The lead or the holding of the saw to its work may be adjusted by its position to the track as already described, or by beveling on the back of the teeth. Assuming that the teeth are properly filed perfectly square in front, then bevel filing on the back of the teeth on the board side will lead the saw into the log, and bevel filing on the lug side will lead it out of the log. A 'snaky' or in-and•out lead indi cates that the rim of the saw is too large for the centre and, therefore, requires to be opened out at the centre. This may be effected by running the saw warm at the centre by creating friction, by reducing the set or spread of the or by hammering between the centre and the rim.
Motion.—The motion of circular saws is one the conditions which requires the most care ful attention. The saw should always be run nearly as practaca a periphery 9010 feet per minute (itt soft woods). and it should be run at uni speed hoth in and out of the cut 1..xcess high speeds generate heat in saw and it so touchy and limber that it will do work only on light teed and while the are usually run at a speed of 450 revolutions per minute, and those for steam feed mills from 600 to 900 revolutions per minute.
The three following rules may be employed for calculating the speed of saw pulleys or drums.
1. To find the number of revolutions of the driven when its diameter is known.
Multiply the diameter of the driver by its number of revolutions and divide the product the diameter of the driven; the quotient be the number of revolutions of the arisen_ 2. The diameter and revolution of the driver being given, to find the diameter of the drisra that shall make any given number of revolutteess in the same time.
Multiply the diameter of the driver by its number of revolutions and divide the product b. the number of revolutions of the drisen. the quotient will be the diameter of the drisen 3. To find the size of the driver.
Multiply the diameter of the driven by the number of revolutions it is required to make, and divide the product by the number of revolu tions of the driver; the quotient will be the size of the driver.
In the case of band saws the most important point requiring careful attention is the tension or the opening up or expanding of the centre of the blade so as to place the strain on the edges of the band. The tensioning of saws may be accomplished by the use of rolls, or by hammering, the last-named method being pref erable as the saws are liable to hold their shape much longer. The accompanying table shows the circles of tension for hand saws of different widths and gauges. These figures give the saw as much tension as it will take and lie flat on the running-hoard, a condition which accords with the best practice.
It is often desirable that the toothed edge should be strained a little tighter than any other portion of the saw. This may be accomplished, and art uniform tension preserved by rolling or hammering the back of the saw to a curve show ing about one-sixty-fourth of an inch rounding in five feet, and then tilting the upper wheel so that the saw has an uniform pressure across the blade. The strains usually put upon saws range from 5,000 to 12,000 pounds, and should be only sufficient to prevent slipping on the wheels. A rubber.tire three-fourths of an inch in thickness on the tipper pulley adds much to the endurance of the blade. This upper pulley should also be provided with a tension link. The following table gives the strains for band saws of differ ent sizes and gauges.
Gauges.— In selecting a saw, the nature of the work for which it is required, and the power available to drive it, should always be taken into account. In this connection the first important point is the proper gauge. Generally speaking, a thin-gauze saw blade lasts much longer than a thick one. For mills of ordinary capacity and operating on a general line of work the most suitable saws are those measur ing seven gauge at the centre and eight on the rim. If the timber is valuable, and the sawyer skillful, saws as light as eight or 10 gauge may be used, but saws lighter than that are im practicable for use in ordinary mills: In large mills where the saving of time is more desirable than the saving of lumber, and consequently greater speed and feed are used, the most suit able saws are those of six and seven gauge, which are sufficiently heavy to stand up to the more severe service. In the case of these heavier saws it is imperative that the pulleys be of large diameter.
The second important point in the selection of a saw is the number of teeth. With high speed and the accompanying high feed, more work is required of a saw in a given time, and, therefore, it should have more teeth with which to do that work and to distribute the strain evenly. The speed, however, should not exceed 3,500 feet per minute with 36-inch pulleys. The number of teeth should depend not only on the thickness of the saw, but also on the character of the timber to be sawed and the speed and feed of the mill. Gullet teeth are preferable for all except very hard woods, and they should not be too close together. Consult Bale, M. P., 'Woodworking Machinery' (London 1914) ; Hasluck, P. N., 'The Wood-Turner's Handy Book' (London 1910) ; U. S. Vocational Edu cation Board, 'Factory Woodworking Trades' (Washington 1919).