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Bob Behnke, a machinist at Busch Precision Inc., a Milwaukee contract manufacturer and machine repair company, said scrape marks are like fingerprints of the person who worked on the machine. “Everybody has their own individual pattern. Over time, you look at enough of them and you’re able to recognize who did the work. We have people who scraped here 25 years ago, and I can still pick out which machines were theirs.”
Behnke said the amount of scraping, the depth of each individual mark and the distance between them (points per square inch) depends on a number of factors. A very worn machine could require grinding or machining to true the geometry, followed by a complete scrape job. A light rebuild may only need to be “freshened up” to make surfaces flat and create the proper amount of oil retention. Machines that will see heavy loads call for deep scraping, where light-duty machines can be effective with shallower scrapes. In each case, the craftsman follows the same basic process, but makes technique adjustments based on skill level, experience and the application needs of the machine tool.
“There’s a lot to it,” Behnke said. “The length of time spent scraping any particular surface or machine comes down to how many high spots there are to begin with, how much resistance is acceptable when the machine tool traverses back and forth, the amount of lubrication that’s needed and the size of the way surface. It takes a lot of experience, but it’s also physically demanding. You have to push on the blade and really dig in if you’re going to get a deep scrape, which is the best way to assure longevity in the machine tool.”
Sticking to the Plan
Aside from strong forearms and a steady grip, successful scrapers have a solid game plan. “You can’t just go at it randomly,” Behnke said. “You have to be very meticulous, scraping first in one direction, then the other. That breaks up the lines and gives a nice, uniform pattern.”
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