VFD for bandsaw?

[ThinWoodsman]

I've been rebuilding a benchtop bandsaw (9" Duro) and it's now at the point where I need to hook up a motor.

I know I can get metal-cutting speeds with a suitable gear or pulley arrangement. I was reading up on using a DC motor with a variable-speed control (RVP Precision has an article and a thread here), and the price for the controller is not that far off a cheap VFD.

Would using a VFD on a 3/4 or 1 HP motor provide sufficient torque for cutting steel? Using a cheap 3-phase motor, the cost would be about the same as a DC with variable speed control. More expensive than pulleys (and presumably less torque), of course, but a convenient way to change speeds would be nice if it doesn't degrade the power of the saw too much.

 

[gr8legs]

I have become an evangelist for the VFD / 3-phase motor retrofits.

Most VFDs will have a feedback loop to keep motor speed constant and provide sufficient torque and the benefit of 'dialing in' the best speed on the fly is golden.

AutomationDirect and DealersElectric both sell packages of matched VFD and motor combinations at what seems like very reasonable prices.

see<https://dealerselectric.com/motor-and-drive-packages-vfd-speed-control.asp>

I would not use a DC motor as the brush/armature requires much more maintenance than a 3-phase motor, and as the speed lowers in a DC motor (for the same torque) the current through the brushes / armature increases which leads to more maintenance.

YMMV

Stu

 

[machPete99]

Depending on the motor setup and wheel size I think you need something like 20:1 speed reduction, and you will probably need a gearbox in addition to any VFD. Need to be running around 100 FPM for steel.

See:

Wood bandsaw into a steel cutting bandsaw.

Question the vertical wood bandsaw has a 1725 RPM motor. I think it is 1/2 to 1 HP. If I put on a 100:1 gear reducer on the motor shaft and pulley it, is the speed at the blade still to fast to use the saw to cut metal. As you can see I'm trying to get away with something. Your thoughts....gear.

www.practicalmachinist.com

 

[ThinWoodsman]

AutomationDirect and DealersElectric both sell packages of matched VFD and motor combinations at what seems like very reasonable prices.

Thanks! Those are indeed a better deal than the individual combinations I was looking at.

Depending on the motor setup and wheel size I think you need something like 20:1 speed reduction, and you will probably need a gearbox in addition to any VFD. Need to be running around 100 FPM for steel.

See:
Wood bandsaw into a steel cutting bandsaw

Yes, I am aware of the slow speed requirements and that PM thread is one of the first things I read.

Wondering if I can do this *without* the gearbox - I'll probably end up kicking it down the road 'til next year if I have to sort that out as well as the motor.

 

[machPete99]

I'd say bag the VFD and spend the money on a proper gearbox. If you have 9" drive wheels you probably need less reduction than say a 14" delta style. Need to figure motor speed into the picture too.

 

[mksj]

A VFD would be useful if you wanted a range of cutting speeds for metals, so something like 100-250 FPS, but you would have to plan for a 60HZ motor to be at around 150 FPS so a variable motor speed of 40-100 Hz. If you look at combination wood/metal bandsaws, they always have some form of speed reduction (belt ratio/gearbox) to achieve the different operating speed ranges. I had considered one of the Grizzly combination bandsaws in the past, the Grizzly G0640X uses a belt reduction for metal along with a VFD for the speed range. I ultimately ended up buying a horizontal/vertical metal only bandsaw with a 3 speed gearbox. Use it all the time, can't imagine not having one for cutting down metal stock of all sizes. If this is a metal only bandsaw, then use a pulley setup to reduce the speed range and use a simple VFD to operate over the desired speed range for metal/plastics.

 

[ThinWoodsman]

If you have 9" drive wheels you probably need less reduction than say a 14" delta style. Need to figure motor speed into the picture too.

Yeah, I've gotta sit down and do the math for a 1750 rpm motor, which seems to be the most readily available. Maybe this evening. Too many other projects competing for attention.

A VFD would be useful if you wanted a range of cutting speeds for metals, so something like 100-250 FPS, but you would have to plan for a 60HZ motor to be at around 150 FPS so a variable motor speed of 40-100 Hz

Right, that was the idea behind using a VFD instead of just slowing down the speed from the motor with a suitable gear or pulley arrangement.

If this is a metal only bandsaw, then use a pulley setup to reduce the speed range and use a simple VFD to operate over the desired speed range for metal/plastics.

Yes, metal only. Only the range of metal cutting speed need be supported, steel on the low end and brass/aluminum on the high end. Could use a step pulley but then I have to also make a convenient way to change speeds, and that again pushes the project off into never-get-done-land.

Found a pretty helpful PM post this morning, also coming down in favor of a 3-phase motor and VFD. Some good points in that thread also about beefing up the table mounts to handler the deeper cuts. Pretty sure the frame of the Duro is cast-iron, should be sturdy enough.

Well, I'll design around a VFD+3-phase and see if the math works out, or if additional reduction (e.g. via gearbox) is needed. If it is, I'll punt this down the road a bit until I have time to research some sort of variable-speed gearbox.

 

[machPete99]

9" wheel should be turning 42 RPM for 100 FPM...

100 FPM / (3.1415 * 9 / 12)

If you use a 6 pole motor it will turn around 1200 RPM, so need 28:1 reduction, ignoring possible pulley arrangement.

 

[DavidR8]

This is a timely post as I’m looking at a 10” Rockwell bandsaw as a metal cutting conversion.
I’m thinking about a brushless DC motor.


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[Winegrower]

I looked at this once for a 14" bandsaw conversion and could not see that a single jackshaft solution would get adequate speed reduction with any realistic pulley sizes.
The worm gear reducer just seemed like too much expense and trouble and I moved on to another vision.