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    How to Shape Metal with Power Grinders

    When you want to shape metal edges and smooth welded joints, having a power grinder — whether a bench grinder or a portable, hand-held sander-grinder is indispensable. The bench grinder is ideal for rounding off corners of hard metals and removing burrs from cut edges; it is your best choice for shaping small iron or steel objects. Choose the portable power grinder when you need to smooth wide areas of flat metal, especially around welds. For grinding nonferrous metals safely, fit the portable grinder with an emery disk or a special ferrous disk because a bench grinder's stone wheel collects metals fragments and hurls them back at you.

    Power grinders are dangerous machines. You must wear safety goggles and heavy leather gloves when you shape metal with either type of power grinder. The grinder may throw hot sparks and metal fragments at you, and it might catch the workpiece and yank it forcefully out of your hands. Whenever possible clamp the workpiece to something heavy and solid. At the bench grinder, use locking pliers to hold small workpieces. With portable grinders, clamp the workpiece to a bench and keep both hands on the machine. Never hold the workpiece in one hand while driving the portable grinder with your other hand.

    Stop if the metal edge you are grinding begins to turn blue — this means the metal is dangerously overheated. Quench the workpiece frequently in water to keep it cool. When choosing a portable grinder, insist on two important safety features: an on-off trigger that turns the machine off when you release it, and either a double-insulated or a three-pronged grounded electric plug.

    Transforming a Square Corner into a Curve

    metal curve

    Marking the Curve

     

    Scribe the desired arc on the metal to be ground, then punch holes along it to make the arc more visible. Use a center punch and a ball-peen hammer to make punch marks about 1/8 inch apart.

     

    Grinding the Arc

     

    Set the bench grinder's tool rest at a right angle to the center of the wheel and 1/8 inch from it. With the grinder on, place the metal on the tool rest and bring the corner into light contact with the face of the wheel. Rotate the corner slowly in a wide, semicircular motion and move the metal continuously back and forth over the wheel. To obtain the correct radius, do not grind beyond the line of punched holes. Quench frequently in water to prevent overheating.

    Beveled Edges Shaped on a Bench Grinder

    Beveling a Pipe

    With the bench grinder turned on, hold the section of pipe at an acute angle to the wheel. Rest the bottom of the pipe end to be beveled on the tool rest and bring one edge into contact with the wheel as it spins. Rotate the pipe slowly clockwise across the surface of the wheel. Quench the pipe frequently in water while you work.

    Beveling a Flat Edge

    Set the bench grinder's tool rest at the desired angle. With the grinder on, place the metal on the tool rest and bring its edge lightly against the wheel. Slide the edge from side to side on the tool rest as you work. Check the angle with a protractor, and quench the edge in water often.

    Using a Portable Grinder to Dress a Weld

    Using a Sander-Grinder

    Secure the welded sheet to the bench with clamps. Turn on the portable grinder, and bring the leading edge of the grinding wheel into contact with the weld. Tilt the grinder so that only the front edge of the wheel is touching the weld. Run the wheel lightly over the joint to remove any irregularities and smooth the surface of the metal.

    How To Sharpen A Cold Chisel with a Bench Grinder

    sharpen chisel

    Setting the Tool Rest

    When the cutting edge of a cold chisel has become dull or nicked, you can resharpen it on the grinding wheel. Take this opportunity to clean up the striking head too. Set up the grinder by loosening the wing nut that holds the tool rest, and place the chisel shaft flat against the rest. Adjust the angle of the rest until one bevel of the chisel's cutting edge is flush with the face of the grinding wheel at a point on the upper half of the wheel. Check to make sure that the front edge of the tool rest is no more than 1/8 inch from the wheel face. Tighten the wing nut and remove the chisel.

    Grinding the Cutting Surfaces

    Turn on the grinder, and when the wheel has reached full speed, place the cold chisel on the tool rest. Bring one of the beveled surfaces into light contact with the wheel face and move the chisel slowly left and right. Grind one beveled surface, then the other. Dip the cutting edge frequently into the grinder's waterpot to cool it and keep it cool. Examine the beveled surfaces for a smooth, uniform finish. Then check the angle of the cutting edge with a protractor. The two beveled faces together should form an angle of 60° (inset).

    Restoring the Shape of the Striking Head

    With the grinder running, hold the cold chisel at a 45° angle to the wheel face and bring the splayed end of the striking head in contact with the wheel. Slowly rotate the tool clockwise to remove the deformed metal, tapering the head inward about 15° (inset). Cool frequently in water to keep the metal from becoming hot. If you were to quench the head when it is too you would re-temper the steel, making it brittle and likely to chip when a hammer strikes it.

    Restoring a Punch Point with a Bench Grinder

    With the grinder on, bring the cone-shaped point in contact with the wheel. Slowly rotate the punch clockwise a full 360°. Then use a protractor to check the angle of the cone. For a center punch it should be 90°, for a prick punch 30°. Continue grinding until the angle is correct. Quench the tool frequently so that it will not overheat.

    Polishing, Buffing, and Texturing Metal Surfaces

    polishing

    As a final step in its fabrication, you need to polish metal to remove deposits, gouges, and imperfections left by welding, soldering, and cutting. By the gradual progression from coarse abrasives to finer ones, you attain a smooth surface that you can then finish in a variety of ways. If you're going to paint the metal, you will generally rough-finish it, providing a grainy surface to which paint adheres. If you plan to leave the metal exposed, you can polish it to a high luster and buff it to a final sheen. Or you can texture it with specialized finishing techniques — brushing, graining, peening, and spot polishing. For other types of specific metals, brushing and graining produce a finely lined satin finish. You'll normally brush aluminum and brass, and you'll find graining a useful technique for restoring the finish on worn stainless steel. Peening, which you do with a hammer, pocks the surface of the metal with light-reflecting dimples. For spot polishing, you'll use a hand-held drill or a drill press fitted with a wooden dowel to cover the metal surface with light-reflecting circles.

    You can usually reduce a metal surface from rough to smooth by hand with sandpaper, emery cloth, abrasive powders, or steel wool. But power tools speed the work. The fastest polishing tool is probably the sandblaster. It is especially useful for rough-finishing large areas that are going to be painted later. Used with heavy sand grit, the sandblaster makes quick work of removing rust, aluminum oxides, and old paint, for example. But when its canister is filled instead with light sand grit, it produces a very fine polished finish.

    The standard hand-held electric drill and the bench grinder also do an efficient job of polishing. You can fit the drill with such metal-polishing attachments as sanding disks, wire brushes, and buffing wheels, and you can replace the hard abrasive wheels of the grinder with wheels of cloth, leather, or felt. You can also use special power buffers, both hand-held and stationary.

    Before you use a soft buffing wheel, coat it with a polishing compound. The four most common are natural materials — pumice, tripoli, rouge, and whiting. Use pumice, made from powdered lava, and tripoli, from decomposed limestone, for preliminary polishing. Use rouge, made from red iron oxide, and whiting, from pulverized chalk, for buffing to a high sheen.

    You can buy these compounds and many commercial variations in block or bar form. Apply the compound by holding it against the spinning wheel. Apply them sparingly, however, to avoid a heavy build-up on the wheel and ultimately on the metal. You can remove excess compound by cleaning the metal with alcohol or with a solution of hot water and washing soda.

    Use a separate wheel for each polishing compound. If you put different compounds on the same wheel, you cancel out their special abrasive effects. For the same reason, you should clean the metal, as described above, whenever you switch from one polishing compound to another.

    Prior to polishing, strip metal surfaces of protective the lacquer coatings that manufacturers put on to prevent metals from oxidizing when exposed to air. Remove these coatings with lacquer thinner rubbed on with a soft clean cloth.

    How to Prepare the Wheel for Polishing

    polishing

    Wearing gloves to protect your hands from the heat that the friction of polishing metal produces, hold the bar of metal polishing compound against the lower front portion of the buffing wheel as it spins. Coat the wheel lightly and uniformly with the metal polishing compound.

     

    How to Buff the Metal

     

    Grasp the metal object to be buffed firmly in both hands, and hold it lightly against the lower front of the polish-coated buffing wheel. Move the object constantly back and forth as you turn it. This ensures a smooth, even finish over the entire metal surface. When the finish appears uniform, clean the metal. If you wish, you can change to a wheel coated with a finer polishing compound. Repeat the metal polishing procedure, again cleaning the metal after polishing. If you want a still higher gloss, change to a wheel coated with a still finer abrasive, then clean the metal again. For the final polish, buff the metal work piece with a leather wheel that has no abrasive on it.

    Polishing Metal with a Sandblaster

    Place a drop cloth at the base of the metal you are sandblasting. Grasp the underside of the sandblaster canister firmly with one hand while holding the handle-and-trigger assembly with the other. With the blast nozzle held 1 to 2 feet away from the surface, aim the nozzle and pull the trigger to blow the abrasive directly against the metal. Move the canister up and down sideways, until one area of the surface is clear, refilling the canister with fresh >abrasive as needed.

    For a smoother finish, refill the canister with the used abrasive that has collected on the drop-cloth, and blast the same area again. As the abrasive gets finer with repeated use, the surface texture of the metal will become smoother.

    How To Change a Grinding Wheel

    Fasten a grinding wheel to its shaft with two metal flanges that fit against cardboard disks, often called the blotters, pressed to both sides of the wheel. A machine nut retains the assembly. The nut on the right wheel loosens counterclockwise; the one on the left clockwise. A wheel guard, screwed to the body of the grinder, protects the outer face of the wheel.

    Before mounting a new wheel, examine it for cracks and chips. To test it for hidden flaws, suspend the wheel on a dowel or a pencil and tap the wheel face lightly with a screwdriver handle. You should hear a solid, bell-like tone, indicating the wheel is sound. A dull tone or a thud indicates a cracked wheel. If the wheel is sound, mount it as above, tightening the nuts just enough to keep it from slipping.

    Dressing and Truing a Worn Grinding Wheel

    grinding wheel

    As a grinding wheel abrades the metal workpiece, so the workpiece also abrades the wheel. After using it for a while, the face of the wheel is liable to become clogged with waste metal, or to develop grooves in its flat surface. The remedy is to re-surface the grinding wheel, a process called "dressing and truing." You can dress and true a wheel with a "diamond dresser", which is a shorter handled diamond tool, or with a "star-wheel dresser", which is a long handled tool with a row of free running, hardened and serrated, discs. In general, you should not continue to dress a wheel that is more than an inch smaller than its original diameter. Install a new wheel instead.

    Adjusting the Tool Rest

    To begin, loosen the tool-rest wing nut and slide the tool rest away from the wheel until the head of a star wheel-dresser will fit in between the tool rest and the wheel. With the motor turned off, make sure the bottom lip of the dresser head hooks over the edge of the tool rest, then tighten the wing nut.

    Dressing and Truing the Wheel

    With the grinding wheel switched on, grasp the dresser handle firmly with both hands and slowly raise the handle, thereby lowering the cutting head until the cutters make full contact with the wheel face. Apply firm, uniform pressure and move the cutting head slowly from side to side, adjusting pressure on the tool so that it produces a minimum of sparks. Continue cutting the face of the wheel until you expose a new layer of grit. Stop the wheel occasionally to check your progress. When properly dressed and trued, the wheel face should be flat and of uniform color, without metallic or shiny patches. Run your fingers across the stopped wheel face to confirm that it has no grooves or high spots.