Pros and Cons of coarse vs fine threads ?

Hi,
I'm just about to start to build a Hemmingway rear toolpost and I will use a 12mm dimension for the clamp post instead of the 1/2 inch in the drawings ('cos
I am Mr Metric). My question is a general one about threads in this type of application :
As it happens, I have taps and dies for M12 x 1.0 (fine) and M12 x 1.75 (coarse) threads and I wondered which is perferable in a clamping application like this ? ISTM that you get the greatest clamp force with the fine thread, but then again, isn't the coarse thread a bit stronger ? What would normal engineering practice say about choosing between these 2 threads ? I actually have a slight preference for the fine thread because my die in that size is adjustable so I can make the post to fit the clamp nut whereas my die for the coarse thread is an el-cheapo non-sdjustable type which I don't have much confidence in.
All advice gratefullly received,
--
Boo


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On Sat, 04 Oct 2008 23:41:49 +0100, Boo

Coarse threads in soft material. Fine threads in hard material. Cast iron. aluminium, brass and EN1A are all soft material.
For the central calming post, it probably makes little difference because the loads on the threads are very small for the size of thread. On the T-nuts, if making your own then use coarse threads unless you are hardening the T-nuts.
Just my thoughts :-)
Mark Rand RTFM
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Delete calming and insert clamping.
Seem to have had a bit of a Chinese manual moment there with the speelingchucker...
Mark Rand RTFM
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.If you are using a Myford, watch your clamping on the rear Tee slots.
Norm
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Just to add another strand to the metric coarse / fine discussion, does anybody know why the choice was made when industry changed over to metric threads that the 'standard' thread was to be a coarse thread for most applications?
In imperial days Whitworth and BSF thread forms were used depending on the circumstances. Although metric fine is available it is not generally used in the way BSF was.
John
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wrote:

Just to add another strand to the metric coarse / fine discussion, does anybody know why the choice was made when industry changed over to metric threads that the 'standard' thread was to be a coarse thread for most applications?
In imperial days Whitworth and BSF thread forms were used depending on the circumstances. Although metric fine is available it is not generally used in the way BSF was.
John In general, it seems to me that a coarser thread is more desireable as long as it will readily develop the necessary clamping force and does not unduly weaken the bolt. It would seem to permit larger fit tolerances. I have read that in the USA, fine thread standards were developed by the Society of Automotive Engineers (SAE) because of the need for greater clamping forces and a increased resistance to loosening caused by vibration . I certainly think fine threads look better, somehow. (Maybe it's like fine wine or fine art! :-))
Don Young (USA)
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Pre WW2 JAP (That's JA Prestwich of Tottenham - not the Nip variety) made motorbike, three-wheeler and small industrial engines. They mainly used cycle thread for steel to steel and Whit for steel to ali. Studs for holding the cylinder barrel to the ali crankcase were threaded BSC on one end and BSW the other.
Bob
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Bob I think that BMC or was it earlier had two different threads on A series cylinder head studs? However, it is also academic. The hiccup comes when one finds that the (say) new secondhand Myford has a nasty warp. Moving on- into today- I did fit a rear tool post onto a 918 or 920 if you must. It went onto a sreel subtable of my own devising.
The sub table then took all sorts of Myford goodies!
Cheers
Norman
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Hello Boo, You're right that you get more clamping force with a fine thread - all other things being equal. But, as implied in Marks reply, it won't happen if the material you are screwing into isn't strong enough to withstand the stress of a greater force over (probably) a smaller area due to a finer thread. Other aspects to consider in general are wear characteristics and the risk of contamination. Coarse threads are more resilient to damage from say grit and dirt. Very fine threads (say on optics) are notorious for being at risk of cross threading. Those are perhaps near the extremes. For your application I can't see why either wouldn't be perfectly servicable , but given the added comment about your dies I'd personally be happy to go with the fine thread. I'm not sure if the design is a fixed stud with a nut-handle or if the working thread is in the base. I'd much prefer the former so that as wear occurs it's only the nut & stud that would ever need attention.
Rgds Richard
On Sat, 04 Oct 2008 23:41:49 +0100, Boo

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Boo,
ISTR that some guru (I think it was Tubal Cain) calculated that most if not all threads in common use will fail by tensile failure of the core long before the threads fail in shear. The margin of this difference is dependent on the thread engagement percentage. Obviously, as you go to larger diameters/finer threads or to shorter thread lengths this will cease to be true. The calculation would be quite complex, but I would hazard a guess that in this case there isn't much in it. If in doubt, use a bespoke extra thick nut.
I can't get into my workshop to get the book to check, but I'm pretty sure you will find this discussed in "Threads, Taps and Dies" in the Model Engineers' Workshop series.
I would guess that either thread is likely to distort your cross slide long before it breaks. I doubt you will have problems either way.
David
--
David Littlewood

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.David maybe right in his source but unquestionably, the late G H Thomas writing in the Model Engineers Workshop Manual wrote of Myford cross slide distortion. To add, I mentioned that there was a problem with tee slots as well. The latter complaint arose from Kenneth C Hart aka Martin Cleeve. He noted the non standard Myford tee slot and write it up in ME. He broke his slide- do'nt ask why. Myford made him a steel one to special order.
There you have it- all had several Myfords and I, too, had a similar problem.
Cheers?
Norm
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Thanks to all for the replies, I think I will go with M12x1 as per the advice I've received here. 1mm is not so fine as to have problems with damage or to be too hard to cut accurately and it does mean I can use my die instead of screwcutting it.
Sorr=y for the delay replying btw, Demon has had major news server problems recently. If anyone knows how to link in my home server into the usenet network then I'd be grateful for a pointer to how this can be achieved.
Thanks again,
Boo
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I had the same problem. Don't know any more than you about how to circumvent the problem efficiently, but you can always go to Google Groups to read the latest posts on any threads you are interested in. It's much more of a chore to do this, of course, and I await any other thoughts with interest.
You might get more expert advice by posting on demon.ip.support.turnpike - even if you don't actually use turnpike.
David
--
David Littlewood

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