Other replies had much good info, but:


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

You might want to try threading at the highest speed you dare run at.
Since this is a very fine thread, you can probably do the threading
at several hundred RPM, at least.


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.

Definitely, the burr is normal.


I touch up the OD with a file after *finishing* the thread. No
point until then.


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).

Tool deflection or workpiece deflection is very common in threading
operations. So, I'd expect it to take more infeed to get down to
final thread diameter. How do you measure it, with wires or triangles?
Without these special tools, measiring the true thread diameter is
almost impossible.


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


At the very end suggests deflection of the workpiece, though it
could be play in the gibs of the cross-slide or the compound. Or a
combination of them all.


Right! Does it make a crunching or snapping sound when threading at
that end? If so, you may need either a tailstock center or a follow
rest to support the workpiece and keep it from climbing up over the
threading tool. Long, thin workpieces will try to do this, and you can
actually see the part jumping up and down as the threading tool takes
bites out of it. Even without the tearing, you'll never get a proper
thread when the work is flexing like that.


Jon