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# calculate missing pulley size to achieve proper saw FPM

#### Peroni

##### Cast Iron
Finishing up a cut off saw that came to me without the original motor and pulley. The replacement pulley is quite small and runs the blade way too slow. I'm not sure how to calculate the correct pulley size I need for the motor.

Motor turns 1725rpm

blade drive wheels are 14.375 dia and 46" apart on center

gearbox ratio is 31:1

original SFM spec for the saw is 60, 90 and 130 using a 3 step pulley.

drive pulley on gearbox is original with approx 6.5", 7.5" and 8.5" sheaves

How would I calculate the size of the motor pulley to achieve the listed blade SFM?

#### johnoder

##### Diamond
Gear box out put is 55.65 RPM (if we assume for the time being that motor is direct driving gear box)

Band Wheels are 3.763 feet in circumference

55.65 times time 3.763 = 209.4 feet per min

to get 60 you divide that into 209.4 and the result is 3.49 to 1 and 8.5" slow pulley has to be driven by a 2.35 dia pulley - because 8.5 divided by 2.435 makes for the 3.49 to 1 ratio

So that is a start

#### MaxPrairie

##### Hot Rolled
Edit after rereading. 4, 3-1/4, 2-7/16 roughly.

#### Peroni

##### Cast Iron
Edit after rereading. 4, 3-1/4, 2-7/16 roughly.

But how did you arrive at those numbers? I really appreciate the response but I'd like to know the math behind it.

#### MaxPrairie

##### Hot Rolled
You have 3 outputs: 60, 90 and 130 sfm. You know the diameter on the drive wheel so you can covert the required rpm using 3.82. Multiply those 3 numbers by 31 to get the gearbox input. Those numbers correspond to your 3 different pulleys. Your 3 rpms must be converted to the 1725 motor output by some ratio. Using this ratio and knowing one of the pulley diameters gives you the other pulley diameter.

#### Peroni

##### Cast Iron
Makes sense now, thank you very much for the explanation!

#### Toolmaker51

##### Aluminum
Bandsawing, and blades in general, don't send the same visual cue like an endmill or drill. The FPM goal is the same, but tremendous gain of circumference and a s**tload of cutting edges just messes up what we're used to judging automatically.
The hints offered are 100% correct. You'll want a step pulley of course; and one more aid. Write the resultant FPM in somewhere visible and durable. Not pencil, felt tip!
I solved the same issue, stepping desired FPM back through wheel diameter, motor speed yaddy-ya. I'm so tempted to work in a jackshaft or a vari-pulley. Flexibility is addictive. If you expect tubing, channel, the structural shapes and solids, among the varied materials, well......

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