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

[Jacob]

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.

 

[Jason]

What about the backs of chisels?

 

[Phil Pascoe]

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

 

[Jacob]

What about the backs of chisels?

What about them?

 

[David C]

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

 

[Jacob]

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

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.

 

[bugbear]

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

 

[bugbear]

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

 

[matthewwh]

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.

 

[bugbear]

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

 

[MMUK]

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.

 

[J_SAMa]

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

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

 

[Jacob]

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

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

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.

 

[matthewwh]

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.

 

[matt_southward]

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:

 

[DTR]

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....

 

[matt_southward]

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

 

[matthewwh]

No sign of it yet....

I hope you packed it well - video.

 

[Phil Pascoe]

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.

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

 

[J_SAMa]

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.

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

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...