Bandsaw RPM

[jwsvandr]

Need to pick some brains. I am converting my POS wood cutting bandsaw to metal cutting. I have built a Rube Goldberg contraption to reduce the speed and it works too well. I wanted to get 100fpm which I believe is the proper cutting speed for mild steel.  Please check my math below:

Circumference of bandsaw wheel  = (14 inches*pi)/12  = 3.63 ft

 100fpm/3.63 =  27.55 RPM

This is way too slow. I then checked my 4X6 bandsaw and it was turning at around 128 rpm with a 7in wheel. That means (according to my math) it is cutting at around 234.54 fpm and that my 14in wheel should be turning at 64 RPM.

Where am I going wrong????

[TriHonu]

Your calculations are correct.  A 14 in diameter wheel needs 27.28 rpm to give a blade speed of 100 ft/min.

(Wheel diameter(inches) x PI) /12 = circumference in feet.

Wheel Circumference (Ft) X Revolutions Per Min = Cutting Speed in Ft per Minute.

I checked both my Vertical and Horizontal saws with an Shimpo electronic Speed Meter that will directly measure both revolutions and surface speeds.

Vertical: 20.25 dia. X PI /12 * 18.9 rpm = 100.2 Ft/min (Set for 100 ft/min on saw)
Horizontal: 11.375 dia X PI /12 * 46.2 rpm = 137.58 Ft/min (Set for 130 ft/min on saw)

Directly measuring blade speed with Shimpo confirmed the speeds.

  Did you or someone else replace the motor on your 4x6?   If it had a 1725 rpm motor and someone replaced it with a 3450 rpm motor you would double the blade ft/min...