User manual 3M GUIDELINES FOR GRINDING

[. . . ] Based on carefully controlled laboratory and field tests, 3M has created a number of equations which indicate the "trend effect" of variables present in abrasive usage. In the tests, we attempted to isolate a particular variable, keeping all other factors constant. Since this is not always possible in an actual production situation, you should use these charts as helpful guides in studying a grinding problem. You should view the results as trends, and not as concrete relationships. [. . . ] Anatural approach would be to use a softer wheel to get the desired coverage on a contoured piece, and also to remove the same amount of stock over the entire width of the sheet or coil. However, it's effective to slot the hard wheel in a way that will give the required coverage. Effect of Contact Wheel Hardness on Surface Finish Generally speaking, the softer contact wheels result in a finer finish. Effect of Contact Wheel Serration on Abrasive Belt Efficiency This chart shows the effect of three different types of wheels on belt efficiency. Both the serrated wheels will remove a great deal more stock than the smooth face wheel. The curves also indicate a longer belt life for the serrated wheels and show that they remove more stock over a greater number of sheets. Effect of Contact Wheel Serration on Finish Serrated wheels do not produce as fine a finish. The more aggressively the wheel is slotted or serrated, the rougher thefinish. Effect of Contact Wheel Area on Pressure The amount of pressure per square inch exerted against a piece of work will affect the rate of cut and belt life. This chart shows how various slotted and serrated contact wheels affect pressure. Given a total pressure of 100 pounds and a tangential contact area of two square inches, the plain face wheel applies a pressure of 50 psi. In other words, with total pressure remaining constant, the pressure per square inch rises in proportion to a decrease in land area. Impact Effect of Contact Wheel Slot Angle The angle of the contact wheel slot also affects rate of cut and belt life. The 90° or straight across slot is the most aggressive grinding angle, and creates the highest impact on the work. The reason is that inertia causes the workpiece to slip into the voids and the impact of the following lands drives the mineral deeper into the work with a very high unit pressure. Because the 45° and 7° angles give more support to the work, this impact is minimized and the rate of cut is lessened, and so is belt wear. One reason the 90° slotted wheel emphasizes belt wear is that the belt gets more flexing with the 90° serration. This results in the shedding of larger grains and faster shedding of working grains. Noise and chatter are other reasons why the 90° slot is seldom used. Effect of Contact Wheel Serration on Mineral Wear Slotting the contact wheel has a unique effect on mineral wear. Notice the several sharp edges on the enlarged grain used on a serrated wheel . This is due to the fact that the grain is presented to the work at a different angle each time it passes over the wheel. The Serr-X Contact Wheel and Features The Serr-X is a special contact wheel developed by 3M for maximum conformability. Unlike a diamond pattern, the Serr-X does not transfer its pattern to the work. It conforms better, cuts faster, and also lengthens the life of the abrasive belt. Contact Wheel Performance This graph shows the results of a test using four different wheels . Actually, Serr-X wheels can, and should be made 10 to 15 durometer points harder than conventional wheels. Effect of Contact Wheel Serration Angle on Finish The smaller the serration angle, the lower the micro-inch finish . Slot angles of 30° or less are usually used in the polishing operation. The 90° angle often makes a siren-like sound and frequently leaves a pattern on the work. The 45° angle is the largest one that will still run relatively quiet. [. . . ] The belts are moved down the line at one-half-belt life to present a more consistent set of conditions than possible when using a belt for its entire life on one station. This system produces more uniform stock removal and finish than a system using twice as many grade sizes on the same number of heads. The Half-Belt-Life Principle Following is an example of the half-belt-life principle at work. Atypical grade 100 aluminum oxide paper belt will produce a finish reading of about 90 when new . If it is moved at half-life, we have the effect of two different belts. [. . . ]