Burrs and rough edges can be caused by several different factors including dull or improperly sharpened cutters, improper speed and feed rates or material that is not "free cutting."
The most common cause and easiest to fix is the dull cutter. Cutters last a long time, but in time the cutting edge becomes worn and rounded. With a sharp cutter, the cutting edge enters the material and slices through it producing a crisp, clean cut with smooth edges. The cutting edge of a dull cutter pushes into the material, but instead of slicing cleanly through it, has a tendency to tear it which causes the burrs and rough edges. The easy solution is to replace or resharpen the cutter.
The other cutter related cause is using a cutter that is not suitable for the material being engraved. A cutter that is sharpened for engraving steel for example will not produce a good clean cut on soft plastic. For optimum results, it is important to use the right cutter for the application. (See Material Classifications)
Cutter speeds and machine feed rates also have a significant effect on the quality of the engraved cut. As it rotates, the cutter needs time to make the cut and clear the chips. If the feed rate is too high, the cutter is pushed into the material at a rate that is too fast for the speed of the cutting edge and the result is rough and jagged cuts.
A simple but appropriate analogy can be made to cutting your grass with the lawn mower. If you push the mower through high grass at a reasonably slow rate, the grass will be cut uniformly and the clippings will be ejected. If you push the mower at a high rate of speed, the blade isn't rotating fast enough to cut all the grass uniformly, the clippings aren't ejected and clog around the blade. This is almost identical to what occurs in the engraved cut. The solution is to decrease the feed and/or increase the cutter speed. Be careful when increasing cutter speeds however, since too much speed can generate heat at the cutting tip which will melt soft plastics or burn the cutter if engraving metals.
Another cause of poor cut quality can often be attributed to the material itself. While most of the standard engraving materials cut very nicely, some of the other materials common to the industry can present problems. Some plastics like vinyl and those with a high rubber content are so soft that it is nearly impossible to achieve a clean cut. The cutter will actually push the material away instead of cutting through it.
Others like some of the computer keycaps and molded plastic lenses used on illuminated switches are very susceptible to the heat generated by the cutter tip and the cutter melts its way through the material rather than cut it cleanly. Some possible solutions would be to use a cutter with increased clearance and face relief, reduce the cutter speed or use a water-soluble cutting fluid (very effective on the keycaps and lenses).
The two most common metals used in the industry are brass and aluminum and how well they engrave has to do with their machinability or whether or not they are "free-cutting." Trophy brass, for example is very hard and is extremely difficult to rotary engrave with any success. Diamond drag or burnishing is the recommended way to go.
For rotary engraving in brass, you need to use "engravers brass" which is a free-machining material with a higher lead content which makes it softer and relatively easy to engrave.
Aluminum is almost the direct opposite of brass in that the harder alloys are generally much easier to engrave than the softer ones. The thin colored material used for trophy plates is usually extremely soft and almost impossible to engrave without the use of a cutting oil formulated specifically for aluminum. The harder alloys such as 2024 and 6061 engrave very nicely.
One of the most difficult materials to engrave is stainless steel. Like any other metal, there are a variety of grades and corresponding levels of machinability. Even the best are difficult to engrave however. Three things that can vastly improve success are slower feed rates, multiple light passes and appropriate cutting oils. Slower feeds minimize the amount of material being removed with each cut thus reducing shock and stress on the tool. Multiple light passes achieve the same thing and the cutting fluids provide lubricity and help keep the material and the cutter tip cool.
Keeping the cutter and the material cool can be extremely important since too much heat can burn the cutter and even harden the material through a phenomenon known as "work hardening." A good example demonstrating how heat can affect the hardness and ultimate machinability is to take a piece of coat hanger wire and bend it back and forth several times. The wire will get hotter and hotter as it is bent making the metal so hard that it gets brittle and finally breaks apart.
When engraving any metal, there are two things that can greatly improve the end results. One is to engrave in multiple passes with the final one a very light cut (only one or two thousandths) which tends to smooth the cut and remove any burrs. The other is to use a cutting fluid which also helps to improve the quality of the cut and will extend cutter life.
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