As stated in a previous chapter, practically any type of saw, hand or power, will cut this material. However, all woodworking saws, and some metal-working blades, have a decided rake, i.e., the front angle of the tooth approaching the work is quite acute, whereas plastics require a negative rake for cleanest results, and that is the reason for the recommendation previously given to use medium - to fine-toothed blades, where the effect of this sharp angle is not so great, and where the angle, in fact, is usually less.

A rip-saw or a combination rip and cross-cut woodworking blade for instance, with the deep gullets, is out of the question, but the finer-toothed back-saws, hack-saws, circular cross-cut blades, and metal-cutting blades in general are all quite satisfactory provided they have a little "set".

No cautions or suggestions are necessary regarding the use of any type of hand-saw.

Circular Saw The only requirements are a firm table and a proper blade in proper condition, using plenty of power. For ordinary purposes, a seven - or eight-inch blade, driven by a quarter or third-horsepower motor at 2500 R.P.M. provides a satisfactory set-up. Blades dull quickly, if a considerable quantity of work is being done, and they also lose their "set", and it is worse than useless to attempt to continue with a dull blade. Excellent results can be obtained with woodworking blades with a few passes of the file across each tooth, changing the angle as indicated in the sketch, Figure 4. Binding must be avoided at all costs, and this can be done by making sure of a true set-up on the saw table, and an adequate "set" on the blade. Otherwise, the blade will heat badly and bind, as the material expands when hot. Cleanest work is done when the blade is set so it just protrudes thru the material.

Proper Angle For Sharpening

Fig. 4. Proper angle for sharpening: circular-saw blades to be used on plastics. Note that the face of the tooth approaching the work is on a direct line with the diameter.

When hot, as a result of dull tools or hard pushing, the material gives off the peculiar acrid odor of formaldehyde, which is one of its chemical constitutents. As far as can be determined, this odor is not harmful in any way, although it is irritating to the eyes and nose, and can be avoided by working slower, with sharper tools, or by installing ventilating hood and exhaust system around the saws, sanding machines and grinding-wheels, or by a water-stream. For occasional homecraft or school work or anything but quantity-production work however, this is not at all necessary. The dust given off when sanding or grinding can also be taken care of by an exhaust system if a great deal of work is being done, but likewise is not known to be harmful in any way, and no more objectionable than the dust given off in woodworking operations.

Dado Heads, Etc. Other types of cutters commonly used on woodworking circular saws can also be used on plastics, bearing in mind the aforementioned requirements as to shape of tooth, and also bearing in mind that for all practical purposes, an increase in speed accomplishes the same result as far as the work is concerned as increasing the number of teeth in a blade. In other words, double the speed and you get the same effect as if you doubled the number of teeth per inch. Smaller diameters of course reduce the effective cutting-speed and larger diameters increase it. This means that small-diameter coarse-toothed dado heads and such equipment commonly used on circular saws may be used successfully on plastics if their speed is high enough to prevent "digging in", or if their points are altered. Sanding-discs, grinding wheels, etc. can also be used in circular-saw frames in the conventional manner at the usual woodworking speeds.