A thread about my 20" Powermatic bandsaw. CONTENT ADDED. Vid!

Salem Straub

Salem Straub

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Oct 20, 2008
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Here's a thread about my newly acquired bandsaw. Mainly it's just to show off my new toy, but I'm going to include notes about making it run smoothly and adapting it to VFD use, which hopefully may help someone someday somewhere.

This is a Powermatic 20” model 81 bandsaw. It's the wood cutting version, the metal cutting version is the model 87 and it's the same saw with the addition of a blade welder and a low speed transmission. Don't need the welder, and I'm hooking a VFD to it, for low metal cutting speeds.

Here's some pics, first of all.

DSC00078.JPG


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I hooked it right up to the VFD no problem, it's got a 3phase 1.5 hp motor and that's enough for moderate wood cutting. Not quite enough for heavier metal cutting- the saw will do it OK, but I want more horsepower.

Everything was in Okish shape when I hauled it home. I got a wood blade with it, and it would rip blocks all right. But, some things about the saw showed up upon further use.

The saw takes blades from 149”-151” which is 12'-5” to 12'-7”. I got a couple of fancy bimetal blades for it, and immediately toasted one cutting some mild steel.

Well, I should have looked at the guides harder first. A $40 mistake considering the blade lost. Duh.

The guides are aftermarket Carter bearing guides, and were not adjusted to the blade right. Furthermore, the bearings were completely or partly frozen due to neglect. So, the blade deflected to one side and tore the corners of the teeth off...

I'll post more with some pics soon about reconditioning the bearings, and replacing some of them.
I will also post more about the horsepower issue. One approach I may take is to go to a smaller motor pulley, to slow the band speed and increase the torque to help the VFD.

For now, it's working fine on wood and metal and already much better than my previous bandsawing setups.
 
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That looks like an awful saw. I'll trade you my 10" Rikon for it.

Seriously though, I'm looking forward to seeing how it works out. I've been getting real tired of the limitations of my Portaband, and starting to think about replacement options.
 
Do what you can to improve the "gear ratio" between the motor and the drive wheel. Besides getting more torque to the drive wheel, you will also be letting the motor run at a higher speed. When you slow down a motor with a VFD, you also slow down the fan that cools the motor and that can cause the motor to overheat. I've learned this one the hard way on a couple different machines...
 
That's a good point. The pulleys are about 3" and 6", I'm looking around for new ones- 2.4" is easy to find for the motor shaft, but I want something bigger than 7" for the driven shaft.
 
Ok. Here's some new info about the saw, which I have worked on a fair bit since posting last.

I had to run the VFD way too slow to get a blade speed that would not destroy blades when sawing steel. So, as mahoney suggested, I changed the pulley ratio.
At first I tried merely upsizing the driven pulley from 6" to 12". That yielded a blade speed of 5110 sfpm at full motor speed (1725 rpm.) That's still FAST! If I ran the VFD at 20% of rated motor frequency, or 12 HZ, that would still be around 1000 sfpm, still way too fast. If I quartered that again, to run at 3 Hz., (not at all workable or powerful) sfpm was around 250, or still twice as fast as optimum for sawing steel.

So, I found a large pulley (17.5") from an old machine, and did the calcs for that reduction. With the 3.4" motor pulley, and a 17.5" driven pulley on the bandsaw shaft, 60 hz yielded 1748 sfpm. At 6 hz, (still not much power or motor cooling) 175 sfpm. Still too fast for steel, and no power.

At this point, I decided to involve a jackshaft for two-stage speed reduction. With some tricky lathe work and pillaging of old machines for parts, I made a working setup for a primary reduction of 3.4 motor pulley to 12" driven pulley, and secondary reduction of 6" jackshaft pulley to 17.5" bandsaw pulley. Then, provision needs to be made for the fact that the actual saw wheel is 20" in diameter, not 17.5" as per the pulley, so a slight increase in speed at the end.

What this gave me, at full 1725 motor rpm, was 869 sfpm. OKish for wood, still. At 15% of 60 Hz, or 9 hz, that's about 130 sfpm, or into the steel cutting range.
(I just redid the math in a hasty fashion at the laptop- IIRC the original calcs came out that I should get a little better performance than that, and I did.)

When I'd finally got the saw running tight, and did some speed testing on the band wheel hub with a digital tachometer, I found that 12 Hz. yielded a blade speed of 108.9 sfpm, and definitely enough power (interwebz research has suggested that 20% of rated Hz is in the lower end of acceptable powerband for a VFD controlled motor.) I also cranked the saw up to 100 Hz to test how it would run on the high end of things, and it did fine. Should be able to rip pretty tall wood on it.

Also, I had to take the Carter aftermarket blade guides apart and free them up. They'd been run past the point of freezing by the previous owners. I really need to order a few more bearings when I get the chance, but they spin again. My friend has a 20" Grob bandsaw that I've run a lot, with solid metal guides- I'd prefer to go that way when I can, and supposedly DoAll guides will bolt onto the Powermatic 81/87 saws. If not, a little modding should make them work. Also, if it turns out I want more power, I'll go to probably 3 hp. (It's got 1.5 hp on it now.)

Check out the vid, though! That's the fun part.

[YOUTUBE]MeTjWK1ByHI[/YOUTUBE]
 
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Nice work. Looks like you have a good setup going!
 
Funny this comes up now.

Saturday evening I was re-sawing up some logs and the carbide tipped blade broke with a big "bang". One of the upper roller bearings had exploded, and locked the blade so fast that the blade tore itself in two. Ordered a new $140 carbide blade and a set of six new bearings yesterday.
 
Nothing fancy, it is a Rikon deluxe 14". Works pretty good for a 1HP. I will someday convert it to a 2HP 3Ph motor and VFD. Carbide blades make up for lower power. They will cut so smooth it looks sanded sometimes.
 
nice conversion, looks very solid. you might want to make a taller key for the 12'' pulley though. if it shears it will gall like crazy and be a bugger to remove.
 
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