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

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

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.

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

remove ns from my header address to reply via email

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.

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

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

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