Inexpensive Blast Cabinet Mods



To help with the rebuilding of my old Triumph Motorcycle, I bought this small benchtop blast cabinets from Tractor Supply.  It was around $100, and is similar to one offered by Harbor Freight, but has the chamber door on the top rather than on the side.
 


 [Click pictures to get a bigger view.  Except this one]




While the cabinet worked passably well, I soon discovered a number of annoyances.  First, the cabinet leaked media.  I was using glass beads, and after using it for half an hour, there was media everywhere on he bench and the floor.  I know now that glass beads on the floor behave a lot like millions of tiny ball bearings when someone steps on them.

Second, the fluorescent lignt in the cabinet is worthless--it was sometimes hard to tell whether it was on or off.  It was also towards the back of the cabinet, putting the front of larger pieces in shadow.

Third, the bottom of the cabinet has two sloping sides to form a trough for the media to collect in. The pickup tube lays in the bottom of the trough.  After blasting for a while, the media tends to collect at the end of the trough away form the pickup end of the tube, causing the tube to suck air.  When this happens, you can't just scoop the media over to the other side because of the mesh screen.  I resorted to tilting the entire cabinet to move the media to the other side.

Fourth, after using the unit for a while blasting flaky rust, the blast nozzle started getting clogged with big flakes of rust.

Lastly, the interior of the cabinet got very dusty after a few minutes.  Some sort of vacuum would help clear the dust for better visibility.


I finally reserved a weekend to improve the performance of the cabinet.

The first thing I did was to take the entire cabinet apart and reassemble it with gaskets and/or RTV to attempt to make it media-tight.

The next thing was to improve the lighting.  I had a set of five 20-watt halogen under cabinet lights that I never used on a remodeling project, and decided to use three of them.  I placed one light in the center of the front wall, just under the lid latch.  The other two lights went on custom corner brackets that aimed the light toward the center of the blasting space.

    


These lights are 120 VAC, so no transformer was needed, but for safety, I put the wiring in flexible conduit.  The conduit connected to an internal box that hooked to the original exterior light switch.




Next up was to improve the way the media collects at the bottom of the cabinet.  Larger floor model cabinets have a four-sided inverted-pyrimidal hopper that seemed a better shape to keep the feed tube supplied with beads.  I cut two triangular pieces of 1/8" hardboard and fastened them in place with Gorilla Tape.



Next, to keep the grit cleaner, I decided to put a screen under the support mesh to keep back the larger crud.  There is a layer of 1-1/4-inch foam tape on the ledge around the perimiter of the box.  On top of that is a 3/4-inch wide foam tape.  I made a framed screen out of standard window screen materials, sized so that it fits snugly on top of the 1-inch foam, inside the narrower tape.  For the pass-through for the media feed hose, I fortified one corner with several layers of Gorilla Tape, then cut a hole for a rubber grommet.





Then made a new, stouter mesh floor out of expanded steel.  Edged it with folded over Gorilla Tape to softent he sharp edges.




A couple of other fittings were an adaptor on the side vent to accept a vacuum hose, and a pressure regulator at the air inlet.




After all of this, the cabinet was much more usable, but I had a little trouble with the media feed being intermittent sometimes.  After looking closely at the feed tube, it occured to me that the double wall tube was that way for a reason.  The only thing I could think of was that the annular space between the two tubes must be a vent to bring air down, either to replace the volume of media sucked up the feed tube, or to assist the flow of media by supplying air for an airstream up the feed tube.  This was confusing, since the configuration before I did anything to the cabinet had the entire feed tube buried inthe media. Possibly this explains why intermittent feed is a common complaint for these units. My feed tube ran up the corner at the edge of one of the new hardboard baffles, but the vent opening was often submerged under media in the corner of the box.  To test my theory, I made a new feed tube arrangement that would ensure that the top end of the vent tube would always be above the media.  It is just two pieces of copper tubing soldered side by side--one 1/2" for the media, and one 3/8" for the vent.  The top ends pass through the grommet where the feed hose for the gun connects to the media tube, and the vent tube is in free air.  This seems to have improved the media feed to the gun.



The only remaining problem was that my large shop vac is too powerful for the exhaust.  It creates enough vacuum inside the cabinet to inflate the gloves like baloons.  The problem was that the existing filtered port on the cabinet was really designed to be an exhaust for the blasting air.  With a lot of air being pulled by the shop vac, the port becomes an inlet, and it's not big enough to supply air for the vacuum, especially with a filter on it, so the pressure in the cabinet drops drastically.  While low pressure in the cabinet is good for keeping the door seal tight, I had too much of a good thing.

Thinking that my main shop vac was overkill anyway, I bought a small vac to dedicate to the cabinet, and also put a speed control on it so I could control the process.  With this setup, I can keep the cabinet clear while maintaining a reasonable negative pressure in the cabinet.  



Right now there is no good way for me to exhaust the vac to the outside, so I'm looking into what filter media I can put in the vac.


Comments to:  elhollin1@yahoo.com