Got me a manual surface grinder last September or so. When trying to adjust the gibs, I found they all were berserked with a mill. So I started to scrap all the ways. That required shaping and scraping a touching prism for the awkward 50.7° for touching the dovetails. One dovetail is completely finished (parallel to within 0.002mm). But the counterpart is so much off that I'm now at pass 117 (I counted that!) and still get contact in maybe only 60%.
That drives me crazy!
Anybody selling a used Biax electrical scraper for little money?
Nick, its not moral support you need its strong medication! You shouldn't be counting as you make the passes that only keeps you awake, try thinking relaxing thoughts of waves gently rolling up the beach ZZzzzzzzzz! Sorry where was I, oh yes you need to keep the faith and you DO know that the very next pass could be the one that jumps to 75% contact and your in the home straight.
If that fails then just imagine all of those superb ground surfaces that the machine will produce for you once finished and that "glow" of self satisfaction not to mention of course the pain in the elbow and fingers but that will go away...... eventually.
Haha! That's right. I intend to write some text about scraping and put it on my homepage. So I thought I'll count the passes for that single surface. I also made a guesstimate and thought that I'll need 100 passes.
Lesson learned here:
I have to improve my rough-cutting technique.
Never ever count and estimate, just scrape along (you are *very* right at that point).
No one taught scraping to me, I just read and tried. The only trick is having the right scrapers (carbide, HSS is complete crap for cast iron; and right length of the handle) and understanding how the spots of the touching blue look like when they are high spots, medium or low spots.
I _have_ got a Biax BL40 scraper. I plan to use it for the roughing work when I rebuild my Beaver mill. I could probably get it shipped to you for about ?50-?80 if you want to borrow it.
The blade holder is not very good. That's because I thought I was getting a BL10 with several blade holders, but when I got home I realised that I had been given the wrong box, with a BL40 and no blade holder, so I had to make one. I should have bought both scrapers form the company that was selling them... It wasn't worth going back.
Have you got 60% evenly distributed? In which case, congratulations.
or have you got 60% at the ends or the middle? In which case should you still be using a file, or just leaning a _lot_ harder on the scraper?
It sounds as if you have progressed well on the job and have done most of the work already. the power scraper might do more harm than good :-)
Do we get pictures when the job is finished to encourage us to do the same?
But you know that I will never ever send it back? ;-) I think I don't want a BL40, but IIRC a BL20 (adjustable stroke, for finer work).
The problem is more "complex": I had to make my own prism, because of the odd angle. Scraping one side of the dovetail required maybe 30..50 passes, so the angle of the prism isn't that much off compared as how it originally was. But scraping the mating part is the pain. The angles are simply way off. I had a gap of 0.15mm on top and a narrow strip of contact on the bottom. So the 60% are a strip lengthwise (contact all over the length). That strip getting wider from time to time. There is a step that was milled in. Seems his cutter wasn't big enough when he was milling.
No, I can controll that very well where I take off. I'm quite proud of having gotten the tilted sides of the dovetail parallel without much pain. Have built a simple tool for (comparative) measuring that.
I wanted one since long. For that specific dovetail, it wouldn't help me much any more. But there is one more left that will be much more work. Steps milled in that you can feel, 600mm long, much wider...
Yes, I'm taking photos and plan to share what I found out. Scraping isn't that complicated if you understood the physics behind it. Most of the trick is (again) reading the blueing marks. Second is getting the "flow" and scraping without thinking to the marks. Maybe 5 hours, and you got that. The rest is tools and sharpening them and not to forget the right light! I really waisted hours with the wrong light.
I'm not sure how accessible these gibs of yours are, Nick, but scraping should only be used for the last half dozen finishing passes after you've filed or machined the bulk of any unwanted metal away. Scraping, like grinding, is not for re-aligning surfaces, it's just for final surface preparation (or fancy finishes) and that's all. If you need to remove much more that a thou, you need to get your files out.
No longer! Finished one dovetail yesterday and assembled and adjusted it. 155 passes. Just put a dial indicator (0.001mm) on it and *tried* to wiggle. Zero! Zilch! Nada! Nix! It doesn't wiggle at all. Not even in extreme positions where the dovetail only overlaps by 50%.
The result makes me soooooo happy! :-))) I didn't expect to be so good. ;-)
Now to the last dovetail with a better rough-cutting strategy.