Machinery's Handbook, 31st Edition
1260 FORMICA MACHINING must be rapid and caution used to prevent the drill from lagging in its work, and the speed must be 1200 rpm. For all machining operations on “Formica” gear material, provision must be made in grinding for the tools to clear themselves. For reaming, the entry of the reamer and the reaming process must be rapid. There must not be a lag between the end of the reaming operation and the withdrawal of the reamer. In turning the outside diameter and the sides of blanks, the tools must be sharp and have 3 to 5 degrees more rake than is common practice for metal. A cutting speed of 750 fpm (228.6 m/min), equal to 720 rpm on a 4-inch diameter blank, is recommended. The depth of the cut can be 1 ∕ 16 to 1 ∕ 8 inch (1.6–3.2 mm), but the feed should be 0.010 inch (0.25 mm), regardless of the depth of the cut. Teeth may be cut on a hobbing machine, shaper, or milling machine. The speed of the cutter should be 150 fpm (46 m/min) and the feed from 0.023 to 0.040 inch per revolution (0.59–1.02 mm/rev). It is advisable to back up the blank to prevent fraying or breaking out of the material as the cutter comes through. The backing plates can be economically made from hard wood. Micarta Machining.— In cutting blanks from sheets of “micarta” a band saw running at a speed of 350 revolutions per minute has been found suitable. The saw should be of the bevel-tooth type, seven teeth to the inch. For large quantities a trepanning tool should be used. In trepanning blanks, the tool should be fed so as to cut part way through all of the “layouts”; then the micarta plate should be turned over, and the cutting completed from the reverse side. Turning tools should be high-speed steel cutting at speeds similar to those for bronze or cast iron. If two cuts are taken, about 0.010 inch (0.254 mm) of stock should be left for the finishing cut. Drilling at right angles to the layers is done with a standard drill, which should be backed off sufficiently to provide plenty of clearance. When drilling parallel to layers, a “flat” or “bottom” drill should be used. In rough-drilling, the hole should preferably be drilled partly through the material from each side to prevent possible splitting as the tool pro trudes. If this is impracticable, the hole can be drilled all the way through the material, provided the material is “backed up” with wood, stiff cardboard, or any other material that is sufficiently rigid to support the under surface at the point where the drill comes through. The methods described for drilling apply as well to tapping, except that when the tapping is done parallel to the layers, it is advisable to clamp the material to equalize the stress on the layers and prevent possible splitting. In milling, a standard tool may be used at a speed and feed corresponding to that used in working bronze or soft steel. The cutting angle of the cutter will give better results if ground with a slight rake. While there is a wide range of practice as to speeds and feeds in cutting gears on hobbing machines, a hob speed of not less than 140 revolutions per minute has given satisfaction. In machining gear teeth on a gear shaper, a speed of about 100 to 130 strokes per minute with a fairly fine feed has given good results. Backing-up plates should be used in machin ing micarta gears. Ultrasonic Machining.— This method of cutting and engraving hard materials such as glass, precious stones, and carbides uses a transducer (vibratory unit) to obtain the neces sary mechanical vibrations needed. The transducer converts the input energy, in this case electrical, into another form of energy, in this case mechanical. A tool of the required size and shape is made of brass or other soft material and is attached to the transducer. The tool is lowered until it just barely touches the work, and current is applied. At the same time, a slurry of water and fine abrasive, usually boron carbide, is pumped over the work. The tool does not actually touch the work, but the vibrations liter ally hammer the particles of abrasive into the surface and chip off tiny fragments. Some wear does take place in the tool, but it is very slight and, as it is equally distributed, it does not change the shape. The method is commonly applied to cutting designs in stones of sig net rings, and also applied to cutting intricately shaped holes in carbide or hardened steel.
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