Truing a disc-sander hub and disc

[jwmelvin]

I’m seeking a few points of guidance on truing my disc sander. It uses a hub with a woodruff key and two setscrews (one on the key, one at 90°), and the disc has three countersunk screws into the face of the hub. The shaft end has about 0.001” TIR (radial), which seems pretty good to me. The hub’s face where the disc mounts has about 0.009” TIR (axial), so that’s where I’m focused.



I'd really appreciate thoughts on the following:

(1) The keyway seems to complicate centering the hub’s bore in a 4-jaw. Can I run my DTI over that gap okay (turning slowly)? Or just go back and forth over the uninterrupted portion of the bore? My other idea was to make a short shaft the same diameter as the sander's shaft, secure it with a key and the two setscrews, then indicate off that, to try and replicate the operational condition. (I could use the actual shaft but it's a disc-belt sander, and would be a little involved to get the shaft out; it feels smooth so I have no need to replace bearings, etc.). I suppose that by using a shaft in the hub, I'm more likely to get the shaft parallel to the spindle axis, which is really what I care about. Addressing (2) below would take care of all this by creating new surfaces with the correct orientation.

(2) There is a little slop between the hub and the shaft; I didn't measure, but I can feel the hub rock a little before tightening the setscrews. improving the fit between the shaft and hub would be nice. My thought was to rebore the hub and press in a sleeve that could then be bored to match the shaft. I'm not sure it's worth it, as then I'd have to cut a keyway. I have no broaches; in a prior project I have made a keyway with the lathe-carriage-shaper approach, though that was aluminum and this would be steel.

(3) Once the hub is improved, I'll mount the disc on it and measure that (it's sitting under the hub in the picture). I'd love to be able to skim the face of the 12" disc. My lathe has a 12.25" swing over the bed. But the carriage's support means I won't be able to get the toolpost any closer than about 5" from the face of the disc. My thought is to use my largest boring bar (3/4" steel with a HSS bit) in a facing cut and hope for the best but I know that's beyond where such a bar performs best.

Thanks,
Jason

 

[cathead]

That's pretty typical of a key way mounted setup. An easy fix would be to run the motor with the
disc attached and put a "sharpee pen" on the face to locate the high side. Then shim the opposite side with the right
thickness of paper or whatever material is appropriate.

 

[jwmelvin]

...shim the opposite side with the right thickness of paper or whatever material is appropriate.

Well there you go. I was all excited to use my new lathe and do something good, but I see your point. Probably a much better use of my time.

 

[cathead]

You could attempt to machine it and possibly make it worse in the process. If you do machine on it,
the best way would be to face it off while the disc is mounted on the motor. Shimming is the easy way.

 

[markba633csi]

Making a snug-fitting arbor and skimming on the lathe would be the best way to get rid of all the runout. You wouldn't necessarily need to cut a keyway in the arbor if you just drill a shallow hole to let the setscrew(s) drop in, then just take very light cuts with a sharp HSS bit.
-Mark

 

[Janderso]

I need to do the same thing.
I keep putting it off but I'm trying to build a slow speed lapping machine for hand scraper blades. The arbor and mounting set up has to be true-no runout.
It's not as easy as it sounds, at least not for me.

 

[Braeden P]

I need to do the same thing.
I keep putting it off but I'm trying to build a slow speed lapping machine for hand scraper blades. The arbor and mounting set up has to be true-no runout.
It's not as easy as it sounds, at least not for me.

I use mine when it has lots of run out the disks that i use are really flexible

 

[Nutfarmer]

I would mount it on an arbor in the lathe . Then if necessary shim or true it on the disk sander. Before you cut anything check it shaft of the disc sander to make sure it's not the problem. It's very likely it may be a slightly bent shaft causing the problem .

 

[jwmelvin]

Making a snug-fitting arbor and skimming on the lathe would be the best way to get rid of all the runout.

Thanks Mark. Though the hub would run truer on a snug-fitting arbor, that would allow it to tilt slightly when mounted to the machine's shaft. Might it be better to replicate the size of the machine's shaft for the arbor, to try and replicate the fit? I was even thinking of adding set screws to the hub to dial it in on the machine's shaft (in which case I'd want the snug-fitting arbor when truing the hub face).

Before you cut anything check it shaft of the disc sander to make sure it's not the problem. It's very likely it may be a slightly bent shaft causing the problem .

The machine's shaft has about 0.001" radial TIR, which seemed pretty good to me.

 

[jwmelvin]

I made a tight-fitting arbor and held the hub on it with pressure from the tail stock using a plastic sleeve I printed a while back. Light cuts, with a new CCGT insert and I was able to take just a thou or so at the end. It worked well as far as the hub, which wasn’t so out of true itself and easily cleaned up.

The mounting with the shaft is the issue. I decided not to shim it yet, as there is some play in the shaft bearings, despite running smoothly. Maybe I can simply apply some preload but I’ll have to look at it further.