recondition a cast iron lapping plate

I need help on the best procedure to have a 16in.cast iron lapping plate reconditioned .
It has never been reworked before ,it has some scratches about .003 deep in one place .We use it to lap pump valve plates,some piston pump bbls.and other hyd.parts
What can i expect in true flatness after it is reconditioned.and what is the proper way to resurface the cast iron plate

to do it properly, it takes three plates to generate one

actually when you are done you got three good plates

can't do it with one or two at least the way i was taught

bob g

Mobile,

So the same procedure works for surface plates and lapping plates both?

I'm pretty inexperienced and could be completely off base, but isn't it hard to scrape with all the slots? Or do you do this like grinding a telescope mirror?

It IS interesting that if you want to keep your own CI surface plate, you need three (or two good friends).

Thanks,

Jim

blanchard grind it, or surface grind it. I'm assuming the embedded grit would make planing or shaping it a non-starter unless the cut was too deep for good conservation.

Once ground, clean it well and spot it on a surface plate. If it is deemed adequate to your needs, it's done. Otherwise scrape it in to whatever flatness & bearing density is desired.

If necessary to do in your own shop, careful surface grinding in 2 or 3 set ups on a 618 grinder would probably get it close enough and under the embedded grit to scrape. Preferably with a biax. A fresh ground surface is kind of slick.

smt

Lapping plate

Clean, clean, and then clean it again. Grind on the best grinder you got and let it "spark out" Really when you think about the circle motion you use, with whatever grit it'll make the piece pretty darn flat anyway. If using for gage blocks get a new one and use this one on your bench.

I wonder if it needs reworking. The 0.003" deep scratch is not a good reason to do anything to it. You could scratch it all over like that and it might work better as a lapping plate. Most lapping plates have a cross hatch of grooves (think giant scratches) to allow lapping compound and swarf to exit the surfaces and allow the lapping process to work. Lapping plates need re working when they get rounded over on the edges or develop other out of flat condition due to improper use such as lapping only in one area which results in a hole being formed. If you have another flat plate you can blue up your lapping plate and rub it with a good flat and find the highs and lows. Is your plate round or rectangular? How large is it? Blanchard grinding it will clean it up and get it flat enough to use if it is in really bad shape. Proper lapping technique will improve the flatness after that.

Scrub it off with a wire brush. Give it a light etch in metalprep solution (loosens retainded grit). Then skim cut it in a mill. Scrape it flat striving for a smooth finish. Go and lap some more.

FWIW A straight edge and a 0.001" feeler should be handy to the plate. Every time it's cleaned the plate should be checked lengthwise, crosswise, and diagonally - that's several times a day if need be. Any high spots thus discovered should be sites for the next lapping. Go about it right and the plate should stay flat indefinitely.

Needless to say attention and cooperation among the people using the plate should be close and enduring. Negligence lead to bad work and a deteriorated plate. If you catch anyone lapping only in the center, give him a crecent wrench shampoo. The whole plate including the edgea should be used uniformly and the straight edge and feeler should gather no cobwebs.

Hand lapping plates are best reconditioned on lapping machines.

Grinding will not impart the flatness necessary for lapped parts.
If you have .003 to remove, you may blanchard grind it to rough it in. Then have it lapped flat on a lapping machine.