SFM Question

Hi All,

Question about SFM, I'm cutting 316 SS on a Citizen M20. The stock is .250" dia, all turning. I'm getting recommended SFM from tooling suppliers to people online for anywhere from 300-650 SFM. The largest diameter on the part is .1565" and the smallest .040". using 300 SFM if I'm doing it right that puts me between 7000 and 28000 RPM! My machines don't do that fast and this is definitely incorrect. For Swiss turning particularly is there a formula I should be using to figure this out correctly?

Talked to Applitec Tooling this morning about a grooving insert they're getting me regarding this turned part. They recommend 120-180 SFM for this tool in 316 SS. This seems much more accurate, puts me about 3000 RPM. I can start with this for turning applications

Hi 55th,
When turning smaller diameters (generally <.125", and especially when you get <.050") SFM calculations begin to break down as a hard and fast rule. We also have citizen M's and in my experience turning small diameters of .020-.050, I generally find that 4000-6000 rpm works best, even though the SFM may calculate out to be very low. I cant tell you exactly why this is, its just what I have observed over time across many jobs, materials, tools, inserts ect. When turning faster, I sometimes see smearing/chip re-weld, bar vibration becomes excessive, or the high point on the insert burns up quickly (if taking a larger depth of cut the largest diameter on the insert sees a much higher SFM than the cutting point)

So anyways, I see 300-800 sfm on many inserts now days, but cant imagine trying to hit 600+sfm even if we had a 16k or 20k spindle. I would bet that unless your barfeeder is specifically designed for high rpms, the higher speed will produce other problems that outweigh any benefits. When a speed calculation gives you a huge number like 28k, just try something in the 3000-6000 range and see how fast you can go without other problems developing. Speeds from 50-250 sfm are much more practical at small diameters.

Thanks MCImes, this is great advice, thanks for your time.