what is my tolerance on this part?

[cherokeechief79]

I made some square internally threaded nuts recently that got rejected.the threads were checked on a gage.

the thread is 3-1/2 16 N3.the id tol for the minor is 3.432 to 3.44.

they say the id is out of round but the most I can measure out is .003.

I thredmilled these so the thread will be the same out as the id bore as they were done at the same time.

what is the criteria for rejecting a part like this?

what is the max out of round something like this could go (and still work fine on the go no go gages)?

[Guest]

From everything that I have ever seen, if it is not called out specifically on the print or in some work specs that a company may issue on quality, typically round within tolerance is acceptable.

 

Naturally, you try and do better but some jobs just wrestle with you all the way,

[Newbeeee™]

+1 to John.

What I would add is to re-iterate the functional check - ie if it acceptable to gauge it is good to go.

[djstedman]

The allowable tolerance at the pitch diameter on a 3 1/2-16 thread from the Machinery handbook is 3.4594-3.4669 that's a range of .0075 .. so unless otherwise specified would mean that in this case roundness should be a function of allowable size.. so the maximum allowable measurable out of round on an indicator would be .00375  however this would only be the case with the maximum allowable diameter and and the minimum allowable diameter.  In reality what you would be looking for is for the entire pitch diameter to fall within the zone between the two diameters.

 

Since a thread gage is really a go and a no go gage, the go is checking to see the thread isn't too small, and the no-go is checking to verify the thread isn't too big, they should be checking to verify that the thread size falls within the .0075 allowable tolerance shown above.

 

There could be something in a spec or something that says different in regards to an allowable out of roundness, if it were me I would ask to see the dimension/spec that controls roundness on the thread.. although if its the customer's inspector rather than an in house inspector it becomes a question of whether you want to argue with a customer or not.

 

(And apparently while I was writing this novel .. I could have just typed.. yeah what John said.. )

Edited April 10, 2015 by djstedman

[cherokeechief79]

thanks john,

I couldn't find anything in the machinery handbook.

what would "typically round within tolerance be"?

is that the min and max difference?

if so would that be the id min max or the pitch min max difference?

[Guest]

thanks john,

I couldn't find anything in the machinery handbook.

what would "typically round within tolerance be"?

is that the min and max difference?

if so would that be the id min max or the pitch min max difference?

 

Yup if you have say .500 +/-.002  then high could be .502, low .498

 

Anything out of that, reject

 

Obviously, the tightest dimension would be most important but all dimensions would have to fall in at some point along the way

 

lol'ing @ djstedman  

[cherokeechief79]

thanks for all of the replies! several of you answered me as I was writing my last post!

[cherokeechief79]

there is only the mating part which checks out to be within the class 3 fit.

they took my minor dia at 2 places 90 deg apart and recorded the size. I had a couple that went .001 under the min in one direction only but ok the other direction.

then they molded the thread form at the places they checked the minor at .with the thread form now in plastic putty they put it on the comparator and figured out the pitch dia by looking at the highest point on the mold and added it to the minor they found.

 

I was wrong in my 1st post on this.

there are no gages,just the mating part which is easier to measure and is within the class 3 limits.

[jlw™]

All the above being said I'm curious... does the mating part fit?

 

IMHO 2 points on a diameter check is weak. I would think any modern machine tool with good tooling would nail the roundness.

[taperlength]

Maybe the part was distorted while the thread was being milled.  Vise too tight?

[Guest]

Maybe the part was distorted while the thread was being milled. Vise too tight?

This ^^^^^

 

It doesn't take much pressure to do that. On parts where this may be an issue, in the past I've put an indicator on my part while I clamp it to make sure things don't go out of round/square. It's an extra step but when you have to be delicate, you need to know exactly how delicate to be.

[Newbeeee™]

I remember working out years ago vice forces with an Abwood 6" machine vice.

By memory (and I'm talking around 1995) 20ib's of force on the handle = 2 tons of clamping pressure. 

 

[Guest]

I remember working out years ago vice forces with an Abwood 6" machine vice.

By memory (and I'm talking around 1995) 20ib's of force on the handle = 2 tons of clamping pressure. 

 

and I bet we have ALL watched someone beat that vise handle down with a hammer too  

[The Cathedral]

That's why we (try) to use torque wrenches on our vises. Leaving it up to touch introduces too much uncertainty, especially when it's multiple operators over several shifts.

[cherokeechief79]

ok but it measures .003 different in 2 directions.still both within the minor range.

the out of round tol is "0"

if I had a no and go gage I bet there would not even be an issue.

do you think that measuring the minor and then making a plastic impression of the area you measured ,putting it on a comparator and then adding those 3 values together would give you an accurate enough measurement to put that down as "the actual pitch dia"?

[Müřlıń®]

ok but it measures .003 different in 2 directions.still both within the minor range.

the out of round tol is "0"

if I had a no and go gage I bet there would not even be an issue.

do you think that measuring the minor and then making a plastic impression of the area you measured ,putting it on a comparator and then adding those 3 values together would give you an accurate enough measurement to put that down as "the actual pitch dia"?

make a couple guages on the lathe.

[djstedman]

First off, you now say your out of round tolerance is "0", is that what the inspector is saying? and if so have they given you a spec or shown you a note requiring that?

 

Anyhow.. so far as the inspection method goes...

 

Plastic impression compound has a shrinkage value to it, also depending on the feature being measured an optical comparator is going to be open to interpretation by the inspector to a certain degree.

 

In inspection, accuracy and repeatability are the most important aspects of the inspection method. If multiple inspectors can reliably inspect the same feature using the same method and achieve accurate results over multiple checks then its probably a good check. If they cannot attain similar accurate results then its probably not a good check.

 

I would guess using the method you described they wouldn't attain the same results if they were to make multiple impressions and if multiple inspectors checked the part, so most likely this is not a reliable method for checking the part.

 

If this is a first piece check, then I would say make the part better to appease the inspector, if all the parts are complete, then I would try and find a better method to check the parts because IMO this method is questionable at best.

 

One method I can think of off hand is, If you have a small ball you could draw up the thread and get a dimension over the top of the ball, then indicate it in several locations to determine actual pitch diameter.