2. Spindles for circular saws exceeding about one foot diameter. - Saws of this magnitude are seldom used on spindles mounted between pointed centers, as represented on page 754, but on those resembling the sections figs. 784 and 785. These spindles revolve in bearings or brasses b b, made in halves, and securely united to the stationary framework of the saw bench. The end-play, or end-long motion of the spindle, is usually prevented alone by the two collars or projections c c, which embrace the one bearing; sometimes, however, the one collar c, fig. 785, is screwed on the spindle to admit of adjustment, and has a side-screw to retain its position; or else the collar c, is in the solid, as usual, and a fixed screw s, exterior to the pulley, is made to bear on the end of the spindle.
Each spindle has a wooden or iron pulley of about one-third the diameter of the saw, for the driving strap, but in mills driven by power, a fast and a loose pulley of equal diameter are placed on each spindle, as in fig. 786, so that the spindle may be disconnected with the engine by throwing the strap on the loose, free, or live pulley.
Saws below about 20 inches diameter, are commonly held like those previously described, between the flat surfaces of the collar or projection c, that is forged in the solid with the spindle, and the surface of the loose collar or washer w, as in fig. 784; one steady pin then suffices, and which is fixed near the periphery of the flange. Large saws require flanges, say from 5 to 10 inches diameter, and which are then added to the spindle, as in fig. 785; the one is fixed by a feather or parallel key, and carries three steady pins; all the steady pins are represented black in the figures.
The loose flange is sometimes pressed up by only one screwed nut n, but it is preferable to have two, of different threads, that the second may prevent the first from being accidentally loosened; as the two then unwind at different rates, and check each other's motion. Either the one nut is right and the other left-handed, as in Collinge's patent axletrees, or else both nuts have right-handed threads, which differ in pitch as well as diameter.
3. Benches and platforms for large circular saws. - These are in general framed together very strongly in wood, in the ordinary manner of carpentry; they measure from about 4 to 12 feet long, 2 1/2 to 4 feet wide, and 2 1/2 to 3 feet high. The bearings for the saw are placed close beneath the platform, and at about the middle of its length; the central part of the bench is represented in plan in fig. 786.
To arrive at the saw spindle for the purpose of changing the saw, there is frequently inlaid in the platform a rectangular frame of cast iron with a rebate on the inner edge, fitted with a loose iron panel in two pieces to form the cleft for the saw. The panel is supposed to be removed to show the nuts and stops for the saw, and before the saw can be changed, it is also needful to lift out the wooden bar, which lies across the end of the spindle and against the saw; the bar is added for the purpose of carrying the stops s s, to be explained.
Sometimes the bench is nearly covered with plates of iron to lessen the friction of the timber upon it; and in benches for heavy work, the half of the platform in front of the saw is occasionally made as a slide, with a rack, pinion and winch handle, by which it is moved endlong. The work is in such cases placed against a ledge or cross piece on the slide, and is carried to the saw with great facility. A few saw benches, for some specific kinds of work, are constructed entirely in iron.
4. Stops to prevent the vibration of large saws. - These are in many cases inlaid in the wooden bed of the machine, beneath the iron plate by which access is obtained to the saw, as shown in fig. 786. The two grooves s s, nearest the periphery of the saw, are in some instances each entirely filled with a block of hard wood, kept in position by the top plate, and set forward from time to time by pieces of card or venter placed behind them, to compensate for the portion worn away by the saw. At other times, the grooves are fitted with blocks of wood or metal, which have mortises for fixing screws, as shown on a larger scale at s' s; these admit of adjustment and fixation. Screwed holes are also used, especially in the iron framings, cylindrical wooden plugs from 3/8 to 3/4 inch diameter are then screwed into the holes and set forward to meet the saw.
Large saw machines have sometimes wedge-form pockets beside the saw plate, which are filled with greasy hemp; the downward motion of the saw carries the hemp into the narrow part of the pocket, and pressing it against the saw, checks the vibration. This method, although it causes more friction, is nevertheless much approved of, as the elasticity of the packing enables the saw to be at all times closely gripped; which on account of its small irregularities, cannot be the case when rigid metallic or wood stops are used; but hemp is less suitable than wood for small saws. Frequently the stops are applied to both the front and back edges of large saws, as shown in the figure.
5. Parallel Guides for circular saws. - The parallel guide mostly added to large saw benches, closely resembles the ordinary parallel rule used for drawing, as will be seen on the inspection of fig. 786. The principle requires that the four centers of the parallel rule should constitute the four angles of a parallelogram, or that the four sides should be exactly two pairs, with which view the two radius bars are clamped together and drilled as a solid bar, and so likewise are the long bars. Unless the centers or pins fit accurately, it will be found that when the bars lie very obliquely, that the front bar or fence will have a rolling motion, as on a center, instead of being firm and parallel.