using sandpaper on a glass sheet to sharpen chisels

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Hi everybody,
This video (http://www.woodsmith.com/issues/165/videos/sandpaper - sharpening/) suggests using a 1/2" sheet of glass under different types of sandpaper to sharpen chisels and plane blades.
My question is about the 1/2" thick sheet of glass, because they seem to be so darn expensive. Why should it be 1/2" thick? Is it to eliminate any flex in the glass that would occur if it were less than 1/2" thick?
I ask because I have several cheap glass panes--from picture frames, mind you--that I'm willing to attach together for a TOTAL WIDTH OF 1/2". (Would this suffice?)
Is there something even less expensive than a 1/2" sheet of glass that anybody here would recommend?
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The video is at www.woodsmith.com/issues/165/videos/sandpaper-sharpening/
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The reason 1/2" glass is often recomended is so it won't flex. It won't flex anyway if you have it on a flat surface. Take your 1/4" glass and put it on a piece of mdf. It'll will work fine.

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CW,
A-ha! I suspected as much!
The lady at the glass shop also suggested 1/4", which is about $50 less than the 1/2". They sand the edges for free, too, all for about $12. Sold.
Thanks for your help, CW, now and in the future, because you'll probably see more elementary questions with my name under them. :->
-Eric (even more of a Freedom Nut now)
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wrote:

dealers in your area .... they most likely have cut offs etc. they would part with cheaply. (go at the end of the day and bring cold beer)
Lenny
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Broken off pieces granite. Check with HD, Lowes, or a local granite/marble supplier.
--

-MIKE-

"Playing is not something I do at night, it's my function in life"
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-MIKE- wrote:

Granite tile for five bucks or so?
--
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--John
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I don't know. I'm talking about pieces that break off of the big slabs sitting in those big racks at the stone suppliers. Sometimes the home stores have pieces from counter tops that have fallen down a broken.
--

-MIKE-

"Playing is not something I do at night, it's my function in life"
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When the local bank was redoing their lobby, a slab of polished granite that was supposed to go on the wall was dropped off the truck. I was able to get a couple of nice chunks from the remains. One is now dedicated to my sharpening station.
Len
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OK, here's my report: the $12 1/4" glass works great.
No doubt all these other ideas are good, too, but this was the most convenient. Last night was the first time I've ever sharpened a tool, so the brand new $20 block plane from HD was the only thing to practice on. It did get very sharp, but I apparently need to (a) get the blade at a perfect right angle in order to get an even grind, and (b) pull out all of the staples that are still in the lumber.
Thank you everybody for your adivce.
Eric
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ups.com:

What you need are 2 things...
1-http://tinyurl.com/y2apx2 2-stop buying lumber where they put staples in it ;o}
Larry
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How expensive? Woodcraft sells granite surface plates for $30'ish. They're unyieldingly flat, but heavy and clumsy to move around. As far as I can tell, they're as flat as real granite surface plates. I own one, and use it for sharpening sometimes, but not one of theirs.
(Woodraft is on my bad list this moment. I flushed my shopping cart contents as of yesterday afternoon. I'm more than a little peeved at the fine print I didn't notice on their promos, first last month, and again a coupla days ago. They boasted "FREE SHIPPING" last month in their before Christmas specials. So I ordered a little corner chisel as a stocking stuffer for myself. Bummer; they charged $8 to ship the $7 item and included a coupon for free shipping on the next order. For sure, I thought, this must've been on their website somewhere and I just didn't notice. No big deal. So, I slowly gathered a few items into the shopping cart over the next couple of weeks. Yesterday, I went to check out, and the coupon code was NFG. Reaching for the goggles, I can make out the small print that says "Expires 1/18". Great. So I figured out how to get free shipping. Didn't pay nothing for shipping on the empty order. F*ckers.)
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Thanks, Mikes.
It's kind of hard to argue with a $12 glass, so I'll mount it to a board (I still need to look up "jig," and therefore not yet sure if I'm using it in sentences correctly) and reply back here to let you all know what I think. I used to use cheap glass sheets from picture frames when doing any kind of tracing, and I sliced my fingers a few times, so I'm guessing that the 1/4" glass with treated edges is going to be like a luxury.
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Re: glass... it's possible to ease edges of glass using a belt sander with a 150+ grit belt. Just go slow and wear eye protection. It works on any glass from 1/8" up.
Re: alternative surface... I stopped in at a local tile store and bought a single 12x12 tile. They labeled it "sample" and it ran me a whopping $2.64 ($2.50 plus tax)
Ed
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Ed, are you sure those are flat enough for sharpening?
--

-MIKE-

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I've got wet/dry paper of successive grits on the 4 corners of the tile (stuck on with 3m spray adhesive). I made some blocks at 25 and 30 degrees My chisels are the sharpest they've ever been. They cut the best they ever have, at least.
Ed
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Cool. I was thinking about this. I'm used to dealing with a surface that has to be completely flat across the entire surface. For the vast majority of sharpening, you're only using several square inches of surface area at a time. It doesn't matter if there's a bump in the middle, as long as the area you're using is flat.
--

-MIKE-

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On Thu, 22 Jan 2009 05:52:11 +0000, MIKE- wrote

If you want astronomy lab flatness, buy two and lap 'em face to face with abrasive paste, lifting and turning occasionally. Tedious beyond belief but you'll get a perfect surface. Don't rock over the edges!!!!
You\re getting close when the cleaned plates stick together with a drop of water, but if the surfaces are evenly matted I'd say that's "done"
Hell, just use MDF... :-)
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wrote:

you need 3 surfaces to produce 3 flat surfaces. 2 will produce 1 concave and 1 convex surface.
regards, charlie cave creek, az
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On Fri, 23 Jan 2009 20:27:37 +0000, charlie wrote (in article

well yeah. I presume we're talking 'scope mirrors here with hundreds of hours of walking round a pedestal-mounted blank ? I was meaning a couple of hours of lapping a couple of tiles but I take your point. I was being a cheapskate AND I said don't rock over the edges. I suppose I should have said...
Buy three tiles :-)
and some labels.
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