If I was standing next to you I would have you increase the stroke length to 3/16", put in a 30-150 blade ground to a R 40 to 6 and neg 8 deg's. Have you go into your bathroom and bring out your bathroom scale and put it on the work bench, lay a red rag on the scale. press down on the scraper blade on the rag which will be 8 pounds down pressure. The do rule 1, 2 and 3 on the plate to give it some new depth of .0004 to .0006" and better texture. This will make the "it looked better then that last time effect" now shorten the stroke 1/8 turn use same blade and do the divebomb cross pressing down the same depth.
What happens and I see it a lot. one sets the stroke to short and uses a 20 rad. blade and as they get better they lighten the pressure down and get the poka dot, looks better last scrape because when they stone the burrs to hard they loose .0001" depth and the bearing disappears. Jeff knows this as he had the same issues at first.
Thanks, Richard. This is helpful. I think the blade width and radius may be causing me more problems than scrape depth, Though. I went into the shop earlier today and measured the depth of individual scrapes on both the straightedge I'm currently roughing in (with a wider blade) and the plate that I set aside (after finishing with a narrow blade). The depth on the plate from high point to adjacent low point averaged around 0.0005" (more than I thought). The depth on the straightedge was closer to 0.0008" to 0.001" (and the bearing percentage looks reasonable, though I'm nowhere close to the desired PPI yet).
This seems to jibe with what I was thinking:
my plan to increase bearing percentage would be to ignore pinpointing for a pass or two and create as even a cross-hatch pattern as possible across the entire surface, with a slightly longer stroke and paying attention not to drop the back of the scraper.
I'll follow your advice to do this with a wider but tightly radiused blade (about R 40 to 60).
To date, I pretty much only use two blades: a 25-150 R90 and a 15-150 R60 (at least those were the radii from the factory).
I think I mentioned earlier that the radius was wonky on my narrower blade. I hadn't held it up to a radius gauge in ages, and was kinda shocked to discover my 15-150 blade was nowhere close to R 60mm like I thought. After an uncounted number of sharpening passes, I'd managed to grind the shape into something almost straight/flat in the very middle section, then falling off rapidly on the sides (like R20 to R30).
My 600 grit wheel must have been more aggressive than I realized. Where I thought I was just polishing the bevel, I was actually changing the geometry every time I touched up on the wheel. (I've since moved to a 1200 grit wheel for honing, one that some kind soul gave to me! <grin>)
I think I was effectively dive bombing with a narrow, but R90mm++ blade.
Anyway, I'll be a lot more careful about blade geometry and depth of cut on my next passes. If nothing else, I'll be checking my blades against a radius gauge more often!
I
think my problem may be mostly related to blade radius, and not to depth or pressure. I'll see what changes when I get back to it, and post the results. Whether or not the bad grind is the culprit for my poor bearing percentage, I'm going to 3D print a fixture for accurately grinding 40, 60, and 90mm radii Biax blades on my Glendo, regardless.
