How flat is a granite worktop?

[Tenson]

Interesting, thanks. Was it fully supported?

I've ordered a slab of 'quartz' worktop which is man-made ~10% resin / 90% ground quartz so I'm hoping it will be pretty flat.

[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.

[Doddy]

Comedy moment: I once used a piece of "granite" in a kiln, sacrificial bed under a crucible of aluminium, not realising the resin nature of it. The black smoke at 550C should have been the warning, and the flames as I opened the door nearly took my eyebrows off.

[Voicecoil]

Interesting, thanks. Was it fully supported?

We stood it on end to try and minimise any bending. I suspect there is a good reason why large surface plates tend to be so thick, if you look at the properties of granite it's not as stiff as you might think.

[Tenson]

What I mean is my 15mm slab of ali is probably the flattest thing I can get my hands on. Gotta be better than a pane of glass or a chunk of MDF for checking 'local' flatness. Probably flatter than the work top material?

What I'm hoping for is a work bench top, not surface plate, but one that I can trust to be flat for general non precision work like checking steel tube for bow when making my next machine. I'm not going to be taking care of it, I'll use it as a general work bench.

If I wanted to check the work top for 'global' flatness, can I expect the edge of a steel rule be reasonably straight?

[JAZZCNC]

If I wanted to check the work top for 'global' flatness, can I expect the edge of a steel rule be reasonably straight?

Not really, it's a ruler, not a straight edge.! . . . if you want to check flatness then you need a proper straight edge.

[Tenson]

I wonder if there is another way to get a reliable straight edge. Such as an affordable laser unit.

How straight do you think supported round rail is? I have some of that.

[Voicecoil]

Aye, rulers are too bendy. A straight edge is deeper & thicker to make it stiffer and stay straight. I got a 1m one for about £50 2 years ago, I seem to remember that the price increased steeply when they got much longer than that.

[JAZZCNC]

How straight do you think supported round rail is? I have some of that.

About as straight as Philip Schofield....

[Tenson]

I just ordered a 1 meter straight edge so I can satisfy my curiosity! Says it is good to 0.04mm across the entire length.