Grinding vs. Scraping
Some years ago I made a small telescope mirror. GROUND it from scratch. Now that I am more active in machinning metal, I have read over and over that precision flats and machine ways are made by SCRAPING. I am curious about the relative merits and problems of each of these processes.
Now I am not talking about grinding with a mega bucks grinding machine. I am talking about grinding with a powdered abrasive, perhaps in an oil or water mix, applied between two of the three surfaces and then they are ground together - by hand. Successively finer grades of abrasive would produce a finer and finer surface.
Obviously, there must be a reason why machinists scrape instead of grinding. But in theory, three flats could be made by alternatively grinding them together with progressively finer grades of abrasive just as well as they can be scraped to flat.
In making my telescope mirror, I started with a coarse abrasive, perhaps 80 grit and worked my way down. Since I was making a spherical surface, only two pieces of glass were needed, instead of three. And I guess that scraping would not work with glass or pyrex because they are too hard. And yes, I know glass can be cut with diamond tools.
But why is scraping prefered with metal? Would grinding with a loose abrasive between the surfaces not work? Perhaps mixed with some oil. Has anyone ever done it this way? I would love to hear some discussion on this.
Make it fit.
You can't win and there is a penalty for trying!