What is it with 27 tpi and old firearms?

[Chill Wills]

I keep running into the need to cut 27 tpi and my lathe does not feed that. Over the years I run into 27 thread and in this case I lost the scope cover for an external adjusted MVA scope and wanted to make one. MVA can supply one but they are pricey and I can do it and would enjoy doing it but for the bastard 27 thread again. OK, OK - maybe not such an odd-ball in its day.

fraction thread - 3/4-27

Ideas for a work around?

[M-Tecs]

Depending on the lathe manufacture you should be able to find info on change gear combinations that will get your there.

[JSnover]

Depending on the lathe manufacture you should be able to find info on change gear combinations that will get your there.

^That!
Can your machine feed .037/rev?

[Chill Wills]

^That!
Can your machine feed .037/rev?

OK - Snover. You are clearly going somewhere with this and I do have something close at 0.0379" which is also equal to the nine thread per inch in the quick change gear box. 0.0379" advances about a thousands too much per revolution but close enough maybe for the three threads I need to cut.
Hmmm - 27 divided by 9 = 3

SO - I see something here; what do I do to make this work? My skill set is like I am still back in high school machine shop or not much better but it is great to be ably to make many of the things I need.

[JSnover]

OK - Snover. You are clearly going somewhere with this and I do have something close at 0.0379" which is also equal to the nine thread per inch in the quick change gear box. 0.0379" advances about a thousands too much per revolution but close enough maybe for the three threads I need to cut.
Hmmm - 27 divided by 9 = 3

SO - I see something here; what do I do to make this work? My skill set is like I am still back in high school machine shop or not much better but it is great to be ably to make many of the things I need.

It works the same as cutting a single-point thread but you don't need to engage the half-nut.
Set up the feed and do not disengage anything until the thread is complete.
Set your stops, back the tool out when the machine stops.
Run it in reverse to get back to your starting point, run the tool back in.
Repeat.

The last time I did this it worked very well on a LeBlonde 1610… but that was in 2007. At the time I only had about 6 years experience and was able to figure this out on my own, so don't be afraid to experiment with some scrap. You'll find it to be fairly simple.
I haven't needed to cut threads this way since then (what I'm trying to say is I'm writing this from memory so if anyone can give better instructions, please do).

[Chill Wills]

Sorry but I am not making the connection.

If I use the gear box lever combination and set to advance 0.379" per revolution; I will get nine (9) threads per inch and I somehow want to morph this into a 3X finer 27 TPI.
So, after I set the 0.379" / 9TPI, what do I do so the carriage does not advance so fast - in other words - we are set at 9 but will cut three times slower at cut 27 TPI?

Thanks for putting time in on answering this... I am sure I am missing something in the answer.

[JSnover]

It's a question of which mode to use; thread per inch vs. feed per revolution.
Without the ability to thread 27 threads per inch, the other option is to feed .037" per revolution and that can only be done if you have the right gears.
The simplest fix in this case might be to browse the web for MVA spare parts.

[labradigger1]

For my south bend with change gears I bought a gear with the correct dp and teeth from an atlas lathe. The arbor was to big so I turned a bushing, cut a key way and it worked for me to cut 27 tpi.

[fg-machine]

Anyone with any machine tools should own a copy of the machinery handbook , if you don't own one buy one .
there is a whole section on calculating gear ratios. And it does a very good job of explaning how in simple easy to understand language . Learn it , love it , live it .
But really you need a copy it will answer almost every question you can come up with

[Moleman-]

A while back I needed to cut a 7/8"-3TPI double start (tandem) square thread. The lowest any of my lathes went was 6tpi. So I replaced the 22 tooth gear on the input side of the gear box with an 11 tooth gear. Set the gear box at 6tpi and the compound 90 degrees to the cross slide. Cut the first thread, advanced the compound and cut the second start.

[Chill Wills]

It's a question of which mode to use; thread per inch vs. feed per revolution.
Without the ability to thread 27 threads per inch, the other option is to feed .037" per revolution and that can only be done if you have the right gears.
The simplest fix in this case might be to browse the web for MVA spare parts.

OK JSnover - I am with you now! You know what they say about assuming things!

I had assumed the TPI and the decimal advancement chart would be the same amount of advancement no mater how it was powered. We had a birthday party tonight so this did not have my full attention until just now. I got a moment and took your clue and did the math for myself.

All this time I assumed wrong about the carriage and so there is a ratio 3-1 for the clutch and the half nuts when power is routed one way or the other - cool - I think you have an idea worth trying!

I may cut a prototype in aluminum first before steel and see how it goes.

Thanks!

[Chill Wills]

Anyone with any machine tools should own a copy of the machinery handbook , if you don't own one buy one .
there is a whole section on calculating gear ratios. And it does a very good job of explaning how in simple easy to understand language . Learn it , love it , live it .
But really you need a copy it will answer almost every question you can come up with

yup - have one and my nose is in it a lot. I do not even pretend to know 1/1000 of what is in there. This machine shop hobby, like cast bullets, is too big for anyone to have it all down pat and mentoring, just like I got is why we are on here.
Thanks for the input!

[cheese1566]

Yep. Magma Star uses 27tpi for the punch and punch lock nut.
Happy that my Grizzly lathe can cut it and sven my new-to-me Craftsman 109 miniature lathe from the early 1950's can as well.
Made several punches and locknuts.

[Kilroy08]

Interesting stuff! Even though I've been a machinist for 14 years, I wouldn't have thought to use a longitudinal feed rate to cut an uncommon thread pitch that wasn't available on some lathes.

[smokeywolf]

I'm wondering if he (Chill Wills) will be able to pick up the thread in the same place if he disengages the feed, backs the carriage up and makes a second, third, fourth, etc. pass. As it's only 3 threads he's cutting, he may want to rotate the spindle by hand and just back his single point tool out and rotate spindle in reverse without disengaging the lever on the apron in order to back out.

[ProfGAB101]

My Grizzly G0602 10x22 will do a 27TPI. My 3X more expensive G4003G gunsmithing lathe won't do a 27tpi. Its problems like this that help me decide I should just buy a CNC turning center and sit back and listen to the splashing coolant and flying chips.

[JSnover]

I'm wondering if he (Chill Wills) will be able to pick up the thread in the same place if he disengages the feed, backs the carriage up and makes a second, third, fourth, etc. pass. As it's only 3 threads he's cutting, he may want to rotate the spindle by hand and just back his single point tool out and rotate spindle in reverse without disengaging the lever on the apron in order to back out.

It can be done but I prefer to leave everything engaged, especially on older machines. All the lash is out and you never lose your place if something goes wrong.

[quail4jake]

Yeah, the old 27 tpi trick...proprietary thread. Notice that large cleaning jags are 5/16 x 27, I think to stop reg'lar folks from spinning them off on their home shop lathes. Notice that Magma engineering uses 27 tpi on their punch. You can chase a 27 tpi thread on a South Bend 9 in. with a special spur gear that South Bend used to make. ever try to find a 27 tpi tap?

[smokeywolf]

Even my WEBB won't do 27 TPI; 26 or 28, but not 27. Had a chance to get a Bridgeport (Romi) lathe that would do just about any thread you could dream up; including 1/2 and 1/4 & 3/4 threads. But, it was 80 inches between centers and would have been far too difficult to fit in the shop.

[M-Tecs]

Both the 1/16" & 1/8" standard taper pipe threads are a 27. Not the most common pitches but its hardly a proprietary thread pitch. Any lathe that has change gears can be setup to cutter 27 pitch. Just because it's not standard on the quick change box doesn't preclude changing gears to cut whatever you want.