How flat is a granite worktop?

[Tenson]

Asked Google, got this thread!

I'm planning (or set on) picking up some cheap granite / quartz worktop for work-bench tops. I wondered just how flat it tends to be, but perhaps that is a question about how long is a piece of string? Is granite and quartz different in this regard? I imagine a man-made material will be flatter than a cut true stone and what they call quartz seems in fact resin based.

Anyway, perhaps I can do a rough check for high and low spots by lapping? I have a 400mm x 400mm slab of 15mm cast ali tooling plate (faced both sides at factory) so I guess that would surface as good beginners lapping plate and give me an idea of my worktop flatness?

[JAZZCNC]

I have a 400mm x 400mm slab of 15mm cast ali tooling plate (faced both sides at factory) so I guess that would surface as good beginners lapping plate and give me an idea of my worktop flatness?

Not really, just because its ground plate doesn't mean it's perfectly flat, it just means both faces are ground flat and parallel so it's a uniform thickness.
I buy loads of ground cast plate and it comes in large sheets which bend under their own weight so you must expect a little bend, far too much to use as a surface plate for lapping.

I have a friend/customer who makes granite counters from both granite and Quartz(man-made) material so have looked at this closely and the granite used for countertops varies wildly, which makes sense because while it looks flat it doesn't need to be precise for a surface counter so they don't lap them more than needed.

Depends on what you using it for but I have a large 4' x 6' calibrated precision granite surface table from which I measured some pieces of countertop from and the surface was so inconsistent that I didn't think it was worth the expense and effort it would take to correct for anything other than a wood router, which didn't really need this level of flatness anyway so would be a waste of money.