Fixing tight tolerance bore in Delrin

JP Machining · Sep 17, 2018

So a month ago I made some delrin parts with a 28mm bore something like 4" deep with a diameter tolerance of +/- 0.001". I am confident they were in spec when they left here, after having sat on the bench for a day and inspected again. Now a month later customer is saying they are 0.0009" under tolerance limit.

Ok, maybe I should of machined them right to the max tolerance anticipating some shrinkage, whatever. I've done similar before and they stayed put. Maybe the current high humidity around here caused it, I don't know

Question is how to enlarge the bore without messing them up, complicated part can really only hold on one end now, just worried about dialing the existing bore in dead nuts and then reboring without ruining finish too.

How bad would a reamer work? I assume ball sizing doesn't work in delrin. I dont think they would like lapping grit embedded in the bore either. Guessing I'm left with boring or reaming and just spending alot of time making sure each part is lined up perfect before starting

HuFlungDung · Sep 17, 2018

Is this a lathe part or a mill part?

JP Machining · Sep 17, 2018

Bore in question is a lathe setup

adama · Sep 17, 2018

You did not hear it from me, but a day in a warm dry environment should bring them back into spec! Return to customer in a vacuum sealed moisture proof bags and jobs a good one!

FredC · Sep 17, 2018

Deleted my whole post.
Your title said Delrin and the text on your post said Delrin. Not sure why I was thinking PEEK. You are a better man than me to measure these and be confident they were correct when they left. Others comments on humidity may be spot on as I have never held that kind of tolerance in Delrin.

JP Machining · Sep 17, 2018

adama said:
You did not hear it from me, but a day in a warm dry environment should bring them back into spec! Return to customer in a vacuum sealed moisture proof bags and jobs a good one!

That sounds like a been there done that answer...

On the plus side I do have an extra part sitting here on display shelf that is good so I have room to goof one if I do have to re bore.

Milland · Sep 17, 2018

Annoying as this is, it's a factor of not having an agreed upon temperature and humidity that the checking is done at. Not to mention that Delrin can change even when kept at a constant T/H.

I'd confirm what your shop temps are, if the part you have is still good then it's likely similar to time of manufacture. Get numbers from the customer as to their measuring and storage T/H, there may be a correction factor that can show the change is purely from that difference.

TDegenhart · Sep 17, 2018

Can't help with the current problem, but in the future it would be wise measure and record critical dimensions that could possible change after leaving the shop. Heat humidity, handling.... Allow the customer a reasonable time from to do their own QC if required, then no further guarantee.

Tom

Perry Harrington · Sep 17, 2018

Perhaps you should share pages 14 and 15 with your customer and then decide what the best course of action is: http://www2.dupont.com/Plastics/en_US/assets/downloads/design/DELDGe.pdf

HuFlungDung · Sep 17, 2018

It might be a PITA to make, but perhaps a round broach would be a way to fix these. It wouldn't need to be very long since it only needs a couple of teeth to skim cut the hole. Make the pilot end a very close fit (even a size on fit) so it follows the original bore axis.

Booze Daily · Sep 18, 2018

I had the same situation with some phenolic parts. Milled part, shallow c'bore, +/- .001.
They were rejected back as undersized. I stuck them in the microwave for a few seconds each (had to experiment with time)gaged them, and sent them back.

Winterfalke · Sep 18, 2018

Milland said:
Annoying as this is, it's a factor of not having an agreed upon temperature and humidity that the checking is done at. Not to mention that Delrin can change even when kept at a constant T/H.

I'd confirm what your shop temps are, if the part you have is still good then it's likely similar to time of manufacture. Get numbers from the customer as to their measuring and storage T/H, there may be a correction factor that can show the change is purely from that difference.

Rule #1 when parts that are not stable and have tight tolerances: make sure you are checking your parts in the same way that the customer will.

If they are not checking the parts in a climate controlled environment, and fixturing them in a way that ensures they are not distorting the parts, how are you going to get measurements that agree? Can they ever measure the part to the agreed upon tolerance? I have had more than one part that grew out of size in the field or distorted when clamped up, and at least one delrin part that grew out of tolerance after it left. We actually had to have one company send engineers out to check our CMM process because they were couldn't replicate our measurements, and I argued for months about a long thin aluminum part that the QC guy kept failing, even though he was letting the thing sit in the sunlight and grow out of spec. I told them it was stupid to have uncovered windows directly over the QC benches (in Florida no less) but the girls in the office wanted their thin drapes.

DMF_TomB · Sep 18, 2018

JP Machining said:
So a month ago I made some delrin parts with a 28mm bore something like 4" deep with a diameter tolerance of +/- 0.001". I am confident they were in spec when they left here, after having sat on the bench for a day and inspected again. Now a month later customer is saying they are 0.0009" under tolerance limit.

Ok, maybe I should of machined them right to the max tolerance anticipating some shrinkage, whatever. I've done similar before and they stayed put. Maybe the current high humidity around here caused it, I don't know

Question is how to enlarge the bore without messing them up, complicated part can really only hold on one end now, just worried about dialing the existing bore in dead nuts and then reboring without ruining finish too.

How bad would a reamer work? I assume ball sizing doesn't work in delrin. I dont think they would like lapping grit embedded in the bore either. Guessing I'm left with boring or reaming and just spending alot of time making sure each part is lined up perfect before starting

.
many plastics get smaller when they cool off after machining.
a 1.1" dia bore of acetal / delrin that gets 16 degrees F cooler gets
.00104 smaller this is besides any water or solvent evaporating from the
plastic.
.
reaming is not easy as plastics often expand when reamer in and contract when
reamer removed
.
sanding or grinding not easy as it can distort bore or give different sizes in spots but if you only need .0005 sometimes it works. most times its measured
with bore indicator or a plug gage. many a time a bore appears ok but when unchucked or it sits for some time it goes out of round and then a simple plug gage or pin wont go in. also a wavy bore or curved bore can do similar when pin or plug gage wont go in all the way.
.
adjustable expansion reamer often will open bore .001" if you can get correct size. most do not expand far. i cannot say if it will mess up finish

4GSR · Sep 18, 2018

400 grit emery cloth will bring it in tolerance easily! Just saying. It worked for me years ago.

Ken

guest · Sep 18, 2018

4GSR said:
400 grit emery cloth will bring it in tolerance easily! Just saying. It worked for me years ago.

Ken

A thou in plastic, a piece of scotch brite might even do the trick. Wind it around a boring bar and tie wrap it in place and program a light pass on the lathe.

Guest · Sep 18, 2018

What is the accepted default temp and humidity to measure parts at? What is the current ISO standard?

JP Machining · Sep 18, 2018

Theres lots of questions I see I will have to get answers to before I run these or similar again. Have a few ideas on how to fix these, thanks for that. I had originally told the customer that it was going to be a tricky bore. I'm also wondering what size the mating part is machined to as they didn't notice it being under till trying to assemble. Its a bigger place that has a QC department and I'm pretty sure knows what they are doing but there still are some things that need to be agreed upon I guess.

DMF_TomB · Sep 18, 2018

get yourself a plug gage. many a machinist measures a bore and swears its to size and yet a plug gage wont go in all the way when part unchucked. sometimes bore is wavy or out of round
.
many parts have to be assembled. if it needs to fit other parts then dont matter what somebody measures so much as it needs to be able to assemble with other parts
.
parts with bores going out of round is common. if chucking pressure is high a bore often will change shape as soon as its unchucked

JP Machining · Sep 18, 2018

Searching for a plug gage that size has come up short so far.

Wouldn't a 3 point bore mic be able to detect out or round as well?

DMF_TomB · Sep 18, 2018

JP Machining said:
Searching for a plug gage that size has come up short so far.

Wouldn't a 3 point bore mic be able to detect out or round as well?

you can make a plug gage if needed. steel gage not going to wear out fast measuring plastic
.
point of plug gage if bore is wavy and or not straight it will start to go in and then bind up stuck
.
many a reamed hole a pin gage starts and hole measures ok and you run 3 other brand new reamers by hand in the holes no problem BUT a long pin gage wont go in all the way. seen it many a time on like 10% of parts. kind of annoying. thats where having a .0005 and .001 oversize reamer is nice to have
.
hard to describe point is though most parts need to assemble. guy aint going to care you measured more than 100 times as ok if he cant hand assemble parts that are suppose to hand assemble (without a hammer) plug gage simulates assembly

Fixing tight tolerance bore in Delrin

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