If the bosses of wheels are to have key-ways formed with a slotting-machine, they are treated in a manner similar to that for levers, being first lined to show the exact shape and place of the intended grooves, and then fixed on the table with the faces parallel to it. But there is a difference in the packing-up of some wheels by reason of the faces of their bosses not being flat, and also because some wheels require to have key-ways without their bosses having been turned. If a wheel, pulley, drum, or such article is entirely lathe-turned, so that the faces of the rim are made parallel to the faces of the boss, the object can be quickly put into a proper position by placing it upon a parallel ring, or upon a few parallel blocks, similar to the mode of placing a lever. If the boss of the wheel is large enough in diameter, and is flat, the packing can be put into immediate contact with the boss-face ; but for a boss having curved faces the packing is put beneath the rim. This arrangement has the same effect of placing the length of the hole at right-angles to the table as if the packing were in contact with the boss, because the edges of the rim were turned in the lathe and made right-angular with the length of the hole.

Placing a wheel with the parallel packing under the rim, instead of under the boss, leaves the boss without anything between it and the table. In this condition the action of the slotting tool would cause a shaking of the boss and arms of the wheel, which would prevent the tool cutting properly, and require very small portions of metal to be removed each time. If the wheel were of cast iron it would also break, unless extra strong, and would separate in some part of the arms. It is therefore necessary to support the boss, which is done after the wheel is finally adjusted and fixed with the plates and bolts; when the boss is packed up by entering a few short hard wood blocks between the arms, and pushing them under the boss to a short distance from the edge. These blocks are rather taper, in order that they may be gradually wedged between the table and the boss, to ensure a proper bearing; and packing-pieces of various thicknesses may be put to raise the wedges to any exact height when required. When the object is thus secured it is ready for grooving with tools of suitable width, and an easy and steady cutting is the result, through the tool being made to cut where there is great resistance by reason of the packing beneath.