bandsawing steel stock - what am I doing wrong?

This my 3 wheel, i cut metal and wood, with different blade of course dc thread mill motor set up.

101_0579.jpg
 
DC motors have a lot of torque at most any RPM, unlike AC so it should be fine. You don't want to "overwork" the blade and generate excess heat. As long as it's cutting well, just let it do it's thing and don't push it too hard. If the motor stalls, it's probably not the right motor. Treadmill motors should have more than enough torque to turn the "correct" band saw blade (designed to carry a 300LB'r).
 
dbassing said:
I have been following this thread and it got me to thinking about working on my Powermatic 14" wood cutting bandsaw (Model 141). I have been looking for a while for a stand alone vertical metal cutting bandsaw and nothing ever seems to become available. I then thought of trying to find a drive system for a Powermatic Model 143, metal cutting bandsaw. Now this idea of using a 90volt DC 1725 RPM motor with SCR speed controller really seems like the smart and economical way be able to lower the blade speed for metal (changing the blade, of course) while still being able to increase to the max for cutting wood.
I did a bit of math to calculate how slow I would need to turn the drive pulley to get the saw blade to be moving at 75 FPM. (This would me the slowest I would expect to have to turn for cutting steel.) I found that the motor would have to be turning at 40 RPM. Here is how I figured this.
Motor speed =X
Drive pulley dia.= 3"
Driven pulley dia.= 6"
Dia. of bandsaw wheels= 14"
Driven wheel speed= Y RPM
Speed of blade = 75 Feet per minute(FPM)
75=Y*circumference of band saw wheels / 12
75=Y* 43.9/12
Y= 20 RPM
20 RPM= X * Drive pulley dia./driven pulley dia.
20 =X * 3"/6"
X= 40 RPM
Now if this correct what I am wondering is would the motor have sufficient torque at this reduced speed to be able to be effective at cutting steel. Assuming correct blade and appropriate feed pressure?
Thanks for all your help,
David
Click to expand...

You may want a seperate cooling fan.

[Edit] I don't think you want to try to cover the full speed range from 75 FPM to 1000 with just motor speed control. I'd add step pulleys to switch between wood and metal.
 
Last edited:
The problem with doing a wood/metal cutting bandsaw is that the speed range is very wide. As stated, the low speed should be about 75 FPM. A good wood bandsaw should run in the neighborhood of 3000 FPM. This gives a 40:1 speed ratio, which is way to wide for a VFD without using an extreme oversize motor. The While VFDs will run a motor very slowly, they loose horsepower linearly from the design speed of the motor down. So if you run a motor at half it's speed, it will be making half it's rated horsepower. You can run the motor to about twice it's rated speed safely, so that means if you want a half horsepower available at the blade, you could use a 2hp motor and run it at 1/4 the rated speed, which would give an 8:1 speed ratio (1/4 the rated speed to twice the rated speed). This leaves a 5:1 ratio that would need to be taken up, either with change pulleys or a variable speed pulley. Single stage variable speed pulleys don't have that kind of ratio, so you would need to use a two stage (accomplished with a special variable speed pulley on a moving countershaft or two normal variable pulleys on a moving countershaft). That's why most of them are either one or the other. Not to mention that most shops don't do much work that crosses over from wood to metal and vice versa. I have a bandsaw that I am planning on putting a combined VFD/variable pulley drive on. The variable pulleys are outrageously expensive if you buy them new, but they can be picked up on ebay for a fairly low price. I put one on an antique craftsman 3 wheeler (its the same kind pictures above by kernbigo) that I have, and it can be varied from about 100-300 FPM and it works pretty good.
 
my thread mill motors have the original fan on them, my lathe and saw
 
Thanks all for everyones input and advise. I will be doing a bit more research and report back with what I decide to do and the results.
David
 
This thread and a couple other projects has led me to believe that I need a non contact laser tachometer. I am also considering putting a treadmill motor on my Craftsman Atlas 101.21400


Jamie
 
You must log in or register to reply here.
Back
Top