Tubalcain converts a wood bandsaw to metal using a VFD, calls it a failure .

[Ken from ontario]

Just when I was contemplating to convert my wood cutting bandsaw to cut metal, Mr. Pete declares his experiment a failure, although he believes it could still work but with a gearbox and not with VFD /3ph motor:

 

[SubtleHustle]

I converted mine using both, a gearbox speed reducer and a VFD. Works great!

 

[JimDawson]

Or you could just use a double reduction with a jack shaft using belts. He's really running the motor way too slow, but it is working. I'm not seeing a real problem here.

I use a gearbox, 2 speed motor, and 4 step pulley on mine. Works great.

 

[cjtoombs]

The problem with VFDs is that they lose power as they slow down, usualy linearly. That means a 1 hp moter run at half it's rated speed will develop 1/2 hp. Wood cutting bandsaws generaly run somewhere in the 2000-4000 FPM range, whereas metal cutting bandsaws need to run in the 100-500 range. One thing you can do is to use a higher hp motor and then reduce it's speed further through belts/gearing to get a reasonable compromise. If you wanted 1/2 hp at the machine, you could use a 3 hp motor and VFD, which gives a 12:1 speed ratio. This would be from 1/6 the rated speed to twice the rated speed. If set up for the low speed to give 100 fpm, that means the high speed would be 1200 fpm. Not a bad compromise, but to get from 100 to 4000 fpm, you need some other variablility, such as change belts or a variable speed belt drive.

 

[cathead]

This design has served me well. The speed can be adjusted somewhat by changing the motor pulley if need be.
.... and no slippage.........

 

[Mitch Alsup]

The real problem is getting the 20:1 reduction without losing TQ.

Mr Pete did multiple tries and the only one that worked 1/2 well was the one with great big sheaves.
2-stage reduction is likely required.

 

[Ulma Doctor]

if mr pete put a 5hp motor on the saw, there would be no problem

 

[pstemari]

...One thing you can do is to use a higher hp motor and then reduce it's speed further through belts/gearing to get a reasonable compromise. If you wanted 1/2 hp at the machine, you could use a 3 hp motor and VFD, which gives a 12:1 speed ratio. ...

You're confusing power and torque. Power, measured in horsepower or watts, is equal to torque (in ft-lb or N-m) times the rotational speed (RPM or radians/sec), plus a fudge factor for the traditional US units.

Power output is the same regardless of gearing. Increased torque is offset by decreased speed.

That said, you might be into something, since a 3 HP motor operated at low frequency might still manage a 1/2 HP of output.

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[GrayTech]

I used a 3.5hp DC treadmill motor with a diy rectifier and speed control on my bench top bandsaw. Works great and cuts very clean even at very low speed.

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[cjtoombs]

You're confusing power and torque. Power, measured in horsepower or watts, is equal to torque (in ft-lb or N-m) times the rotational speed (RPM or radians/sec), plus a fudge factor for the traditional US units.

Power output is the same regardless of gearing. Increased torque is offset by decreased speed.

That said, you might be into something, since a 3 HP motor operated at low frequency might still manage a 1/2 HP of output.

Sent from my Pixel XL using Tapatalk

Perhaps my example wasn't clear, but I understand the relationship between torque, RPM and hp. The VFD pretty much keeps the torque constant below the rated motor RPM, which means power drops proportional to the RPM as you slow it down. Reducing the speed through gearing or belts will not reduce hp, as it increases the torque (minus the small loss from efficiency). What I was talking about was reducing the speed through gearing/belts such that the speed at the 1/2 hp speed of the motor (in the case of my example, 1/6th the rated motor speed) would run the saw at 100 fpm. This means that the saw would be 1/2 hp at that speed. When you speed the VFD up, the saw would be at the rated hp of the motor (3 hp in this case) between 600 and 1200 fpm, and would range linearly between 1/2 hp and 3 hp between 100 fpm and 600 fpm.