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This article is the third in a four part series designed to bring to light some common routing problems and the tooling and/or process changes that became the solutions to the problems. By Van Niser As more and more CNC routers are sold for the purpose of machining plastics, companies are running into problems that never existed when they were hand finishing or machining plastics and other materials. The scenarios here look at four real-world problems that happened at fabricators in the United States and the solutions that were found. Material Cut: Aluminum skinned acrylic What happened with this fabricator is a very common problem in the signage and point-of-purchase industries. A favorite material used for sign displays is a clear or colored acrylic covered on one side with a thin sheet of aluminum that is attached with glue. This material is cut out for displays and letters and a premium finish is required on both the top (aluminum) surface and the edges of the material (the acrylic). In this particular application, the material was 3Ú8-inch-thick acrylic with a 0.015-inch aluminum skin. The fabricator was machining the material with the aluminum side down for better hold-down and surface finish and had already selected the correct tool for the job. The problem was that as the aluminum and its attached adhesive were cut and the chips flowed through the tool's flute, the chips would heat up and weld or stick to the acrylic. This required a hand finishing operation that the fabricator was looking to remove from the process. This scenario is similar to a situation written about in the last issue in which a fabricator was cutting letters in the same type of material with a 3Ú16-inch diameter cutter. In that instance, switching to a tool with better geometry was able to solve the problem. In this case however, the glue and aluminum weren't as well behaved and the machinist was already using the optimal geometry. The solution for this problem was to use a modified two-pass system for machining the part. The aluminum side remained face down. The first pass of the cutter was set at a depth of 0.030 inch above the aluminum/acrylic interface and was set to leave the part 0.015 inch oversized. This enabled the cutter to remove the bulk of the acrylic material without cutting into the soft aluminum or the glue line. The second pass was machined at full depth and on-size. This gave a clean cut to the aluminum and acted as a finish pass to the acrylic without generating an excessive amount of heat or leaving a line at the depth step. Because the finish of the first pass was inconsequential and the finish pass was removing so little material, the feed speeds were increased to 250 ipm from 60 ipm and the total cycle time was less than the original process. This method eliminated the hand finishing operation, reduced the cycle time and increased the cutter life. A win-win situation. Material Cut: Cast acrylic This was a case of too much material for too little tool. The company was using a 1Ú4-inch diameter cutter with optimal geometry for cutting 1Ú2-inch thick cast acrylic. In many cases this is a perfectly acceptable method of achieving a premium edge finish if the equipment and fixturing is well maintained and very solid. Unfortunately for this company, time had taken its toll and the equipment did not have the rigidity required to remove that depth of material and still maintain a premium edge finish. Even with the machine reprogrammed for a rough and finish pass, the finish did not improve enough. The second attempt to solve this problem was to use a multi-fluted acrylic finishing tool. In many instances, adding flutes can lead to an increase in surface finish, but it can be at the expense of heat buildup and tool life. The 3-flute finisher was used as a finish pass tool after the single flute spiral removed the bulk of the material. Unfortunately, this still did not produce the desired edge quality. The final solution was to use a 1Ú2-inch diameter acrylic finishing tool and to take a single pass. The added stability of the 1Ú2-inch diameter combined with the increased surface speed of the cutter edges produced the premium edge the company was looking for. By changing their fixturing and programming, the company eliminated two of the polishing operations. Material Cut: ABS and acrylic Product: Awards Router Type: 3-axis CNC Feeds & Speeds: variable Initial Tooling: 1Ú8-inch diameter spiral "O" flute Problem: Tools were breaking when materials were switched. In many cases, router operators feel comfortable with a particular tool and do not want to be bothered with a setup change or the expense of testing and operating multiple tools. This case was no exception. The operator was very happy with the finish that was generated by the 1Ú8-inch spiral "O" flute in both 3Ú8-inch thick acrylic and 1Ú16-inch thick ABS. Unfortunately, the tools kept breaking when they were used to machine the thin ABS. The problem originated from the fact that the cutting edge length (1Ú2 inch) required for the acrylic was too long for the ABS. By using the same type 1Ú8-inch diameter spiral O flute tool with a 1Ú4-inch cutting edge length, the breakage problems were eliminated. The operator was able to use the same tool for both jobs by taking multiple passes (at higher feed rates) on the thicker acrylic material without going through the multiple setups. This article is the last in a series describing actual routing problems and the tooling, fixturing and programming methods used to solve them. The points that should be taken from these articles are: • Pick the Right Tool for the Job: The single most avoidable mistake that users of CNC routers make is picking the wrong tool. There are hundreds of tools available for cutting hundreds of different types of plastics. Diameter, geometry, chip evacuation and flute count are all variables that need to be considered for each machining job. • Employ Solid Fixturing: It is difficult to achieve premium edge finishes (measured in millionths of an inch) if the fixturing allows the parts to move a few thousandths of an inch. This is a critical issue and should be evaluated for each job setup. • Be Willing to Change Programs: If the finished part parameters are not acceptable or optimal after selecting the correct tool and fixture, the answer can frequently be achieved through programming. Whether it is a rough/finish pass combination, multiple depths of cut, conventional or climb cutting, or changes in feeds and speeds, there are many options available to increase part quality and throughput. If these three issues are evaluated and solved, productivity, quality, and efficiency should be greatly improved. Van Niser is director of Plastic Application Engineering at Onsrud Cutter. Readers are invited to send questions to Van Niser at Onsrud Cutter, 800 Liberty Drive, Libertyville, Il 60048; e-mail: vniser@onsrud.com.
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