MoldMaking Technology

DEC 2018

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Page 20 of 50 19 stone. However, polishers realize quickly that they must remove that hard layer first, which means that the soft, finer stones will not work. It is better to begin with a 400-grit stone. Basically, this hard layer requires an abrasive stone, but as polishers remove the hard layer, they start to scratch the softer steel, ripping and tearing the steel as they get through the hard layer. So, although high-speed machining benefits the machinist with cutting, that process does not help the polisher. For EDM, the hard spot particles can sometimes flip out as the polisher works on the surface, leaving a big pit. Other times, operators finish grinding an area and then use EDM over the grind area until it sparks out to match. Those EDM pits are then below the grind surface. "It's counterproduc- tive," Smith says. Another example is when machinists do not provide enough overlap with the finisher to remove all of the rough- ing burn. This requires the polisher to remove deep pits and the recast layer below if the goal is a good polish. Today, polishers need to learn how to work with these sur- faces differently. Experience and having the ability to gauge how much material the polishing removes are essential. The answer is training and practice. If a polisher only polishes something once a month as a minor part of the job, the pol- isher is never going to become proficient at polishing. Learning Culture Another aspect of polishing proficiency is creating a culture of learning. "You need a culture of learning to figure out dif- ferent polishing strategies and to troubleshoot the cause of a condition," Smith says. Polishers need experience and common sense to solve problems and to do things faster and more efficiently. This means learning from mistakes and successes and building that experience into documented strategies. Smith suggests taking a 3D print of the part, inserting an arrow on the surface and noting what was done to achieve the surface finish. He says not to focus on individual steps. Instead, focus on the actual finish because the finish that the machinist provides to the polisher determines the start- ing point for the polisher. The process is what the polisher uses to achieve the customer's required surface finish. The customer might specify the tool or the method, but not the process. "Take, for example, the SPI A1 finish from a felt buff. Ask yourself—How are you using that felt? Are you doing it by hand, on a machine, etc.? If you are using a machine, you need to understand that different speeds will give you differ- ent shines. Are you using hard felt or soft felt? All of this is part of your internal strategy for each project," Smith says. That being said, polishers should not document their pro- cesses for the customer, Smith says, because it will lock them into a particular process. An example is heat-treating steel. Just one point from 50 to 51 can make a difference in how a polisher works on that job. This does not sound significant, but it is. A hardness range from 48 to 52 is a whole world of difference in polishing to the polisher from one part to another. "So, if you have parts that are 48 and others that are 52, the polisher is going to polish differently," Smith says. As long as the shop is achieving the customer's desired finish, polishing documentation is for internal purposes only. How the polisher goes about it is the shop's proprietary information. "For example, we had a syringe mold, and the customer wanted to mold polypropylene, which polishes well with a 600-grit stone or a light- glass bead. But, this gave a frosted part, and the cus- tomer wanted to be able to see through the syringe. We had parts sticking in the mold. Our solution was first to draw-stone the part (the direction the part pulled off). We then bead-blasted it and brushed it with dia- mond compound," Smith says. This polishing process was documented internally to improve his shop's polishing efficiency. If shops want to produce a quality part every time, they should establish an environment for learning that includes internal process documentation. In the end, that will help polishers overcome challenges such as the impact of HSM and EDM on polishing. "How you do it is up to you. Just keep practicing," Smith says. The secret to improving polishing efficiency and part quality is improving the front end, what's coming into the polishing department. FOR MORE INFORMATION Gesswein & Company / 203-366-5400, ext 280 / / About Steve Smith In 1964 after high school, Steve Smith started working at Wilkin's Mold Polishing, which was the only mold polishing shop in the western part of the United States that trained polishers. In 1972, the company closed its doors. Smith moved to Samson Mold, a 20-person shop where he worked hand in hand with the EDM and grinding departments, learning the value of good communica- tion. In 1979, he got an offer from Caco Pacific to run its polishing department. Five years ago, Smith retired but accepted a position as a consultant and instructor for Gesswein Co.'s polishing course, which to date has taught 400 students across North America.

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