Did you grind a proper side relief angle? Don't forget your tool travels much faster than when normal cutting, so it needs the normal relief angle plus the thread pitch angle. If too small, the tool will scrub. This angle must be larger for low pitch threads. The one in the turning set usually is good only for high pitch threads. If you hold the tool by hand with the left side relief against a good thread, the top should be tilted 5-10 degrees to the right.
Ken Shea's advice ensures that the tool will only cut on the edge facing the chuck. Set the dial on the cross slide to zero so it's easy to find the same setting for each pass. If you feed with the cross slide it will cut on both faces, which gives the result you describe. The last shaving should be done with the cross slide though to take off any steps on the trailing flank. This must be a very fine cut.
Use cutting oil.
Good luck. It's satisfying when you get there. |