I've got a used iron or steel lapping plate 10 inches diam. by 1 1/2 inches thick which has roughly concave faces - I measure the dip in the middle of each face to be 0.25mm and 0.1 mm. I was hoping to use it as both a surface plate and lapping plate (lapping mainly by using it to support lapping film to avoid wearing it) but it obviously needs work. Can I fix this myself by hand? Thanks,
Thanks Steve, What about when you only have one plate? The techniques I have in mind is grinding the lap on a sheet of float glass first with silicon carbide grit to get rid of the worst of the error and then scraping the surface whilst using a sheet of float glass as a reference with engineers blue to spot the high points. Does this sound practical? Or would the lapping plate and two sheets of glass work with the three plates technique? Thanks,
The problem may be with the differing hardness, and embedability of the two materials. Cast iron to cast iron does work, but as I said, 3 are needed. each plate needs to be lapped against the others. you also need some way of checking so that edges do not get rounded. Best to read up on it first, as it has been many years since I have done any optical work.
I think you'd be there till Doomsday trying to lap or scrape that much out by hand, certainly on the side with 0.25mm wear. It needs either truing up on a lathe or mill first and then get it spot on by hand or surface grinding which should get it good enough to use straight away.
If it were mine I'd run a flycutter over it on the mill which should get it to within a fraction of a thou from true. Better than I'd ever need from a surface plate anyway not that I actually use them.
I was going to say that on my old nail of a J&S 1400, that's old enough to have the controls labeled in runes, I'd expect to achieve about 3 tenths on the size of that lapping plate. Say .0075mm. I'd need to do it in two halves, because the grinder's only 8"x24", but that's doable.
Downside is that at the moment, there's a couple of months work stopping me getting to the grinder (literally! I ran out of space to stack bits of lathe while I rebuild and paint the cabinet, so it's all piled in front of the grinder :), then I've got to repair a stripped thread on one of the grinder's hydraulic hose connections.
If you haven't had any luck by mid-late March, I should be close to a position to sort you out. I'm in Rugby, but could probably prevail upon a colleague to bring it up from Borehamwood on one of his twice a week commutes if you could get it that far.
Hopefully you'll get a better offer, but at least you can fall back to that.
What I would suggest is to lay out for one of Fordeight's (AKA Rotagrip) rocks off Ebay. Invest in the 18x12 rather than the 12x9. Save up the money if necessary, it'll last you a long time. Use that as your main layout and marking tool. Get some sucker (like me) to surface grind the lapping plate. Then either use it purely as a lapping plate or use it as a practice piece to learn scraping. From experience, it'll be close enough to make quick, satisfying, progress on when scraping for final finish.