how to clamp and machine this part (UMHW)

[HEAVY METAL]

Stock is 4 inch thick by 17.5 by 25 . Part finishes at 3.75 by 15.9 by 20. I know it is going to relieve after taking off all that material. Does not matter what side I start with it is going to be hard to hold to clamp . If i have to flip it a couple times it will compound the problem. Anyone have a workingholding  soilution idea or process order.

Thanks

[HEAVY METAL]

cant seem to get a piture to upload.

[The Cathedral]

We do quite a bit of plastic work and using dovetail fixtures helps a lot. As long as you keep the dovetail cut properly, all the clamping pressure is in and down on the dovetail, leaving the material above in a free state, so it doesn't spring when you unclamp. That's about all the advice I can give without a picture.

[HEAVY METAL]

I posted a file and a picture top FTP X8 folder UMHW is the name

[EX-wccprogrammer]

What type of material?  Quantity?

[CJep]

Plastic?

Double Back tape.

 

Did that all the time workin' with delrin.

Clean it real well with alcohol, don't use coolant, air works well for clearing the chips 5psi,.. 

[EX-wccprogrammer]

If it is metallic, and you have a low quantity, you could bond rails on it with epoxy resin.  Kind of slow, but very good at holding difficult geometry.

[djstedman]

The following is how I would do approach it to maintain the best flatness .. I am assuming since there is no pic your just looking for a rectangular cube given the dimensions above..

 

My thought is that it is easily big enough over finish size that you could mill a slot around the outside using a t cutter to allow clamping on while still being able to cut the entire top.. you want to the entire bottom surface supported .. especially under your clamps since plastic will deform easily.

From the times I have had to cut plastic sheet I think your likely going to have to cut equal amounts off either side in order to bring in thickness, I could be wrong but I think if you attempt to take it all off of one side you will end up with a part warped worse than a potato chip when your done.

 

If the stock is supposed to be 4 inch it will probably come in about 4.04 so you would have like .290 total to remove from thickness..

For best results you probably want to cut both sides a few times taking progressively less each time.. like .07 per side then like .045, then .020 and finally finish with a pass of .01

 

So.. cut .07 off side 1 then .07 off side 2 then .045 off side 1 then .045 off side 2 etc etc..  

 

Once you have it to thickness you can then cut the excess off the sides without much concern for distortion.. if you have access to a table saw to get it close its actual a great way to trim fairly close to size.. otherwise vertical bandsaw then mill ..

[The Cathedral]

Heres is picture

 

 

[djstedman]

That's just ugly.. lol.. anyhow very basic idea of what I would try is.. I would probably mill one side leaving stock and leave some tabs you can hold onto it with, then mill the other side.. holding onto tabs, work back and forth until its done then last remove tabs.. would require some fixturing though..

[HEAVY METAL]

Thanks for the replies. I have thought about leaving tabs. Just wanted  to bounce it off everyone. I might look into the tape maybe for the light cut finish work. I agree this is a ugly part.

[mc since v3]

The link gives stress relieving info for UHMW. I worked at a shop where this was a common practice.

 

 

http://www.garlandmfg.com/plastics/annealing.html

[greenxman]

  i am sorry to hear you have to work with that material.

 

i would machine it and get qc to sign it off asap. because the next day it will not be even close to what you started. that stuff moves every day by temp

i hate working with it.