Very slight improvements in vibratory deburring

My homebuilt vibratory deburrer is still a far cry from a commercial unit – I went to the local machine tool tradeshow this past week and gazed longingly at the big industrial units on display there…  until I found out the prices.  I’ve got maybe $200 into my homemade version (a really good scrounger would have been able to do it for far less, I’m sure), which is still a good order of magnitude cheaper than one of the biggies.  Still, it leaves a lot to be desired – among its deficiencies are poor media movement, extremely long cycle times (probably related issues), splashing water in ‘wet’ cycles, and acoustic exuberance (“noisy as all hell”).

I also don’t get much ‘scrubbing’ action on part surfaces – while burrs and sharp edges do get knocked down admirably (though needing a bit of time), surface scratches and tiny dings still remain and I can still see the tool marks of the lathe tool on Roundhead bodies.  I’m seeing if using a Scotch-Brite wheel on the bodies will help remove the dings and allow for a better surface finish in the deburrer.  Well, calling it a Scotch-Brite wheel is pretty generous at the moment – since I wanted to try something ‘right now’, I took a pair of 3M abrasive nylon pads, tore a hole through their centers, slapped them onto the shaft of my grinder/polisher and had a go at it.  Not too bad, though I’ll have to get a proper wheel soon, as the pads wore down quickly.  It remains to be seen if this Scotch-Brite pre-treatment will improve post-deburred surface finish.

As mentioned, one issue with the vibe deburrer is that it flings droplets of water upwards and outwards every so often when running wet deburring media.  I had naively thought “eh, it’s just water, and the basement floor drain is like a foot away” when originally making the deburrer.  I had intended to put a cover on it (honest), but there were no suitable covers for the laundry tubs I used.  So I ran it as is, which wasn’t too bad at first.  Sure, the supporting beam, chains and basement floor just got a light sprinkling, but the problem is with the suspended microscopic grit and removed metal suspended in the water droplets.  When the water evaporates, it leaves a gray coating of grime that leaves your hands with that sensation of “eyuugh” when you rub it between your fingers while running for the nearest sink.  I’m assuming it’s like dried slip, but worse.  So it was with this in mind that I created a cover.

Vibe deburrer lid

Nothing fancy, but it turned out nicely.  I used some cheapo plywood and 2″ thick foam insulation from Home Depot, cut into 24″ sqaures and then ‘assembled’ with some 3M spray adhesive (I doubled up the foam for 4″ of thickness).  I then drew a 20″ circle on the sandwich and ran it through the bandsaw at work (blade must have a massive weld on it – note the vertical lines around the perimeter of the foam caused by the weld that closes the loop of the bandsaw blade).  Topped off with a door handle, and beveled the edge of the foam a little to fit the tub.  Now the nasty grimy droplets will stay put (note my attempts to wipe away the gray grime from the edge of the tub).  I had hoped that perhaps the cover might abate the noise a bit, but no such luck.

The second improvement made regards the media flow in the tub.  The ceramic media that I deburr with mostly just spins around the tub, like the water in a draining sink (which really doesn’t provide any deburring action, and is wasted movement).  Beyond this, the media moves in a toroidal fashion – media moves downward around the outside edge of the tub, and also downward in the very center (naturally the media must come upwards between these two locations).  I mentioned in a previous post that parts tended to pool in the center, and long parts like the Roundhead bodies would stick halfway out of the center, contacting media on only one end.  To counter this, I figured it was a good idea to try to fill that center area with something else.  I filled a Gatorade bottle up with water, and placed it in the center.

Better media circulation

Incredibly, it worked!  I no longer had parts pooling in the center – the movement of the media kept the bottle in the center, and the actual parts were free to circulate.  This explains why all circular commercial deburrers I’ve seen have a cone or cylinder in the center of the tub, or a donut shaped bowl – the center is simply a dead zone in all circular tubs.  I’ll have to see about modifying a laundry tub with a bottle or PVC pipe in the center, and perhaps some ribs on the outside edges to impede media from swirling when it should be bouncing.

The third improvement for this post regards not the deburrer itself, but preparing the parts for deburring.  A problem that I had with the first batch of Roundhead bodies was with the threaded holes for the thread inserts that need to be screwed into place (think Heli-Coils, but better).  The outer end of the threaded hole was getting peened over by the ceramic media, and I had to use a tap to chase the threads.  This was less than ideal, as the threads are fine and very shallow, so I had to use a bottoming tap.  Trying to get a tap straight on a cylinder (even with existing threads) is not exactly easy, and I’ve ruined several bodies in the attempt.  One thing I discovered was that my thread milled threads weren’t actually large enough in diameter to accept the thread inserts, so I changed the the CNC program so that the thread milling operation was technically 0.006″ over the nominal 1/4″.  At least now the inserts fit in place without needing the threads chased, but the peening issue remained.

Peened top of thread
Peening of top thread edge due to deburring media. Brown particles and residue are the rouge treated walnut and corncob polishing media, though the peening is caused by the earlier used ceramic media.

The above is an admittedly poor photo, but you can sort of see how the top edge has been beaten down a little bit, squashing the thread.  I figured the best way to prevent this would be to use a plug of some sort in the threads.  Unfortunately, there are no set screws available in the 1/4-36 thread needed (that I could find, at any rate).  I guessed using the thread inserts themselves as a plug would be the best method, but the centers of the inserts (which have an 8-32 thread) needed a plug of some sort themselves.  An 8-32 set screw was the logical choice, but the trick was in securing the two together.  I considered using Loctite, but I wasn’t sure that even red Loctite would hold up to the torque of really cranking an insert into place.  Plus, I didn’t have any on hand.  I asked Frankie about soldering the set screws into place, and he said that for stainless, Stay-Silv 45 and black flux would be good to use.  However, he noted that on stainless, silver solder doesn’t ‘wick’ into joints very well.  This worried me, as the only way to really get a solder joint between a set screw and a thread insert was by getting the solder to wick between the two.  I tried heating a set screw and thread insert with a propane torch and applying some rosin core electronics solder to the joint, but the solder simply balled up and made no attempt at capillary action.  I figured I wouldn’t have much improved luck with the silver solder.  While idly torching the metal to a red hot state, I suddenly had a thought – what if I could forge the two together somehow?

The idea is simple enough – I threaded the insert onto the ‘head’ end of the set screw so that perhaps .010″ of set screw stuck out past the end of the thread insert.  I then heated the unit up to red hot while it sat on the anvil of my bench vise.  Once sufficiently heated, I smacked the tip of the set screw a few times with a hammer, squishing it slightly and binding it to the insert (while hopefully keeping the insert itself dimensionally unchanged).

Heating the ser screw and thread insert
Heating the set screw and thread insert - it doesn't actually glow as brightly as the picture makes it look, which leads me to think that the extra brightness is due to the camera's CCD sensitivity to IR radiation.
Set screw and insert on left, mashed version on right.
Set screw and insert on left, mashed version on right.

The resulting plugs seem to do the trick – I’ve inserted them into several bodies, though I still have to run them through the ceramic media.  The set screws on a few plugs will spin with enough torque, but a little more heat and hammer should fix that.

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