I have scraped a few machines to get where it was not practical to grind or machine and have had very good results but if machined right I have not seen where scrapping is needed. I have a hard time with flaking part of it because all it does is limit surface contact which glides easy but also can chatter more and wear quicker like a golf ball surface. I dont know I am up in the air about it I read this and thought I would share he has some good points to keep in mind ideas and opinions can go either way there is good and bad to everything I guess.
The scraping process is incredibly crude, and totally incapable of producing anything like a smooth surface or precision flatness. It is simply impossible for that process alone to produce the result desired.
The scraper gouges a swath of material away. The scraper is not flat on the end, so the gouge it produces is rounded. The resulting surface is therefore a series of gouges, in a random pattern and of random depth.
You cannot simply scrape a surface flat. End of story.
Because of that, scraping is inevitably associated with various other items besides the scraper. These are necessary to correct the horrible distortions of the surface which the scraping process produces if uncorrected.
That is the secret of how the scraping process ends up with a flat surface.... The extra equipment tells you where to apply the incredibly crude process, and where NOT to. By correctly following the directions given by the comparison straightedge etc, as shown by the blue (or red, etc) spotting material, you can remove the high spots, and bring the surface down to where the "surface" is a series of flat spots with the "residual gouges" between them.
The net result is a surface defined by those flat spots, which is as flat and planar as your comparison device is. If the straightedge is curved, you can scrape a very accurate curve to match, but never a flat surface.
Luckily, geometry allows the creation of a flat surface of any desired flatness by the principle that if A=B, and B=C, and C=A, then A=B=C. A may have a curve "up" , and B may have a matching curve down, but C cannot then match both. Only when all are flat can C match A and A match B AND B match C.
A side note:
The existence of scraping marks on a machine may either mean that scraping was used to bring the machine into alignment, OR that someone made marks all over the surfaces in order to make you assume that a proper re-alignment was done.
This is usually obvious when a surface with visible linear wear marks also has "Nike swoop" marks on it that cut across the wear marks...... Most of the tricksters don't put a full surface of scraping marks on... that would be too much like work. They are satisfied with crude "flaking" to fool the unwary.
