Originally Posted by
Toolmaker51
The lapping process works in a particular manner. It is dependent on the lap [form] being softer than the part being fitted. Abrasive embeds [term is 'charged'] in the form, to work it's profile on intended part. Consequently a gib employed as lap would not be desirable as a mate to the fitted part. One way or the other, a duplicate is required. Soft steels, cast iron, brass, and copper are commonly used as laps, as mentioned selected for hardness less than the fitted part. Cleaning is with same oil as used during lapping and kerosene. Proof of complete removal? Microscopic particles x how many square inches!
This why many refrain from toolpost grinders. The problem is some think a pile of rags filter grit and metal fines. NO! Oiled brown paper or plastic wrap are far better suited when properly draped.
Scraping can be viewed as a cutting or finishing operation, per time invested. As machinery is typically cast iron scraping is superior to lap for several reasons. Surface of iron isn't very dense, trapping detrimental particles. The result is wear seen in well used machines. Scraping does not produce a 'flat' surface as such. Ideally, it presents 18 to 25 contact points per square inch. They are surrounded by 'divots' intended retain lubrication. With two such surfaces in contact, it is very nearly hydroplaning on lube, and isolated metal to metal. Properly cleaned and maintained, scraped-in work has incredible longevity. it also presents less stiction, the resistance to movement being initiated. Evidence of this relates to bumping a lathe carriage handwheel to transit the ways a few desired thousandths. All sliding surfaces have it in different degrees; generally desirable if consistent across entire range of travel. Automated equipment use linear guides to increase positioning speed and accuracy.
There are a variety of epoxy-like materials bedded to worn surfaces to supplant traditional metal to metal. Rebuilders use it with varied results, but an industry standard for quite some time now. Generically, they are known as "TurciteŽ"