In this operation it is necessary to work with three at the same time. For the sake of making the explanation clear, they will be called A, B, and C. After the plates have been planed, a straightedge should be laid on each. A straightedge is merely a piece of flat steel having one or more edges true and straight. Set the straightedge on the plates in all directions. If it touches over its whole length in all positions, then the plates are ready for scraping. If it touches at the edges of the plate and is clear in the center, the former are high and should be filed down. If it touches in the center and rocks to and fro, the plate is convex and the center must be filed down. After the plates have been filed to trueness as far as trueness can be indicated by the straightedge, they are ready for scraping.

Now take plates A and B and place them face to face. Strike a blow on the upp r one, and it will cause a jarring sound to be heard. This shows that the two are not in perfect contact. Smear the surface of plate A with a thin mixture of red lead and oil. Cover the surface evenly and thinly. Then rub the two plates together, and where the red lead comes off onto the surface of plate B, the two come in contact. Take the scraper and scrape off a little of the metal from each of the plates where they have been in contact. Wipe off plate B; and again smearing plate A, proceed as before. Continue this process until the two surfaces are in contact over their whole areas. This does not prove, however, that they are flat. They may be in contact, as required, if A is convex and B is concave. To test this, the third plate is necessary. Smear plate B with red lead, and scrape C to fit it. Do not touch A, It is evident that A and C will then be alike. Bring them together. If they are both convex they will roll over each other. If they are concave they will bear at their edges, and not touch in the center. They will appear to be out of true by twice the actual amount. Scrape off the contact points of A and C. Remove as nearly as possible the same amount of metal from each. When these two plates have been brought so as to be in contact over their whole areas, lay plate aside, and scrape B until it fits C, but do not touch A. Try A and B together. If they do not touch over their whole areas, treat them as before described for A and C. Then introduce C again. Continue this alternating process until each of the three plates forms a bearing over the whole of the surface of each of the other two.

During the latter part of the process, use alcohol instead of red lead. This will leave clean, bright spots at the points of contact.

Fitting Brasses

This is a piece of work now usually done on a machine, but sometimes done by hand. Brasses which are to be used for connecting rods, and which are made in two pieces, as shown in Fig. 255, have a tendency to warp after the machine work has been done on them. The difficulty arises from their closing along the diameter A. Thus, if the brass is finished, and the hole bored out to the proper diameter, and is then cut apart on the line CD, it will be found, shortly afterward, that the diameter A is less than the diameter B. It may therefore be necessary to bore the hole somewhat larger than the working diameter. The kerf made by the saw will usually allow the parts to be drawn together along the diameter B, so that it will more than make up the shrinkage at A. The hole can then be scraped to fit the pin. The brasses should always be keyed solidly, metal to metal. This avoids a wear of the sides and edges of the metal, due to the thrust of the rod.