Thread: Measure twist with a pendulum ?

Originally Posted by RayG

You should check out the price on a 300x230x50 chinese granite surface plate, I don't know about the UK, but here they sell for around $70 http://www.carbatec.com.au/granite-surface-plate_c20499

Floor tiles are best left on the floor.

Ray

PS.. This one looks to be the UK equivalent, http://www.axminster.co.uk/small-granite-surface-plate

Thanks Ray,
I've seen that one, they are very nicely presented. But in all honesty, for the amount of use my tile cum surface plate gets, I can think of other things I'd rather spend £40 on.

Originally Posted by BaronJ

Thanks Ray,
I've seen that one, they are very nicely presented. But in all honesty, for the amount of use my tile cum surface plate gets, I can think of other things I'd rather spend £40 on.

Hmmm... I'd disagree, there's just too many things you can't measure without a flat surface as a reference. Like testing your level, for instance, one critical test is to flip the level 180 and see that it reads the same, that test won't work on a bumpy surface.

I know you are going to try and tell me that your porcelain tiles are flat, I can be 100% confidant that they are going to be nowhere near the sort of flatness that's needed.

The measure of flatness is the distance between two parallel planes that will completely enclose the surface, in the case of that cheap surface plate that distance between the planes going to be about 5 microns or so.

Also everything flexes, glass porcelain, granite, cast iron.. For a granite surface plate the thickness required to meet the standard depends on the plate size, for a small plate, you still need it to be fairly thick, a few inches at least, and even then the surface plate ideally needs to be mounted on the same three points used when calibrated to make sure it's still accurate, because of the flexing of the granite. So you can understand that I am extremely skeptical that a thin porcelain tile is going to be anywhere near rigid enough for reproducible results. As soon as you put anything on it it will flex and distort.


Ray

Originally Posted by RayG

Hmmm... I'd disagree, there's just too many things you can't measure without a flat surface as a reference. Like testing your level, for instance, one critical test is to flip the level 180 and see that it reads the same, that test won't work on a bumpy surface.

I know you are going to try and tell me that your porcelain tiles are flat, I can be 100% confidant that they are going to be nowhere near the sort of flatness that's needed.

I wouldn't dream of it.

The measure of flatness is the distance between two parallel planes that will completely enclose the surface, in the case of that cheap surface plate that distance between the planes going to be about 5 microns or so.

Also everything flexes, glass porcelain, granite, cast iron.. For a granite surface plate the thickness required to meet the standard depends on the plate size, for a small plate, you still need it to be fairly thick, a few inches at least, and even then the surface plate ideally needs to be mounted on the same three points used when calibrated to make sure it's still accurate, because of the flexing of the granite. So you can understand that I am extremely skeptical that a thin porcelain tile is going to be anywhere near rigid enough for reproducible results. As soon as you put anything on it it will flex and distort.


Ray

I absolutely agree. Whilst a good surface plate would be nice to have, I haven't got around to working to the level of precision that would need the use of one.