How to do fine threads

blue_luke

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Hello all!
The things I intend to fabricate are audio microphones accessories. (I am a sound engineer)

This pose a few problems.... I own a Precision Matthews PM1127 lathe. This is a nice machine and I am happy with it, but the most standard thread size for mounting microphone bracket is 5/8"-27tpi On the charts given on the machine, there is no gear combination to cut this thread!
Also, I build my own microphones and I would like to learn how to cut fine threads like the ones used in photography lenses and filters.
This way I can use some nice capsules I have (Schoeps, AKG etc) to preamps and contraptions of my design or design accessories circuits like pads and low-frequancy roll-offs...
For those who don't understand this giberish,,, it all boils down to be able to thread fine threads on thin tubular brass or aluminium tube.
Right now I have no specifications to give, it is more the technique of holding the parts in a chuck, what geometry the cutting tool should have, how to avoid distortion in the finished parts...

Sincerely, Luc
 
Did your lathe come with a change gear set? If so, you can calculate the gears needed and hopefully find the right gears in the set.

As for geometry, if you use a HSS tool then a standard 60 degree tool should suffice.

Holding the stock depends on the size. A collet will hold it without distorting or damaging the tubular stock. A 6 jaw chuck will work better than a 4 jaw will but that gets expensive.
 
Fine threads are easy unless you are trying to put threads on really small diameter rods. Putting fine threads on larger diameter pieces is no different than any other threading operation. If you need a refresher on threading, I'm almost certain there's info about threading here: https://www.hobby-machinist.com/forums/noteworthy-threads-info.290/

If you are trying to put threads on really small diameter rods (say, 1 mm or less), let me know when you get the process perfected...

Ray
 
If you set your quick change for 30 TPI and have a 30 T gear on the stud gear and a 27T gear on the change box, and any number of teeth on an idler, you should get 27 TPI; I did a post on this a couple of days ago.
On my Grizzly G4003G I take the setting for 18 tpi and slow it down 50% to get to 27 tpi. That is my gears for 18 tpi (with the QCGB set @ 18) are 40/86/40. Switching to 40/86/60 (with the QCGB set @ 18) gives me 27 tpi.
Are you sure? I do not see how dealing with even numbered change gears can result in an odd numbered thread.
 
Hmmmm! I think I have opened a can of worm here! :-D

For the 27 TPI threads, Yes I did receive quite a few gears for cutting threads, but no combinatons seems to arrive to 27 TPI. If memory is right, I can get 26 or 28 tpi but no 27!

For Ray C, no the diameter I have to thread, both inside and outside obviously, are more in the 0.750 and over up to about 2"
The threads are fine pitch and metric for sure, but again, I have no specs as of yet.

Usualy, a microphone (modular) is made of a tube shaped thing that contains an electronic circuit and at one end, the output connector, and at the other end, a single pin connector. This end of the tube is threaded with a fine pitch and is meant to receive the capsule element mounted in a special housing. This housing is threaded internally to match the receiving threads on the tube.
The idea behind this approach is that, using the same électronics in the tube, we can insert different pickup patterns capsules like omnidirectional that picks sound from everywhere, cardioid, that picks mostly up front, and many other types for different purposes.
Then using the same system, we can also insert modifying signal elements, such as 'pads' that will make the microphone less sensitive, when you record things that are very loud, like a huge concert bass drum, or a canon!, or filters that can eliminates very low frequencies that add nothing to the music but creates mud... like a big heating system fan, or nearby trafic rumble.
Here is a link to one microphone kit I currently own... https://www.oktavausa.com/mics/product/mk-012-msp6-factory-matched-stereo-pair-microphones
The diameter is 27mm, the thread is unknown but I can fin out...
Anyway you see where I am heading.
Another question, Usually, north american threads are cut at 60°, is it the same for metric threading?
 
Yes, metric threads are 60 degree...

I cut a lot of threads but am not familiar with your machine. The theory is the same on all modern manual lathes that are designed to cut both SAE and metric threads. FWIW, the finest pitch I've ever cut is 32 TPI. I can certainly see how anything finer than that would present some different challenges. There's not much room for error...


Ray
 
Yes metric threads are also 60 degree; whitworth are 55 degree and there are a few other threads with other (odd) angles. You would need a 27 tooth change gears to accomplish 27 threads.
 
Thank you Ray and Benmy, obviously I will have to dig more into th subject.
For the time being I found taps and dies for 5/8-27tpi threads (these are a bit rare). This might get me by for a while, but I foresee some cases where it would rather cut the threads on the lathe.
Thank you, Luc
 
Yes, I think if you dig into it a bit, you should be able to sort out what gears are needed to get you 27 tpi. I have two pretty standard machines - one has the option to select 27 tpi (with the standard end gears), the other does not - just needs some different change gears (the change gear set is made up of 20 gears - added to the selections in the Norton box, that's a lot of thread pitch options).

No holding the tube is another issue. If it is not too soft / thin - then Mikey's suggestions of a 6J or collet will be fine. Be gentle and it will work out.

You may have to turn a close fitting plug to slip inside the bore (and then you'd have no trouble with using the 3J or 4J).
 
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