Verify my Bandsaw SFPM Calculations Please

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:

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).

On 20 Nov 2004 12:21:02 -0800, snipped-for-privacy@hotmail.com (Wally Blackburn) vaguely proposed a theory ......and in reply I say!:

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I am going through the throes of the same process.

You are right. As has been said you need ratio reduction. For metal you need slow speeds and high pressures/torque.

For stainless and other harder metals, the slower the better for your blades. I have be3en advised to start at about 40-50 SFPM for what I am doing (bit tougher than stainless)

You could use a gearbox, or belt for the first reduction, and then chain for the final drive. I was going to use belt to do this. But belts get more and more slippy as they get slower on smaller and smaller pulleys. So gears or chain.

You would also benefit from at least a spray bottle with some cooling oil, which you spray on at regular intervals. This lubricates and cools. Not a bad idea to have something on the lower wheel to brush/scrape the residue off the tyre, either.

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

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