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threading metric gears on lathe

silencerman

Plastic
Joined
Nov 7, 2011
Location
Pennsylvania U.S.A.
i have a project i am in the planing stages of that requires a 14x1 LH internal thread. a tap is over 30 dollars and i only need to make one or two parts that use the thread. i plan on doing the project on my dads south bend 12 inch, flat belt drive, motor under the bed with quick change gear box lathe ( sorry i don't know the model number) he said he has the metric conversion gears but can't remember how the pitch then correlates to the gear box setting. can anyone help me out here? i'm sorry if i have not given enough information let me know what you would need to help me out and i will try and obtain it. also i am assuming the threading dial is useless with metric and i will have to reverse the lathe to reset for the next pass.
 
Hello,

You're in luck, 1mm pitch is very easy:

1. install the transposing compound gear, the 127/100 combo
2. set the QC box to 20 TPI

Shazam, your lathe now cuts exact 1mm pitch threads :D.

The math -- 127/100 * 20 = 25.4, there are 25.4 mm in 1".

Switch the QC box to 16 TPI, and you get exact 1.25mm pitch threads.

Remember, with metric threads, you cannot disengage the halfnuts once you start your cutting passes. You have to stop and reverse the motor to go back to the start for the next pass.

Regards.

Mike
 
Silencerman,

I am always in the boat that you are describing. My lathe does not have a threading dial, so I always reverse my lathe when cutting threads. Make sure to fully retract the cross feed so the tool does not bind in the threads when you reverse. I do one full turn out, easy to keep track of.

I just keep advancing the compound and taking passes until the matching part just starts to thread on then I advance with the cross feed instead of the compound so my cutter takes a cut off both sides of the threads and cleans things up. I am talking about a few thousandths at most for this final bit.

If I don't have a matching part to try on the threads I use thread measuring wires and just keep taking passes while advancing the compound with each pass. If you keep measuring with the wires as you go you will get an idea about how much a turn of the compound takes off in depth so you can anticipate when you will be done.

I would strongly discourage you from taking a full depth threading pass. That would probably result in a less than pretty thread.

BTW, what part of PA are you in?
 
did i mention it is an internal thread? the mating part is an ak-47 threading that on and off to check the fit would be a pain. what i might do is make a test plug to spec and use that to test the fit.
 
I test plug would be exactly what I would make since I have no good way to measure internal threads.

I am not aware of another low cost method other than making go/no go gauges for the threaded hole. I think I can do at least a decent job making go/no go gauges since I can measure the male threads of my gauges with wires.

I don't know how much clearance to recommend when you make your gauge so you don't have to put things together with a wrench (or maybe you do). For the one-off work that I am doing I work a lot one how things feel, since I have the luxury of having the entire job in front of me. No need to make a piece for a job that is 100's of miles away.
 
morsetaper2, your links aren't working for me. mattsix, you are right i am waiting on my forum 1 packet from the atf so i can fill it out and send it in. when i get it back i'll be working on a 30 cal can for my ak and mosin nagant scout rifle. in the mean time i'm working on a new comp for a customer and i want to try it out on my ak before i sell it to him.
 
morsetaper2, your links aren't working for me. mattsix, you are right i am waiting on my forum 1 packet from the atf so i can fill it out and send it in. when i get it back i'll be working on a 30 cal can for my ak and mosin nagant scout rifle. in the mean time i'm working on a new comp for a customer and i want to try it out on my ak before i sell it to him.

The links work for me so I don't know what to say. The images are in my photo album hosted on this forum. So locate my photo album under my username and parse through my photos and you will find them.

In this album.... click here for SHOP STUFF

Click on my username and open up my profile, look in the "shop stuff" album if the preceding link doesn't work for you.
 
ok that works for me. my question now is does the transpose gear replace the idler gear or work in addition to it? considering it's size and that little chart in the photo i would guess it replaces it but i just want to be sure.
 
silencerman,not to change the subject of the thread,how do you manage to get a 30 cal subsonic without cycling problems
 
The transposing gear replaces the 80 tooth idler. The gear on the gearbox is swapped to the outer position to cut metric. It is left on the inner position to cut imperial. The 80 tooth gear can stay in the drawer henceforth.

allan
 
kitno455, thank you. mattsix, short answer: it's really hard. long answer: first thing you can do is lighten up the recoil and hammer spring, after that open up the gas port. you need to use a 200 -240 grain bullet which requires a 1 in 8 twist barrel (stock ak is 1 in 14) to stabilize at sub sonic velocities. really it requires a purpose built gun. i don't really plan to shoot subsonic out of my ak but i do plan to shoot them out of the mosin nagant. you can still shoot full velocity rounds through a suppressor. it will quiet the muzzle report making the sonic crack the loudest part of shooting the gun.
 








 
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