In August 2016, I had the opportunity to attend a weeklong machine-tool scraping seminar run by Richard King. Rich is the owner of King-Way Scraping Consultants, and is one of a small handful of folks with deep experience in scraping for precision alignment. Thankfully, he’s willing to teach what he knows, and he runs seminars around the world for companies and individuals.
Over the course of the week, our group picked up the fundamentals of scraping, using both hand and power scrapers. In addition to completing practice parts (scraping grey iron blocks to 20 PPI), every attendee brought a personal project to work on. My project was an 18″ master straightedge, used for scraping hard-to-reach areas on machine tools such as the interior of dovetail ways. After machining the raw casting down, I worked under Rich’s supervision to scrape the primary reference surface to 55 PPI/40% coverage, and the dovetail reference surface to 45 PPI/50% coverage.
With scraping completed, I wanted to check the work I’d done. With help from Peter Stahle (MIT NSE), I used a Zeiss Eclipse CMM to characterize the flatness of both the primary and dovetail surfaces, as well as the angularity of the dovetail. The primary reference surface is flat within 0.015 mm (0.0006″), and the dovetail reference surface is flat within 0.020 mm (0.0008″). The angularity of the two surfaces relative to the idea 45º is poor (0.201 mm or 0.0079″), but this doesn’t matter in practice – the dovetail surface is intended to be used just as a flatness reference, rather than as a precise angular reference. Most usefully, the CMM provides a visual depiction of the topography of the scraping surfaces – fun to see, and useful feedback on my scraping technique.