tend to cut off the blood circulation to your hand) Once you are sealed up, get after it. DO NOT direct your spray towards the bag because the grit will eat through eventually. Once you are done, unwrap the bag and remove the blasted part.
Fri Sep 07, 2007 3:10 pm
that is a very nice set up
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