# need some help with cutting 20 tpi thread on hf lathe



## gt40 (Mar 23, 2010)

I have been trying to cut 20 tpi threads with my new lathe. I am not sure what I am doing wrong. I am using the following gear combo which I got from the Lathemaster site:
tpi: 20
Top inside gear: 72
Top outside gear: 90
Middle inside gear: 45
Middle outside gear: 50
Bottom gear:75

Is this correct? I haven't been able to decypher the factory manual so I am using the gears called for on the lathemaster site.

Here is what I did:
First I turned the rod to .4675. Then I set my compound to 29.5, used a 60 degree cutter perpendicular to the stock. I tapered the edge of the stock slightly where the thread would start. Then I engaged the feed to cut the threads and continued till they were .435 deep. When I compared it to a 1/2"- 20 bolt, the threads don't match. 

I am waiting on tooling etc so I made a cutter which is a 60 degree triangle shape. It seems to cut okay. For measuring, I am still waiting for my thread gauge I ordered so I just used the bolt. I don't know what I am doing wrong unless either the cutter I made is too thick or I am using the wrong gear setup. Any suggestions or verification of the correct change gears to use would be appreciated!


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## 65535 (Mar 23, 2010)

Check the distance to centers of the teeth. a 20tpi should measure out very close to .05"

You are .0325" shy of a proper 1/2 -20 thread. I would not expect the bolt to match up.

You are basically splitting a 7/16 -20 and a 1/2 -20 thread.

That may be why they don't match up.

Turn the OD to .5000" then thread to .4387"

If you have a proper 20 tpi thread the bolt will fit nicely. With smaller threads such as 1/2 -20 chasing threads with a tap isn't a bad idea.


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## darkzero (Mar 23, 2010)

On my HF 8x14, for 20 TPI I use

_ 70
60 80
50

Not sure if your manual is different but these are the gears it states to use along with the chart on the drive cover which is pretty clear.


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## gt40 (Mar 23, 2010)

Thanks. I should have spent more time on the manual and outside of the machine than reading something from a 3rd party. I just received my indexable thread cutting tools so I will give it a try with your gear settings.


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## will (Mar 23, 2010)

There are 2 issues that should be checked

1. Are you actually cutting 20 TPI? this can be checked by lining up the peaks of a known 20 TPI bolt with what you are cutting on the lathe. should look like this:
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2. Are you cutting deep enough?


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## darkzero (Mar 23, 2010)

gt40 said:


> Thanks. I should have spent more time on the manual and outside of the machine than reading something from a 3rd party. I just received my indexable thread cutting tools so I will give it a try with your gear settings.


 
One thing that is not covered, when you install the 70 gear on top, it specifies to be on the inside (to your right). On the outside (to the left) you will need to use a "dummy gear". I just reinstall the 35 gear that was originally there (assuming you were set for .005" feed before the change). Doesn't matter which gear you use for the "dummy" as long as it's not big enough to interfere with the 60 gear that will be directly below it.

At the bottom where the 50 gear needs to be. Originally the 100 gear (again assuming you were previously set to .005" feed) is on the inside with the spacer on the outside. The 50 gear will need to be on the outside so it engages the 60 above it so you would install the spacer on the inside. 

The do a cut at .002" & check the pitch. The .002" pass won't affect the part if you're off & need to recut but with those gears you should be spot on.


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