It could depend on the size lathe you are working on. It is hard to bump knurl a coarse pattern on a light lathe, such as a SB9 or SB10K. Sometimes people reduce the width of the knurl for light lathes (grind the teeth off much of the width on a spindex or similar on a surface grinder). Another way is to cant the knurl slightly so it can penetrate more easily.
Having said that, I never had much luck with bump knurling on my 10K & second op Hardinges. But the scissors type works great for me.
As far as diameter, as others have said, it is not that critical, if the lathe is heavy enough to get deep penetration in only a few revolutions; or if a scissors type holder is used. If the knurls cannot be made to penetrate quickly, they may well tend to double and sometimes triple track. Feed the knurls in steadily, positively, and relatively quickly. Dwell briefly and back out.
I always set up a practice piece to get the knurl opening (on scissors type) and the stop or dial reading on the infeed. I have used the longitudinal feed (as opposed to lead screw) for longer knurls, but try to design parts so it is not necessary. There are many metals that will start to spall off the points (work harden and shed the knurl) if it is done with too much dwell. So a slow rotation, relatively heavy infeed, and relatively coarse longitudinal feed are wanted. But again, on smaller lathes not all of this may be possible. I avoid using the leadscrew in order to preserve accuracy for threading, and protect the half nuts from wear with the heavy knurling forces.
smt
