For a lathe, as far as I can tell, there are only two PC software based controllers available which can do electronic gearing (synch an axis to the spindle). MACH2 is a WIN 2000/XP controller with a very good reputation on the CAD_CAM_EDM_DRO new groups, but needs a pretty powerful PC (like any windows NT based OS). The other is Dave K's TurboCNC.
Initially I tested my controller with KellyCAM's free demo under WIN 2000, but the pulse train was horrible.
EMC is interesting because it is Linux based, free, and probably most powerful of the PC software controllers having cutter radius comp, constant velocity conturing and lead screw error mapping built in. Unfortunately there is not electronic gearing built in yet.
I switched to TurboCNC V3.24 because it had a free unlimited trial, and registration was only $20 and got you the source code. Being a DOS program it will run on anything with a maths co-pro (basically 486DX and up). I got to scrounging and got offered a 486DX33 Toshiba laptop by a colleague who was happy to see a once expensive purchase get a new lease of life. (Thanks Norbert).
The toshiba gives me about a 13kHz, very clean, pulse train on this slow processor, which is plenty for my half stepping drive and low torque motors. The program is very powerful, having a number of basic drilling canned cycles, a good turning cycle, and a single point, single pass threading cycle on top of the standard stuff.
I wrote a little Excel Macro to generate the standard Metric threads using multiple passes of G33.
So far I mostly run the Lathe in MDI mode, or just hand write the programs in notepad, or excel.
I am on the beta testing team of the TurboCNC revision V4.00. This is a huge leap forward in the user interface, with a built in text editor and the ablity to jump in and out of programs, MDI and jog easily. A new file handling system is also a great improvement. The expansion of single pass threading (G33) to tapered threads, and a new code (G76) for multipass threading add to its convenience.