How do you produce a sliding or mating fit using filing, scraping and lapping techniques?

Filing does not produce fine surface finish.

A scraper is an unusual blade with a burred scraping edge rather than a sharp cutting edge. A scraped surface is preferable to a sanded one, as it is extremely smooth and it avoids the slightly clouded appearance of wood that has been sanded. Normally, scraping does not produce good
surfece finish needed for sliding fits.

Lapping is a fine finishing procedure which has two main functions. Firstly, it produces a superior surface finish with all machining marks being removed from the metal surface. Secondly, it is used as a method of obtaining very close fits between mating parts such as pistons and cylinders. The parts to be lapped may either be flat (as in the case of locomotive slide valves) or cylindrical (for piston valves or IC engine cylinders). It is very much more difficult to lap a taper fitting for reasons that become obvious when the relevant methods which are available to the home machinist are described.

To digress a little, with the right technique it is incredible the accuracy to which the home machinist can work with some effort. I have an interest in astronomy and lapping is a fundamental process in the art of telescope mirror making. Here, a chunk of low-expansion plate glass is first ground (by hand) using a second plate of glass moved across the top with abrasive between the two. This produces a spherical depression. This rough (relatively speaking) shape is then lapped with jeweler's rouge and a pitch lap (again all by hand) to a precise paraboloid surface. The degree of accuracy is described in terms of fractions of the wavelength of light (we are talking 0.000005" variation over, say, an 8" diameter mirror). This is an extreme example and there is no other practical use for such fine limits in normal model engineering.

Honing becomes desirable when rings and liners are suffering from scuffing. In such circumstances, the first priority is to find and eliminate the cause of the scuffing. After this is accomplished, the question is whether to recondition the liners economically.

For the initial honing diamond, stones are used to break up the hardened surface in the scuffed areas. For the main honing, very coarse hard stones are used to produce a very desirable rough surface all over the liner.

The advantage, especially for the 2-stroke engines, is the possibility of saving the liner after a seizure, scuffing or blow-by. Even eliminating the ovality of the liner is possible. Another advantage is that it is possible to machine a rough linerwall to obtain a well oiled surface.