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Truing a 12" sanding disc: YYC

I've recently acquired a Old Rockwell/Beaver 6x48" belt and 12" disc sander combo that I'm rebuilding.

Unfortunately, some time in the past 50 years since new, the disc was dropped and is no longer running true to the bore

I'm wondering if a YYC member could true this on a lathe. I need the disc face running perpendicular to the shaft bore. Its cast iron as far as I can tell.

When I lay the disc face flat on the machined table of my 12" jointer, the bent/warped section is about .030 clear of the table. This is a rough estimate as the disc face is covered in old glue/adhesive. Disc thickness is about 7.2mm. I'm hoping a .030 to .050 cut would clean up the face. It doesnt need to be absolute perfect either, within a couple thou would be more than sufficient for my woodworking and occasional metal sanding needs.

Obviously its going to cost me something. PM me and we can discuss. There's no rush to get it done either.



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RobinHood

Ultra Member
After turning the face flat, it will need rebalancing.

I’d have to measure, but I am not sure my grinding wheel balancer can handle this 12” disc.

I can turn it for you on the lathe. At least you’d be that much closer to having it operational again.

Some other thoughts:
I presume you have checked it for cracks by doing a ring test? CI does not like to bend much, it tends to crack. You could try and heat it to straighten it back out. A heat gun or a propane torch might be enough to bring it back to flat.
 
Some other thoughts:
I presume you have checked it for cracks by doing a ring test? CI does not like to bend much, it tends to crack. You could try and heat it to straighten it back out. A heat gun or a propane torch might be enough to bring it back to flat.

Im unaware of a 'ring test' until now. I'll do some reading.

I hadnt thought of heating it... HEat while on a 'reference' surface like my jointer bed or tablesaw and see if it returns to 'flat'? I realize a jointer table isnt as precise as, say, a surface plate, but my jointer tables are pretty darn flat as checked with a lee valley reference straight edge and feeler gauge
 

RobinHood

Ultra Member
It should sound like a bell (or tuning fork) when struck lightly with a metal object like the shank of a screwdriver or with a light tap from a small hammer. Best to run a string / rope through the hub and suspend the plate like a gong.

If the sound is crisp and slowly fades away, the plate is integral. If the sound is dull and fades away quickly, there is a crack in the plate.
 
I've done the ring test, but all I get is 'thud' any spot on the disc. Maybe its a forging?

I'll try holding it with some rope and give it a light tap with a ball peen
 

YYCHM

(Craig)
Premium Member
I've done the ring test, but all I get is 'thud' any spot on the disc. Maybe its a forging?

I'll try holding it with some rope and give it a light tap with a ball peen

If you clean off a spot down to shiny metal and rub it with white printer paper CI will leave a grey smudge. Steel does not.
 
Attached is a zipfile containing an mp4 video of me holding the disc with some twine and tapping it lightly with a hammer. It just goes thud anywhere on the disc.

I also cleaned the sandpaper surface of the disc with laquer thinner and sanded it with an orbital+80 grit to get it clean. I did a close visual inspection and cant see any visible cracks - not to say there isnt any...
 

Attachments

  • IMG_2118.zip
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YYCHM

(Craig)
Premium Member
I had a printed invoice from Gregg's and used the clean backside. It picked up a bit of dirt from the pores, but it didnt go solid
gray

Doesn't go solid gray, looks more like a dirty finger print. It's pretty obvious when you compare the same test results to that done on a steel surface. Post an image of your paper smudge.

Here is a paper test I did today.

CISmudge.JPG


CI on the left, Steel on the right.
 
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Janger

(John)
Administrator
Premium Member
Testing Zip archive of video from my phone. I don't think we've done that on the forum before - for shorter clips this is simpler than uploading to youtube or vimeo etc. Thanks Chris good idea.
 

Attachments

  • IMG_0952.MOV.zip
    5 MB · Views: 28

Janger

(John)
Administrator
Premium Member
Attached is a zipfile containing an mp4 video of me holding the disc with some twine and tapping it lightly with a hammer. It just goes thud anywhere on the disc.

I also cleaned the sandpaper surface of the disc with laquer thinner and sanded it with an orbital+80 grit to get it clean. I did a close visual inspection and cant see any visible cracks - not to say there isnt any...

Is that thudding or ringing - what says the forum?
 
Is that thudding or ringing - what says the forum?
It definitely doesnt ring like a tuning fork. My wife is a amateur musician and it sounds nothing like the couple of tuning forks she has.

EDIT: I did the tap test again with a small ball peen and light but sharp taps. There is a faint ringing that lasts for a few seconds after the tap. I repeated this all around the circumference (8-10 spots) and got a faint but consistent ringing each time.

The rub test I did again comes out like the cast iron rub @YYCHM posted above.
 
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RobinHood

Ultra Member
So that indicates that you have integrity throughout the disk ==> no crack. If you support the disc in the center by hand, you will get a better ringing sound as the string, the way you have it attached, touches the circumference and attenuates the sound.

So I think we can conclude that your disc is made from CI, has no crack, but is slightly bent because of an impact force deforming it (or the CI has stress relieved over time causing the observed warp).

facing the surface will true it to the bore ==> what you are after.

I can turn it on the lathe, but my balancer is too small (I can balance a wheel up to 8” on my set-up).

You would need to find a way to balance the disc.
 

Janger

(John)
Administrator
Premium Member
Try this. Look at your “downloads” round icon with a down arrow very top right of the screen. Open the zip file and the open the .mov or .mp4
 
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