Levers having three bosses each can be easily shaped on shaping-machines if small, and on slotting-machines if large; the several operations for lining, adjusting on the tables or on arbor-chucks, and removal of the metal, being similar to those detailed for shaping levers having only two bosses each.

Every three-boss lever which is to be reduced along the entire lengths of the arms' broad sides, requires the junctions at the boss-ends to be reduced to the finished thickness by the turning or boring, which reduction is similar to that for a two-boss lever, with the necessity of exercising more care to properly reduce the metal from around all the six boss-ends. These junctions are usually shaped with the same machine that finished the holes. When thus carefully treated it is ready for a shaper, or for a slotter, according to the size of the article. The order in which the several surfaces are produced is the same as that observed for other levers if they are slotted; the bosses and junctions being first shaped, and next the narrow sides of the arms; after which the producing of the broad sides is effected. This comparative easy operation of planing with a slotter, as it may be termed, is the last performed, and of course requires extra care during adjustment, because four broad sides are to be produced on each lever instead of only two.

Because the junctions of the boss-ends with the arm's broad side have been thinned on the boring-machine until they are smoothly finished to the specified dimensions, and because eacn pair of junctions are made equidistant from the primary centre line or periphery around the arm's narrow side (see pages 220 and 221), it is needful to fix the lever on the slotting-table with regard to these finished parts at the time the broad sides are to be produced, in order that the places where the junctions terminate and the straight sides begin may not be ridgy after shaping, and require considerable filing. Instead of entirely depending on the accurate shapes of the bosses for placing the broad sides into the exact right-angular position with the table, it is sometimes preferable to place the lever in front of an el-chuck and put parallel packing-pieces between the chuck's face and each finished junction. This arrangement will cause the lever to be correctly placed, if the bottom edge of the el-chuck is put exactly to one of the straight gauge-lines on the table. When it happens that the surfaces of the junctions are too small for being thus packed against the chuck, the faces of the bosses should be put to the chuck, packing-pieces of proper thickness being put to whichever boss may need them, in order that the requisite parallelism of the arm with the traverse may be attained.

During the adjustment of a lever or bar on a slotting-table for producing its broad side, or during the adjustment of the article for shaping its narrow side, a tool-scriber may be used. Such a tool consists of a stock similar to that of an ordinary cutting tool for the machine, in the lower end of which is a small hole of sufficient depth to hold a scriber; the scriber, or pointer, as it may be termed, being a piece of pointed steel wire like a scriber for hand use, but shorter. This pointer is held tight in the stock with a wedge or with a little screw, and when fixed it constitutes an index to indicate the relative situation of an object on the slotting-table beneath.

The pointer may be used for adjusting either connecting-bars, two-boss levers, or those having three bosses. As soon as the article is put somewhat near its intended place on the table and some holding plates attached, also some poppets placed, the tool-scriber is to be fixed in the tool-clamps, and the lever or bar is gradually shifted until one end of the gauge-line by which the lever is to be adjusted is put directly under the scriber-point, and very near to it. The lever is now advanced along in the direction of its length by rotating the traverse screw, and when the lever is moved along under the point about as far as the length of the gauge-line, the observer looks to see whether the point is now exactly over the line as it was while at the other end. If not, the amount of shifting necessary to make the gauge-line parallel with the traverse of the slide is plainly shown, and therefore effected.

Adjustment by means of a tool-scriber has the same effect of placing the gauge-line into the desired position, as if it were adjusted while the article is against an el-chuck whose bottom-edge is on a gauge-line on the table, or as if it were adjusted to a scriber-block point whose bottom edge is on a similar gauge-line; because the path in which the lever is now made to move during adjustment is parallel to the path in which it will move during the cutting off of the metal.