I hope somebody has the patience to read through this :-) I am trying to
include all the details because I don't know what I am doing wrong.


I recently tried to cut a 3/8-40 thread on a piece of 1018 HRS. First
pass was smooth. With later passes the surface seemed to get rough,

then before reaching final depth small sections of the thread tore out...
so I tried again:

I ground a new 60 degree HSS threading tool with 12 deg. relief on both
sides and 10 degrees side and back rake, and stoned the top and sides

smooth. It is mounted directly in a turret type tool post, (i.e. the tool
bit is horizontal, not sloped upward as in a rocker type tool holder).

I turned a 0.500 diameter section on a short bar. It is held in a three
jaw chuck, (no tailstock support).

I set up the threading tool on center height and perpendicular to the
surface.

I set up the QC gearbox for 40 TPI, spindle for slowest speed (about 40
RPM), compound slide at 30 deg.

First pass was just barely grazing the surface to ensure that there were
no surprises (i.e. mistakes in my setup), zeroed the collars.

I started by feeding in the compound 0.002", reducing progressively to
0.0005" for the last pass.

I measured the OD occasionally. It increased by 0.001" after about three
passes, i.e. it appears that a "burr" is being raised - is this

normal? I didn't expect it. I touched up the OD with a file before
proceeding.

I seem to be reaching the final thread form before I have cut as much as
I thought I needed from the Machinery's handbook, (0.01534" deep,
0.0177" on the compound at 30 degrees).

Finally on one pass I got a rather rough appearance to the thread at the
right hand end, although the rest of the thread still appears to be

smooth.

In case it's relevant I am doing this on a 10" Atlas lathe.

1) What can I do to reduce/eliminate the "burring"

2) Any ideas why I ge the rough thread, especially at the right end?

Rob