On Oct 7, 5:55 am, Stealth Pilot
wrote:
I'm having my second go at a piece of torture.

The item in question is the screw threading for the Nano.
The 0.1cc diesel published in Model Engineer some years ago.

I've built one of these before that should still be sitting on the end
of Craig Wall's desk as a paperweight. In that instance we spark
eroded the ports and stuffed them up with all ports ending up at the
same height in the cylinder.
This incarnation wont make the same mistake.

I dont recall just how I did it before but in this torture I'm having
trouble getting good threads. If I look at them under a 10 power
magnifier the threads look like a wire pulled through clay. tear smear
tear smear tear smear, you know the stuff.
the real killer is that I'm using 1214 free machining steel this time.

apart from having the tool exactly on centre height, sharp edges to
the cutting tool, and experimenting with cutting speeds there are no
other tricks to getting polished threads at 7mm x 48tpi are there?
I'd welcome a description of how you get really schmick threads at
this size.

(other than the steel threading it all comes out ok.)
Stealth Pilot

I've done 50 tpi for some oddball gun parts. I made sure the HSS tool
edge was SHARP and polished the working faces with a black Arkansas
stone. Need to run the lathe at high speed, too. This might be the
time to think about a rear tool post and thread things in reverse so
the carriage goes from left to right. Otherwise, make your threaded
section much longer to allow for reaction time and trim to length
after threading.

Stan