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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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Verify my Bandsaw SFPM Calculations Please
I have one of the old Delta 14" wood bandsaws that I am using for
metal cutting. I am using a 3-phase 1HP motor with a VFD - so I can slow the motor way down for metal. To be honest, I've never used a bandsaw before, so I am learning the hard way, and I think I just huffed my first blade (14-18TPI Morse Bi-Metal). Anyway, I was just guessing at speeds to run the motor, and I thought I would calculate the SFPM to see where I really needed to be. Looking at the speeds metal bandsaws are advertised at, I think I need to slow down. Could you guys just check my math? - Bandsaw drive wheels are 14" diameter. 2 * pi * r = 44" or approx. 3.7' - Motor has 2.5" pulley, pulley on bandsaw wheel is 6" for 0.42 reduction So, if a rev of the drive wheel moves the blade 3.7' and it is driven at 0.42 of motor speed, SFPM should be (3.7)(0.42)(Motor RPM), or approx. 1.5 * Motor RPM. If that's true, getting the fastest speed of a Wilton metal bandsaw (278 SFPM), I would have to run the motor at 185 RPM! If my calcs are correct, then I need more reduction between motor and drive shaft. Does this sound right, or am I missing/misunderstanding something? Thanks, Wallace |
#2
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Your numbers seem reasonable. Your original premise, that you can slow
a motor down 90% and still use it normally, is not. 185 fpm is a little high for metal cutting but it will work. 80 is a better all-purpose target. Slowing a little 1hp motor down 50% is about the limit, and be aware that many motors are fan-cooled so when you slow them down you are also removing their cooling capability. A nifty 5:1 gear reducer would help you a lot. A 10:1 would be better. GWE Wally Blackburn wrote: I have one of the old Delta 14" wood bandsaws that I am using for metal cutting. I am using a 3-phase 1HP motor with a VFD - so I can slow the motor way down for metal. To be honest, I've never used a bandsaw before, so I am learning the hard way, and I think I just huffed my first blade (14-18TPI Morse Bi-Metal). Anyway, I was just guessing at speeds to run the motor, and I thought I would calculate the SFPM to see where I really needed to be. Looking at the speeds metal bandsaws are advertised at, I think I need to slow down. Could you guys just check my math? - Bandsaw drive wheels are 14" diameter. 2 * pi * r = 44" or approx. 3.7' - Motor has 2.5" pulley, pulley on bandsaw wheel is 6" for 0.42 reduction So, if a rev of the drive wheel moves the blade 3.7' and it is driven at 0.42 of motor speed, SFPM should be (3.7)(0.42)(Motor RPM), or approx. 1.5 * Motor RPM. If that's true, getting the fastest speed of a Wilton metal bandsaw (278 SFPM), I would have to run the motor at 185 RPM! If my calcs are correct, then I need more reduction between motor and drive shaft. Does this sound right, or am I missing/misunderstanding something? Thanks, Wallace |
#3
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On Sat, 20 Nov 2004 12:21:02 -0800, Wally Blackburn wrote:
I have one of the old Delta 14" wood bandsaws that I am using for metal cutting. I am using a 3-phase 1HP motor with a VFD - so I can slow the motor way down for metal. To be honest, I've never used a bandsaw before, so I am learning the hard way, and I think I just huffed my first blade (14-18TPI Morse Bi-Metal). Anyway, I was just guessing at speeds to run the motor, and I thought I would calculate the SFPM to see where I really needed to be. Looking at the speeds metal bandsaws are advertised at, I think I need to slow down. Could you guys just check my math? - Bandsaw drive wheels are 14" diameter. 2 * pi * r = 44" or approx. 3.7' - Motor has 2.5" pulley, pulley on bandsaw wheel is 6" for 0.42 reduction So, if a rev of the drive wheel moves the blade 3.7' and it is driven at 0.42 of motor speed, SFPM should be (3.7)(0.42)(Motor RPM), or approx. 1.5 * Motor RPM. If that's true, getting the fastest speed of a Wilton metal bandsaw (278 SFPM), I would have to run the motor at 185 RPM! If my calcs are correct, then I need more reduction between motor and drive shaft. A wood cutting bandsaw is going to run in the 2500-3000sfpm range. Your's as set up is running at 2650sfpm. So to get into the right range you will need to be running the motor a bit above a tenth of its normal speed (180rpm). So yes your calculations are okay. At that speed you'll have pretty much no torque avalailable from the motor (if it will even run that slow). So to do metal cutting at those speeds you need a 10:1 speed reducer in the powertrain. And given the torque needed it really should be a gear reducer directly coupled to the lower bandsaw wheel. I can state from experience that you can't push enough torque through the 6" drive pulley with a belt (it slips). -- The instructions said to use Windows 98 or better, so I installed RedHat. |
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#5
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Jim Levie wrote:
At that speed you'll have pretty much no torque avalailable from the motor (if it will even run that slow). So to do metal cutting at those speeds you need a 10:1 speed reducer in the powertrain. And given the torque needed it really should be a gear reducer directly coupled to the lower bandsaw wheel. I can state from experience that you can't push enough torque through the 6" drive pulley with a belt (it slips). Machinery's Handbook has all the formulae for limits of horsepower that can be transmitted by belts and sheaves. One thing they do *not* say is if you need to transmit more torque than a belt can do, you can always use two belts in parallel. Or three .. multi-belt step pulleys are mighty rare if existent, though. Sheaves are fun to turn if you have a lathe. GWE |
#6
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Why don't you try it? If it is unsatisfactory, you'll know why. I run
my 3 ph 3/4 hp drill press motor at 5 Hz sometimes. Indeed, the torque is a lot less than if I'd changed the belts, but it's ok. If you do run it slow, for extended periods, you should put independent cooling on the motor. E.g., a muffin fan. Bob |
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