# Disc Sander RPM ???



## jj777746 (Jan 17, 2015)

Hi All,a family member recently renovated his bathroom & gave me the pump/motor from his spa bath (hot tub).I've been wanting to make a disc sander for some time now & thought the motor would be perfect for the job,but the RPM is 2800. A backing plate made of Melamine would surely fly to pieces at that rate of speed...Is there a method of reducing the RPMs without a gearbox,or is 2800 rpm O.K ? Thanks for any/all info.James


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## Cherryville Chuck (Sep 28, 2010)

It may depend a bit on what exactly you set up James but these table saw attachments are meant to turn at a minimum of 3450rpm because the the slowest any table saw that diameter would run. Buy Sanding Disc 10in. at Busy Bee Tools


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## cocobolo1 (Dec 31, 2015)

Cherryville Chuck said:


> It may depend a bit on what exactly you set up James but these table saw attachments are meant to turn at a minimum of 3450rpm because the the slowest any table saw that diameter would run. Buy Sanding Disc 10in. at Busy Bee Tools


Thanks for the link Charles. I was thinking of doing something similar on my table saw, until a 1725rpm motor came into my hands.

But James, if you make a disc out of a good quality plywood, rather than Melamine, I think you would be OK. At least that's what I will be doing. I want a rather large disc, as compared to the usual 9" variety, something closer to 14".

If you want to gear it down, just use a small pulley on the motor and larger pulley on the disc.


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## old coasty (Aug 15, 2014)

Is there a problem using this on a table saw? The saw expects the pressure parallel with the blade. The sanding disk pressure is at right angles to the blade. Will the bearings be happy with this?


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## Cherryville Chuck (Sep 28, 2010)

I wouldn't be pressing that hard personally. For me it would be more of a finishing "cut" than stock removal. I have a 108" x 6" vertical belt sander and I don't push that hard on it.


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## old coasty (Aug 15, 2014)

Thanks Charles. I thought that might be the answer. I had one in a previous lifetime on a Radial Arm Saw but did't use it much.


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## Gene Howe (Jul 10, 2007)

My disks are all 12" and they are mounted on my table saw. I've been using them for 35 years. Two are steel and two are ribbed aluminium plates.
There has never been a problem with bearings. But then, it's a Shopsmith. 
I've never seen a need to run them any faster than 1725.


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## DaninVan (Jan 1, 2012)

An issue James didn't mention is the shaft of the motor. I too have a 1 1/2HP spa motor lying around (I didn't drain the hot tub and an unexpected deep freeze destroyed all the plumbing parts including the pump).
The shaft isn't like a normal a/c motor...it's threaded on the end to couple with the pump. No keyway and no flat.  
I think grinding a flat on it may be the simplest solution ?


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## jj777746 (Jan 17, 2015)

cocobolo1 said:


> Thanks for the link Charles. I was thinking of doing something similar on my table saw, until a 1725rpm motor came into my hands.
> 
> But James, if you make a disc out of a good quality plywood, rather than Melamine, I think you would be OK. At least that's what I will be doing. I want a rather large disc, as compared to the usual 9" variety, something closer to 14".
> 
> If you want to gear it down, just use a small pulley on the motor and larger pulley on the disc.


Thank you Keith,good quality 3/4" ply should do the job eh ? Re gearing it down,I was just going to fit the disc straight onto the motor shaft as seen on several you tube shows & I should have been more precise in my initial post. JJ


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## cocobolo1 (Dec 31, 2015)

jj777746 said:


> Thank you Keith,good quality 3/4" ply should do the job eh ? Re gearing it down,I was just going to fit the disc straight onto the motor shaft as seen on several you tube shows & I should have been more precise in my initial post. JJ


I would say so, I'm going to use some 11/16" birch ply.

In your case I would keep the size of the disc down to about 10", otherwise I think the speed of the outer edge of the disc might be so fast as to cause burning, especially once the sandpaper started getting worn out.

At 10" and 3450 RPM you have an edge speed of slightly over 150 feet per second, or 9000+ feet per minute. Plenty fast for sandpaper. I may just stay down to a 12" disc and 1725 RPM, which would have an edge speed of 94+ FPS, 5650 FPM. More than fast enough to remove skin!


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## cocobolo1 (Dec 31, 2015)

Gene Howe said:


> My disks are all 12" and they are mounted on my table saw. I've been using them for 35 years. Two are steel and two are ribbed aluminium plates.
> There has never been a problem with bearings. But then, it's a Shopsmith.
> I've never seen a need to run them any faster than 1725.


Undoubtedly your discs are perfectly balanced and run smoothly. Wood often doesn't have identical density throughout which may cause an imbalance. That is where I think one could run into trouble.

Cutting the disc with a router should make it nice and round to start with.

I was in KMS Tools yesterday and they had a good selection of 12" sandpaper, which has prompted me to drop down to 12" from 14".


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## Cherryville Chuck (Sep 28, 2010)

I think balance may be an issue for sure. A lot of things like this have been balanced, you can see where holes were drilled on one side to get the weight even. The slower you spin, it if it is out of balance, the less problem the uneven weight causes.


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## Herb Stoops (Aug 28, 2012)

I agree with Chuck. Some of the plywood discs I have seen in that size have a tapered back where they are thinner at the outside rim and have a smaller diam hub mounted on the back. Then they are trued up on a wood lathe.

Herb


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## cocobolo1 (Dec 31, 2015)

Herb Stoops said:


> I agree with Chuck. Some of the plywood discs I have seen in that size have a tapered back where they are thinner at the outside rim and have a smaller diam hub mounted on the back. Then they are trued up on a wood lathe.
> 
> Herb


I too agree with Chuck. 

I wonder how the thinner edge would be. Do you think it might flex a bit? Not that that would necessarily be a bad thing. I like the idea of the extra hub on the back...seems to me I saw one on a Youtube video recently. If you don't have a wood lathe, you could probably trim the edge right on the disc sander itself once it was up and running.


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## Cherryville Chuck (Sep 28, 2010)

The lower the mass at the outer edge the better as far as balance goes. One thing that can be done is a static balancing where you put a shaft through the hub and then set it down on two thin parallel rails. The heavy side will roll down. You could keep playing with adding or subtracting weight until it either doesn't want to roll or at least rolls very slowly. That should get it close at least.


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## cocobolo1 (Dec 31, 2015)

Cherryville Chuck said:


> The lower the mass at the outer edge the better as far as balance goes. One thing that can be done is a static balancing where you put a shaft through the hub and then set it down on two thin parallel rails. The heavy side will roll down. You could keep playing with adding or subtracting weight until it either doesn't want to roll or at least rolls very slowly. That should get it close at least.


That's how we used to balance the little electric motors on our slot cars when we were kids. Remember those? I used to take the skinny wire off the motor and replace it with thicker wire and fewer windings. We used two razor blades to set the armature on.


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## Cherryville Chuck (Sep 28, 2010)

I remember the cars but I didn't know that about the motors.


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## Cherryville Chuck (Sep 28, 2010)

Ho Nancy and welcome to the forum. Some woods tend to burn at higher speed rates (like maple) so going slower is certainly preferable in those cases. Plus I find I screw up at a much slower rate when I slow things down. One of the reasons I often switch to hand tools when I start getting close.


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