I can't cut a straight thread, on my lathe

Cut tapered threads?

Really, not enough info, lathe, thread, how much taper, etc

The only thing that makes since to me is that you have to much stick out from the chuck and are getting a lot of tool push off. What size material are you threading, what material is it, what type threading tool are you using? If it is something that has to stick out beyond a 4 to 1 ratio then support it with a center in the tailstock.

is your compound set to 29.5 degrees?  How big a cut are you taking? Have you tried to support end with tail stock?

sounds like the compound is loose and being pushed off the work.. Heres a test .. Do a radial thread , with the compound parallel to the work .. So stop the modified flank cutting ( 29 degrees) and just go straight.. lock the compound  because its now eliminated from the process. Lower the tool height just below perfect , and I mean by .010 or so , nothing dramatic..keep in mind infeed will now be adjusted at the crosslide in this configuration

james, this also takes the compound backlash out of the equation completely so you can monitor the crosslide backlash and tool deflection

No stick out, I cut right up at the collet or jaws.

I don't the 29 degrees, I always cut straight in.  The carriage, cross slide, compound, and tool post are tight. The lathe is tipped back a bit, I'm going to level it up, and see if that makes a difference, I would be surprised if it did.

well I think you said this machine has always been a problem for threading..

I think the problem is free play in the head stock spindle bearing. I can push and pull on the chuck, and get a couple thousandths, movement each way. I'll look at adjusting the bearing.

Are you using the tailstock to hold you work in center? It could be the the tailstock is not centered. There are aluminum bars that are certified for checking tail stock center for under $100.00 and they are worth every penny, and will save you a lot of headaches down the the road. A tapered thread or any type of taper cut usually means tailstock is out of alignment, that would be the first place I would look. You can try this  https://www.amazon.com/BLAKE-Blake-Co-ax-Indicator-Set/dp/B005PX7QV8/ref=sr_1_2?ie=UTF8&qid=1518316761&sr=8-2&keywords=coaxial+center+finder       or  you can try this     http://www.penntoolco.com/27-000/?_vsrefdom=adwords&gclid=EAIaIQobChMItNXdj-uc2QIVgx2BCh1jkwDREAQYASABEgLWAPD_BwE. The second is a cheaper option the the blake co-axial but I don't know how much machine you want to really d=get into. I would get one from penntoolco personally. You can look for others out there just make sure they are certified.

Good luck

Pat

you can't cut threads straight in have to have the 29 1/2 angle on compound or you will never accomplish anything

Eddie, you are mistaken.  Threads cut by only cross slide advances will cut threads just fine.

If a dies to know only goes on a short ways, it seems to me James the wrong pitch is selected.

Did you ever get this fixed?

pat

Does your lathe cut a taper when you just take metal off bar stock, no more then a inch or two? Are you cutting between 3 jaw and live center in the tail stock? are you cutting between centers? Important did you indicate anything in before you started your threading, or just skipped the most important part of the threading job? This is what kills most operations on the lathe is the lack of setup. 30 minute set up to do a 15 minute job equals 1 hour shop rate or can make 2 for roughly same price :), all ready set up. Training goes a long way here when it comes to threading. Are you using hand ground tool steel or carbide and who ground the tool blank? Type (name) of lathe would be helpful to.