How flat does the glass need to be for scary sharp?

[Phil Pascoe]

Yes, if the 3mtrs was a series of 360mm pieces - but I'd have thought that a deviation of .75mm would make the surface of the glass an arc, so if you held one edge down to a dead flat surface, I'd have thought the end of the arc 10mtrs away would be more than 5mm off. The error would be accumulative, surely? Please feel free to prove me wrong - it's only conjecture.

 

[matthewwh]

Just got the glass back (whole) and it is straight along the long axis. The curvature is on the short axis starting from halfway across and reaching a maximum deviation from planar of 0.12mm / 5 thou at the edge. The material thickness is consistent - i.e. concavity one side convexity the other, and as Phil mentioned above it does seem to be progressive rather than regular.

It is surprising how visible this is with the naked eye and just by looking at it I would probably have guessed that it was more pronounced than it actually is too. Matt, your refund including shipping should be in your account by the time you read this.

This is the first instance we have ever had of a piece of glass being anything other than immeasurably flat, but it shows that it can happen. The degree of flatness we usually get probably is overkill for the job of sharpening, but it is useful it is to have a decent (and comparatively cheap) reference surface in the workshop to compare things to.

Now that I come to think about it, a known fractionally concave and fractionally convex surface is going to come in very handy too.

 

[Jacob]

......
It is surprising how visible this is with the naked eye and just by looking at it I would probably have guessed that it was more pronounced than it actually is too.....

Not surprised at all! Haven't I been saying (over and over again :roll: ) that if it looks straight/flat than that's good enough for woodwork. Although winding sticks are useful of course.

 

[David C]

If you can see straightness, I'm amazed that you can't see wind.

Something is not adding up.

David

 

[Jacob]

If you can see straightness, I'm amazed that you can't see wind.

Something is not adding up.

David

Yes - you aren't!
Wind on an edge is difficult because the two arrises may well be straight but not in the same plane - the wind is hard to see. Easier on a wide board.
How are you getting on with the inner protractor Dave? Practice, practice!

 

[matt_southward]

Thanks Matthew, for the info and the refund. I guess you could say it's been a interesting diversion into 'flatness' :wink:
Funnily enough I didn't even check it by eye - it never occurred to me, but then I am a newbie to all this and it I guess that'll come in time. I think the convexity was becoming pronounced to me as I was using my straight edge on the worst diagonal and then holding it down on one end (thus doubling the error I suppose - which puts me a little closer with my final guestimate of 0.3mm).

What do you plan on using the glass for now then Matthew?

 

[bugbear]

This is the first instance we have ever had of a piece of glass being anything other than immeasurably flat, but it shows that it can happen. The degree of flatness we usually get probably is overkill for the job of sharpening, but it is useful it is to have a decent (and comparatively cheap) reference surface in the workshop to compare things to.

I guess a straight-edge check will now be part of the shipping procedure...

BugBear

 

[Richard T]

A pessimist may consider his glass to be 'not flat' whereas an optimist may see his as 'nearly flat'.

 

[Phil Pascoe]

Almost nearly perfect!

 

[matthewwh]

What do you plan on using the glass for now then Matthew?

I'm thinking that the convex side (across) might be useful for grinding the backs of chisels correctly, the concave side (along) for initial honing of cambered plane irons a'la Toshio Odate.

 

[matt_southward]

Ah...yes canny thinking - now why didn't I think of that!? It's good that you'll be able to put it to productive use after all.

 

[Cottonwood]

Almost nearly perfect!

Imperfectly perfect?
or
Perfectly imperfect?