Bandsaw pulley calculation question

I could not find the specifics in the search.

I'm converting a wood bandsaw into a metal bandsaw, and I need to slow it down to, I believe to 200 to 300 feet per minute. I found in the Grainger catalog various size pullies, to slow down the RPM.

The band saw uses a 80" blade, the motor runs at 1725 rpm and has a 2" pulley, what size pulley do I need on the bandsaw side?????
I can fit an up to 11" pulley in there.
Thanks, Chris

 

You are running a 2" motor pulley and a 12" saw pulley, so for every 6 motor revolutions the saw pulley turns one turn. 12/2=6 1725/6=287.5 revolutions per minute. circumference of 12' pulley = 37.68" 37.68/12 = 3.14ft per revolution. 3.14 x 287.5 = 902.75 ft per minute 287.5/902.75 - .314x2"=.636" on the motor.

If my calculations are correct, that seems impossible to do, so the next best idea is to get a motor which turns around 600 rpm, then you would scale the 902 ft per minute down to around 300. Hope that helps

 

I'm running a 2" motor pulley but need to know what size saw pulley I need.
The 12", comes from the "wheels" where the 80" blade is attached to.

 

With 12" Bandsaw wheels 1 RPM = 1 FPM blade speed

Therefore you need to change the driver / driven pulleys to give 200 to 300 rpm.

Motor Speed x driver pulley dia / driven pulley diameter = final RPM

2" motor pulley to 8" driven pulley
1725 x 2/8 = 431.25 RPM/FPM

2" motor pulley to 9" driven pulley
1725 x 2/9 =383.33 RPM / FPM

2" motor pulley to 10" driven pulley
1725 x 2/10 = 345 RPM / FPM

2" motor pulley to 11" driven pulley
1725 x 2/11 = 313.6 RPM / FPM

this would appear to be your best combination

1.5" motor pulley to 10" driven pulley
1725 x 1.5/20 = 259 RPM / FPM

Hope this Helps.

 

Your the man. Thats exactly, what I was looking for.
Thanks alot

 

Dreadman please make sure you post what you come up with and how it works as I would like to do the same thing. I was thinking of doing it in steps (an intermediate set of pullies) but if it will work with one that would be even better. Thanks

 

one 12" bandsaw pulley that turns one RPM there is 37.69 inches or just over 3 feet of blade travel

remember your friend PI

 

For my calculations i have assumed that the pulley driven by the motor on the opposite side of the shaft to the 12" blade driving puley is presently 8" in diameter.

 

so if you want 300 FPM on the blade the 12" wheels the blade is on will need to turn about 100 RPM

 

Many appologies for such a basic oversight.

 

So your saying, it's not as easy as I thought.
What would my FPM be if I had a 1.5" motor pulley and a 12" saw driven pulley???
Sorry for so many questions, I'm just more confused now.

 

FWIW: I got an old Chinese wood bandsaw, bought metal blades for it, set it on the slowest speed and have been using it steady for about 10 years. Granted it is a little fast, and I cut mostly thin stock but I love it for patch panels and such.

 

to make it easy , let's say the motor is 1700 RPM...to get 100 RPM on the 12" wheels you will need 17:1 reduction. a 1-1/2" drive pulley with a 12" driven pulley is only 8:1 reduction

you will probably need to use a jackshaft..that's what i did on the wood bandsaw i converted to metal

 

Dreadman, check out www.metalmeet.com


I believe someone there hooked up some sort of variable control to slow the motor down.

 

Edit.

 

I can't find the specific thread i remember, but I did find this...

http://www.metalmeet.com/forum/showthread.php?t=5059&highlight=BAND

 

Let's just reverse engineer your problem:

The 12" diameter wheels that drive the blade will feed the blade at a rate of 1 ft. (12") times Pi (3.1459) = 3.1459 ft. per revolution (note that this is per drive wheel revolution, not the entire blade system). This is simply the circumference of the 12" drive wheel.

To make it simple and clear, if you rotated the drive wheel at 100 rpm, your blade speed would be 314.59 feet per minute. Not too far from your goal of 200-300 fpm.

The units work out like this: (feet/revolution) x (revolutions/minute) = (feet/minute). The revolutions part cancel each other out.

So to get an exact blade speed and an exact rpm, your formula would be:

For 200 fpm: (3.1459 ft. / 1 revolution) x (y revolutions/minute) = 200 (feet/minute)
Where y is your variable.

This simplifies to y= 200/3.1459 (rpm) which is about 63-64 (rpm) in real life

and similarly, for 300 fpm:

y=300/3.1459 (rpm) which is about 95-96 (rpm )

So you are looking for a realistic rpm that would fall between 63 rpm and 96 rpm at the driven blade wheel. The middle of this mess is (63 + 96)/2 = 79.5 rpm. Let's just call it 80 rpm for simplicity.

To get your pulley size, just think of it as a ratio of the motor speed (1750 rpm) with one pulley size (2") vs. the desired 12" drive wheel speed (80 rpm) and it's pulley size (which is our unknown, or y):

(1750 rpm) / (80 rpm) = (2" pulley) / (the unknown pulley---> y)

simplifying this mess yields: 21.875 = 2/y or approximately, y = 2:22 ratio. (same as 1:11)

So with a 2" motor pulley, you would need a 22" pulley on the band saw, which isn't an option, but if you can put a 1" pulley on the motor and an 11" pulley on the band saw you would be fine.

 

bigvindaddymac....that's some fancy math there. i was just rounding things off

the 1" drive pulley in theory may work , but i think it would not give the V- belt enough surface to drive the belt the under load of cutting..i think it would just slip

 

I doubt you there is a 1" pulley available. Shaft is most likely 3/4" and the belt would not like it too much either.
You can use an intermediate pulley with two diameters - maybe salvaged from on old drill press for example to increase your speed. It will require a little more fabricating but nothing you can't handle for sure if I look at the stuff you make.

 

Many times they will use an adjustable speed treadmill motor. You kind of need to have adjustability in case you want to cut aluminum with it.
Justin

 

The smallest pulley available is 1.5" and after some additional measuring I can fit a 12" pulley at the saw.
I will try that and see how it does.

What would the FPM be with 1.5" and 12" pullies?

 

Not an electrician nor a mathematician, but how about a rheostat for the motor to slow it down?? Somewhat like a heater motor speed control but for heavier usage??

Just a thought, maybe?

 

That's a 1:8 ratio

works out to a tad over 688 fps.

I have to confess, my calculation was off by a factor of 2...I screwed up. The ratio you seek is 1:22 not 2:22 as I stated earlier. I have the flue which is why I'm here instead of work and my thinking cap isn't on too tight.

 

The jackshaft idea with a 3:1 reduction would solve all your problems

 

llonning: Rheostats and AC motors don't get along too well, You can buy a Varistat for this very purpose, and my God they are expensive. Even with a Varistat, the AC motors tend to burn up quicker than normal. Now a DC motor, they do fine using speed controllers.

 

Just for shits and giggles because I have the flu and can't do math, I went out and measured the pulleys on my band saw. On mine, I have a 2" pulley on the motor and a 7.5" pulley on the drive wheel. (1:3.75 reduction) which roughly translates into 1468 feet per minute with a 1750 rpm motor.

I use mine for cutting wood/masonite patterns mostly, but I have cut 1/2" aluminum no problem with a metal cutting blade and TapMagic cutting fluid. It does suck for sure trying to cut steel, so I use a plasma cutter for steel. Which is why I need to make patterns.

This brings me to my question: What kind(s) of metals are you wanting to cut with your band saw?

 

I want to be able to cut up to 1/2" cold rolled steel. Brackets etc.

So with the jack shaft set up, what would a good combination pulley system be.
I found a stronger 1.5HP motor with a 3" pulley, 1725 rpm.

So for the math people on here, what size pulleys would I need to get the FPM to 200-300, using a jackshaft.
4 pulleys involved now.

 

Just get the 600 RPM motor and be done with it!

 

No, I have a $400 motor, and pullies are cheap, like $10 each

 

Like I said not an electrician, it was just a thought.