How to Hold and Make This?

Yes, your observation is correct. I did use a 1/2" endmill. That facing mill was homemade by someone, welding a 1/2" shank to a cutter that was cannibalized. I've never quite trusted it since overlapping cuts sometimes sometimes seem to leave a ridge. I may have to do a better job of making sure all is in alignment on the mill. I have flycutters, but never used since I do not know how to grind the hi-speed steel tool bit for use. I tried, but the cutting result was not so good. I can grind my own on the lathe, and they work OK there.

I will use the facing mill next time, and make sure the mill is dead nuts trammed.
 
Phil3 said:
I may have to do a better job of making sure all is in alignment on the mill.
Click to expand...

Phil3 said:
and make sure the mill is dead nuts trammed.
Click to expand...

That is absolutely required, but I guess was not mentioned as we were only talking about the order of cutting operations.
Without knowing the mill was in tram/alignment it would be very difficult to control thicknesses.

Would you be able to save the part, or are you already below your minimum thickness?

-brino
 
I felt the mill was trammed as well as I know how, but I suspect my methods are not so good.

I saved the part, as I left a tad extra. Ond dimension is a bit under, but hope it will be OK. I still do not really understand how my channel thickness was so far off. I could see slightly with my naked eye.
 
Phil3 said:
Yes, your observation is correct. I did use a 1/2" endmill. That facing mill was homemade by someone, welding a 1/2" shank to a cutter that was cannibalized. I've never quite trusted it since overlapping cuts sometimes sometimes seem to leave a ridge. I may have to do a better job of making sure all is in alignment on the mill. I have flycutters, but never used since I do not know how to grind the hi-speed steel tool bit for use. I tried, but the cutting result was not so good. I can grind my own on the lathe, and they work OK there.

I will use the facing mill next time, and make sure the mill is dead nuts trammed.
Click to expand...


When overlapping cuts leave a step it is almost always a tramming problem. If it was a chip under the part, the surface would still be flat, just not level.
The only other thing it can be is if the machine and or part holding fixture is flexing that much under the load of the cut giving the effect of out of tram.

Even with the welded face mill, if the weld was not square and or straight, it would still cut flat, It would just be only using only one of the cutting edges instead of all of them it would act more like a fly cutter.
 
OK, I will make sure the mill is trammed perfectly. Can you steer me to tools needed to best do this? Please note, my mill cannot hold a shank larger than 1/2" diameter, and the table is only 5.125" deep. I have dial and test indicators, but not a good fixture for sweeping the table.
 
Can you map the thickness variation you are getting? That could provide some clues as to what's going on. For instance, if it's repetitive and has a period that is 1/2" (or whatever the displacement was you used for each pass) it is pretty likely to be tram related. If the variation is something else, the problem probably is something else...
 
For tramming you can use a piece of 1/4 dia wire with one end in the chuck and bend the other end to get your indicator as far out as you need it. To help with the Y you can add a known flat plate to extend the table.
 
Phil3 said:
Yes, your observation is correct. I did use a 1/2" endmill.
I will use the facing mill next time, and make sure the mill is dead nuts trammed.
Click to expand...
Tramming beforehand is often a good idea, but (if you aren't in a hurry) the usual way I'd approach
a flattening operation, is with a fly cutter. One cutting edge, you can touch up with a stone as needed...
and a few feeler gages plus a fly cutter is an easy way to check for tilt(s).
 
For tramming your mill you say you don't have the right tools but you do have a dial indicator, and you also said you have fly cutter, can you clamp your dial ind. to the fly cutter and use that to sweep the table?
 
Like all materials, Delrin can warp when you relieve internal stresses by facing. The 1/4" thickness which results from cutting the channel is fairly flexible. You didn't say which direction the .030" went, but I suspect the it thinned by the .030" in the center. This would happen if the Delrin lifted in the center.

One possible solution would be to use siper glue to hold the part to a backing plate. The plate should be flat and and thick as practical and preferably aluminum as wood can deflect under clamping pressures. To ease removal of the part, a layer of painter's tape can be applied to the aluminum and the superglue applied to it and the workpiece attached with some pressure to ensure good contact. John Saunders (NYCCNC) has some good You Tube videos on the process. The clamps will still be needed to prevent the workpiece from shifting. The purpose of the glue is to prevent lifting of the part. Super glue can be removed from the part with acetone which will not affect the Delrin. An alternative to gluing is to use double sided tape. I have used that to hold down 1/16" printed circuit board while milling.

A wacky way to do it is to freeze the part to a backing plate with water. I did that once when I was trying to mill some epoxy/phenolic composite material and successfully made pieces of .010" thickness. The final piece was thin enough to read print through it. A large piece would need a refrigerated plate but ice at -10ºF is a great adhesive. When we got into production of the part, we used a vacuum plate which worked well enough to produce parts with thickness variation of a few tenths of thousandths.
 
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