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Help fixturing tiny, brittle parts?


Nov 1, 2004
Today, I am making some small parts, little toroidal cores, out of some rather brittle material.
They are under .12" in diameter, and are done except they need to be faced off to thickness.

Making them so far was easy, I profiled them out with a 1/32" end mill, then cut them a little long with a 1" .014" slitting saw, almost through. Then I could carefuly snap them off of the parent material.

Grabbing these tiny little donuts has me stumped, though.
I starting with making a bore in a set of soft jaws, and clamping onto them, no matter how gently I try to close down on them, they shatter into pieces as soon as the jaws touch them.
I was planning to nest them into the pocket, and face them off, again with the 1/32" end mill, very slowly, and carefully.

I considered using as skived strip from a rubber band as a facing for a "rubber collet" but I don't know if that will flex, and twist the parts out of spec for the facing operation.

Maybe an open pocket, .125" in diameter, where the part can "float" and then using a sanding disk to face them off...
Don't know if that will work, or rattle the parts into shattering.

Again, this is some brittle stuff to work with, I can't give Any more detail than that, I really can't.

I don't want to run too many experiments, the material is expensive, and I couldn't get any more in time for the delivery date anyway.

Any ideas? Or maybe this is just interesting reading this morning...
In situations like you have described, I've had the best results from vacuum fixturing. You would need to have a rotary union to run vacuum on your lathe but you could make a vacuum plate fixture for your mill and face multiple pieces in one go. No vacuum pump? In a pinch, double sided tape is surprisingly consistent in thickness. Tricky part is getting your part off the tape without breaking it ;)
No way you could do anything with a vacuum. It is a toroid, and under .120 in diameter. very, very, little serface area. All of my machines even have connections to a central vacuum system, I use it that often, for other parts.

Tape? I don't know if they have enough surface area for that either.
And the material is Hard, and brittle. Recomended SFM is 90, using carbide.

A 10X to 30X microscope is what I use to look at these parts. To give a better perspective.

An interesting(?) project...
I am not sure of your tolerance but I would head the direction of plastic vise jaws with a drill hole right at the intersection of the jaws. If you drill pretty close to the diameter of the part, as you clamp, you should get just enough bite to hold them. This also should help prevent over torquing of the vise to some degree.

Also consider chucking them in a .125 collet and running them over a cutting tool. Depends totally on the how many we are talking here. 5-20 of them, That would work well but I would go the direction of the vise jaws first. We have had good luck with Nylons fixturing tiny parts.
I have had good results "potting" small parts in woods metal, an alloy that melts below the temperature of boiling water. In my case I was making small aluminum parts which I profiled into an aluminum plate leaving a "floor" at the bottom with the parts still attached. Then I poured in the melted alloy until the parts were almost submerged. Next I put the plate upside down back into the milling vise and faced off the "floor" separating the parts from the aluminum but still potted in place by the alloy. Last thing was to put the plate and it's little cargo of tightly potted in parts into a pot of boiling water and voila, a bunch of little finished parts. And the woods metal is salvaged for the next job. It sounds like your parts are all separate. If they are denser than wood's metal, then perhaps you could just mill a shallow dish with a flat bottom out of aluminum, stand your parts in the dish on the finished ends, pour in the metal, and face them all off at once to length. If they are less dense than woods metal then this idea won't work as they will just float to the top. I hope you can find one or two useful inspirations in this story. Your problem sounds like a goodie.
A flat plate with #400 wet or dry. 1" diameter aluminum disk with .06 deep pockets in a qty 8 bolt circle. Lightly sand them? Kind of a lapping process.



IMHO suface area is a relative thing. If you have a .120 diameter donut with a .04 hole (I'm guessing) and you create a matching donut nest groove with as many .015 vacuum holes as you can make you will easily have enough surface area to hold the part for a light facing cut. I'd be skeptical of the tape thing too if I hadn't seen a guy do it.
Another alternative is if you can control the outside diameter to +/-.0003 and have a 5c spindle, you can use an emergencey collet with the pins in the collet slots and control the gripping force by changing the diameter of the pins.
You could use Mitee-Grip from http://miteebite.com/products/mitee_grip_e.html.

Make yourself a little aluminum plate with wells for each toroid and walls around the perimeter to prevent the Mitee-Grip from flowing out when heated. Put the toroids into the wells, sprinkle some Mitee-Grip shavings in-between, place a weight on top of the toroids (which will force them flat against the bottom of the wells), and put the lot into the oven or onto a hotplate. Let it cool and bring it to the mill. After you're done, remove the toroids with heat.
Miteegrip looks like something I would like to try. After removing the parts with heat I presume some sort of film remains on the parts. Can this be dissolved away with some common solvent like acetone? I tried hot melt glue (sticky wax) once and it worked fine but I never found the right solvent to get all the residue off my finished parts.
Two othr suggestions, Mitee-Grip has been covered. You can use Super Glue, it dissolves after a soak in acetone. You can also use dop wax aka diamond setters wax. It's cheap & sold thru lapidary suppliers, you have to use heat to melt it, it's very hard & brittle, not like wax in the normal sense of the word. Warm whatever plate you are using to fixture the toroids to, rub the dop wax stick on the hot plate to get a good thick syrup like coat, kinda like melting butter in a skillet. Position the toroids on the fixture plate, warm them with a heat gun while keeping the plate warm until you see a meniscus form. Allow it all to cool, machine, & soak in denatured alcohol to remove the dop wax.
I usually remove Mitee-Grip with WD-40 and then a degreaser to remove the WD-40 residue. Base cleaner (used for removing old wax from the bases of snowboards and skis before waxing) might work well as a one-shot solution. I've been meaning to give it a try, but I keep putting off digging out the bottle in my waxing kit. Whenever I need to remove residue, the WD-40 is right there...

I've found that paraffin wax doesn't hold as well as Mitee-Grip, however I have used it successfully.
Brass Collet

Here's how I'd try to hold them. Make a brass "collet" out of stock using a theaded brass ring, fine pitch, as the closer. Slit the brass collet, de-burr, put ring on to bore collet to diameter. Then you control how much to tighten by finger, not the closer. You can even get fancy and have a spacer behind the ring to the collet shoulder when boring to size and replace it with one that is .001 less to clamp on the part. Just a thought.

Quick update. Then I have to run.

V Block would crack them.
Finished length (thickness) is .060"

They have to done tommarow (!)
I don't think I can use glue, then acetone for example. I don't think wax will work.
Well it will work great but, I have serious questions about cleaning these parts up afterward.
No heat, no chemicals, AFAIK.

I made a pocket where the part could nestle in one side.
I sliced an .030" wide strip, .045" thick from a rubber band.
Even using a razor blade, it was interesting. I laid the rubber band onto a strip of sandpaper to be able to "slice" it, without it just stretching.

Teh pocket had .035" clearance for the rubber band segment.
I got a dot of superglue onto a small wire, and spotted a tiny drop into the pocket.
Luckily, the rubber band fragment fell into place.

Now, I could grip the part with the rubber band having a max of .010" crush.
( The pocket was split across two soft jaws )
It grabbed it nice, I clamped onto it, then held it down and backed off the jaws, then clamped again, while holding down, to make sure that the part was sitting perfectly flat, and not squeezing up from the rubber band.

It held perfectly, and it was faced off......
Except that it chipped slightly from the End mill.

A new tool path, so that the cutter was pushing into the part as it went around, eliminated the chipping. Took .002" DOC max. at 4 IPM, 90 SFM.

It looks like it is working.

Now, how to clean the parts?
Under a 30X scope, they can accumulate some grit from cutting, and blowing them off with air is a little violent, since my fingers can barely grip them.

Should work out OK.

Lots of great ideas here, for anybody with some small parts to fight with.

Thanks again.