Re: A couple more machinist questions
SFM stands for Surface Feet per Minute. This is the speed at which the workpiece material is passsing the cutting tool edge. For a give diameter, the higher the RPM, the faster the SFM, because the workpiece material is moving past the cutting tool at a much higher speed.
The quick and easy formula for calculating SFM is RPM X DIA in inches X .26. This should get you close enough.
Different workpiece materials can be cut at different speeds due to the hardness of the material, the heat generated in the cut, the rigidity of the machine and set up, etc.
On a manual machine you will usually be limited by the speed at which you can react and manipulate the controls. On a CNC machine this all happens automatically, and operator dexterity is not a factor. Most manufacturer's higher speed recommendations are made for the CNC applications.
On a manual lathe you will probably find 200-400 SFM on alloy and stainless steel will be your limit, and a little faster on aluminum. Run the aluminum as fast as you can control the machine.
If your speed is tool high on turning or milling operations for the work piece material and cutting tool, too much heat will be created and the tool will burn up or dull quickly. If the speed is too slow, the tool will not effectively cut the material, but will tear it off and leave a lousy finish on the work piece surface.
For most gun smithing applications, you will want to use PVD or uncoated lathe inserts. These inserts have a sharp edge and will make lighter cuts without creating as much push off as CVD or honed edge inserts.
An old machinint's rule of thumb was "Double the speed and half the speed on a finishing cut", or "Half the speed and twice the feed on a roughing cut". The lower the feed, the better the surface finish (less threading on the part OD).