Do RPM's matter when cutting on a lathe?

[MychesterCNC]

I'm modifying a taig lathe to turn a large diameter (12"+) wheel and since the motor is small and I will use step down pulleys to increase the torque but of course this will slow the speed of the material turning...

Any disadvantages as far as quality of the cut, heating up, noise, etc... that I should be aware of while cutting a round part (out of steel) that is turning so slowly?

(BTW, I realize I will be needing to take small amounts of material off at a time)

Thanks in advance!

[MattTheNoob]

The "SFM" (surface feet per minute) or speed of the material surface relative to tool is what you are looking for. The same applies to any cutting operation, be it on a lathe, mill, shaper, saw, drill... Use your google-fu to find the recommended speeds for the material you are cutting, and the type of tool you are using (HSS vs. carbide)

[MychesterCNC]

Yes, but will a slow speed in itself cause major problems? I'm not looking for efficiency, I am already expecting to take of minimal amounts at a time, I'm wondering if the fact that the wheel will only turn 200 rpms or so will cause some obvious problems that I'm not aware of yet...?

Thanks!

[lordedmond]

take a zero off and you will be somewhere near 20 rpm on a 12 dia

assume 60 fpm

them 60 x4 /12 =20 rpm

the 12 is you work dia the 4 is a constant

[MattTheNoob]

I think the best answer I can give you is "sometimes". Most of the time, slow is ok. If you are using carbide cutters, they sometimes like higher speed and will give a better finish than at lower speeds. Not knowing what it is you're making, that's the best I can do.

[Shotout]

Originally Posted by MychesterCNC Yes, but will a slow speed in itself cause major problems? I'm not looking for efficiency, I am already expecting to take of minimal amounts at a time, I'm wondering if the fact that the wheel will only turn 200 rpms or so will cause some obvious problems that I'm not aware of yet...? Thanks!

You want a cutting tool to cut and running a SFM that is to low will cause the tool to rub rather than cut. With a tool of any material if it isn't cutting then excessive heat is being generated. Excess heat will cause the tool tip to overheat and break down. Surface finish will suffer and some materials will harden. The excessive heat in the work piece can cause it to move after it cools leading to run out. The heat can cause the part to grow so after cutting it to dimension it will shrink after it cools, how much depends on a few factors, material; temperature; how deep the heat has soaked into the work piece; etc. The idea is to cut at a recommended SFM range based on work piece and tool material, RPM is how you achieve that SFM, so that heat is carried out of the cut to allow the tool bit tip to remain at a temperature that it is stable and below it's plastic state temperature.