Questions

1) When you sanded the plastic sliders, I did not know if you went through the same sandpaper grits and then the last was the #400? You didn't comment on that. So what grits on the plastic did you use?

2) After the graphite, the final grit on the wood was what grit.....#400?

3) The e-mail exchange was what I could not find again.....That was what I wanted to read again. Thanks

4) You mentioned that the DVD contained "sealing the block" I did not see that. Were you meaning the process with the phenoseal, or did you only mean with the "duct tape"? I'd love to see the phenoseal process.

5) Do you have anything about checking the individual bellows? Do you ever seal the bellows channels with phenoseal? How do I know if I need to recover the bellows? Is shellac used at all on the air motor, or am I just seeing hot hide glue in places?

I have an observation for you (please don't believe I am trying to criticize...not in the least....I am very grateful for the time you have taken to help others). My observation is the result of a telescope reflecting mirror I worked on some 45 years ago. The method of grinding the round mirror is to have the "sanding substrate" sitting on a surface the SAME size of the Pyrex glass to be gowned. The idea is to move the glass back and forth and each time rotate the glass a bit. One does this over and over and over. I must have worked on this after work several hours a night for several weeks. The glass become concave...because the glass spends increasingly less time away from the edges.

The point is.....I believe, if the slider area for the plastic valves is to be perfectly flat, then the sandpaper you use needs to be at least as long as the full movement of the length of travel of the sanding. Now maybe it really is not all that important, but because of my experience with the telescope mirror, I think about that. Just a thought for you. In your video, the ends are spending less time on the sandpaper as is the center section. Since you are so detail in your work, this is something you may want to think about?

I know several years ago, I reported a broken web site link for you, and you were very open to this, so I thought you might be open to hearing about my thoughts with sanding.

My Reply

What I would really like is a piece of sandpaper that is 2-3 feet long and a piece of glass that is perfectly flat. That way I could drag (or push) the block and/or the sliders in only one direction on each pass. In other words, drag the piece across the paper from (say) top to bottom, then lift it up and start over again from the top... etc. However, I don't do enough of this type of work to justify buying the rolls of paper that would be needed. I resurface one or two air motors a year...

Naturally, one could keep using a higher and higher grit and less and less pressure on each pass until the block is 100% perfectly flat, but the truth is that the graphite itself is an abrasive. That's why there is a 'burn-in' period after the motor has been re-assembled. And, if it looks like any of the sliders aren't making good contact all the way around the vacuum holes, then more work is needed.

Most shops only sand the block down to 220 grit....

Then there's the question of 'which way do you sand?' With the grain or against the grain?

The point is, the method isn't nearly as important as the result. There's an old saying that goes, 'the closer you get to perfection, the easier it it to spot imperfection'. My view is that as long as people understand what the goal is, they can use whatever method they prefer. (And this is one of the reasons why I don't plan to ever write a book. I've seen how people will tear the author apart because he picked one method over another method or one material over another material and others spent a great deal of time and effort to discredit them for their choices.)

As for the sliders, I believe I started and ended with #400. In regular air motors with wooden sliders, the wood that the slider is made of is much harder than the block, and they typically need very little attention. Here again, I just keep checking the pattern and the shadows.

As for sealing the main vacuum channel, I just seal the holes, pour in the phenoseal, slosh it around, and pour it back out. If I'm in a hurry (which is only the case if I'm on the road at a customer's house), I'll use a hair dryer to speed the drying process. Otherwise, I let it dry overnight.

I do not pour the phenoseal back into the bottle, but I do save it for other jobs, such as sealing the internal wooden surfaces of bellows or valve chambers.