Math Help

[Richard McComas]

My shaper spindle has 3 speeds, 3700, 6500 and 7800. My shaper cutters are stamped with the maximum safe RPM's. I assume maximum means RPM's at the tip of the cutters.

I do I calculate this?

[Scott Rollins]

The numbers probably indicate RPM. However if you want to calculate Inches per minute of cutterhead speed:
Diameter Cutter head (inches) X Pi (3.14) X RPM = Inches per minute of tip speed on the cutter. If you want it in Feet per minute convert inches of the cutter head to Feet, or metric.

[Rick Moyer]

The RPM's (revolutions per minute) would be the same at the center as well as at the edge. I think you would not need to know the difference of the traveling speed (not sure of the term, angular velocity I think) at the edge compared to near the center of the shaft.

[Rod Sheridan]

Hi Rick, as has been posted you can calculate the rim speed in feet per minute ( I assume you're using Imperial units due to your location).

For shaper/moulder cutters you calculate the required cutter RPM based upon tip speed in feet per minute.

For feed rates, you calculate your feed rate in feet per minute based upon how fast your cutter is spinning, the number of knives, and the RPM.

The following range is what I've used sucessfully.

HSS Cutters

Softwood 10,000 to 15,000 FPM, Hardwood 8,000 to 12,000 FPM

Carbide Cutters

Softwood 12,000 to 17,000 FPM, Hardwood 10,000 to 16,000 FPM, MDF 12,000 to 16,000 FPM.

Cutters may be marked MAN which is the Euro designation for Manual feeding, and these cutters have built in chip limiters to prevent them from cutting chip thicknesses larger than the safe amount. These are the safest cutters for manual feeding, cutters without chip limitation are best used with a power feeder.

I also have a note in my workbook that kickbacks increase below 8,000 FPM tip speed, which probably came from a course I was on, although I'm not sure of the source.

Spindle Speed


So if you have a 5" diameter HSS cutterhead and you want 10,000 feet per minute tip speed for softwood, you would calculate shaper spindle speed as follows;

(10,000FPM X 12Inch/foot) / (5" X 3.14) = 7,639 RPM


Feedrates

Fine work is in the area of 0.010" to 0.030" chip thickness.

So if you have a two knife cutter at 5,000 RPM with a 0.015" chip thickness you get

2 X 5,000RPM X 0.015" = 150 inches per minute or 12.5 feet per minute feed rate.

I use the above points as good starting settings, then adjust them up or down as the wood requires. The cutter manufacturer will be able to supply you with chip load and tip speed values for their cutters, although the above info should be pretty standard.

Regards, Rod.