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Gordon, thread check please

Pete F

Titanium
Joined
Jul 30, 2008
Location
Sydney, Australia
I wrote a post in the metrology section to Gordon, who many here consider the resident thread expert, to check some thread calculations as we were getting different results. However for reasons best known to himself, the "moderator" in the metrology is deleting posts of mine. I frankly don't have time for such childish BS, and it's easiest to just abandon that forum and repost the question here.

Can you (Gordon) or others please check my figures for a 40 x 1.5 mm 4h as they seem to be different and I can't figure out why. Furthermore, the figures provided didn't make sense to me, and I want to become more confident with using metric thread tolerances.

Assuming you are referring to tolerance 4g both for major (OD) and pitch diameter (PD) then:

OD max 39.974 and min 39.862
PD max 39.324 and max 39.244

Actually I was referring to 4h and my understanding is the maximum major diameter on an h thread will be the nominal thread diameter, so in this case is 40.00 mm (I'm not keen enough to work to 3 decimal places on a thread, as that's better than I can cut! :D). So the figures would be:

OD max 40.00 min 39.85
PD max 39.03 min 38.93
Minor max 38.38

However what's more concerning to me is that I'm not getting the same figures as Gordon on a 4g thread as I'm getting:

OD max 39.97 min 39.82
PD max 38.99 min 38.90
Minor max 38.34

I'd be grateful if Gordon or others could steer me straight on where I'm going wrong with my figures, as some of mine are quite different to Gordon's. I've stopped the job until I can figure out what's going wrong here.

Thanks in advance.

Pete
 
Here's what I wrote as my reply in the metrology sub forum:

You're absolutely correct that with both H and h they will both be nominal diameter. What might be causing some confusion is the fact that the max diameter of an internal thread very rarely has the max tolerance given, only the min dimension.

In theory (and even IRL) the point of the thread cutting tool could be a sharp V and the internal thread wouldn't be incorrect.

This is where having the relevant standard is worth its weight in gold as when giving the pitch diameter tolerance as all other tolerances are a given. When exceptions are necessary the thread tolerance for an external thread (never for an internal) would for example be given as 40x1.5-5h4h. The first one given (in this case 5h) applies to the PD and the second (4h) to the OD.

No matter which letter applies (h or g) then both the OD and PD tolerance should be the same to have the same allowance from nominal. Of course, again, h doesn't have an allowance from nominal. In other words it's a NoNO to give for example 4h5g as a tolerance.

Putting this on the table as it applies to both OD and PD.
http://f-m-s.dk/1.03.pdf

I hope this helps clear things up.


To be very specific then I'll add this:

The thread in question is M40x1.5 and an external thread.

Nominal pitch diameter (PD) is 39.026.

Using ISO 965-3 the tolerance for M40x1.5-4h is max 39.026 and min 38.931.
-4g can't be found in ISO 965-3 or any other written standard I have.

My conclusion is that having as fine a tolerance as 4g doesn't make sense as the purpose of 4h is to make a thread with as tight a fit as possible and allowing an allowance (as g does) kinda defeats the purpose as a 5h or even 6h would do the same while allowing more to work with.

Now if I use ThreadPal - dimensions and tolerances for inch and metric screw threads
then I get max PD for M40x1.5-4g as 39.324 and min 39.244

It's amazing what can be calculated using the relevant formulas but 4g probably isn't given in a standard for a good reason.

I think this is one of those times when what appears to be a simple question can't be given a simple answer but hell, I've tried LOL

Pete, keep the questions coming as I thrive on this stuff :D
 
Anyone wanting my free opinion, advice or just tolerances on threads can email me - don't PM me as my IN box just gets filled up too fast.
Click on my name (which is also my aka) to get my email.

I don't claim to know everything but if I can answer I will. I'm not afraid to admitting what I don't know about threads ;)

The easiest thing is to ask on PM as there are others that can answer thread questions just as well as I can plus the answer(s) might just help other members with a similar question or problem. As to which sub forum is best for asking thread related questions is another matter. This one "General" is probably best for that.
 
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Pete, keep the questions coming as I thrive on this stuff :D

I know you do Gordon, I know you do ;)

So you think I'd be better cutting to 4h where nothing is specified except application, just to give a bit more margin for error?

For info, there was a bit of a delay on the external as I'm cutting an internal imperial at the moment and decided to poke a runout groove in there and every tool I had ground specifically for that purpose rubbed in one place or another :rolleyes5: I'm having just one of those days today ... AND it looks like I have the flu :ack2: I managed to dodge it up until now at least, but apparently it's been a shocker of a season down here for it.
 
So you think I'd be better cutting to 4h where nothing is specified except application, just to give a bit more margin for error?

Try and get as close as you can to nominal PD as you can to within at most 0.02mm to 0.03mm under and you'll be good to go. Just my opinion :cheers:

Try a whisky for your flu. It might not take away your flu but OTOH it won't hurt :)
 
I know you do Gordon, I know you do ;)

So you think I'd be better cutting to 4h where nothing is specified except application, just to give a bit more margin for error?

For info, there was a bit of a delay on the external as I'm cutting an internal imperial at the moment and decided to poke a runout groove in there and every tool I had ground specifically for that purpose rubbed in one place or another :rolleyes5: I'm having just one of those days today ... AND it looks like I have the flu :ack2: I managed to dodge it up until now at least, but apparently it's been a shocker of a season down here for it.

Switch the lathe off and get to bed. You shouldn't work any machine feeling ill. Regards Tyrone.
 
Switch the lathe off and get to bed. You shouldn't work any machine feeling ill. Regards Tyrone.

For some that's good advice but personally I'd rather be doing something to take my mind of my flu (I don't have one) than lying in bed feeling miserable. If it's one of those flues that come with a headache then a couple of headache pills helps that.

OTOH having a flu and working around people that don't have a flu is just plain inconsiderate.

"Is that water on your nose?"
"No it's snot"


Should be said aloud :)
 
Tyrone was right, I scrapped a part and decided it was time to finish for the day. No biggie, I'm not feeling too bad, just that work expects me not to come in and give it to everyone else. Even the scrapper was avoidable, I was just trying something different to see how it would work out, and ... well it didn't.

Not working at machinery while tired/sick/drunk/stoned/all of the above is however good advice and I appreciate the thought.
 








 
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