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



## matt_southward (2 Oct 2013)

Hello all, this is my first post - so be gentle 

I have lurked on here for a few weeks and have been really impressed by the content and advice, and general banter. Having recently started a re-skilling of my oldish bones (I'm 42) into furniture making I've found the info on here invaluable already - so thanks for that.

Anyway, introductions aside - onto my questions: 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. How significant a deviation is this in terms of it's use? Should I get an exchange? I picked up a piece of granite tile for a fiver from Topps tiles and that's flatter! I'm completely new to scary sharpening (but it does look good), up to now I've messed about with waterstones - with mixed success.

Thanks, Matt


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## Phil Pascoe (3 Oct 2013)

Test your straight edge against another one - float glass doesn't deviate that much.


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## matt_southward (3 Oct 2013)

Thanks for the reply. This is a brand new Veritas steel 24" straight edge - it's been out of the packet for 2 days - do you really think it would be out by that much?


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## Mike.S (3 Oct 2013)

Suggest you hold your glass against a pane of float glass e.g. in a window. If its flat then it should almost stick to it with suction but any crowning or twist should reveal itself. If faulty then I'm sure Workshop Heaven will be happy to replace it.


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## Jacob (3 Oct 2013)

matt_southward":360k7ixf said:


> Hello all, this is my first post - so be gentle
> 
> I have lurked on here for a few weeks and have been really impressed by the content and advice, and general banter. Having recently started a re-skilling of my oldish bones (I'm 42) into furniture making I've found the info on here invaluable already - so thanks for that.
> 
> ...


You have to bear in mind that scary sharp (and other modern methods) can be a bit ritualistic and obsessive about details. In reality your un-flat piece of glass won't make the slightest difference to your sharpening, but if your tile is flatter why not use it. Anything flattish will do, a piece of mdf frinstance.
Similarly with your straight edge - the closer you look the more inaccuracies you will find. Simpler to make your own from wood; as accurate as you can is accurate enough, but don't look to closely! And it's an exercise in making things straight, independently of bought gadgets and devices.


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## Cottonwood (3 Oct 2013)

If you want FLAT then I sudgest you get a cast iron AAA grade reference plate off ebay. Otherwise dont worry about it, use the topp tiles. FLAT is like the scarlet pimpernel, elusive, the more you try to find it the further away it gets. :shock: £64.50 for a piece of non FLAT glass and a few sheets of sandpaper..... =D> Well, anything with 3M wrote on it will cost an arm and a leg. Actually I'd be inclined to send the whole kit back, stick with the topp tile and emory paper off ebay, to see if you like that method. That has worked for lots of folk before the £££'s industry took over.....The thing I have found with sharpening is dont worry about it, its not a quasi religious, esoteric existential excercise (homer)


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## Dangermouse (3 Oct 2013)

I use a piece of polished granite offcut from my local stone mason. Got it for a fiver and then use ordinary wet and dry paper down to 2000 grit, everything I sharpen on that cuts like a laser beam though butter.


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## Random Orbital Bob (3 Oct 2013)

or.....just get a Tormek


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## Bigus (3 Oct 2013)

Cottonwood":3l5ohgiq said:


> If you want FLAT then I sudgest you get a cast iron AAA grade reference plate off ebay. Otherwise dont worry about it, use the topp tiles. FLAT is like the scarlet pimpernel, elusive, the more you try to find it the further away it gets. :shock: £64.50 for a piece of non FLAT glass and a few sheets of sandpaper..... =D> Well, anything with 3M wrote on it will cost an arm and a leg. Actually I'd be inclined to send the whole kit back, stick with the topp tile and emory paper off ebay, to see if you like that method. That has worked for lots of folk before the £££'s industry took over.....The thing I have found with sharpening is dont worry about it, its not a quasi religious, esoteric existential excercise (homer)



I just went to my local glass works and asked them for a very flat 18x16" piece of glass and they cut me some 6.4mm laminated for just under a tenner which is spot on flatness wise.

My scary system is based on this chaps:

http://www.wwgoa.com/no-tech-sharpening-system/

I already had a honing guide from an oilstone kit I bought for a few quid off eBay, some sheet rubber to stop it sliding around and got some 3M PSA / honing fluid from Workshop Heaven.

Got to assemble it yet but if it works well then I might try to find a cheaper sandpaper than the 3M stuff.

Bigus


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## bugbear (3 Oct 2013)

Jacob":kx164qt4 said:


> matt_southward":kx164qt4 said:
> 
> 
> > Hello all, this is my first post - so be gentle
> ...



He''s paid for straight - why should he accept anything other?

BugBear


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## Jacob (3 Oct 2013)

bugbear":1iwepm7x said:


> Jacob":1iwepm7x said:
> 
> 
> > matt_southward":1iwepm7x said:
> ...


Certainly, send it back. All the kit in fact. And the straightedge. Non of them are needed.

PS Matthew's site says scary sharp is "affordable" which implies cheap. 
This is misleading - the system is probably the most expensive of all. Look at the price of honing fluids - more expensive than very good malt whisky! 
If you want to spend, spend, why not use whisky as honing fluid? Bells will do, and save you money. 10 YR old Laphroaig is cheaper than Honerite Gold.


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## woodbrains (3 Oct 2013)

Hello,

Why were you not having satisfactory results from the waterstones?

I agree that scary sharp will be more expensive in time, I think even Workshop heaven mention this at some point. The idea is, though that you can get an ultimate edge quickly and for only a little money initially. Your glass should be flatter than it is, though. The only advantage that I can see with scary sharp is you can always rely on a flat surface as there is no wear involved to the glass, granite, mdf whatever. If it is not flat in the beginning though, all bets are off.

Mike.


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## Jason (3 Oct 2013)

Unlike water stones and oil stones you can't flatten glass. So you need it flat from day one.


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## J_SAMa (3 Oct 2013)

0.75mm sounds like a lot. Veritas straight edges should be quite straight as they have good quality control. I'd suggest you find a piece of flatter glass, almost any float glass will work (the one you got is probably a 1 in 100 exception), including mirrors and windows.

That said I'd suggest you move on to sharpening on stones followed by leather sooner or later. Keep using 3M sandpaper and you'll have spent too much money before you realize it. I'd say 2 diamond stones+thin leather with a bar of honing compound. They'll last at least 10 or 15 years 
If you don't want to spend that much on diamond stones then go for a Japanese waterstones. A 240/1000 combination stone will cost you half the price a diamond stone does. Problems are you'll have to some something to flatten them (or just buy two separate stones and flatten them with each other) and that they don't last as long. 
Sam


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## matt_southward (3 Oct 2013)

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.


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## Wiley Horne (3 Oct 2013)

Hello all,

Just one small point. Float glass. Float glass is not a specialty glass. All window glass is float glass. That's how they make it. English man named Pilkington invented the process back in the 1950's. Everywhere you look, you see float glass. If you google 'float glass', you find info like this:

http://www.glassforeurope.com/en/indust ... rocess.php

If you go to your local glass shop and get an off cut out of their shorts pile, it'll be float glass. Now there's tempered and untempered--if you can find salvaged glass shelves from a refrigerator, they're tempered. But it's all float. 

Wiley


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## nanscombe (3 Oct 2013)

How about using something like the glass out of a photo frame or would that be too fragile?


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## woodbrains (3 Oct 2013)

Hello,

But in theory, the molten tin the glass is floated on should be pretty flat due to gravity, so float glass is generally reliably flat enough for what we need. Tempered glas however, has been heated again and cooled rapidly, which makes it less flat. I think laminated glass is probably best, in fact and a bit safer than float if you drop it with wet hands.

Mike.


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## Cottonwood (3 Oct 2013)

woodbrains":1api2g2j said:


> Hello,
> 
> But in theory, the molten tin the glass is floated on should be pretty flat due to gravity, so float glass is generally reliably flat enough for what we need. Tempered glas however, has been heated again and cooled rapidly, which makes it less flat. I think laminated glass is probably best, in fact and a bit safer than float if you drop it with wet hands.
> 
> Mike.



I've said it before and will say it again, tempered (toughened) glass is _never_ flat because of the way it gets processed in the kilns and cooled afterwards. If you were to put your vertitas edge accross it, you would sometimes almost fit a matchstick in the hollows. 
If your getting a laminated float piece, better check what they laminated it in, how FLATT was the press and how even was the pressure to squeeze the 2 panes together to get an even film and elinimate air bubbles. We used a clear UV curing gel adehsive to join glass to glass. Obviously for upvc window applications mega flatness isnt critical, but if you want sharpening FLAT it might be a no no....


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## matthewwh (3 Oct 2013)

Hi Matt,

I would run some simple tests before you send anything anywhere. 

The Veritas aluminium straightedges are only made to a tolerance of 3 thou, if my memory serves correctly. This is quite broad, you couldn't use it to check the flatness of a plane sole for example. Your glass should be substantially more accurate than the straightedge, and neither should be out by as much as 0.75mm so it becomes a question of what is being checked against what?

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. If you lay the glass and the straight edge on the countertop and apply pressure at one end you should be able to use a feeler gauge (the corner of a piece of paper would do) at the opposite end to establish which of the two items has a big gap under it. 

The point of using plain float glass is that it is very, very, flat, at 10mm thick it is also very stiff. Toughened or laminated glass goes through further heat treatment which removes the natural accuracy of plain float glass.


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## Cottonwood (3 Oct 2013)

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

Theres the rub (hammer) Its all relative.


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## Cottonwood (3 Oct 2013)

Wiley Horne":22hb7l72 said:


> 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..


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## Phil Pascoe (4 Oct 2013)

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


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## bugbear (4 Oct 2013)

matt_southward":pi3e0r3j said:


> 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


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## Sheffield Tony (4 Oct 2013)

matthewwh":3g6otosg said:


> 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 !


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## Cottonwood (4 Oct 2013)

phil.p":27n9nk2j said:


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


I know it!


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## Jacob (4 Oct 2013)

Sheffield Tony":dzkdw0gl said:


> matthewwh":dzkdw0gl said:
> 
> 
> > 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.
> ...


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


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## Scouse (4 Oct 2013)

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...


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## Jacob (4 Oct 2013)

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!


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## Cottonwood (4 Oct 2013)

Jacob":2grint1x said:


> 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)


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## bugbear (4 Oct 2013)

Cottonwood":34j85ia8 said:


> Jacob":34j85ia8 said:
> 
> 
> > 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.
> ...



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


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## Corneel (4 Oct 2013)

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.


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## bugbear (4 Oct 2013)

Corneel":14frqn7e said:


> 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


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## Corneel (4 Oct 2013)

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


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## Phil Pascoe (4 Oct 2013)

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.


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## J_SAMa (4 Oct 2013)

matt_southward":2twrbvai said:


> 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.
> 
> ...



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


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## matt_southward (5 Oct 2013)

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:


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## iNewbie (5 Oct 2013)

matt_southward":e13n1r1x said:


> 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...


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## Jacob (5 Oct 2013)

matt_southward":fqvk7o9n said:


> 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.


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## Jason (6 Oct 2013)

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


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## Jacob (6 Oct 2013)

Jason":2wzrf3i0 said:


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


No I don't bother. I flattened an oil stone once and found it suddenly difficult to get a camber on a blade. Haven't bothered since. Have to pay special attention to shoulder/rebate plane blades which really need to be straight and square as they cut right to the corner, but it's not a problem.


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## Jason (6 Oct 2013)

What about the backs of chisels?


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## Phil Pascoe (6 Oct 2013)

As long as the leading edge is shiny, they're not bad... Jeez, how long is this thread going to run?


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## Jacob (6 Oct 2013)

Jason":12lv8ok8 said:


> What about the backs of chisels?


What about them?


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## David C (6 Oct 2013)

Now we know why so many old blades are bellied.

Hollow oilstones.

I once read that "only a careful craftsman would take the trouble to keep his stones flat".

How true.

David


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## Jacob (6 Oct 2013)

David C":31mj1lsu said:


> Now we know why so many old blades are bellied.
> 
> Hollow oilstones.
> 
> ...


1 My blades aren't bellied. (I am, but my chisels aren't!)
2 It's perfectly easy to take off the burr on a less than perfectly flat stone (within reason) without "bellying" it. 
3 I keep my stones flat (ish) by attempting to use the whole surface.
4 For most purposes it doesn't matter a jot if chisel faces are slightly convex, but mine aren't unless they arrived in that condition
5 You shouldn't take everything you read in books too literally.There are too many would-be experts wasting peoples' time by spreading alarm and despondency about how to do things! Witness - this thread and loads of nonsense about flattening.


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## bugbear (7 Oct 2013)

Jacob":24vynhyc said:


> I flattened an oil stone once and found it suddenly difficult to get a camber on a blade. Haven't bothered since. Have to pay special attention to shoulder/rebate plane blades which really need to be straight and square as they cut right to the corner, but it's not a problem.



Neither is making a convex edge on a flat stone. It's so easy that beginners often do it without meaning to. In your case, use the same technique you use to sharpen your strongly convex scrub plane blade on a mildly hollow stone. The relationship is the same; the stone is less curved than the edge shape you're aiming for. I hope this tip helps you.

I assume that your straight edges (when needed) are worked on the flat regions of your stones (ends/corners) avoiding the hollow in the middle, which would obviously make the edge convex?

BugBear


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## bugbear (7 Oct 2013)

matt_southward":1nwgzqhy said:


> 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).



Is it hollow on the opposite side?

BugBear


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## matthewwh (9 Oct 2013)

Out of interest as much as concern for quality control, I have inspected 30 sheets of float glass this morning against a numbered Bowers Metrology straightedge and a Moore & Wright 1.5 thou feeler and have not been able to get the feeler under on any of them at any point.

Matt, if you would be kind enough to give me a call, we will happily arrange for your glass to be picked up at your convenience and sort you out with a refund or replacement.


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## bugbear (9 Oct 2013)

matthewwh":r7m2hv1l said:


> Out of interest as much as concern for quality control, I have inspected 30 sheets of float glass this morning against a numbered Bowers Metrology straightedge and a Moore & Wright 1.5 thou feeler and have not been able to get the feeler under on any of them at any point.
> 
> Matt, if you would be kind enough to give me a call, we will happily arrange for your glass to be picked up at your convenience and sort you out with a refund or replacement.



Indeed - that's rather interesting.

BugBear


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## MMUK (9 Oct 2013)

phil.p":qeq7513x said:


> float glass doesn't deviate that much.




Having worked within the glass industry I can tell you that just isn't true. Saint Gobain will pass QC on a full 3m x 1.5m sheet with a 5mm deviation and up to 0.5mm thickness deviation. Glass industry QC isn't up to the standards of other industries.


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## J_SAMa (9 Oct 2013)

bugbear":36h10gzf said:


> Corneel":36h10gzf said:
> 
> 
> > 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.
> ...



This is unrelated, but why is it that the lower edges of old windows are often thicker than the rest of the glass?


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## Jacob (9 Oct 2013)

J_SAMa":20zxsn2g said:


> bugbear":20zxsn2g said:
> 
> 
> > Corneel":20zxsn2g said:
> ...


Because that's how they were fitted - if the variation was conspicuous. Stronger the way up. Imagine an exaggerated section shaped like an onion. It'd rest happily on the fat end but be very weak on the thin end. Some pieces of crown glass can be from 5mm (or more) down to 1mm.


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## matthewwh (9 Oct 2013)

Going back to the original question, how flat does it need to be?

If Matt's piece is out by a 3rd of a mm over 360mm and I sharpened one plane iron on it and another on a different piece would anyone be able to discern which plane iron was which?

I'm still happy to change it / refund it etc Matt, this is purely academic, but on one hand we have Jacob happily using an oilstone that could double as a spoon and on the other we are worrying about the thickness of a Sainsburys receipt over 360mm.


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## matt_southward (10 Oct 2013)

Sorry, I didn't realise this thread had continued somewhat!

In answer to bugbears question - yes, with my admittedly basic measuring equipment, the opposite side is indeed dished, though not by as much as it is crowned. I have actually sent it back to WH via myhermes (sorry Matthew I was offline a few days and didn't see your post on here). Perhaps when Matthew gets it back he can check it with his equipment and let us know what the deviation actually is. (if you deem it too far out Matthew then perhaps you could refund me the return postage as well?).

Now as to whether it's degree of 'non-flatness' would be a sharpening issue...I don't know, maybe it is all in the mind, but then the mind is a pretty powerful influence on the outcome generally - sharpening voodoo :wink:


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## DTR (10 Oct 2013)

matt_southward":21zib906 said:


> I have actually sent it back to WH via myhermes



Whenever myhermes deliver here it normally gets lobbed over the 6' fence :shock: If it wasn't flat before....


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## matt_southward (12 Oct 2013)

Sheesh - I hope it does turn up in one piece, the OCD in me wants to know how flat it is(n't)! :wink:


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## matthewwh (12 Oct 2013)

No sign of it yet....

I hope you packed it well - video.


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## Phil Pascoe (12 Oct 2013)

MMUK":1sycrilc said:


> phil.p":1sycrilc said:
> 
> 
> > float glass doesn't deviate that much.
> ...


The op thought it deviated by up to .75mm - unless your full sheet was wavy, that would give far greater deviation than that.


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## J_SAMa (12 Oct 2013)

phil.p":1kyov5og said:


> MMUK":1kyov5og said:
> 
> 
> > phil.p":1kyov5og said:
> ...



Workshop Heaven's glass is 360 mm long, and let's say that a 3 by 1.5 m sheet (3000 by 1500 mm) really had a 5 mm deviation along its length:
5/3000*360=0.6 mm
^ that means a 3 by 1.5 m sheet with a 5 mm deviation can yield a 360 mm sheet with 0.6 mm deviation... Sounds close enough to that 0.75 mm estimate...


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## Phil Pascoe (12 Oct 2013)

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.


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## matthewwh (14 Oct 2013)

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.


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## Jacob (14 Oct 2013)

matthewwh":32tjqaaj said:


> ......
> 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.


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## David C (14 Oct 2013)

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

Something is not adding up.

David


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## Jacob (14 Oct 2013)

David C":2p6te6k4 said:


> 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!


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## matt_southward (14 Oct 2013)

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?


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## bugbear (15 Oct 2013)

matthewwh":3d0l5gf9 said:


> 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


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## Richard T (15 Oct 2013)

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


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## Phil Pascoe (15 Oct 2013)

Almost nearly perfect!


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## matthewwh (15 Oct 2013)

matt_southward":a5fbqvrj said:


> 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.


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## matt_southward (15 Oct 2013)

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.


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## Cottonwood (16 Oct 2013)

phil.p":14oa7743 said:


> Almost nearly perfect!



Imperfectly perfect?
or 
Perfectly imperfect?


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