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Will Changing a Class of Thread Change the Major Diameter?

04/08/2013 8:27 PM

I have a customer whom I supply a shaft, on the end of the shaft is an M65 x 1.5 4h thread. Now the customer messed up because the next diameter over is the bearing journal and the major diameter of the external thread is the same size as the bearing journal. Now, here's the request, change the thread to M65 x 1.5 6g. I've always been under the impression that the class of fit was just another way to tolerance a thread. So, here's what the Machinist Handbook says. A 4h fit is to make the thread to mean, but the 6g says the root can be up to -.032mm and the major diameter had ~.080mm more tolerance. The customer says this will allow the bearing to fit over the top of the thread without damaging the threads. Can that be true? If I buy a die to 6g, to chase the threads that I've already made, will the die remove material from the O.D.?

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#1

Re: Will Changing a Class of Thread Change the Major Diameter?

04/08/2013 8:51 PM

Check the coefficient of thermal expansion of the bushing and shaft materials.

Maybe a cold shaft will fit inside a hot bearing journal?

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#2

Re: Will Changing a Class of Thread Change the Major Diameter?

04/08/2013 11:01 PM

In a way you already answered your own question.

With the class change the allowable diameter changes and you could machine it to a different diameter which maybe would be enough to fit the journal.

Maybe worth asking what the engineer who did the drawing was thinking about it. To shrink fit the shaft might be what was meant to be performed, which I think is a standard practice for this.

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#3
In reply to #2

Re: Will Changing a Class of Thread Change the Major Diameter?

04/09/2013 10:11 AM

I have my doubts. I have 40 or more parts already machined to the 4f spec and my customer want them "re-worked" before I ship.

Do you know if I purchase a 6g die if it will cut the major diameter of the thread? From what I got out of my Machinist Handbook, it would cut the pitch diameter lower by about 15 mircons, 32 microns max.

Thoughts?

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#4

Re: Will Changing a Class of Thread Change the Major Diameter?

04/09/2013 1:26 PM

It's only in pipe threads that the sharp peaks of threads are useful. Many bolts have these peaks not sharp to avoid galling. I would shave off the peaks on the male threads to allow clearance for the bearing. The peaks really don't contribute much strength to the thread.

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#9
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Re: Will Changing a Class of Thread Change the Major Diameter?

04/10/2013 8:44 AM

I'd also have the customer change his drawing to specify the max limit of the threaded section OD, or some type of written documentation. It seems like they want to use the looser fit designation to allow a smaller OD, but the current too large diameter is also to spec. Otherwise you could be responsible if you don't shave enough or too much.

Note that with Mike K's solution you don't need a 6g die.

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#5

Re: Will Changing a Class of Thread Change the Major Diameter?

04/10/2013 1:10 AM

The major diameter for the thread you are forming has a maximum allowable diameter for the class of thread you are generating, as well as a MINIMUM allowable diameter for that class of thread. (tolerance)

The customer (or print) should have the information on the "class of fit" required by design. (4h) No idea what the part or assembly goes on, or how critical the part/assembly requirements are?

In effect, the answer to your question is yes. The class of thread is determined by the allowable tolerances for the specific "class" of thread. So in effect, by changing the class of thread, you are changing tolerances.

If you can form the major diameter of the existing thread to the "minimum" allowable diameter tolerance for the listed class of thread and the bearing ID will slip over the threads, you're set.

Or, as someone else suggested, a thermal fit may be appropriate.(freezing the shaft) Hope this helps.

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#7
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Re: Will Changing a Class of Thread Change the Major Diameter?

04/10/2013 7:10 AM

It does, in short, I can tell my management that by changing the class of fit, they only changed the tolerance and we would currently meet the requirements without changing anything. In short, they called out 4h and we made them to 4h, now they changed it to 6g and that doen't change the parts, they still meet the 6g requirement.

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#6

Re: Will Changing a Class of Thread Change the Major Diameter?

04/10/2013 6:45 AM

With such a small amount to come off the die may not cut any metal off but may burnish the top of the thread. It would be much better to grind the thread portion of the shaft. Better still to shrink fit the bearing. With such a relatively large bearing the expansion on the bore is quite big.

Jim

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#11
In reply to #6

Re: Will Changing a Class of Thread Change the Major Diameter?

04/10/2013 12:17 PM

If the bearing is shrunk fit to the shaft over the interfering threads, the bearing may damage the threads if it is ever removed. Removing would be done by pressing. Damaged threads may interfere with replacing the nut, risk of galling.

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#8

Re: Will Changing a Class of Thread Change the Major Diameter?

04/10/2013 8:08 AM

When we go back to the basics of any metric thread, there is another way to handle this tolerance / interference problem.

And here I am talking from the top of my head. Please look up the exact numbers, if that is important to you.

A metric thread such as yours has a load bearing surface on its central 90%. The top and bottom area is used to cut it short to avoid any interference, and to round off to avoid stress concentration that would arise without it. These values do have relatively loose tolerances.

With that in mind, you can make the threaded end O.D 0.01-0.02mm undersize for a loose fit thru the bearing, without getting close to the load bearing surfaces of the thread itself. And this maintains the original fit of the nut too.

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#10

Re: Will Changing a Class of Thread Change the Major Diameter?

04/10/2013 11:20 AM

I guess the bearing journal diameter is 65mm.
An M65 metric thread has naturally an OD of around 64.9mm which is under the 65mm and the bearing will move over without damaging the threads.
What is the OD of the thread you are having now?

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#12

Re: Will Changing a Class of Thread Change the Major Diameter?

04/10/2013 8:33 PM

Looking at the machinery's handbook, you will notice a J class thread.

These special threads are used in high stessed areas and what makes these threads so special is that the root has a larger than normal radius and the peak is rounded off.

Maybe you can consider what they did on the peaks for the imperial thread and apply that concept to your metric bolt

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#13

Re: Will Changing a Class of Thread Change the Major Diameter?

04/10/2013 10:22 PM

It appears, that my #8 K.I.S.S. note is more appealing to readers. I know, it is not a voting contest. But, slicing part of a very secondary piece ought to be a no brainer to anybody examining the basic Metric standards.

I understand, you need the concurrence of the customer to execute a nonstandard, and bloody simple reduction of the outer diameter by a few hundreds (within the tolerances of a threading die), to make an easy slip fit thru a bearing.

I acknowledge, that interpreting vs. reading tolerances is not a simple thing.

Nonetheless, you have to put you foot down somewhere, firm. And help out the customer at the same time.

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