What is flat in the home shop?

[Don Young]

At a Harley shop years ago we used a piece broken from a solid glass door, about 1 1/2" thick as I recall. Nothing we measured or measured with could detect any lack of flatness in it. I have often wondered if a bullet-proof window might be pretty good.

[GNO]

used to be the fuel inj. guy for a gen plant big diesels, used heavy glass for a lapping plate.Could wring the parts togetter after lapping & cleaning [that's how I knew they were flat enough]

[Evan]

The main problem with glass is that it scratches so easily. Rather than going to the trouble of setting it in concrete maybe float it on some plaster of Paris. If you can find some in 12mm or so you won't need to do that. Of course, surface plates are pretty cheap. Another possibility is some countertop granite. It isn't as flat as a surface plate but is usually better than .001" over a good distance.

[.RC.]

surface plates are what? $50 or so...

Why would you bother with glass then they are that cheap...

[Scottike]

Why not just spring for a granite surface plate? they're not that expensive unless you get into inspection grade. And from the sounds of it, your not looking for that much accuracy. (I know it would be nice, but I know I don't need that much accuracy)

[Mcgyver]

Evan says float glass is real flat so I guess if one was to put a couple of inches of concrete in a tray and 'float' some float glass on top that would make an easy surface plate, it would support plenty of weight but still not like things being dropped on it.

depends what you're working on...if its really flat it'll work for all but the most challenging tasks, although sometimes you need very flat. Often quite flat is good enough; some say you can get by with pretty flat

come on guys, these descriptions are meaningless...the top of a table saw is really flat to a carpenter.

AB, what matters is what you're trying to do.....layout with a scriber in a surface gauge is different from checking squareness with a 10th's indicator or scaping.

imo small shop grade import plates are cheap enough its not work going for any thing less, they'll do most everything you might encounter.

[The Artful Bodger]

I dont really need to spend anything on a surface plate, my question was what is the most flat surface out of the stuff home shoppers typically have on hand. In my case that seems to be my mill table.

Float glass on concrete, well I am rather partial to concrete right now...

[darryl]

I've done a couple different things for a 'flat', including embedding a transmission valve body into cement. The best size and most trustworthy is the surface plate, 12x18 in my case, and it was less than $50 including taxes. It's kind of a no-brainer once you have it- are you going to get a glass plate to be as flat? How would you know? Why not buy a small surface plate-

Other things I've used are pieces of granite countertop and floor tiles. Either one can be checked to a degree of precision by looking across it and angling it to catch a reflection of things in the background. As you angle the flat back and forth you can see whether there's any rippling in the reflection of straight lines on things. If things look good visually, it might be more than good enough for the application. But that' the thing- how accurate do you need it to be? A piece of glass is probably flat enough over a half square foot or so, but maybe not over the entire surface, corner to corner both ways. How can you know- for fifty bucks a working grade surface plate is a pretty good deal. Mine has a certificate saying it's good to .0002 or something over the entire surface. You will have some peace of mind using something that you can feel sure is flat.

As a backup for sandpaper, etc pretty much any of the alternates would be good enough. I trust my piece of granite counter top for that, and the same for the floor tile, although the tile needs to be mounted to a 'flat' surface to help it stay as flat as it is.

[Bill736]

Stone surface plates are inexpensive, and I bought mine during " free shipping" specials. I have two small, 9 x 12 inch plates, and I'm amazed how often I now use them. I put wet or dry sandpaper on top of them, and use them to surface carburetor parts, touch up ruler edges, and with an angle jig I sharpen my chisels . In the process I also found out that my table saw top was not very flat after all.

[dp]

The flattest thing in my shop is a granite surface plate I bought from Forrest Addy when I took a scraping class. 18"x12"x2", it wasn't all that expensive.