This is a little update regarding my ongoing scraping adventure. I used my previously-described diamond lapping setup to shape 3 carbide scrapers I purchased from Ebay (Coulter Precision, prices ranging from $9.99 to $18.99). They're too thick to work on dovetails so I'll have to come up with something else when I get to that point, or maybe risk grinding one down so it fits.
As received, the scrapers had a lot of positive rake so they needed to be re-ground, hence the setup I made. My tool holder is designed to accommodate home-made brass inserts with various radii, so I could easily and reproducibly form the end radius on my scrapers. For my widest scraper (3/4") I started with a radius of 4.5" but the point of this post is that there seems to be an upper limit on the radius for hand scrapers. The 4.5" radius was too hard to use, at least for me. When I tried my 3/8" wide scraper with a smaller 2.5" radius, it was much easier to use so the difference did seem to be related to the curvature.
I did a little online research and noticed that the "standard" Biax scraper has a radius of 3-3/8", so I made another brass insert with that radius and re-ground/honed my scraper. That made quite a difference -- now the scraper is much easier to use and control. The cut it makes is still fairly wide so it should be OK for its intended purpose, which is for doing the initial "rough" scraping work.
I'm now scraping the bottom of my surface gauge. As it came, that thing was really out of whack -- I think its base was "ground" on a belt sander. It's slowly becoming flatter but still needs a lot of work.
Mark
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