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AC 43.13-1B 9/8/98 should be hollow-ground or have some set to prevent binding. After the teeth are set, they should be side-dressed to produce a smooth edge on the cut. Band saws are recommended for cutting flat acrylic sheets when the cuts must be curved or where the sheet is cut to a rough dimension to be trimmed later. Close control of size and shape may be obtained by band sawing a piece to within 1/16 inch of the desired size, as marked by a scribed line on the plastic, and then sanding it to the correct size with a drum or belt sander. (2) Unlike soft metal, acrylic plastic is a very poor conductor of heat. Make provisions for removing the heat when drilling. Deep holes need cooling, and a water-soluble cutting oil is a satisfactory coolant since it has no ten dency to attack the plastic. (a) The drill used on acrylics must be carefully ground and free from nicks and burrs that would affect the surface finish. Grind the drill with a greater included angle than would be used for soft metal. The rake angle should be zero in order to scrape, not cut. (See fig ure 3-18.) FIGURE 3-18. Drill having an included angle of approximately 150°, used to drill acrylic plastics. (b) The patented Unibit (see fig ure 3-19) is good for drilling small holes in aircraft windshields and windows. It can cut holes from 1/8-to 1/2−inch in 1/32−inch in crements and produces good smooth holes with no stress cracks around their edges. FIGURE 3-19. Unibit drill for drilling acrylic plastics. h. Polymerizable Cements. Polymeri zable cements are those in which a catalyst is added to an already thick monomerpolymer syrup to promote rapid hardening. Ce ment PS-30 and Weld-On 40 are polymeri zable cements of this type. They are suitable for cementing all types of PLEXIGLAS acrylic cast sheet and parts molded from PLEXIGLAS molding pellets. At room temperature, the cements harden (polymerize) in the container in about 45 minutes after mixing the compo nents. They will harden more rapidly at higher temperatures. The cement joints are usually hard enough for handling within 4 hours after assembly. The joints may be machined within 4 hours after assembly, but it is better to wait 24 hours. (1) PS-30 and Weld-On 40 joints retain excellent appearance and color stability after outdoor exposure. These cements produce clear, transparent joints and should be used when the color and appearance of the joints are important. (2) PS-30 and Weld-On 40 should be used at temperatures no lower than 65 °F. If cementing is done in a room cooler than 65 °F, it will require a longer time to harden and the joint strength will be reduced. (a) The cement should be prepared with the correct proportions of components as given in the manufacturer’s instructions and thoroughly mixed, making sure neither the mixing container nor mixing paddle adds color or affects the hardening of the cement. Page 3-18 Par 3-23PDF Image | AFS-640
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