CIRCULAR SAws.—In taking up the subject of circular saws, we may first consider the log mill, board-mill, and resawing machines, these being the first in order of action upon the wood, in its conversion from the log to the finished prodnet, no matter what it is. As circular saw mills have been treated at considerable length in the preceding volumes, it may be desirable in this place only to note special forms of this wonderful factor in wood conversion, and to mention some of the appliances and attachments which give it greater range of dimen sions and character of output, and better quality of work, coupled with great increase in the amount of material that can be handled in a given time.
very great advance in the circular saw-mill is making it double— that is, with two saw arbors, one above the other in the same vertical plane, the upper one bear ing a smaller saw than the other, both saws cutting in the seine vertical plane. The upper arbor is given vertical adjustment on the housing, to enable it to be raised and lowered to suit variations in the diameters of the saws. The upper saw is driven from the arbor of the lower one, usually by an open belt, so that both saws, as regards the spectator, rotate in the same direction ; but as regards the lumber, the teeth of the upper one enter it in the direc tion opposite to those of the lower one, the teeth passing each other in opposite direction. The saws are set so that the periphery of each one intrudes a trifle upon the kerf or channel made by the other, one of them being a little in advance of the other to enable this to pre vent the teeth of one saw interfering with those of the other. The upper arbor is for saws having the sante holes as the lower ones, so when the lower one is worn too small for effective service it may be used as an upper one, and the upper one moved to a smaller mill. As smaller and thinner saws are used than on single saw-mills, they can have, and really require, faster feed ; they cut a thinner ke•f, are more readily kept in order, are less liable to accident, and cost less to replace when broken. As the speed of the smaller saws is higher than that of one large saw, the feed and gig motion of the double mill are higher than those of the single.
As some sawyers desire that the upper saw in a doable circular mill shall run reversed, and as a quarter-twist belt would be impracticable, by reason of the short distance between arbor centers, such a direction of motion is got by having the belt run from a pulley on the lower ar bor, over an idler pulley above the upper mandrel, down under the pulley on the upper arbor, up over another idler, and down under the pulley on the lower mandrel. This produces the effect of a quarter-twist belt, with full facility for varying the tension, and gives better contact upon the pulleys, as the idlers are quite close together, so that the belt gives more than 180° wrap on the upper saw pulley. It is desirable to have a device for guiding the rim of the saw near the cut, to prevent it from straying out of the true plane ; and this guide must be adjustable toward or from the saw arbor to suit various diameters of saws, and also must have adjustment to snit the varying gauges of different saws, and also the varying thickness of the same saw, as it is worn down in diameter. In addition to this, there must be a certain amount of adjustability to and from the line of the carriage, to accommodate different thicknesses of collars, etc., as well as different conditions of saw tension. All these adjustments should preferably be made without the use of a special wrench, and should be of such a character that they may be done quickly.
One of the best of these consists in effect of two horizontal and parallel hollow cylinders, in each of which turns a wrought-iron pin, adjustable lengthwise of the bore containing it, by a screw and milled nut. One of these, when nearest the saw edge, is terminated by a short arin bearing art anti-friction piece, which guides the inner edge of the saw disk. The outer bears a longer arm, having a smaller anti-friction piece, which may be brought into contact with the first, or withdrawn by means of the saw or milled-nut arrangement, This second bar and arm engage and' guide the outer face of the saw disk. The entire device is fastened by screws and milled nuts to a slotted piece borne on the saw frame, thus permitting length wise adjustment to suit saws of varying diameters.