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
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
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.
Not without a reference flat.
If you have to buy one of those, you may as well
just use the new one, of course!
If you can borrow a reference flat, you can scrap
(or otherwise cut) your concave plate to the reference
in the classic way.
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 think surface grinders vary, but well under a thou is normal.
However, if you "just" want a surface plate, buy one!
(unless you can get free use of a surface grinder)
That's INSPECTION grade for Pete's sake.
That second one is very cheap! I was keen on the dual use aspect of a
lapping plate, as I can also use it for occasional marking out.
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.
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