After reading some of the answers to your post (and your other responses):

Point of honor: If you arbitrarily choose 80 sfm for the cutting speed of
your
1018, it yields a RPM of approx. 853 for 3/8 diameter. The stated 40 RPM is
extremely slow. Go as fast as your reflexes/skill will allow. Practice
until you
can approach 300 rpm. Your current 40 RPM translates into SFM of about 4??

Get some cutting oil, and brush on liberally for each pass. The old
fashioned
sulphur based cutting oil is quite good for single pointing.

Study the machinery handbook a bit more on thread form. A sharp vee
is not considered standard. With that in mind, and after stoning a slight
radius
on the end of your tool, it should be clear that you will achieve proper
thread
form before a sharp vee occurs on the OD. Once you have done a few threads
you will learn to gage the flat on the OD as a clue that you are almost
there.

Burrs happen when machining. Suggest removing with file or emery cloth
before reaching final thread depth. (Which you did!)

As to tool angles: When threading and using the compound rest at 30 degrees
a positive rake at right angle to the left side of the thread profile will
give best
results with the tool mounted horizontally. (for 1018) The back rake only
serves
to put everything except the tip of the tool below the center line of the
work.

As to the final pass producing roughness, it is VERY common. So before you
get
to that point, you start babying it. Extremely small infeeds when you
approach
full thread depth. Perhaps even a couple passes without any infeed at all.
(Known as spring passes)





"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