These wheels are composed of an abrasive grit, bound with various types of synthetic resins. They are from 1/32" to 1/8" thick, the latter being considered a very thick wheel. They are so thin that they are quite flexible in the hands, and it is difficult to understand how they can cut hard metals at such high speeds, but the stiffness when cutting is provided entirely by the centrifugal force created by revolving the wheel at high speed. This force is terriffic, and its proper application provides a powerful tool, but on the other hand its improper application creates extremely dangerous hazards. For this reason, the cautions outlined below MUST be followed carefully, as otherwise the machine is not only useless, but very dangerous. The machine must be so constructed and handled that there is absolutely no side-play in the mandrel or in the swinging arm, and the work must be so held that there is absolutely no movement of the material in any direction while the saw is in operation. The following design has provisions providing for these factors— any alteration in the design MUST take these requirements into consideration. The blade must not be allowed to "whip" or "chatter" or "whine" like the average amateur's buzz-saw does —a few seconds of this, or a slight twist of the material in the midst of a cut, will find the saw in a thousand pieces, and if by any chance this happens while the blade is without a guard, some of these pieces will be found IN—not on—the far wall. Many of us, not excluding the author, are prone to leave the guard off a circular saw once in a while, but that once might possibly be the last time you would leave it off a high-speed abrasive cut-off saw. The explosion of a wheel UNDER the guard of course means nothing but the expense of a new one and the trouble of changing.
The provisions insuring a rigid and non-vibrating mounting shown in the illustration, Figure No. 8 include: The wide bearings for the swinging arm and the collars for taking up play, the rigidity of the arm itself to prevent twist, and the take-up provisions on the wheel-arbor. Rigidity of the work is provided by the work-table and backrest. These are normally made of bronze, machined and quite heavy, and with provisions for setting in various positions and for attaching various types of jigs, but the fixed table shown in the sketch will serve most amateur purposes satisfactorily, and the purposes of an adjustable "stop" for automatic measuring of cut-off pieces are served by a block of wood clamped to the left table with a C-clamp. It will be noted that the right table on which the work is held firmly by hand, comes up as close to the wheel as possible, while there is a gap of 2 1/2" between the blade and the left work-table, leaving room for cut-off pieces to drop down into the drain-well. The range of this saw, with a 12" abrasive wheel, is sufficient to cut the larger cylinders (3 1/2" diameter) or larger, and it is rigid enough to be safe. It may look like a "wood-pile" in the sketches, but this weight and strength is necessary to take the place of the heavy castings used in the commercial models.
Fig. 8. Construction—Diagram for Hi-Speed Abrasive Cut-off Machine.
1 1/2" well-seasoned oak or hardwood is used in the construction, and since most of the lower portion will be exposed to water-spray, it will be necessary to waterproof it to prevent swelling and warping. This cannot be done perfectly, but the nearest approach to it is to heat the wood and then paint it with melted paraffin, heat it again so that this paraffin will be sucked into the pores, and then paint it again while hot. All iron parts, bolts and screws will likewise require treatment, and the treatment of the wood parts should be done after all saw-cuts and bolt-holes are made. Construction and assembly, except for the following comments, is clear from the drawings.
The wheel-bearings should be as good as you can afford, ball - or rollerbearing if possible, due to the high speed and good fit required. The back flange must be this large diameter, a force-fit on the spindle, fastened with a taper pin, and faced as well as turned all over on the lathe. This is absolutely necessary to avoid vibration and consequent blade-breakage. The outer flange likewise should be machined and a good fit, and the threads for the hex-nut should stop about 1/8 inch before they reach the back flange, so that the blade will rest on the flat spindle-surface, and not on the threads. The spindle is 3/4" diameter, which is a common blade hole-size. The collar and fibre washers provide for taking up end-play.
The hinge-spindle can be lighter, 1/2" in this case, and ordinary babbit pillow-blocks used for bearings, provided they are rigidly bolted down. Collars and washers here also permit a tight fit and no play. The motor acts as a counterweight to hold the saw off the work until it is pulled down by the handle carved in the front end of the arm. The guard is soldered up from galvanized iron, and is screwed to the arm by two angle-pieces on top and bottom. This also must be rigid. It will be noted that the dimensions and shape of the guard and the length of the spindle are such that blades can be changed without necessity for removing the guard. The back splash-guard gets 95% of the water, the speed of the wheel being so high (about 3000 R.P.M.) that very little water is picked up by it, and the drain-holes are directly under and in front of this guard. The entire assembly is arranged so that it can be set on a bench, in a galvanized iron pan about 1" deep and a foot larger than the machine all around. Practically no water is thrown in front or at the sides.
Tap-water at low pressure is led as close as possible to the point of contact between the blade and the work thru lead or copper tubing which can be bent to accomplish this purpose. The saw-dust is so fine and the amount so small that there is no danger of clogging the drains.
A quarter-horsepower motor, 1750 RPM is satisfactory, with a 6" motor-pulley and a 3" arbor-pulley, driven by a 1/2" V-belt, which passes thru a slot cut in the arm as indicated. The position of the motor can be shifted slightly so as to provide proper belt-tension, bearing in mind the motor's secondary function as counterweight. Depth of cut is regulated by the stop-link shown, which consists merely of a 1/4" carriage-bolt passing thru the rear of the arm behind the motor, and the stretcher below it, having a pair of nuts which can be adjusted and locked for various depths.