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



## Ken from ontario (Feb 6, 2019)

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:


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## SubtleHustle (Feb 6, 2019)

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


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## JimDawson (Feb 6, 2019)

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.


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## cjtoombs (Feb 6, 2019)

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.


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## cathead (Feb 6, 2019)

This design has served me well.  The speed can be adjusted somewhat by changing the motor pulley if need be.  
	

		
			
		

		
	



  .... and no slippage.........


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## Mitch Alsup (Feb 6, 2019)

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.


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## Ulma Doctor (Feb 6, 2019)

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


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## pstemari (Feb 6, 2019)

cjtoombs said:


> ...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 (Feb 7, 2019)

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 (Feb 7, 2019)

pstemari said:


> 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.
> 
> ...



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.


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## Eddyde (Feb 7, 2019)

I started down the same path as Mr Pete when I began my bandsaw conversion. The VFD alone wasn't enough to get down to the real slow speeds needed for cutting steel. I used a bolt on planetary-gear reducing pulley and replaced the direct drive motor with a shaft and step pulley arrangement. Works great, heres a link for anyone interested:








						Bandsaw speed reducer build.
					

Hi All, I recently picked up a nice mid 50's vintage, Boice Crane, 14" vertical bandsaw.  It has a three phase motor so I figured I'd simply throw a VFD on it and be able to get down to metal cutting speeds, wrong!... The motor is rated at 1150 rpm and running it at 15hz only gets the blade down...




					www.hobby-machinist.com


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## just old al (Feb 7, 2019)

Agreed on the gearbox requirement. When I converted the wood bandsaw I bought I simply added an add-on gearbox to the setup, bolting it and the motor to a sub-plate which was then bolted to the saw base using slotted holes for adjustment. That gets the speed down to a comfortable if fixed level and is more than adequate to my needs for the sheet and plate work (less than 1/2" plate) that I use it for.

Anything that requires heavy hogging of thick materials gets given to the horizontal bandsaw.


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## jwmay (Feb 7, 2019)

cjtoombs said:


> 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.


I'm losing you in the conversions.  How is making the motor 6 times more powerful affecting the speed?  The only way to get a speed ratio is with compound pulleys or gears, right?  input rpm/output rpm.     What am I missing there? 

Running a 3 hp motor at 10 hz to get 1/2 hp at 100 sfm seems to be what you're recommending.  And then also I'm thinking in rpm, so maybe that's what's got me in a bind.  Does that negate the recommendations from manufacturers stating not to run a VFD at less than 50 %?  I guess it's ok outside of industrial applications?  I'm not attacking.  I really want to understand.  And I do know of several motors that have been running 30 hz for at least a couple years with no ill effects so far.  So maybe that little nugget of wisdom I picked up somewhere doesn't hold water?


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## JimDawson (Feb 7, 2019)

jwmay said:


> I'm losing you in the conversions.  How is making the motor 6 times more powerful affecting the speed?  The only way to get a speed ratio is with compound pulleys or gears, right?  input rpm/output rpm.     What am I missing there?
> 
> Running a 3 hp motor at 10 hz to get 1/2 hp at 100 sfm seems to be what you're recommending.  And then also I'm thinking in rpm, so maybe that's what's got me in a bind.  Does that negate the recommendations from manufacturers stating not to run a VFD at less than 50 %?  I guess it's ok outside of industrial applications?  I'm not attacking.  I really want to understand.  And I do know of several motors that have been running 30 hz for at least a couple years with no ill effects so far.  So maybe that little nugget of wisdom I picked up somewhere doesn't hold water?



Running a standard motor at much lower than the rated speed can cause overheating.  But unless you are trying to pull near maximum torque this is normally not a problem.  The problem is the fan on the motor, as the RPM is reduced, the airflow is reduced.  This can be negated by installing a seperate fan.  Industrial motors that are designed to run over a wide speed range are equipped with separate cooling fans, spindle motors for instance.

Depending on the VFD used, the torque may drop off as the speed is reduced.  Modern sensorless vector VFDs will maintain full torque down to near 0 RPM, and thus constant torque up to the rated RPM, and constant HP above the rated RPM.  The older V/Hz VFDs drop off the voltage as the speed reduced and thus the torque is reduced as the RPM is decreased.


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## cjtoombs (Feb 7, 2019)

Yes, at 10 hz you could run into heating issues as stated above depending on the motor you use.  If you use one of those antique motors with no fan, it shouldn't be an issue.  In the example I spoke of above, you are using the VFD to vary the speed, but using gearing/belts to get the saw into the right speed range.  A 12:1 speed ratio on the VFD would give that idealy at 100 to 1200 FPM, but if you changed the gears/belts you could adjust that to 200 to 2400, etc.  A 3 hp motor to get 1/2 hp at the blade is a bit of an extreeme example, as the cost for the motor and VFD would be pretty high compared to what you could do with change belts, but it just depends on how anoying it is for you to change belts.  Personaly, I think combining a varialbe speed pulley set up with a VFD motor is the way to go for wide speed ranges without changing belts.


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## Submachine (Feb 7, 2019)

I have a 30-1 baldmor gear motor I will be selling (cheaper than VFD and LOTS of torque.) It was for my bandsaw project which I never had time for.  I ended up buying a Swag table with a variable speed Milwaukee and really love it.  Foot switch is great on these.


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## tcarrington (Feb 7, 2019)

another pitfall at low speed is a motor not designed for a VFD doesn't seem to work as well. In the applications with which I am familiar needing a large speed range, meaning going below 10 Hz, an inverter duty motor is a great asset, maybe even a must. Even unloaded, it takes a change to the normal volt / hertz curve to make it work at all.


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## pstemari (Feb 7, 2019)

Yeah, when you run a motor at lower frequency you run the risk of saturating the iron core. 
The iron core effectively acts as an amplifier for the magnetic field up to the saturation limit of the core material, and past that point you get much less force increase for a given current increase. The peak magnetic field in the core is proportional to the current, and therefore the torque, so the power output (torque*speed) at the low end is likewise limited.


What that limit is depends on the motor design.

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## HarryJM (Feb 7, 2019)

What a timely topic for me as I have been researching how to convert my recent purchase of a 1939 Delta 14” wood band saw to a metal one.

Over the past several weeks I have searched this forum, Practical Machinist and OWWM for wood to metal band saw conversion's and it seems to me that the consensus is that VFD buy itself is not the best solution due to the torque loss as slow speeds (as Mr. Pete declared). Jack shaft reduction and/or gearbox speed reducer seem to be the best solution along with VFD or not. I purchased a copy of The Home Shop Machinist January/February 2004 for their article “Converting a Wood cutting Band Saw to Cut Metal” and that was done with a jack shaft.

I purchased the band saw along with a Craftsman 10” table saw for $50 and then turned around a sold the table saw back to the PO for $23. So for a $27 investment I can now to spend a little $ to convert it to a metal cutting one. The frame is sturdy enough as Delta also manufactured a wood/metal version of this saw with a gear box reduction. The saw was a little beat up and I have so far I have purchased two missing pieces from OWWM BYOD for about $76.

Not sure at this time as to what my final solution will be, although the Home Shop Machinist article sounds doable as I like the idea of being able to convert from wood to metal with just a belt change.


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## jwmay (Feb 7, 2019)

So from what I’m reading here, the damage we try to avoid with regard to running a vfd at below 30 hz is not damage to the vfd, but to the motor. Is that correct? This is assuming a max hz in this example of 60. 

I don’t know a thing about sensorless vectorless vfd’s. It sounds pricey though. Is this what we’re getting on EBay for 100 dollars or less? Or are those the old volts /hz models?


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## Dabbler (Feb 7, 2019)

I have a VFD driven bandsaw.  It uses a 1.5 HP motor, and has 18" drive wheels... It has more than enough power for things like 1/2 inch thick steel.  think 2" tool steel would bog it down, but hey, I'm a hobby shop. I'll cut a little slower.

Now my motor is VFD rated, but the whole contraption is an offshore, low bid machine.  It has never let me down in 8 years of use.


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## stupoty (Feb 7, 2019)

Ulma Doctor said:


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


just put a bigger motor on it they said


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## Ulma Doctor (Feb 7, 2019)

stupoty said:


> just put a bigger motor on it they said


That is awesome!!!


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## hman (Feb 7, 2019)

Just saw an ad for a vintage Walker Turner wood/metal bandsaw on the Phoenix Craigslist - $950.  It's actually in Elgin, about halfway between Tucson and Mexico.








						Walker Turner metal or woodcutting bandsaw
					

This is a great running Walker Turner bandsaw with 2 speed transmission and multi step pulleys for even better choice of speeds depending on material your cutting. Very heavy, everything works as it...



					phoenix.craigslist.org
				



Here's a screen shot of one of the photos in the ad, showing the speeds available with the 2-speed "transmission."


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## pstemari (Feb 8, 2019)

jwmay said:


> So from what I’m reading here, the damage we try to avoid with regard to running a vfd at below 30 hz is not damage to the vfd, but to the motor. Is that correct? This is assuming a max hz in this example of 60.



Yes, the major risk is overheating. I forget the specifics of how the motor appears electrically when the core saturates. It might draw more current due to reduced impedance, but the VFD should limit the current.



> I don’t know a thing about sensorless vectorless vfd’s. It sounds pricey though. Is this what we’re getting on EBay for 100 dollars or less? Or are those the old volts /hz models?



I don't know about the eBay specials. The Hitachis are sensorless vector drives, and not terribly expensive.

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