Bandsaw torque improvement methods

AllenV

AllenV

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H-M Supporter Gold Member
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This post is meant to be an educational effort in advance of modifying my bandsaw to improve low-speed torque and prevent stalling the blade when cutting steel. I have seen that a number of the HM community have made similar improvements. Perhaps we can collect that experience, specific to bandsaws, here in great detail.

My specific challenge is that I own a Grizzly G0621 wood/metal bandsaw. This is extremely similar to the current model G0261X

https://www.grizzly.com/products/grizzly-13-1-2-1-1-4-hp-vertical-wood-metal-bandsaw/g0621x

The saw uses 1-phase 240VAC power on a 30A circuit. I’d like to keep that as-is.

The saw has an AC induction motor, 1.25 HP, 3-phase at the motor, 1725 RPM.

It has a built-in electronic inverter/VFD that I have nearly zero information about.

The VFD display reads directly in blade speed FPS, a nice feature but not required.

If turn the speed knob to its limits I get a
-> max speed of 3022 FPS
-> min speed of 50 FPS or so before the blade just stops

When cutting 0.25-inch steel plate at about 150 FPS, it cuts well but I can easily stall the blade by pushing moderately hard on the piece. A gentler push keeps the cutting moving along at a gentle pace.

Further FYI: I have a horizontal 5”x6” bandsaw, Grizzly G9742, that is a champ. If I can cut it there, that is what I do.

That is the set-up. I’d to apply more pressure on the work piece without stalling.

I see a few general options and I’d like this community to help me select the best path for my situation.

Possibilities are (please add any other options that I have missed):

1) Further mechanical drive ratio reduction. Allow the motor to spin faster at 100-150 FPS blade speed and thus stay higher up on the torque-vs-RPM curve. This could be accomplished either by changing the idler pully or including a gear reduction at the motor shaft. I believe that a further drive ratio reduction of 2:1 or 3:1 might be adequate as this would double or triple the motor speed at a given blade speed. Of course, the max blade speed would drop to 1000-1500 FPS, still nice for metal. Is my thinking correct here?? Swapping out the idler pully might run into limitations in maximum size due to clearance against the frame of the saw (see image, yellow line marks the frame of saw).

2) Boosting the HP by swapping in a larger AC induction motor and suitable VFD. How much HP is enough? This is experience-based and intuitive. What works for the hobby-machinist? Any specific guidance on motors and controllers. Is vector control needed or desired here? Please educate me as to what is practical and achievable.

3) Shifting to DC motor and controller. The high torque at low RPM seems attractive. What are the pros/cons of this approach? Again, can you suggest motors and controllers?

here is the image, taken from the manual, of the belt drive system.

IMG_1006.jpeg
 

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AllenV said:
This post is meant to be an educational effort in advance of modifying my bandsaw to improve low-speed torque and prevent stalling the blade when cutting steel. I have seen that a number of the HM community have made similar improvements. Perhaps we can collect that experience, specific to bandsaws, here in great detail.

My specific challenge is that I own a Grizzly G0621 wood/metal bandsaw. This is extremely similar to the current model G0261X

https://www.grizzly.com/products/grizzly-13-1-2-1-1-4-hp-vertical-wood-metal-bandsaw/g0621x

The saw uses 1-phase 240VAC power on a 30A circuit. I’d like to keep that as-is.

The saw has an AC induction motor, 1.25 HP, 3-phase at the motor, 1725 RPM.

It has a built-in electronic inverter/VFD that I have nearly zero information about.

The VFD display reads directly in blade speed FPS, a nice feature but not required.

If turn the speed knob to its limits I get a
-> max speed of 3022 FPS
-> min speed of 50 FPS or so before the blade just stops

When cutting 0.25-inch steel plate at about 150 FPS, it cuts well but I can easily stall the blade by pushing moderately hard on the piece. A gentler push keeps the cutting moving along at a gentle pace.

Further FYI: I have a horizontal 5”x6” bandsaw, Grizzly G9742, that is a champ. If I can cut it there, that is what I do.

That is the set-up. I’d to apply more pressure on the work piece without stalling.

I see a few general options and I’d like this community to help me select the best path for my situation.

Possibilities are (please add any other options that I have missed):

1) Further mechanical drive ratio reduction. Allow the motor to spin faster at 100-150 FPS blade speed and thus stay higher up on the torque-vs-RPM curve. This could be accomplished either by changing the idler pully or including a gear reduction at the motor shaft. I believe that a further drive ratio reduction of 2:1 or 3:1 might be adequate as this would double or triple the motor speed at a given blade speed. Of course, the max blade speed would drop to 1000-1500 FPS, still nice for metal. Is my thinking correct here?? Swapping out the idler pully might run into limitations in maximum size due to clearance against the frame of the saw (see image, yellow line marks the frame of saw).

2) Boosting the HP by swapping in a larger AC induction motor and suitable VFD. How much HP is enough? This is experience-based and intuitive. What works for the hobby-machinist? Any specific guidance on motors and controllers. Is vector control needed or desired here? Please educate me as to what is practical and achievable.

3) Shifting to DC motor and controller. The high torque at low RPM seems attractive. What are the pros/cons of this approach? Again, can you suggest motors and controllers?

here is the image, taken from the manual, of the belt drive system.

View attachment 447340
Click to expand...
That's a clean and compact way to drive the blade, imho.
 
I have a G0640X, also has wood/metal capability, with 2HP which I find has plenty of torque for metal when configured for metal cutting. It has a 2 speed setup, where the belts are changed between metal and wood cutting speed ranges, with a VFD control within those ranges. I keep mine in dedicated metal cutting configuration, I have a different bandsaw for wood.
 
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The challenge with some VFDs is that at low hz values the torque drops off rather abruptly. That's likely the reason you are able to stall the blade at low speeds.
@mksj will hopefully chime in with his expertise.
 
IMHO, that design doesn't have enough mechanical reduction and relies too much on the VFD for speed control. The torque will drop off significantly at lower Hz. Though, I know there's not an easy remedy...

That being said, I find slow and steady is the best way to cut on a bandsaw. Feeding work at too high a rate creates excessive heat and chip clearing problems, thus shortens the blade life. It also causes blade tracking and walking issues. I was taught to "let the blade do the cutting" You shouldn't need to push hard.
 
I think Ed nailed it- too little torque at low rpm.
The DC motor approach is my personal favorite, just cuz I'm familiar with the old fashioned scr controllers
They have an elegant simplicity and reliability that's hard to beat
 
I agree with all of the above, first off, it is almost impossible to have a motor cover that speed range, and not have a dramatic fall off in motor performance at the extreme ranges of operation, this can also be seen in mill and lathes where you will you will often see 2 speeds with an 8:1 or 10:1 reduction ratio between the upper and lower speed range. A 1.25 Hp is not going to do you much good, and very doubtful that it has a constant torque ratio more than 4:1, where inverter motors are typically 10:1 and upwards to 1000:1. Even with a flat torque curve at lower speeds you loose the multiplying power of the mechanical advantage. Going to a bigger motor say a 3 Hp, would not gain you much on the low speed end, unless you changed the motor drive ratio to something like 4:1, and then over speed the motor to say 200 Hz. You would need a new VFD, does not sound very cost effective I think you pretty much have a wood saw, and a lightweight metal saw.

So what are your other options, either a gear reduction or belt reduction on the drive using the existing motor. A 8:1 or 10:1 gear reduction would put you in the 300 SFM and below speed range which would be much more ideal for metals. You might be able to fit another double pulley to get maybe a 4:1 speed reduction. Either of those options should allow you to use the current motor as opposed to replacing the motor and drive.
Tigear 17Q10R56 Gear Reduction Box Speed Reducer Gearbox 10:1 Ratio 1.67 HP
https://www.ebay.com/itm/304223326040
 
Trying to get a small HP motor to operate satisfactorily at both slow metal working speeds and high wood working speeds is asking a lot. Probably the easiest solution to more torque for metal would be to add a large outboard sheave to to the idler pulley by bolting or welding. Also add another outboard sheave to the motor pulley. This will allow slow speeds for metal and still allow you to switch back to the original ratio when more speed is needed. Lots of smaller lathe have a setup like that from the factory. Just put the belt on the pullies that give you the ratio you want.
 
rabler said:
I have a G0640X, also has wood/metal capability, with 2HP which I find has plenty of torque for metal when configured for metal cutting. It has a 2 speed setup, where the belts are changed between metal and wood cutting speed ranges, with a VFD control within those ranges. I keep mine in dedicated metal cutting configuration, I have a different bandsaw for wood.
Click to expand...
That is a nice machine, and not much more expensive than the new version of mine. I'll allow myself to believe that this was not the case back in 2008, when I bought my saw.
 
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