I noticed a local granite countertops place has some free scraps available; any chance that stuff is flat? smooth? enough to use as a surface plate? Too good to be true, I imagine.
Why not check it out with some plastigauge between two pieces? I've got a Corian or synthetic granite like table and its pretty durable and non-porous. Cleans up nice after paint, etc. Good enough for jobs like rolling axles and the like to check for straightness. A local shop I worked at briefly had a chunk for decking small heads on sandpaper.
It might be good enough, if you know someone with an actual surface plate you could have them check it.
I don't know how big of one you are looking for but small granite plates aren't really that expensive, I'd personally just buy one of them. If what you are doing warrants a surface plate do you really want to trust the free scrap from the countertop place?
On Edit, I guess it really depends on what your needs are and what is acceptable to meet them.
It may not have to be perfectly flat as long as you keep rotating the head and using a figure 8 pattern.
Quote from: 1976RD400C on April 04, 2021, 10:31:50 AM
It may not have to be perfectly flat as long as you keep rotating the head and using a figure 8 pattern.
That's assuming he's using it for lapping...
Wade, I use a piece of 3/8" glass, it's poured, so it is very flat......................D
All good points. A surface plate is on my list of tools or resources I'd like to have for checking flatness, lapping and precision measurement, but damn, good stuff is expensive! Much like parts and services for these motorcycles, right? I saw a pallet with granite scraps available for my favorite price and my little wheels started turning. Maybe I'll just grab a piece and check it out.
having have been a quality manager at 3 different companies.. no. it's not even close to being a surface plate. as far as it being a reference for flatness? meh. maybe for quick reference sure. but, if you want a real one, even a grade b one, check local auctons when business' close. used ones come up cheap. .02
If you had a 12" micing bar and a mag base dial indicator, you could probably fashion up a ghetto surface flatness checker and sweep the surface, kind of like the same tool they use to test/cal surface plates. I'm willing to bet it's not that far off, but there are also different grades of surface plates, so as long as it's accurate enough for what you're measuring.
If you get a piece of that granite and want to check it, clean it first with lanolin hand cleaner and wipe it with a lint free cloth.
If you're lapping, there are plates specifically for lapping, that have a precision ground surface with grooves/channels ground into it to let excess compound and grindings to fall into.
And don't expect an old lapping plate to still be as flat as a referance plate. For lapping, I have a 12"x12" piece of plate glass that I use removable spray adhesive to glue 600 grit to. I use isopropyl as a lapping lubricant. Restored a seriously warped GL1000 head and block with that, single cut mill files, Prussian Blue and Dykem.
I have a granite cutout from a kitchen sink that I use to clean up gasketed surfaces on engine panels after scraping off the gaskets. Works great for that but I don't know about more critical mating surfaces.