# Machining Delrin



## Mosey (Sep 20, 2012)

I am milling some black delrin pieces and am experiencing growth of the part after machining. That is to say, the holes I mill and drill are smaller that the tool that cuts them. Is this due to spring back of the material, or does it grow afterwards?


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## old-and-broken (Sep 20, 2012)

I know it can warp when a hole is drilled near an edge.  The difference in material thicknesses around the hole are responsible.  Large parts are most likely to do this, and very small parts are fairly stable.


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## sunworksco (Sep 21, 2012)

I use Graupner u-joints, if you are not scratch-building u-joints.


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## Mosey (Sep 21, 2012)

Obviously, I didn't explain my question well. What I want to know is if anyone has experience machining Delrin, and particularly with dimensional accuracy approaches.


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## Bluechip (Sep 21, 2012)

Mosey said:


> I am milling some black delrin pieces and am experiencing growth of the part after machining. That is to say, the holes I mill and drill are smaller that the tool that cuts them. Is this due to spring back of the material, or does it grow afterwards?


 
I have had a similar experience, not with black Delrin but just generic poly-acetal bar, when reaming it.

Drilled some 18mm OD 11.5mm and reamed 12mm in a ER25 collet.

No hope of getting 12mm bar through, so I reamed again and it took a fine shaving out and was OK.

Brand new UK made reamer, works well to size on brass or steel.

No idea why, did 4 bushes, all did exactly the same.

Just touchingly grateful they did all fit correctly in the end.

Sorry Mosey, no help really, but you're not on your own ... 

Dave BC


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## maverick (Sep 21, 2012)

Mosey,
Delrin seems to "wander" a bit, Sharp tools, good chip removal and a flood of water soluble coolant are the best bet. Excessive RPM will also cause problems due to heat build up. For many plastics, I use dedicated cutters
that have never touched metal, not even brass or aly.

Best of luck,
Maverick


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## Darren English (Sep 21, 2012)

I cut a lot of Delrin at work. It does have a tendancy to spring back after drilling. Either use loads of coolant or take a spring cut (pass the drill through again) this might work!
 When i need to keep tight tolarnces with any plastic I will only use boring bars and not drills or reamers when turning for the reasons you've found out.
 When milling I interpolate the hole, this is easy enough when using CNC but probably not much help for you.

Back to your original question it will may grow back slightly as the material cools down but will probably still be undersize.

I hope this helps in some way.


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## Mosey (Sep 21, 2012)

Yup, it helped a lot. Cancelled the Shrink appointment (no pun intended).
I found reference on the Professional Machinists forum to growth of Delrin 24 hours after machining.
Now I have to anticipate oversizing my cuts to fit.


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## cwelkie (Sep 23, 2012)

Your findings on the other forum are consistent with my experience ... for what it's worth.
When machining delrin for use as go/no-go gauging on automatic inspection machines the only way to get to the +0/-0.001" tolerances was to "let it rest" at least over night.  A final finish cut could then be taken that would hold to tolerance for the life of the gauge (at operating temperature).  We were usually taking 1 1/4" round stock down to the 0.6" to 0.75" range and left 3/4 thou for the finish cut.
Cheers
Charlie


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## Mosey (Sep 29, 2012)

According to Dupont, Delrin must be allowed to rest for about 24 hours after initial machining, and then the last finish cut (0.001") to precise dimension can be taken. Very sharp tools, lots of coolant, and temperature limits are recommended. Don't you just love to flycut the stuff?
Thanks to all for help given.


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## Bastelmike (Oct 3, 2012)

Mosey said:


> According to Dupont, Delrin must be allowed to rest for about 24 hours after initial machining, and then the last finish cut (0.001") to precise dimension can be taken.


This special issue for Delrin left aside, my finding is that all soft elastic materials tend to be undersize after cutting. Plastic, rubber, wood...

Probably the materials are so soft that some deformation occurs during cutting and a bit of the material escapes from the cutter. Sharp tools, cutting speed high enough for acceptable finish but not too high producing too much heat is helpful. Some materials like also lots of coolant.

Mike


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## terrywerm (Oct 3, 2012)

I used to make quite a few parts from Delrin and other similar UHMW products. In most cases there was enough tolerance in the size of the finished parts that a couple of cleanup passes was all it took to get to spec size, but in cases where tighter tolerances were required, the overnite rest period was the way to go, but as others have said, sharp tools and high cutting speeds were a big help also.  Over time I also found that in most cases the multiple cleanup passes were all that was required and I could usually hold pretty decent tolerances without the rest period. 

Machining rubber was an entirely different subject, however, and could darn near drive a person to drinking.....    heavily......    for extended periods!


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