Tearing hair out with disc sander

This topic has 9 replies, 6 voices, and was last updated 20 July 2011 at 10:38 by charadam.

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9 July 2011 at 19:18 #71583
charadam
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@charadam
I happened upon a 1/2 hp 240V electric motor and decided it would form the power source for a disc sander.

A 12″ alloy disc was sourced, a shaft bush was inserted in the boss of the disc and there was .060″ runout on the disc surface.

On checking on the surface plate, the disc is flat within .008″ and the mounting hole is concentric within .008″.

The motor shaft is 12.5mm (sorry to mix conventions) and runout is less than .001″

One issue is that the motor shaft has a 2″ overhang from the mounting surface plus a threaded end and I had to make the shaft bush with an internal shoulder to square itself on a step in the shaft. a retaining nut and penny washer was used to retain the disc and tension the shaft in the shaft bush. The bush and the shoulder show no discernable runout.

The disc mounting hole is therefore, probably not square to the disc surface.

My lathe is a 10″ Chipmaster and obviously cannot swing the disc.

Other tools are a Boxford mill and a pretty solid pillar drill.

Any suggestions how to proceed?

Thanks in advance for any thoughts.

Charles
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9 July 2011 at 19:18 #5576
charadam
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@charadam
9 July 2011 at 20:11 #71587
JasonB
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@jasonb
Can you fit say 3 grub screws around the bush equally paced and use these to tweak the alignment.
9 July 2011 at 20:23 #71588
Les Jones 1
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@lesjones1
Hi Charles,

First I will sate what my interpretation of the way the disk is mounted.

My understanding is that the bush is a good fit on the unthreaded part of the motor shaft and the alloy plate fits on the motor shaft between the bush and the retaining nut and washer. If the centre part of the plate is thick and a good fit on the motor then it will be how perpendicular this hole is to the surface of the plate that determines the axial run out of the plate. If the centre part of the plate is fairly thin and the hole a loose fit on the motor shaft then the run out will be more influenced by how parallel the front and back surfaces of the plate are and the axial run out on the face of the bush. You could true up the hole in the alloy plate using a boring head in your mill and make a bush so it again fits the motor shaft.

Regards Les.
9 July 2011 at 21:31 #71592
Ian P
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@ianp
Charles

A 12″ disk sounds quite large to hang on a 12.5mm dia shaft but it would be OK if you can get it close in to the motor bearings. From your description 12.5mm is not a normal motor shaft size so presumably its not a standard motor and you say its shaft has some run out anyway.

I would be tempted to make a new boss more or less permanently fastened to the motor shaft which you true up in situ, possibly by putting the whole lot between centres on the lathe (you could temporarily power the motor instead of the lathe) If the face of the hub is reasonably large, say 50mm you could fasten the disk with 3 or 4 countersunk screws.

The existing boss on the disk could be milled off to give flat face parallel to the disk face.

Ian P

Edited By Ian P on 09/07/2011 21:31:38
9 July 2011 at 22:08 #71596
Pat
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@pat
Hi Charles

As I see it you probably need to make a boss on which to mount the alloy 12 inch disc rather than a simple bush. IMHO the disc needs proper support to run true and the sanding forces at the perimeter would be too large for a simple bush and I think the 0.06 run out is to be expected without adequate support.

IMO the boss for a 12 inch disc should be several inches thick to make maximum use of the motor’s shaft length. Step the bore as necessary but ensure it is a good fit on the motor shaft. Having bored the boss make turn a dummy shaft and without removing the dummy shaft mount the boss and turn its out side diameters and face off square on the large diameter end that forms the support for the disc. test run.

To centre the disc on the new bush make a stub alignment bush that is a good fit in the disc and the new bush. Drill the disc and secure to the boss. I assume the alloy disc is a minimum of 12mm thick so you could use counter sunk screws of at least 6mm thread. Clamp the disc and boss together when correctly centred and drill for the securing screws. If the alloy disc is thinner than about 5 mm you may need to make the boss larger in diameter to add to the rigidity of the disc if you are going to use a heavy sanding pressure but the motor does not have enough power for sustained heavy sanding at the edge. You may have to trim the disk on the mill as a last resort.

The boss should be and 4 to 6 inches in diameter reduced to around 1.5 inches to permit three 6mm grub screws to be placed at 120 degrees to secure the boss to the motor along with the use of Locktite. Make sure you have left 12 to 20 mm thickness on the large diameter on which to mount the 12 inch disc. A disc that comes free during use is not a nice workshop companion! Hence the locking screws and Locktite after a brief trial run.

Secure the alloy disc to the boss using the dummy motor shaft which can protrude through the disc to help ensure centring is correct. Use c/s screws to secure 6mm dia along with Locktite between the two flat faces. Suggest you make the c/s heads recessed and fill with twin pack Araldite and sand flat. Also fill the centre hole. Secure sanding discs with Cow Gum or similar.

Try and make all screws symmetrical and use three or four as this will tend to make balancing less critical. Check that the motor is not going to spin the disc too fast 1,400 rpm is OK but 2,800 is too fast for a disc of this size. A one horse motor would be nearer the mark for a 12 inch disc and 1/2 horse may limit you to light sanding or fettling but would still be useful.

Hope this helps Regards – Pat

Edited By Pat on 09/07/2011 22:12:19
10 July 2011 at 00:28 #71607
charadam
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@charadam
Great stuff,

A lot to think about.

Many thanks – I will revert with more data.

Forumial regards,

Charles
20 July 2011 at 09:30 #72152
charadam
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@charadam
Thanks to all who replied.

Things have moved on and I have decided to mount the disc on a short shaft and provide the motor with a pulley for belt drive.

I happen to have a pair of 3-groove V-belt pulleys lying around so limited speed control will be available.

Picador rules OK!

Charles
20 July 2011 at 10:03 #72160
DMB
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@dmb
Piador is dead, I believe.
20 July 2011 at 10:38 #72163
charadam
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@charadam
http://www.picadoreng.co.uk/
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