Working with plastics

Plastic may not possess the character and beauty of wood, but you'll never find a species of wood that's 100% waterproof; splinter-resistant; stable; and, depending on the type of plastic, transparent. When you need some or all of those characteristics for a jig or project, let this material see you through.

Here we'll focus on three types of plastic—acrylic, polycarbonate, and phenolic laminate—and how to machine, bond, and finish them. Acrylic is crystal-clear and rigid; polycarbonate looks like acrylic, but offers much greater resistance to impact; and phenolic laminate is opaque, has more strength than the other two, and won't melt as you machine it. When you need plastic, choose the best type with the help of the chart below. Then adjust your cutting and shaping operations accordingly.

You can cut plastic with any of your power saws, or shape it with a router. Here are the keys to top results.

Tablesaw: An 80-tooth, triple-chip blade does an excellent job of cutting acrylic and polycarbonate on your tablesaw, as shown in the photo below. When you cut phenolic laminate, however, avoid fine particles by using a 40-tooth or coarser blade, wearing a dust mask, and providing good ventilation, because the dust irritates the lungs. The coarse blades also cut acrylic and polycarbonate but leave a rougher edge and can chip the surface. If you have to use a coarse blade on those plastics, cut the workpiece oversize and trim it on the jointer. Or, use a straight router bit and router table; set the fence for a ¹⁄₁₆ " cut.

Bandsaw: Use a skip-tooth blade, and match the coarseness to the thickness of the plastic. A blade with 10–14 teeth per inch (tpi) works great with ¹⁄₈ " stock, while ¹⁄₄ " material calls for only 6–8 tpi. If the plastic melts as you cut, you need a sharper blade or one with fewer teeth.

Scrollsaw: Melting can be a problem here, because you're always cutting with the same portion of the blade, and the friction heats it rapidly. Scrollsaw expert Rick Hutcheson recommends a speed of 1,000 strokes per minute or slower, a #5 double-skip-tooth blade, and two or three layers of masking tape or clear packing tape to absorb heat.

Router: You can shape plastic parts quickly with templates and a handheld router, as shown in the photo below, where we're using an old router-table insert plate to shape a new one. Use bushings and straight bits or bits equipped with pilot bearings. Carbide bits spinning at high speeds give the smoothest results.

Slightly modify any twist bit that you want to use with plastic by carefully reshaping the cutting edges at the tip of the bit. Work both sides of the bit equally on the side of a fine grinding wheel, as shown in the photo below, to form a vertical scraping surface, as shown in the inset photo below. Now the bit will bore without chipping the edges of the hole.

Lubricate the bit with a light oil, such as WD-40, and run your drill at a low speed. To drill a ¹⁄₄ " hole, for example, use a speed of about 1,800 rpm; for a ¹⁄₂ " hole, set the speed at 900 rpm. These steps prevent heat build-up that can melt the plastic.

Check at hardware stores, home centers, and specialized plastics outlets (look under "Plastics" in the Yellow Pages or Google "specialized plastics outlets") for methelyne-chloride solvent, labled as a cement for acrylics. It bonds acrylic to itself or polycarbonate to itself by dissolving a thin layer of plastic on the adjoining surfaces. The plastic flows together and hardens to make a joint.

When joining an edge to a face, make the edge smooth and straight. As shown in the opening photo, rest one piece on the other, with a backer board behind each, and clamp in place. Keep the vertical backer board separated from the joint line so it doesn't contact the solvent.

Use a solvent applicator, like the long-needled model shown, or a syringe to place a small amount of solvent all along the joint. It will flow between the pieces, dissolving plastic as it goes.

You can handle the assembly after it has hardened for several minutes, but the bond continues to get stronger for about a week. We left a small tongue protruding, as you can see in the photo, then shaved it off later with a bearing-piloted flush-trim bit in a router table.

If you're making a quick jig for the shop, the edges that you saw, joint, or rout should be satisfactory as they are. But if you want a more finished look for a display piece, you easily can smooth and even polish the edges.

Smooth to a matte finish with a hand scraper. To refine further, use a sanding block and fine wet/dry sandpaper, starting with 320 grit and moving up through finer grits until you're satisfied with the appearance. For total clarity, charge a buffing wheel with a tripoli buffing compound, and polish the edge as shown in the photo below.