Granite, flatness, and straight edges

[GraemeVW]

Trying to get a bit of a feel for what's considered straight.

Looking, for arguments sake, at 1m straight edge.

Maun say their tolerance is 0.1mm per meter. That sounded alot, but what do I know.

Quick look for old ones and a Moore and Wright one I say was grade A, which should be the same as grade 0 I think. Which had a tolerance of 2+(L÷100). The resulting figure is in microns.
So, a 1000mm rule, divided by 100, plus 2 is 12 microns. 0.012.
A fair bit better than 0.1 then.

Do I need that accuracy? Probably not. But 0.1mm seems large.

Is a second hand, 70 year old, grade A straight edge still going to be anywhere near it's tolerance? Who can tell! It might be, it might not.

Seems you either stump up a few hundred quid for a new one with guaranteed accuracy, or you just buy whatever and stop thinking about it.

 

[GraemeVW]

Classical way of generating REALLY flat surfaces, get out the spotting blue and lap them against each other - but supporting them so they didn't deflect under their own weight would be tricky... look up Airy Points.

Alternatively... live with it, and/or assess with a LASER level or similar - there's a few ways, one is to use a horizontal (like the levels produce) grazing the surface projeting onto a screen - i've never gone that far though...

Dave H. (the other one)

I have a laser level, I could have a play I guess.

This whole thing just made me realise I have no reference for anything and it hasn't really occurred to me before.
Most accurate items I own are some old engineers squares, but they have been kicked around workshops by various owners for 70 years so only any good for basic woodwork.

I could probably do with at least 1 thing that I know is straight, or flat, or something.

 

[hotponyshoes]

0.1 would need holding up to the light to see as a gap between two edges.

Last cylinder head I checked I did with an uncertain stright edge as the workshop manual said the allowance was 0.2mm

 

[zzr1200]

I guess the most serious it would get is things like checking manifold flanges or engine covers for flatness.

A piece of plate glass 6+mm with a sheet of fine emmery paper as the cutting surface, many many an old bike head re-surfaced that way.