Plate Glass Flatness

[Kevin in Keswick]

Good morning,
I am going to be heading down to my shop not long after posting this but I would like post this question/thought that I have wondering for a while.
I know alot of comments I have heard about the Norton Flattening stone have been negative but my experiences with it have been quite positive, but am now at the point where the flattening stone needs to be flattened. I have read alot of comments about just using a piece of plate glass with snadpaper adhered to it to flatten waterstones but it just seems to expensive in my experiences with it. In this case though I would like to use this method for flattening my flatenning stone.

I have only ever heard that plate glass is flat and can be used for this application. Currently, I am lucky enough to have obtained a large 1/2" thick piece of tempered arena board glass that probably measures around 36x36. Would this glass be flat enough for flattening stones? I am assuming it will be. Is there any clear plate glass that would be flat enough for this or is it basically all flat because of th eway it is manufactured.


Would really appreciate your thoughts on this,
Kevin

 

[WellsWood]

I could be wrong, but I'm pretty sure I remember reading once that float glass is inherantly flatter than tempered - something to do with the re-heating of glass during the tempering process reducing its flatness.

 

[TheTiddles]

If you go to your glazier and ask what thick float glass they have in offcuts you will probably find they will be willing to part with it for very little, I have a variety of sizes none of which cost me more than 50p

Aidan

 

[Woody Alan]

It may be flat but even that thickness will bend if not fully supported by a "flat" :roll: surface. That could of course be your workbench with a piece of mdf on. There is a reason granite flat blocks are 3 inches thick and more if you pay the price.

Alan

 

[bigjoe]

Normal annealed float glass is flat to within a very fine tolerance although i would always lay it on a sheet or mdf or similar,toughened glass is never as flat as by its process it is put into stress during toughening process and it looks rippled when reflecting light off it.

Bigjoe

 

[dchallender]

A lot of model engineers use plate glass / floating glass as a cheap alternative to a surface plate, as mentioned you do have to keep it supported though.

There are different ways of doing this but the ones I have used include making a small tray for the glass to fit into and then filling it with sand and then resting the sheet on top so it is fully supported. Or do exactly the same but instead of sand use silicon and 'glue' the thing in place.

In terms of it's flatness you can get it accurate to about - equal or less than 0.001" I used to lap in compressor valves when i was in the navy on a piece of glass so you should be ok :lol:

The ripple effect you often see on plate glass is often due to the thermal imprint of the stress rather than a physical dimensional difference, similar to the effect that you get in tempered steel.

Hope that helps

Darren

 

[jonnie]

Hi

Float glass should be the flatest I believe.

Regards support - the glass "wants" to lie flat but it has its own weight to contend with.

The glass should achieve optimal flatness when supported by a substrate which offers the same resistance per unit area regardless of the amount of depression, carpet on an uneven floor is no good - you sink into carpet until the upward force on the soles of your feet balance your weight - the resistance depends on the depth of depression - not so good.

The ideal support in this respect is a large air cushion - it can sit on a slightly bumpy or uneven surface yet the pressure witihin is uniform - this means that regardless of how it deforms to the ground or work bench the top layer of fabric or rubber can remain flat ( since it is in contact with the glass ) and since the pressure is uniform then the glass is evenly supported.

Just how close an airmatress or other readily available air bag conforms to the ideal is another matter - over inflation will not be a good idea as in the limiting case the air cushion will adopt a shape more dependent on its seam lines etc, I think a loose floppy level of inflation will suffice, ideally the air cushion should also be about the same size as the glass so as to avoid edge effects.

An under inflated overnight matress should be pretty good but ideally it should be the same size as the glass - if the matress is much larger and the glass "sinks in" then the edges will receive additional "pull".

I reckon sand as suggested by someone else might also form a good bed for the glass, as long as it is dry and not compacted and free flowing then I think it will support the glass with equal upthrust everywhere, ideally you would want to agitate the sand so that it does flow underneath the glass, perhaps just repeated lifting, raking and reseating would eliminate any high points.

I am interested in this as I do not have a 1m x 2m slab of flat granite but I do have a sheet of toughended glass ( nonfloat so not as flat ) this size and I need a good reference surface this size to align some precision parts. Air bag support is probably not a whole lot of use if you want an active work surface but for passive alignment issues its worth a thought - of course if the components placed on the glass have a small footprint or are sufficient to deform the glass then you need thicker glass or its time to abandon this method.

 

[marcros]

might be better starting a new thread on this Jonnie- the OP is from 2008!

 

[Jacob]

Or just get out of the flattening fashion thing altogether and save yourself a lot of bother. It's not necessary and nobody used to do it a few years back (within reason) - a piece of wet n dry on a board being the most you'd need.
It's a by product of the honing jig - they don't work too well unless everything is flat and then plane blades end up with dead straight edges which is a boogger to plane with unless you only do edges narrower than the plane.

 

[SammyQ]

1. "Or just get out of the flattening fashion thing altogether and save yourself a lot of bother. It's not necessary and nobody used to do it a few years back (within reason) - a piece of wet n dry on a board being the most you'd need.".

Boards have tension - a stark, undeniable fact. That means they can be uneven. They can also be soft and therefore deform under point loading such as you get at the tip of a blade being sharpened. Glass etc do not have these disadvantages to anything like the degree wood deflects.

2. "It's a by product of the honing jig - they don't work too well unless everything is flat and then plane blades end up with dead straight edges which is a boogger to plane with unless you only do edges narrower than the plane."

"dead straight edges "..................wot? On a plane? Surely not! I prefer mine as curved as a political statement!

C'mon Jacob. Consult your copy of "Planecraft" and every article using it as a source since: "straight edge, curved corners, unless it's a scrub, in which case curved is good" (paraphrased).



i have no intention of indulging in a flame war over sharpening. Frankly, I'd rather be sodomised by a diesel locomotive. Equally, I have no intention of letting an alternative, perfectly acceptable way of creating be rubbished because it seemingly is outside the rigidly applied code as established, sustainedly trumpeted and argued sophistically, tediously and perpetually by someone who should know better.

Fellas, there is more than one way to skin a cat. If, like me, woodwork does not supprt you, a honing guide helps replace the work hours spent establishing muscle memory. If you do earn your crust by shaving and carving and have acquired (through years of practice) the rhythms and stances needed to sharpen freehand consistently, then, by all means do it. JUST DON'T BL**DY WELL THINK YOU ARE THE ONLY RIGHT ONE ON THE PLANET!

Sam, looking for the 'don't feed the Troll' sign he mislaid.

 

[Racers]

There are only two ways, Jacobs way and the wrong way.


Pete

 

[Jacob]

Only trying to be helpful and save him a lot of bother. People do seem anxious to suppress alternative ideas. :roll: Not that alternative either - is pretty universal and I'm not the only one.

...., a honing guide helps replace the work hours spent establishing muscle memory. If you do earn your crust by shaving and carving and have acquired (through years of practice) the rhythms and stances needed to sharpen freehand consistently, then, by all means do it. JUST DON'T BL**DY WELL THINK YOU ARE THE ONLY RIGHT ONE ON THE PLANET!

Sam, looking for the 'don't feed the Troll' sign he mislaid.

Wos all this "muscle memory rhythms and stances"? You learning to tango or something? Not sharpening as I know it, which should take a about 20 minutes to get the idea and another 20 to perfect.

 

[Racers]

I have a dyspractic son who would really struggle to learn sharpening in 40 minutes, we aren't all the same Jacob.

Pete

 

[Cottonwood]

Toughened glass is not flat. The float glass (which is "flat"-whatever that means to the individual in a mathematical, philosophical or metaphysical sence), is heated up with a furnace until it starts to glow, by which time it is soft like toffee. It runs back and forth over fireproof ceramic rollers so it doesnt sag in one place like pastry in a dish. After a few minutes, it gets quenched very suddenly with a huge blast of cool air from above and below. This hardens the outer surfaces. The inside (still soft after the initial cooling blast) is tempered with a further blast of air. Despite the action of the rollers, it is inevitable that you are left with small ridges and valleys, it is impossible to recapture the pristine float flatness.
If I wanted to use glass (I dont) I would maybe set a piece inside a rigid steel frame with a concrete infill base, and a self levelling screed between the glass and concrete.

 

[Bale]

The last time I flattened an oilstone I did it with a slurry of builders' sand and water on a patch of concrete. No need for glass, plate or float.

Pete

edit - I thought the OP was referring to Norton oilstone - doh! Sand on concrete could still work though. I can't imagine that wet and dry paper would have much impact on coarse silicon carbide, or have I got this completely wrong?

 

[Graham Orm]

Or just get out of the flattening fashion thing altogether and save yourself a lot of bother. It's not necessary and nobody used to do it a few years back (within reason) - a piece of wet n dry on a board being the most you'd need.
It's a by product of the honing jig - they don't work too well unless everything is flat and then plane blades end up with dead straight edges which is a boogger to plane with unless you only do edges narrower than the plane.

Conti board or mdf on a flat bench with wet n dry works perfectly. Could have had it done in the time it took you to write the post :wink: If anyone can show me a piece of modern glass that's not flat enough to use I'll show my a**e in Burtons window.

 

[bugbear]

Not sharpening as I know it, which should take a about 20 minutes to get the idea and another 20 to perfect.

Steve Branam tried your idea for 2 years, and the poor schmuck still getting poor edges until Deneb Puchalski put him right.

BugBear

 

[Jacob]

Er, so what?

 

[bugbear]

Er, so what?

So your brash assertion of a method that can be learnt 20 minutes is shown to be yet another overstatement,

BugBear

 

[Jacob]

Er, so what?

So your brash assertion of a method that can be learnt 20 minutes is shown to be yet another overstatement,

BugBear

He's in America somewhere. If he'd dropped in here I could have shown him how to do it.
He was a bit too fussy and cautious judging by his video. You have to relax and not take it seriously. See cottonwood's post here post797134.html#p797134

Not everybody can get it quickly. Most people can but some can't. I guess you can't! :lol: :lol:

Fussiness, complexity, caution have become the norm for modern crazy sharpeners. Have a look at the sheer difficulty of using a Kell jig (one in the for sale section with photos). De-programming the brain washed is sometimes not easy!