How flat is a piece of glass?

[justinpeer]

Ok I hope this isn't too daft a question. I'm putting together my sharpening setup and amongst the things I need is the ability to flatten the base of a couple of old planes I inherited from my grandfather, a Stanley no.5 and an old wooden smoothing plane. The sole of the Stanley needs a lot of work and I've read that the best way is various grades of sandpaper on a sheet of plate glass.

Now, in the back of our barn I found a decent size sheet of thick glass, it's 70cmx40cm and ¼" thick, heavy chunk of stuff. Apart from the edges needing taping over as they're sharp and a chip out of one side, would this be flat enough? I've no idea how old it is or what type of glass but it seems flat and I thought most heavy glass is made by float process which is inherently flat.

I also have a spare glass shelf that's thinner, about 3mm, but has ground edges and is marked as safety glass. This piece is very slightly flexible but on a piece of something like mdf would probably also be pretty flat. I'll post a picture in the hope it helps.

 

[julianf]

Float glass is called float as it's floated on top of, iirc, a layer of molten tin.

Id suggest that it is adequately flat for your application.

 

[gcusick]

Agree with @julianf - the float glass is flat enough for this purpose, though it does need to be supported. I’d guess that the thinner toughened glass would be poorer, both beca it is more flexible, and because the tempering process may have affected the flatness.

 

[justinpeer]

Thanks gents, my plan was to put the big sheet of glass on a tea towel for support and work from there. Looks like I have a plan now

 

[Jacob]

I'd use the big sheet on a well supported piece of MDF same size. Don't cut it down, the size is handy.
Best for plane soles is a coarse grit say 80, wet n dry, paper backed. Just plonk it down on the glass in a pool of water or white spirit, splash more on top and off you go with your plane. Keep it wet and after a few passes it lies down flat and stuck to the glass, though you can peel it off and move/reuse it. Bigger plane use two sheets end to end. Cloth backed doesn't stick down and is thicker, less precise. Paper backed and wet is designed for the job.
Having a big sheet of glass means less water or white spirit runs off, if you set it up dead level to start with.
PS you can clean up the glass edges with a diamond pad such as 3m Diapad

 

[justinpeer]

Brilliant, thanks Jacob.

 

[Ollie78]

If its been toughened it will no longer have the flatness of the float. When it's re-heated for toughening it develops a wave. You can see it at an angle.

Ollie

 

[Dee J]

Silly question maybe, but is the lack of flatness causing functional problems in use?

 

[Bingy man]

You can also use a flat tile - (not rippled ) marble or natural stone or porcelain- you can buy 1 tile as a sample from wickes and will only cost you between £5 and £10 a lot cheaper than float glass as with float glass the tile will need support-I used ply wood.

 

[Sideways]

I'll bet if you support the glass at the ends and press in the middle you'll be able to flex it.
Try 1, 2, 3, etc thicknesses of paper under each end and push the middle down until it touches the benchtop.
That will demonstrate the need to support it on something that is at least straightedge flat.

Not knocking the idea by the way. I got an offcut of 8mm float glass from the local glazing co. myself for sharpening.

Another option is a piece of thickish "Ground Flat Stock" - a type of steel available from people like Cromwell. That's precision ground to uniform thickness and usually comes decently flat. A thick piece isn't easy to bend...

 

[Phil Pascoe]

Much as perfectly flat soles are desirable, in the real world a few thou out doesn't matter a jot.

 

[Cozzer]

A few years ago somebody tipped me off about a company on Rutland Road in Sheffield who placed "offcuts" of marble outside their premises.
Sceptical, but seeing I was driving past it a couple of times a week visiting my m-i-l in the Northern General Hospital, I kept my eyes open....and sure enough, there they were!
I nabbed the biggest piece - a full 3cms thick - and nearly gave myself a hernia carrying it to the car!

 

[Orraloon]

Any planes I have flattened I did on the cast iron top of the tablesaw. May not be to the nearest thou but flat enough. I know that wont flex.
Regards
John

 

[Drifter2406]

Should be fine, before you start good old fashioned sand paper for the edges until it's safe to use, (used to work in a fish tank manufacturer and they did it this way all day long) place the glass on a flat service as it will break when you push down on it.

 

[Phil Pascoe]

A diamond plate will do it in seconds.

 

[pgrbff]

Everything I have seen about sharpening on glass has suggested thicker. As as already been said you can often get pieces of 10mm, offcuts, from suppliers for relatively little money. I use both glass and the cast iron top of my saw table. The longer the abrasive area, the quicker you will flatten the sole.

 

[Fordgate1]

Some time ago I visited our local glass supplier, they gave me an A4 size piece of glass, I then used self-adhesive wet and dry paper, from Axminster, used it for my planes. I also sharpen my planner blades.

 

[justinpeer]

Silly question maybe, but is the lack of flatness causing functional problems in use?

Oh the plane's unusable, I think it sat in a box for years. My mum had it after my grandfather died and that's 20 years ago, it's pretty badly rusted but it's an old Stanley, I'm sure it'll come up just fine once it's been sanded clean and flat again.

 

[justinpeer]

Should be fine, before you start good old fashioned sand paper for the edges until it's safe to use, (used to work in a fish tank manufacturer and they did it this way all day long) place the glass on a flat service as it will break when you push down on it.

Ah now that's great to know, thanks very much, I'll do that rather than taping the edges.

 

[justinpeer]

A diamond plate will do it in seconds.

I have one for flattening my Waterstones but I don't think that would be big enough for a long smoothing plane