SFM

[Roy BirchParticipant @roybirch29994]

Hello All

The Boxford is up and running now and all is good, I have a question regarding SFM for EN steel, my lathe is a metric lathe so in order to get the correct RPM do I have to do any conversion of SFM to metric?

Kind Regards

Roy

[Roy BirchParticipant @roybirch29994]

[DiogenesIIParticipant @diogenesii]

No, SFM cutting speeds expressed as RPM for given diameter will be the same for all lathes irrespective of which units they use for linear measurement.

If you want to covert SFM to metric form, the term used is metres per minute – "m/min". Searching "SFM to m/min" should locate conversion tables / formulae on the 'net.

[SillyOldDufferModerator @sillyoldduffer]

Yes, but I prefer working from an easy rule of thumb.

For steel, rpm = 10000/diameter of work in millimetres. (Cutting with HSS. Multiply by 5 for carbide.)

It's a good start point from which best results can be experimentally searched for.

SFM numbers are aimed at industry, who optimise metal removal, tool-life and power consumption for maximum profit. We don't need to do that, so the rule of thumb works well. I can't remember the last time I looked up a SFM recommendation.

RPM for other metals can be derived from steel; cast-iron is slower, say half or a third, and Aluminium is faster, say 2 to 4 times steel rate.

In a small workshop, the optimum is often impossible because our machines aren't fast, rigid or powerful enough. It's usually necessary to experiment with rpm, feed-rate and depth of cut for best results. I'm sure your Boxford could be run harder and faster than my Chinese Lathe, but it's powerful enough that I don't care to run it flat out because it sprays smoking hot chips everywhere. No fun at all! I generally back-off to suit me rather than push the lathe to it's theoretical limit. I like thge machine to sound as if it's working, rather than just spinning, but not labouring or spitting metal. A mini-lathe would be operated even more gently, but they still get good results, just takes a bit longer. RPM about the same, but feed-rate and particularly depth of cut reduced.

Dave

 

 

Edited By SillyOldDuffer on 13/06/2022 21:57:46

[JohnFParticipant @johnf59703]

A handy set of charts here in FPM with the "type" of steels etc as well as the US spec numbers, but the type is more appropriate for most use

Cutting speeds

[HopperParticipant @hopper]

For us imperial guys, the formula for cutting steel is even easier: 400 divided by the job diameter in inches. So a 1" diameter job spins at 400rpm and a 4" inch diameter at 100rpm. A half inch diameter job at 800rpm etc.

For soft metals such as brass or aluminium, double the rpm. For hard stuff such as alloy steel and cast iron, halve the rpm.

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