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

[Cottonwood]

"so it becomes a question of what is being checked against what?"

Theres the rub (hammer) Its all relative.

 

[Cottonwood]

If you go to your local glass shop and get an off cut out of their shorts pile, it'll be float glass.

If your able to find a glass shop that has toughened (tempered) glass offcuts in the skip, you did better than Tommy Cooper... :arrow: That is why upvc (vinyl) d/g unit's have to have the toughened glass made to measure-cut to required size first, then toughend and made up into sealed units, because its impossible to cut toughened glass in the same way float is cut with a diamond wheel and snapped to the line. LOL I once saw a bad batch of toughened panes start to spontanoeusly pop one after the other like a chain reaction, pop, pop, pop; after a minute or so there was a huge mound of tiny square fragments, nothing could be done to stop it..

 

[Phil Pascoe]

:-k He didn't say you'd find tempered glass in the skip.

 

[bugbear]

I received the scary sharp 'starter pack' from Workshop Heaven today (yes I know Matthew posts regularly here) and got out my Veritas straight edge to confirm the flatness of the float glass only to find that it's not flat. As far as I can see it 'crowns' in the middle and my best guess is that it's out on each corner by (in the order of) 0.25 to 0.75mm.

I have had a belated thought.

Glass is normally not only flat, it's parallel. In fact, from the float glass process, I would trust the parallelness more than the flatness, given the relative flexibility of glass.

This leads to the observation that if one side of a piece of glass (A) reads convex, the other side (B) should read concave to an equal extent.

Of course, if B also reads concave, you might very reasonably assume that your straight edge is (in fact) convex, and your glass near enough flat.

BugBear

 

[Sheffield Tony]

A third flat thing is needed - your kitchen worktop is probably flat to better than 0.75mm so it will at least be able to point you in the right direction.

So there you go - forget all the fancy glass plates, just stick some wet or dry onto the kitchen worktop. (If you dare !)

One other thought about the flatness of glass - don't try using an old sealed unit; they are normally concave because they are partially evacuated !

 

[Cottonwood]

:-k He didn't say you'd find tempered glass in the skip.

I know it!

 

[Jacob]

A third flat thing is needed - your kitchen worktop is probably flat to better than 0.75mm so it will at least be able to point you in the right direction.

So there you go - forget all the fancy glass plates, just stick some wet or dry onto the kitchen worktop. (If you dare !)

One other thought about the flatness of glass - don't try using an old sealed unit; they are normally concave because they are partially evacuated !

A bit of mdf is also likely to be flat enough. Good for a laugh though, these crazy sharpening threads. :lol:

 

[Scouse]

But even if you have a flat piece of glass, it will move depending on it's support apparently

plate-glass-flatness-t25931.html

So what I have learned from this is not to worry and that engineering tollerance flatness might be a touch futile outside of engineering and academic interest, especially given the wood you are working will move out of the afore mentioned academic flatness the second you have worked it...

Test the quality of the tool on wood not straight edges.

Just sayin...

 

[Jacob]

Yes the glass will move so it needs turning regularly to balance surface temperature and also supporting on something flat. Frinstance another piece of glass, which will need supporting.... etc etc.
So you have a big stack of plate glass, and turn and shuffle them between each stroke with a honing jig.
Don't drop them!

 

[Cottonwood]

Yes the glass will move so it needs turning regularly to balance surface temperature and also supporting on something flat. Frinstance another piece of glass, which will need supporting.... etc etc.
So you have a big stack of plate glass, and turn and shuffle them between each stroke with a honing jig.
Don't drop them!

And dont leave a bit of sawdust or a shaving in between either (hammer)

 

[bugbear]

Yes the glass will move so it needs turning regularly to balance surface temperature and also supporting on something flat. Frinstance another piece of glass, which will need supporting.... etc etc.
So you have a big stack of plate glass, and turn and shuffle them between each stroke with a honing jig.
Don't drop them!

And dont leave a bit of sawdust or a shaving in between either (hammer)

Interestingly, long ago on OLDTOOLS, someone was doing scary sharp, back by glass, backed by MDF.

There was a price sticker on the underside of the glass.

After a couple of weeks use, the shape of the price sticker could be seen in the wear pattern on the SiC paper.

BugBear

 

[Corneel]

I've heard often that glass isn't really a solid but more like a very viscous fluid. It gives slowly away to pressure. Being no glass expert I don't know how serious this is but it could explain the pricesticker incident.

 

[bugbear]

I've heard often that glass isn't really a solid but more like a very viscous fluid. It gives slowly away to pressure. Being no glass expert I don't know how serious this is but it could explain the pricesticker incident.

No, glass does not flow at normal temperatures. (popular) Urban myth.

If it flowed at the rates people suggest, we'd be finding glass puddles in Egyptian tombs, not glass drinking vessels.

BugBear

 

[Corneel]

Ok then regard my message as utterly nonsence. And pretend it was never written.

 

[Phil Pascoe]

Panes of glass were originally cut from large spun discs, and were obviously lighter and thinner at the outside - the old "bullseyes" that were found in old doors and windows were the remains of that process. They were cheap enough for everyday use, while the flatter stuff was used in churches, manor houses etc. The myth arose because the panes were always fixed with the heavy edge down because it was easier, and people then assumed that it had slumped. Modern "float" glass is so called because it is floated on molten tin in manufacture - there is actually a difference in shine between the top side and the underside, this can sometimes be shown by putting a drop of water on it as the surface tension is slightly different.

 

[J_SAMa]

Thanks for all the replies, and different points of view.

I had almost decided to keep the glass as the underside was a bit flatter, but then that meant that it rocked about a bit on the bench :shock: so I will be sending it back after all.

I certainly don't see scary sharp as cheap! The reason I went with it for the moment is partly that I couldn't afford the initial outlay for some diamond stones (which I'd have preferred), and partly circumstantial in that I wanted something fairly quick to set up for the various things I've got to sharpen. I had a bit of trouble with my combination waterstone because of it's softness - I was sharpening whittling knives on it and kept catching the edges, and so it was getting a bit beat up and uneven. Plus I can only just fit my plane irons on it. As I'm using a honing guide at the moment so that I can get some consistent results for the time being - there's not a lot of room on the stone AND I keep having to true it. Also my 'workshop' is actually a spare bedroom so sloshing water about is also a bit of an issue. So scary sharp was really just an interim way of getting my tools ready for work whilst I'm learning all of this. When I can afford it I intend to get some decent quality, decent sized diamond stones, for plane irons, chisels and knives and any 3M paper I have left will be handy for odd shaped sharpening needs.

I wasn't really obsessing over flatness, I was just a bit surprised that the glass was out by so much after all I've read about the flatness of float glass (after a closer look I'd say it's about 0.3-0.5mm out in each corner, though I can't find my feeler gauges to check).

Anyway, it's all part of the learning curve I guess - thanks again for all the input.

Try EZE-LAP. Quite cheap compared to ATOMA and DMT. Buy a 6'' by 2'' 250 grit and a 8'' by 3'' 1200 grit and you're good to go. As for honing compound, one bar usually costs 10 euro/quid, or just use the metal polish that you probably have lying around your garage. BTW I started freehand and only use a honing guide for chisels narrower than 1/4'' (can't keep them square...).
Having some sandpaper lying around is indeed good. Diamond stones will become quite slow after a few months of use and restoring old blades is a nightmare on them. I have to go through this right now with my newly bought spokeshave blade

 

[matt_southward]

Sorry, I've been busy the last few days...

From Ron Hock's 'The Perfect Edge': "if you are ever at a loss for something to talk about with a woodworker, just start talking about sharpening. It's a conversation that can go on an on."

I must admit that when I read that today it made me smile! Anyway, I tried Matthew's suggestion of testing on the kitchen worktop (though for the record Matthew I have a Veritas steel straight edge which I think is supposedly accurate to a thou), but then I realised the stupidity of what I was doing - at this point remember I'm basing what I'm doing on the assumption that I can't trust my straight edge OR glass and then I throw in the extra unknown of a kitchen worktop (is it flat or not - how would I know!!). Then I had a bit of a Matrix moment where I thought - 'there is no spoon' and gave up :?

Luckily enough, yesterday I received a Veritas glass plate from Axminster and this turned out to be actually flat - that is unless those guys at Veritas are making their glass and straight edges out by the same amount. So I'm now pretty sure the float glass is crowned as I first thought, and so I'm concluding that you can't just rely on float glass being flat by way of it's production process after all (and my final best guess based on the thickness of Sainsburys receipt paper (all I had) is about 0.3mm - so down a bit from my first wild, half-blind guesses). Now whether or not it's lack of flatness would affect it's actual performance...this thread at least has taught me not to go down that rabbit hole :lol:

 

[iNewbie]

So I'm now pretty sure the float glass is crowned as I first thought, and so I'm concluding that you can't just rely on float glass being flat by way of it's production process after all (and my final best guess based on the thickness of Sainsburys receipt paper (all I had) is about 0.3mm - so down a bit from my first wild, half-blind guesses).

I doubt the manufacturer relies on it 100%, either - and most people at sometime have bought something thats slipped through a companies quality control dept...

 

[Jacob]

Sorry, I've been busy the last few days...

From Ron Hock's 'The Perfect Edge': "if you are ever at a loss for something to talk about with a woodworker, just start talking about sharpening. It's a conversation that can go on an on."

I must admit that when I read that today it made me smile! Anyway, I tried Matthew's suggestion of testing on the kitchen worktop (though for the record Matthew I have a Veritas steel straight edge which I think is supposedly accurate to a thou), but then I realised the stupidity of what I was doing - at this point remember I'm basing what I'm doing on the assumption that I can't trust my straight edge OR glass and then I throw in the extra unknown of a kitchen worktop (is it flat or not - how would I know!!). Then I had a bit of a Matrix moment where I thought - 'there is no spoon' and gave up :?

Well you missed a trick there. You have three straight (ish) edges to compare/contrast (glass, veritas, worktop). This is the way to ultimate straightness!! 2 not enough, 4 a crowd.
http://home.comcast.net/~jaswensen/mach ... _edge.html

and is howwould let you
Luckily enough, yesterday I received a Veritas glass plate from Axminster and this turned out to be actually flat

How do you know? Personally I doubt it very much.

..... So I'm now pretty sure the float glass is crowned as I first thought,

Doesn't matter, won't affect your sharpening, unless it's so far out you can see it with the naked eye. The eyeball is the ultimate arbiter in woodworking - if it looks straight than that is good enough.

 

[Jason]

Jason has cause to wonder if Jacob has ever attended to the flatness of his sharpening stones for any reason...