First off, lose the rake.
The complex geometry of the tool cutting into a curved surface, makes it
almost impossible to get a tool with rake to have the proper thread profile.
Back rake is only good for roughing out coarse threads and will need to be
followed by a finishing tool.
Side rake is only needed for course threads or very tough materials.

If you are in a situation that needs side rake, like production cutting
coarse threads, where a tool change would be non productive, or cutting
threads in copper or inconel, then do this...

Use only side rake. It should be perpendicular to the leading edge. This
way the leading edge
remains horizontal and cuts an accurate angle
on that flank of the thread. Then set your compound very precisely to 30
deg.
This way the accuracy of the trailing edge of the thread is formed
by the travel of the compound.

Either way, for 40 tpi you do not need any. Your tool should be flat.

Next, be sure that your tool has the proper clearance. The leading edge of
the tool needs more clearance than the trailing edge,
Look at the end of the tool with the cutting surface up. The junction of the
facets should appear to lean clockwise on a right hand external tool. For
left hand, it should lean counter clockwise.

I suspect this clearance is your problem. The lack of clearance causes the
tool to heel or rub against the thread before it can cut it.
It causes you to use excessive pressure because the edge of the tool cannot
cut and is instead forging the thread into the steel. Eventually you get
enough pressure, that the work rides over the tool snags it, and forces it
to cut where there is enough clearance, Usually, in the middle of the
previous thread.

Paul K. Dickman

Rob McDonald wrote in message ...
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