Bandsaw torque improvement methods

[AllenV]

Thank you all for the well-considered responses. Allow me to pull it together in summary and expand on the DC motor.

Again, please correct my mistakes and feel free to offer further advice.

1) Further mechanical drive ratio reduction.

@mksj has pointed out that a significant further gear reduction might be needed to drop the blade speed down to 300 SFM at max with electronic speed reduction from there. This would keep the motor up in higher torque values at speeds between 75-300 SFM. (Using a constant torque ratio of 4:1, common in many AC motors) He has further offered up a 10:1 Gear reduction box that could serve the purpose. This is the Tigear 17Q10R56. The cost is $200. See his post for details and a link.

This is most certainly the most simple and economic approach. Some machine work would be required to fabricate brackets to mount the motor and gearbox. The existing motor and VFD are retained. And, if the 10:1 reduction is used the SFM display on the VFD remains useful by dividing the display value by 10

2) Putting in a larger AC motor and VFD.

It looks like again we are limited by a constant torque speed ratio (CTSR) of around 10:1 for a good inverter motor. @mksj suggests a larger motor would still need a further gear reduction of maybe 4:1. This would bring down the max blade speed to around 750 SFM. Then 75 SFM is within reach with electronic speed reduction. A new motor and controller are required, along with some resizing of pulleys.

Here I have not picked out good example motors and controllers. The marketplace is vast and objective, quantitative, advice is hard to find. This is one reason for my generating this thread.

Here's a thought. Seems like 100:1 CTSR would do wonders for this saw. Is it possible to pick up low-speed torque by swapping the existing “unkown” motor with a better quality inverter-rated motor having 1000:1 CTSR and perhaps 1.5 HP? This is based on the idea that the root issue here is loss of torque at low speed. I don’t need more power, just need to retain torque at lower speed. Is it a given that doing this needs a new VFD?

Useful link I found:
https://www.woltersmotors.com/do-you-need-a-general-purpose-motor-or-an-inverter-duty-motor/

3) Going with variable speed DC

DC motors are famous for constant torque over a wide speed range.
http://www.drivesandautomation.co.uk/useful-information/dc-motor/#:~:text=DC motors can develop a,with a simple DC drive.

In fact, back in the early 70s my father put variable speed DC motors on his small Hardinge mill and Craftsman drill press. Oddly, his bandsaw got a complicated V-belt scheme.

My own hunting has found a number of motors offering 20:1 “constant torque rating” and 1800 base RPM operation. Such a motor can be expected to run with near constant torque from 1800 RPM down to 90 RPM with steady torque by altering motor voltage while holding the armature current constant.

In my case this translates to blade speeds of 3000 SFM at 1800rpm down to 150 SFM at 90 RPM. A modest pulley change (smaller pulley on motor shaft) to drop the high-end speed down to 2000 SFM would shift the blade speed range to 2000 SFM on the high side and 100 SFM on the low side. If I can get away with a still-smaller pulley we can imagine 1000 SFM maximum and 50 SFM minimum.

While I am not sure how much DC HP is enough, I did some shopping at 1 and 1.5 HP.

1.5 HP

• DC motor from Automation Direct, 180 VDC, 1800 RPM $410
• 20:1 constant torque rating
• MTPM-1P5-1L18
• https://www.automationdirect.com/adc/shopping/catalog/motors/dc_motors/general_purpose/mtpm-1p5-1m18
• 
  
• 
  
• DC Motor Drive, analog, from Automation Direct $329
• Rated 0.25 – 2 HP @ 180V
• GSD5-240-10N4
• https://www.driveswarehouse.com/s-150a

1.0 HP

• A combo package from Drives Warehouse
• SP-101 analog controller & E101AM-P motor $615
• Motor is 1 HP, 180 VDC, 1750 RPM
• 
  
• https://www.driveswarehouse.com/sp-101-e101am-p

 

[AllenV]

Everybody knows about the treadmill conversion, but have you seen the (patented) stationary bike bandsaw?

View attachment 447374

@pontiac428 is thinking outside the box. Let’s put up some numbers.

I am a cyclist of a certain age. My indoor cycling set up routinely measures my Functional Threshold Power (FTP). This is the power I can generate and hold steady for 1 hour. After that 1 hour I fall off the bike and need 24 hours of recovery. My FTP is 220 Watts. That’s about 1/3 HP. I can do much more but the duty cycle drops significantly. Think 600 Watts for 30 seconds in a sprint. The pro cyclists half my age can do 400 Watts or more while being chased by coaches riding in BMWs. Let’s assume the link from bike to saw is 100% efficient and I can use the gears to maintain a pedal cadence of 90 RPM +/- 10%. Since 1 MPH is roughly 90 SFM at the tire outer circumference we can imagine a super low-geared 33-speed touring bike set-up might yield 180 SFM to as much as 1000 SFM. So, blade speed is not so bad but power is limited

Here is a 1 HP bandsaw motor. It is fully asynchronous and a VFD is not required.

 

[AllenV]

here is a nice write-up of a DC motor installation on a Central Machinery 14" saw, converting it for metal.

Bandsaw conversion to DC drive...Wood to Metal Cutting Conversion | RVB Precision

I have a horizontal bandsaw for cutting steel stock to length. It is an amazing time saver and a great asset if you do a lot of metal fabrication. This type of saw can be used in the upright position to cut metal on a small table. But it is inconvieneient to convert to thisRead More

rvbprecision.com

I found a similar 3/4HP motor on amazon priced at $160 and an SCR controller at $185.

So that approach can be done for about $345 plus ancillary items.

He seems quite happy with 3/4 HP in a DC motor

 

[AllenV]

And some comment by the person who did the 3/4HP DC conversion, @rbertalotto

Wood Band Saw conversion to DC Metal Cutting

Had some time today so I wrote up a little show and tell on converting an inexpensive 14" Wood Cutting band saw to cut metal with a DC motor conversion. It is posted on my web page as it contains lots of photographs www.rvbprecision.com I hope you enjoy it!

www.hobby-machinist.com

 

[AllenV]

Well, I began to question if the saw might be over-speeding the motor to hit the indicated 3000 SFM. I don’t have a tachometer so removed the lower wheel and uncovered the pulley mechanism to measure diameters and calculate the reduction.

I measured with belts in-place, trying to estimate the inner edge of the belts. Probably got diameter to within 1/8 inch. Referring to the figure in the first post of this thread

• Motor pulley. 2-3/16
• Idler large. 4-7/8
• Idler small. 2-7/8
• Wheel pully. 6

For my reference, the motor shaft looks like 5/8”

Went to The Engineering Toolbox
https://www.engineeringtoolbox.com/pulley-diameters-speeds-d_1620.html
to calculate the resulting wheel pulley RPM for a given motor RPM

At 1750 motor RPM the blade wheel will turn 376 RPM

The speed reduction factor is 4.65 ( maybe 4.5 if accurate pulley sizes used??)

Now, at 3000 SFM blade speed, the motor would need to spin at 3800 RPM, roughly 2X of 1750 rpm. Over speeding seems very likely.

Now it’s clean-sheet time:

Using the 4.65 speed reduction

At 1750 motor RPM the blade speed is 1350 SFM

Plugging in a 20:1 motor speed reduction yields 87.5 motor RPM on “low” speed

At 87.5 RPM motor speed, the blade speed is 70 SFM

Plugging in a 10:1 motor speed reduction yields 175 motor RPM

At 175 RPM motor speed, the blade speed is 140 SFM

This implies that as-is I have been running the motor at ~10%, or less, of rated speed to cut steel

The fog is starting to clear

 

[AllenV]

Still working this issue, at a lower priority.

This whole discussion has been relying on suspect data. Specifically the digital readout on the factory-installed VFD. Grizzly programmed the VFD to display directly in feet-per-minute (FPM) blade speed. Until now, I have had no way to independently measure this.

I went to E-Bay and sourced a used "pre-owned" contact tachometer. These have a wheel of known circumference which can be held against a moving surface, such as a belt or bandsaw blade. These are widely available as inexpensive units of unknown pedigree or calibration. I managed to find a higher quality used unit that came with NIST traceable calibration when new. This is a SHIMPO DT-105A with a 6" wheel. See the image below.

I first verified quantitative functionality by measuring the 6" chuck on my lathe. Those numbers agreed to within 0.5% of the calculated SFM. I am willing to say that this instrument is within +/-2% accuracy. (The factory specification is +/-0.2%. This may well still hold true but I lack and independently calibrated tachometer to measure against.)

Then I set about measuring both motor and blade speed at various VFD-indicated blade speeds. See the detailed data and graph below.

The upshot here is that the VFD-indicated speed is significantly lower than the measured speed. The error is about 12% to 25%. Also, the VFD runs the motor as high as 4571 RPM. the motor label indicated a speed of 1720 RPM. Thus it runs up to 2.65X rated speed.

At rated, 1720 RPM speed, the blade speed is roughly 2200 FPM.
At 25% of rated speed, 430 RPM, the blade speed is roughly 550 FPM.