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Thread Timing - Ring Gage Frustration

S

steve68steve

#1
We have a part with a thread timing spec:

A cylindrical part has a lengthwise fin/ groove which - when threaded into the mating part - has to have the fin/groove align to within so many degrees of a point on the mating part.

We've been inspecting the cylindrical part for thread timing using a threaded ring gage. On the face of the threaded ring gage, a mark has been placed where the cylinder's groove should stop when the cylinder snugs up against the face of the gage.

The issue: this arrangement doesn't seem to be repeatable worth a lick.

I have a "master cylinder" which was used to mark the ring gage in the first place. Finding that it no longer lines up, I created a new mark with the master cylinder. Now that mark is not lining up and the old one is. The difference between the marks is on the order of the whole tolerance for timing - my gage R&R is ****.

Daunted, I've abandoned the ring gage and now use the "master cylinder" to qualify a mating part, then use the qualified mating part to verify production of the cylinders.
The issue with THAT is that the mating part is Aluminum, soft, and wears quickly. Management is aware of this shortcoming and has insisted that the ring gage be used because it's thought to be the height of "accuracy" - since it's an expensive gage, and it gets calibrated.

Experience to this point suggests the ring gage is fine as a GNG for thread, but has demonstrated itself unreliable for timing. In my defense is the fact that ring gage is small (heat sensitive), has relief slots cut into it, etc.

So:
1. is there a more reliable, accepted way of verifying thread timing? Web searching has turned up nothing.
2. is there anything known/ documented about the variability of using a thread gage in this way? I keep telling Mgt. that the results speak for themselves - they reply, "it's a gage. It HAS to be trustworthy. Figure it out - you're missing something."

Sorry for length.
 
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J
#2
OK - I'm intrigued....and have my famous "dumb as a rock" look on my face....:confused:

Question, what stops the gage? end of thread? Part Shoulder?

As number 5 says...."need input"

 
S

steve68steve

#3
The part is shouldered; the shoulder snugs up against the gage (or the mating part). Elsmar won't let me upload photos because I'm too noob.
 
J
#4
The part is shouldered; the shoulder snugs up against the gage (or the mating part). Elsmar won't let me upload photos because I'm too noob.
Well my first thought then would be to look at variations in the shoulder location as possibly being the culprit.

Not sure I even have a second thought....:notme:

James
 
S

steve68steve

#5
The confusing part about it is that the variation is evident when I run my MASTER cylinder into my GAGE. The same part, run into the same gage, does not read the same timing on subsequent runs.

I'm guessing this has to be an issue with temp or with the gage flexing.

Again, I'm wondering if there's a better way to verify thread timing.

Also: I have pics hosted at: steve68stevework.blogspot.com but I can't link here because I don't have ten posts yet.
 
J
#6
The confusing part about it is that the variation is evident when I run my MASTER cylinder into my GAGE. The same part, run into the same gage, does not read the same timing on subsequent runs.
Not sure I understand this statement....Same part...same gage....subsequent runs??? "Subsequent runs" sounds like it these are different parts run onto the same gage (or mating part on the original master...)

I'm guessing this has to be an issue with temp or with the gage flexing.
Thanks for the link to the pictures. :agree1:
Dealing with aluminum temperature can certainly be a component. You might try taking a part, get it hot to the touch - gage it, then put it in a refrigerator overnight and gage it again...A bit down and dirty but will give you some indication if temperature is a significant issue.

Another variable could be play in the threads though I don't see this as being significant.
My old machinist brain is still focused on the the mating shoulder as being the primary culprit here.
Assuming that this is produced on a CNC lathe capable of starting the thread in the same location relative to the chuck position, then the variables have to lie in the machining of the part. If there is little variation from part to part in a run, but there is variation between runs, I'm looking at possible issues with tool wear, variations in tooling location at set-up and other variables that can effect the precise location of that shoulder relative to the timing of the thread.
For example, On a 16 TPI thread, each degree of turn = only 0.000174 inch of linear movement. So if there is even 0.001 inch variation in the location of the shoulder, this will translate to about 5.7 degrees difference in the seated location of the mating part....

I know that this is kind of wordy and I'm really just tossing out ideas....I hope it's being of some help....

Again, I'm wondering if there's a better way to verify thread timing.
I'm no expert in this area...my guess is that getting anything truly precise would require some sort of magnifier...optical comparator type tooling....But I've been out of the field for so long...
Also: I have pics hosted at: steve68stevework.blogspot.com but I can't link here because I don't have ten posts yet.
Those are Great....

James
 
S

steve68steve

#7
James, thanks for all your time, patience, and effort.

If you had a thread timing spec, how would you measure it?
 
S

steve68steve

#8
Looking back at your post, James, it's clear to me that I've done a terrible job of explaining this. I'll give it one last shot:
1. Take master cylinder. Spin master cylinder into threaded ring gage until the shoulder snugs up against the gage's face.
2. Mark the thread gage's face where the master cylinder's "fin" has stopped traveling AROUND due to it's being screwed tight up against the gage's face.
3. New gage in hand, go measure a bunch of production cylinders.
4. When production starts to seem odd, verify the gage by spinning the master cylinder into it again - since the master cylinder was used to MAKE the mark on the gage, it should line up. It doesn't.
5. Assuming wear or screw-up, remark the gage at the NEW spot the master cylinder stops. Go measure a bunch of production.
6. Repeat step 4 and 5, except now the master cylinder is aligning with the mark made in step 2.
7. Scratch head. Post on elsmer cove.
 
J
#9
James, thanks for all your time, patience, and effort.

If you had a thread timing spec, how would you measure it?
Just saw your last post...after posting this one...Decided not to delete this one and will read and respond to the latest in another post.

To be completely honest my first idea would be to do just what you are doing. Ring gage/mating part check.
I say this because it is the most practical and functional to the usage of the parts.
Now - given my limited experience and having never dealt with this type of spec...I can only imagine that I might try putting the part on an optical comparator and try to check the distance from the last full thread height to the shoulder of the part. Of course this would require very precise mounting of the part. However I'm not at all sure how accurate this could be given how little change in shoulder distance can effect angular alignment.

On a more practical note, I think I would try working with the set-up people to determine the variables in the machine and make adjustments at set-up...
As long as there is little change within a given run - but variation is seen between set-ups, that would seem to me to be the logical place to go for a solution.

James
 
J
#10
Looking back at your post, James, it's clear to me that I've done a terrible job of explaining this. I'll give it one last shot:
1. Take master cylinder. Spin master cylinder into threaded ring gage until the shoulder snugs up against the gage's face.
2. Mark the thread gage's face where the master cylinder's "fin" has stopped traveling AROUND due to it's being screwed tight up against the gage's face.
3. New gage in hand, go measure a bunch of production cylinders.
4. When production starts to seem odd, verify the gage by spinning the master cylinder into it again - since the master cylinder was used to MAKE the mark on the gage, it should line up. It doesn't.
5. Assuming wear or screw-up, remark the gage at the NEW spot the master cylinder stops. Go measure a bunch of production.
6. Repeat step 4 and 5, except now the master cylinder is aligning with the mark made in step 2.
7. Scratch head. Post on elsmer cove.
Thanks that DOES help clear it up....Can't say that I have any brilliant answers for you at this point....kind of back to my "dumb as a rock" look...

Still thinking....

James
 
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