Truing up an SGA contact wheel

[Hubert S.]

I just built a surface grinding attachment and noticed some slight waviness in the surface finish of the test pieces I ground. The amplitude of the waves is very small and barely makes the needle on my test indicator move, but it is pretty visible by eye, even though it is very difficult to capture in a photo. I remember reading some posts about this issue being caused by the contact wheel being slightly out of round. There is a video by Reeder showing how they true up their contact wheels to overcome this issue, but it is a pretty elaborate rig. I thought about trying to true up the wheel on the lathe, but it seems like you have to have a tool post grinder to machine the rubber. So, before doing something complicated I thought I'd try something really simple. I took a piece of precision ground 3/8" thick by 2" wide A2 tool steel and glued on a piece of 220 grit sandpaper using 3M 45 spray adhesive. I then put the A2 on the magnetic chuck and spun the wheel by hand as I very slowly moved the sandpaper covered A2 towards the wheel using the feed mechanism on the SGA. I could tell that the wheel was slightly out of round when it started to make contact in one spot during its rotation. I kept spinning the wheel and could actually get it up to a pretty good speed by advancing the chuck very slowly. The total feed on the SGA's dial indicator from first intermittent contact to where the wheel made contact throughout the entire rotation was 1.5 thou, and I went a bit past that for a total of just over 2 thou. I locked the translation stage at the end and kept spinning it for maybe half a minute or so. There was a small amount of rubber grinding dust just below the contact point. I did move the chuck laterally throughout this procedure to expose fresh grit on the sandpaper.

After doing this, I ground in the chuck again, which hardly took off any material. Then, I ground another test piece using the same 120 grit ceramic belt as before and the difference in surface finish was just amazing. There was almost no waviness left. In the picture below, the top piece is the one I ground before truing up the chuck and the bottom is the one after. Unfortunately, the picture does not really show the difference very well, but the vertical bands in the top piece on the left are caused by the surface undulations.

Thought this might be useful to somebody. If you try it, please post your results.

 

[Ken H>]

That's a darn good idea, I might try that. I know my wheel isn't all that "round". I have used a lathe to true up a 10" poly wheel I have. First put wheel in freezer overnight so it's real solid, then chuck up in a 4-jawed centered on the wheel axle. A frozen wheel turns pretty good.

Hubert, You've done a really fine job on that SGA.

 

[Hubert S.]

That's a darn good idea, I might try that. I know my wheel isn't all that "round". I have used a lathe to true up a 10" poly wheel I have. First put wheel in freezer overnight so it's real solid, then chuck up in a 4-jawed centered on the wheel axle. A frozen wheel turns pretty good.

Hubert, You've done a really fine job on that SGA.

Thanks, Ken. I've read about freezing rubber before turning, but I did not want to chance it since it takes so long to get a new wheel if I mess it up. I figured sanding it by hand in small increments was a safer option. Also, setting up the lathe for this would probably take me at least an hour before making the first cut. The method I used took less than fifteen minutes. I have another 12" contact wheel that is slightly out of round that I think I'll try this with. I think I can just stick the tool arm for the 12" wheel in the tool slot next to the SGA, but I would have to take the 8" wheel off the SGA to do it. I'll try to take some pictures of the process next time.