How flat is a granite worktop?

I've been shopping for kitchens (yes SWMBO has finally persuaded me that our 20y old kitchen is in need of some refurbishment) and got chatting to the designer guy in the shop about stuff such as CAD (as you do :)) and the use of granite worktops as a base for aligning things (he's a model RC builder). He reckoned they're pretty flat (at least by by his standards) and promptly offered me two 12" square offcuts of 1.5" thick granite worktop which I couldn't refuse, much to the annoyance of SWMBO ("I can't believe you did that!") :D

A quick test with the edge of an engineers square (about the only thing I have close to a true straight edge) suggests they are pretty good although I can see daylight in places between the edge and the top face, but how could I measure the flatness?

If I had one :)

Well put a 1.5thou (.02mm) feeler gauge under it and I can push it through in places, so I suppose the answer is 'not very flat'

I have 2 pieces roughly the same size. I was wondering about lapping them together to get it flatter. Thoughts anyone?

A 12" sq 3" thick granite surface plate is about £35-£40 new. So it might be worth the effort.

You need three surfaces to scrape/grind/lap something perfectly flat.

By only using two, you can end up with one perfect concave surface, and one perfect convex surface.

I remember seeing a mate checking his surface plate, not sure why he had a surface plate as he painted cars;-) he had plotted the surface out into squares and was checking between squares by resting a spirit level across a pair of cylinders placed in two squares.

I gather gravestones, surface plates and worktops are all finished on the same machinery. I asked in a stomnemason and they said they often had a skip full of bit's, I keep meaning to call back and get some bit's.

I would look into suface tension, if you could get a bit of plate glass and seperate the plate glass and surface plate at one end with a feeler gauge the surface tension of a thin liquid would show as a straight line, if the surface was flat.

with a section of plate glass you can do the old machinist trick of printers ink and paper. You just have to figure a way of lowering the glass completely evenly (spirit levels, even of the torpedo version, can help here) anything that touches first needs to be ground down slightly and go from there. There is also taking the pieces to a stonemason and having them level and polish. My question is how much dampener are you going to put between any Mill/Router that you plan on mounting on it and the tool. Reason being that stress will build up in the stone and it WILL split along any inherit fault lines within the stone (ask me sometime about a number 1 diamond wheel and Obsidian). Great idea if you can control the vibration reaching the stone or give it an ability to transfer it.

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Originally Posted by irving2008

If I had one :)

Well put a 1.5thou (.02mm) feeler gauge under it and I can push it through in places, so I suppose the answer is 'not very flat'

I have 2 pieces roughly the same size. I was wondering about lapping them together to get it flatter. Thoughts anyone?

A 12" sq 3" thick granite surface plate is about £35-£40 new. So it might be worth the effort.

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You certainly shouldn't be able to put a .02 feeler under it even at 30mm means you got Chinese or indian granite.
as for granite a 30mm sheet 3 metres long will bend 30mm or more.
lapping will bring a higher polish but will wear the softer parts of the granite.


If you are near Marlborough Wiltshire Give me a call I,ll give you a couple of pieces for free at 30mm or 40mm.
Google: ramsbury stone for contact details

Did you not see the date on this post.? . . . . 2008.!! .. . Lol

No just aw latest activity 4 hours ago, RAFLAO thanks for letting me Know.

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?

I checked some 30mm thick pieces of granite maybe 1m long at my local stone place and found about 0.8mm variation in height. Mind you they didn't surface/polish it themselves, it was bought in.

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.

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Originally Posted by Tenson

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?

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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.

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.

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Originally Posted by Tenson

Interesting, thanks. Was it fully supported?

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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.

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?

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Originally Posted by Tenson

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

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

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.

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.

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Originally Posted by Tenson

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

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About as straight as Philip Schofield:hysterical:....

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.

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I have 6 Compositor stones (two together attached) which, in my humble opinion, are about as good a surface as you can get, my company used them for die making, where flatness and consistency of height is required for making the best tools

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Originally Posted by JAZZCNC

About as straight as Philip Schofield:hysterical:....

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.....or the people in the government doing the procurement during the pandemic :disgust:

Update!

I got 30mm thick quartz, 1m x 0.6m and 2m x 0.6m

I created a steel support frame with screws around the top that I can adjust to support the quartz nicely.

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I got myself a nice straight edge and I'm glad I did!

So far I only mounted the 1m piece, but the results are that it is pretty flat on the longer axis but cupped on the shorter axis. Damn, it's almost 1mm!

I expect it was flat when created as it comes in a far larger sheet, but I'm not sure if this cupping is due to storage or internal stress released when cut. Being cast, I expect the former.

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Now I'm wondering how much I can help the situation by supporting it only at the middle points and clamping down on the outside edges. It has helped flatten it while the clamps are there.

I know quartz can sag if not well supported, but I don't know how long this might take?? Days, months? Any ideas?

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If that slab is only supported at the outer edges I would be very careful what you put on the top as it won't take much of a knock for it to crack.!

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Originally Posted by JAZZCNC

If that slab is only supported at the outer edges I would be very careful what you put on the top as it won't take much of a knock for it to crack.!

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Its supported around the perimeter, but inset about 10cm.

When I worked with Corian,and similar, worktops we dreaded having to lift them once a sink cutout was in place.They were prone to cracking or even breaking from the corners of the cutout to the edges.We made a point of carefully sliding a similarly sized piece of plywood or MDF under them.I would think you ought to be providing a lot of support to a sheet of that size as objects of any significant weight may cause deflection.For aeromodelling,it may not be a consideration.

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Originally Posted by Tenson

Its supported around the perimeter, but inset about 10cm.

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No where near mate,you need to fully support it no matter what you will use on it.

Phill

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Originally Posted by Tenson

Its supported around the perimeter, but inset about 10cm.

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I asked my mate who makes counters from Granite and quartz about using quartz unsupported and said wouldn't recommend it, just one tap in right place and it could shatter like glass. Apparently, it can crack under its own weight if wide enough and not supported well enough.

It's supported as below. Top is 1m x 0.56m and 30mm thick. Most quartz worktops are 20mm as it is stronger than pure granite or corian.

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