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Checking tight nylon bores?

DeadlyToad

Plastic
Joined
Feb 3, 2018
I have a tolerance of +/-.0005 on dark nylon casting. Temperature in the building seems to be pretty stable. I setup gage blocks for a bore gage and ran a part to size, then I used the CMM to check my bore and I am getting a variance of a .001 between the gage blocks and the cmm. Gage blocks have been verified on the cmm and a height gage, my guess is the spring loading on the bore gage to metal jo blocks. I was just going to set my bore gage up to the part and using it as a ring master. How would you guys check tight tolerance nylon casting bores?
 
What size are the bores? How deep? How big is the part? Are you needing to check taper? Circularity?

Your temperature can't "seem to be pretty stable", you have to know it's stable at 68º, same for your humidity since Nylon is hygroscopic.

We make small plastic stuff, tight tolerance bores are never larger than .098" so this may be very different than your situation, but we find non contact measurement (video) is best in the Ø.0005" range and for any location measurements, and when tolerances get tighter, our only option is .0001" increment gage pins. It takes a LOT of practice to get the feel right since you can pretty much shove any gage pin you want in the hole and call it good.
 
ø1.6583 +/-.0005 .500 deep. I don't have video measurement system. I know even if you had the best shop in the world you are still going to see the nylon move.
 
I would be concerned about the probe tips of a bore gage deforming a soft material like nylon. I'm thinking gage pin, but it too is going to be very sensitive as to feel. Matt hit a lot of the worry parts regarding nylon in general. but to be honest, you need to have a conversation with your customer. How do they plan to measure them etc. I would definitely ask if they feel that tolerance is realistic for their intended application.
 
That is what I am thinking do is the probe tip is deforming the nylon. I made the part itself into a ring gage after measuring on the CMM and it seems to be repeating. You know after a day it will change.
 
The bore size and part size have a great deal to do with how the part will change in absolute values on the dimensions. As others have noted already, have some discussion with your customer. If you are running these parts with a water-soluble coolant, they will likely be close to their maximum water absorption (~10% by weight). If they sit in a dehumidified QA room or a hot dry warehouse for a few days, your tolerance is likely done for.

I would use pin gagaes as well for this.
 
Expensive in time and money, but if I could afford it, I would let the parts stablize in controlled environment for 48 hours, then use an air gage. No contact force issues, and with appropriate sensor you can get more than basic diameter,
 
yes, op answered the question himself: " I know even if you had the best shop in the world you are still going to see the nylon move." nylon will move for extended periods of time for some wierd reason i dont understand, even if temp. has long equalized and humidity didnt change. it took me 3 days to get a firm sliding fit on a few bushings.

i wonder what procedure was used to get the parts on size in the first place. lots of coolant?
 
I have a tolerance of +/-.0005 on dark nylon casting. Temperature in the building seems to be pretty stable. I setup gage blocks for a bore gage and ran a part to size, then I used the CMM to check my bore and I am getting a variance of a .001 between the gage blocks and the cmm. Gage blocks have been verified on the cmm and a height gage, my guess is the spring loading on the bore gage to metal jo blocks. I was just going to set my bore gage up to the part and using it as a ring master. How would you guys check tight tolerance nylon casting bores?
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i made some nylon wheels about 4" dia and i press in a bearing about 1.6" dia with .001 interference fit and put 1/2 dozen of the wheels in a drawer.
...... a year later i grab a wheel and bearing falls out. i check rest and all the same bearing bore bigger no longer .001 press fit. nylon dries out and goes to center of mass bores get bigger and outside dia gets smaller.
 
my point is this might happen overnight with humidity not having changed and having machined them dry.
 
i had 6" round nylon on a lathe drilling 1.5" hole down the center and when i broke through it cracked radially like a wood log. tried another piece same thing. as nylon get wet and dries its like a piece of wood warping shape.
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really nylon not the most stable material any more than milling a piece of wood to .0005" and being surprised the size changes over time or the part warps just sitting on a bench
 
Knowing the diameter would help but as suggested contact your customer and ask:

1. How will you yourselves measure/inspect it?

First thought that went through my mind when I read the first post...

2. Why the stupid tolerance? OK you might have to be more diplomatic than that :)

I'm wondering about the practicality of such a tight tolerance. It's plastic for cryin' out loud; get the hole anywhere within
five thou and the mating part will "fit". No matter how close you get it today it's going to be a different size in a different
location on a different day...
 
i read somewhere nylon changes size up to .010" per inch from fully wet to fully dry
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so 1.6" bore would change .016" as it dries. obviously normally see less than this as most dont keep nylon under water for weeks then machine then wait months for it to dry out. still nylon does change size from water and temperature changes quite a lot
 








 
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