OK, if you are new at this, two things.
1) cutting parameters: take off no more than a thousanth at a time. Run the machine at the slowest speed possible. Keep the work brushed with cutting oil - NOT lube oil.2) for a novice practicing threading, give yourself a runout groove beyond the end of the threading area - that is to say a groove that the tool can land in, that is well deeper than the minor thread diameter. Only after you have gotten some time single pointing threads should you try to thread to shoulders or stop the threading at some particular point.
Simply opening the half nuts and allowing the tool to inscribe an ever-deepening groove on your part is not such a bad way to end a thread. The problem with it is that it a) looks ugly and b) creates a stress riser at that point, a bit larger than the rest of the stress risers that the actual threads represent. This can be eliminated by making the runout groove with radiused corners. Actually a thread that ends *with* a section of stock that is below the minor thread diameter, and has all of its edges radiussed, is much stronger than a thread that simply ends in mid-stream.
Also remember that when threading to a shoulder, with the compound set to 30 or whatever degrees, that the endpoint of the thread will walk to the left as it gets deeper, as you increase the cut depth with the compound dial.
Finally (and this is much more than the two important points I wanted to make above) when you are trying to do ID threading or threading to a shoulder or specific point, use a DIAL INDICATOR on the carriage. You should set it up so that when the carriage reaches the point where you have to open the half nuts, the pointer sweeps by zero on the dial. I use a one inch travel indicator that clamps to the bed ways.
The reason that cutting with the compound parallel to the ways, plunging straight in, will encourage inaccurate threads is that even if you do properly load up the compound screw in the direction of travel, the cutting forces on the tool are fairly ballanced left to right. Granted the cut is happening right to left but those forces can be mild compared to the infeed forces involved. The infeed forces ultimate resolve into left/right forces.
So a small amount of inequality in the cutting tool angle can actually cause a force that *unloads* the compound screw and can cause it back off, ruining the lead accuracy of the thread.
Single point threading has one main advantage over die cutting threads, and that is lead accuracy. The thread form tends to be a bit sketchy unless the tool is really set up with gages and all. But the lead will be spot on and it's a shame to throw that away - othewise a die cut thread will be just as good.
Many will single-point a thread almost to completion, and then take off the last thou or two with a quality die, to get the best of both worlds. Lead from the lathe, and form from the die.
Do remember however that unless a thread is rolled, it will be quite weak. Single pointed and die cut threads invariably show all kinds of stress risers and tearing at the thread root that make them many times weaker than a rolled thread.
Jim
================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================