Hello Bill,
I just found my information on the Atlas 6” lathe and it also shows 27 TPI as standard.

I am know my 7x16 MicroMark minilathe can also be configured to cut 27 TPI. I have the mathematical formula to calculate the gear tooth requirements for any thread. Very handy to have.

Tim,
The Unimat thread masters I have had were not buttress thread form but “V” form. I would assume the metric thread masters are 55 degree thread form but I am not sure and the US version is most likely UNF 60 degree (unified national fine).
Perhaps someone here on this group can clarify this
.

I think uncommon TPI configuration(s) were done on purpose to keep the rookies (like me) from trying my own repairs.

This is an interesting thread though.... couldn't resist.

Speaking of rookies, this one generates for g-code, I don't have a CNC lathe.... (still.)
https://sct-usa.com/thread-mill-code-generator/

Tamra

Were they hardened? More to the point, since the followers are brass, can you get away with not hard? And are they cut with a buttress thread form? I've never actually seen a threading attachment.

Tim in VT

I just ran out back to look at my Clausing 4902. It has 27 tpi right on the QCGB. Also 22,23,24,26,28, then jumps to 32. No 25, no 30.?

OTOH, I looked up the Atlas threading supplement and a 10F will do 25,27,and 30. Atlas for the win! But you shouldn't need a 32T gear to do this.?

This has piqued my curiosity—what equipment used 27tpi? It must have been common enough somewhere b/c I just googled it and found it's stupid easy to get 3/4-27tpi taps and dies. Not even that dear, really, the tap is under $25.

Tim in VT

On Tue, Feb 28, 2023 at 11:06 PM, OldToolmaker wrote:

Given the high price of vintage Unimat thread masters, some thought might have to be given
into machining some thread masters to order.

I'm not sure it would be easy to set up many modern lathes to cut 27 tpi. Not sure about some of the older ones either. I'll have a look at our old (1950s vintage) Cardiff to see if that can do it, and how much hassle it would be.

Given the high price of vintage Unimat thread masters, some thought might have to be given
into machining some thread masters to order.

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Hi Gang:

In Trade School they recommended facing from the outside to the center. The rationale is if you start in the center and one corner is high, it will be too heavy to finish the cut. Light cuts are the way to go on a Unimat, the parts are small and the cuts are usually short.

Do get the proper Tommy Bars, and keep the cuts light.

Carl.

Regarding your question about the stock moving in the chuck. ?You should get a proper set of Tommy bars to tighten the chuck as well as cleaning it as others have mentioned. ?Also check your facing tool center height and that the tool is sharp.?

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Interesting… thanks!

Hello John,

If 27 TPI thread guide are not available for the Unimat SL I can think of at least two ways to make a thread master to fit the SL.
(1) If you have access to a thread cutting lathe you can make an exact copy that will mount right on your SL. This would be my preference.
(2) You can buy a 1/8-27 straight pipe nipple from a hardware store and use it as a master. These can be purchased in fairly long lengths and they are often used in lamps. Of course, you will still need to make a follower to mesh with the master. A 1/8”-27 pipe fitting could be pressed into service for this.

On Mon, Feb 27, 2023 at 08:55 PM, clankennedy2004 wrote:

there's a 1/8" BSP tap that's also 27tpi

1/8 BSP is 28 tpi, not 27. It is a constant problem here that the USA and UK thread standards often differ by only 1 tpi thus avoiding any hope of compatibility while being similar enough to cause confusion.

I've looked through my thread pitch tables and can't see anything 27 tpi in the usual British range, nor in any of the other ranges I have listed, I recall it was used on some pipe threads, for example on 5/8" diameter microphone stands. It doesn't work out to be anything close enough in metric threads either. Mert, late of this parish, had a method that worked if you had a smaller thread of the correct pitch, say a nut and bolt. I would think that a bit of 1/16 or 1/8 NP should be usable, but it will take a bit of setting up, and getting internal threads right is a bit of a pain. I hope you will be working in a good material like hard brass!

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Thanks Jeff.?