How flat is a granite worktop?

**Voicecoil** · 10-05-2021

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.

**Tenson** · 10-05-2021

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.

**Voicecoil** · 10-05-2021

Originally Posted by Tenson

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.

**Doddy** · 10-05-2021

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.

**Tenson** · 10-05-2021

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** · 10-05-2021

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?

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

**Tenson** · 10-05-2021

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.

**JAZZCNC** · 10-05-2021

Originally Posted by Tenson

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

About as straight as Philip Schofield

....

**Voicecoil** · 12-05-2021

Originally Posted by JAZZCNC

About as straight as Philip Schofield

....

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

**Voicecoil** · 10-05-2021

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.