G33 or G76 threading?

I have a 10,000 ppr spindle encoder hooked up for thread sync and I want to produce the most accurate thread I can with a very light final pass using a Sandvik threading insert - I have a couple of questions:

1. The LinuxCNC manual states that G76 is based on G33 and I'd like to double check that G76 does indeed use spindle synchronised motion (I can't see how else a multipass thread could be cut so I guess it must).

2. Infeed per pass - should I use the recommended Sandvik catalogue infeed per pass (in which case I guess I've got to write a custom program using G33), or is it sufficient to use the G76 'R' depth degression parameter (not sure exactly how this works)?

3. Infeed type - for smoothest thread flanks, should I be using radial infeed, or some sort of flank infeed as described in the Sandvik catalogue, and if so, what G codes do I use to implement this?

Hi

I will have a go at answering, but the I and R values is as tranparent as bricks.

1) Yes based on G33

2) The I value covers the initial cut depth and I have always taken it to be a rough guide to infeed thereafter
If R is set at 1, it seems to pretty much feed in by those increments, If set near 2, the deeper it gets the smaller the increment, because more of the tool is touching metal
You can end up just rubbing and not cutting much

3) You can select Q to get angled infeed type, as you would on a manual lathe.
However I have a slant bed lathe with an 8 tool ATC at 90 deg to the drive line, so in fact I always have a 90 deg infeed, use light cuts with the tool upside down in the holder and using M3, for
coventional rh threads
I also normally use a tool steel bit freshly honed, which I find plus neat lube gives me the best threads
(but then I don't do a lot of them)

I would suggest that you run a sim and experiment with different settings to see how many passes of what depth result.
If you need a constant infeed depth, then writing a routine using G33 which loops until depth is reached by set increments is pretty simple

regards