Adamsgt,
Don't give up, you'll need to do something with your lathe.
Unless your lathe is missing teeth, or so worn that it is jumping a cog in the gearing, and not having a broken shear pin in the feed screw, any lathe will cut a thread. 60 deg threads can be just about any pitch you desire, usually 4-80 tpi, so if your gearing is wrong, only will the pitch off. It will still cut a pretty thread, just will not be the right pitch.
If your chuck is in real bad shape, IE, the jaws worn to the point that they will not clamp equally around a round part, Though you would notice this as the part "barrel" would be jumping around.
What you are experiencing is a problem with the backlash in your lathe, All machines have it, even very pricey CNC's. It's the nature of the beast.
On a lathe, you will find you have backlash in the carriage feed gearing, the lead screw, the cross slide screw, and the compound screw. These all have to be eliminated per se before you start to cut and before you set the dials to "0"
As an example, turn the cross slide screw, counter clockwise until the cross slide moves a little, then turn it in to where you want it to be zero then you turn the dial to set the "0". The same thing goes for the compound. Any time you retract the tool you need to be sure you have zeroed out the backlash.
Now the Lead Screw, as shown in Bob's video, you need to engage the half-nuts at the proper mark on the thread dial. When doing this you sometimes "most always" need to use your left hand as a brake on the carriage feed wheel, as you engage the half nuts. This will ensure that your backlash stack up, driving the carriage is reduced to zero. After a while you will get good enough to chase down the mark you want on the thread dial by using the feed wheel.
A lot of machinists will pull on the tool post before they start a cut to make sure the backlash, with regards to the cross and compound is out.
Also check your Gibbs on the slides to reduce any excess play.
On some lathes the compound is so shot that operators will just set them at zero, then lock them down tight, and cut their threads straight in, "Radial Infeed". It's really a lot less stressful for a novice,,,,,, doing it that way.
Read that article from Sandvik that is posted above. Don't worry about the techie stuff, but it will help explain the different ways threads are cut.
It's really easy once you catch on to it, don't give up.
IndianaJames,
Carbide, for all practical purposes does not cut material, but instead removes metal by rubbing it off. There are exceptions to that rule, with very high positive inserts, but generally they do not cut. Thats why rigidity and horsepower is necessary to utilize it properly. Like I said there are exceptions, like Swiss Tooling and Nonferrous tooling. Light weight machines, Like most speak of here are better off with HS as they cut.
Having said that, in our CNC's we cut, "rub" usually around 50 thou as a finish pass. Not for threading, but turning. The reason they recommend a minimum depth of cut is that Carbide will super heat if run too shallow, and will knock the edge off.
Sandvik recommends a high positive insert for cutting stainless. It does not like being rubbed on.
