Unusual Metric Thread Size(s)

[danielleoh]

I'm working on what is more or less a reverse engineering job right now. The original uses some stock parts, which is, I believe, how they might have ended up with a couple of strange thread sizes that I can't seem to find a spec for.
The most important one is M24.5 x 1.0. I measured the part a dozen times before accepting that this is actually the size of the thread, then went online to see if anyone showed any resources and/or specifications for it (the parts will be turned on a lathe, so at least we won't need to come up with taps or dies; but we will need to come up with a spec for the threads to be made).
Does anyone here know of a resource for it? I did a thorough search and found that it is, in fact, a real thread size; but the one product I found for it was an old thread gage on eBay, listed in Australia. So it's likely an obscure, maybe military or optics type of thread. Any help at all would be appreciated.

Thank you,

Danielle

 

[3DDave]

A one millimeter pitch on a 1 inch nominal diameter? So it's 25.4 threads per inch?

The main thing missing are the tolerance requirements, which no one can help with. I believe the typical thread standards have ranges to choose from.

 

[zeusfaber]

Worth questioning that. The OP said M24.5 - not M25.4.

A.

 

[gearguru]

3DDave is referring to pitch, not to the OD of the (external) thread. His answer is correct for any metric thread with the 1mm pitch.

 

[gearguru]

Couple clicks and google found

https://amesweb.info/Screws/metric-thread-dimensions-calculator.aspx

Be aware: because of the tolerances, the actual dia of the external thread is always smaller than the nominal ( for example the M25 thread has the OD slightly less than 25mm; it allows to safely slide parts (like bearings) with nominal hole size 25 over the thread).
Metric threads profile is based on equilateral triangle with the side length equal the pitch.
Tu create the exact profile you have to apply the tolerances and tip/root shape to that triangle. There is plenty info about it online.

 

[zeusfaber]

As a sparky by training, I'm willing to be corrected - but "nominal diameter" doesn't sound a right lot like "pitch" to me.

A.

 

[3DDave]

I was pointing out that the OD was the same numeric value as TPI, sort of a weird thread and probably chosen to reduce competition; maybe the side effect of having a metric lathe and single-pointing on inch diameter stock.

 

[robyengIT]

could be : ISO metric M25x1 (all mm) tolerance 6g/outside 6H/inside (probably manufactured by rolling system and not by lathe)

 

[GregLocock]

Any number of industries use fixed tooth profile/pitch machine made threads, either with chasers or some other device. Watchmakers and bicycle makers were keen on this. Obviously this isn't watchmaking, but the principle applies.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[zeusfaber]

Still not getting this. Why do we think it's 1"?

A.

 

[3DDave]

25.4 mm is defined as one inch.

 

[zeusfaber]

OK, that's what I thought. The OP was talking about a different number though. If she ever comes back, we'll have to ask whether she meant what you said.

A.

 

[3DDave]

Ha - i cants reed.