What is flat in the home shop?

[The Artful Bodger]

I do not have a flat reference such as a surface plate, I dont even have anything better than a hardware store straight edge, neither do I really see the need with the sort of work (play?) I do.

I assume my mill table is likely to be the most flat surface in my shop, how would that compare with, say, plate glass?

Presumably I would be able to buy something like a stick of key steel and check that against my mill table for use as a portable straight edge?

[gary350]

I need a flat surface too, to cut glass so I guess I will just have to build me a special table with 3/4" plywood top. The hardware store wants $1.50 per cut 324 mirrors 2" square = $486 plus the mirror glass plus sales tax. I have cut glass before the table can not have any humps, bumps, dips or defects. It needs to be just as smooth and flat as the glass itself. I wish I could borrow a professional glass cutting table for about 2 hour.

[KiddZimaHater]

Probably your mill table.
Give it a good stoning to remove any burrs.
I had to use my mill table as a surface plate when making a few rods today.
Customer wanted them 15 inches, +/- .005.
And I only have 12" calipers. (oops!)
So I mounted an indicator in the mill spindle, set it at zero on 5 stacked - 1,2,3 blocks, and BINGO!! Instant height gage!

[firbikrhd1]

One of Dave Gingery's books talks about making a "surface plate" from pieces of glass. His method is to obtain two pieces (thicker is better) and lap them with valve grinding compound. I think using three pieces might be better; as with scraping, when all three can be interchanged and mark the same they are flat. I made these years ago of 3/4" plate glass but unfortunately eventually dropped and broke them.
In any case, it could be done cheaply. Valve grinding compound is very inexpensive and I see glass table tops for free on Craigslist often. One table top could easily be cut to make two or three pieces if decent size.

A quality table saw probably has a fairly flat surface as well.

[Your Old Dog]

I need a flat surface too, to cut glass so I guess I will just have to build me a special table with 3/4" plywood top. The hardware store wants $1.50 per cut 324 mirrors 2" square = $486 plus the mirror glass plus sales tax. I have cut glass before the table can not have any humps, bumps, dips or defects. It needs to be just as smooth and flat as the glass itself. I wish I could borrow a professional glass cutting table for about 2 hour.

You're over thinking this one gary! It just ain't that big a deal. I worked for a glass shop and can tell you your work table doesn't need to be nearly as flat as the original poster is talking about More important you remember to snap the glass immediately after scoring it with the cutter and not to let the cut get "cold". Don't forget the drop of oil on the cutter before making the cut.

[Evan]

I assume my mill table is likely to be the most flat surface in my shop, how would that compare with, say, plate glass?

Plate glass isn't common anymore. Plate glass is made by grinding the surfaces fairly flat but not to very tight specs. Nearly all window glass is now float glass and it is nearly optically flat. It is around 4 wavelengths flat (530 nanometres per wave) within any 2 cm radius and over longer distances it has a curve equal to the curve of the Earth.

Float glass is very flat but that doesn't mean the thickness is constant. Thickness may vary by 10% or so but will be very consistent for any particular piece. If evenly supported it will be flat as a surface plate over similar distances.

[boaterri]

I generally use my table saw as a flat surface when using a height guage etc. It may not be as good as a granite toolmakers plate but it works well enough.

Rick

[The Artful Bodger]

Plate glass isn't common anymore. Plate glass is made by grinding the surfaces fairly flat but not to very tight specs. Nearly all window glass is now float glass and it is nearly optically flat. It is around 4 wavelengths flat (530 nanometres per wave) within any 2 cm radius and over longer distances it has a curve equal to the curve of the Earth.

Float glass is very flat but that doesn't mean the thickness is constant. Thickness may vary by 10% or so but will be very consistent for any particular piece. If evenly supported it will be flat as a surface plate over similar distances.

Thanks Evan, I do have some glass, I assume it is float glass, on a desk top which I can use if I want to get 'fussy', otherwise it appears the mill table will be my 'shop reference'.

Thanks for comments everyone.

[J Tiers]

Float glass is likely to be pretty flat, indeed, depending on how made (how fast and how fussy they were).

The problem is that it is so thin that it is bendy, where a reference flat should be solid and unmoving.

It all depends on whatcha need. For me, no way even float glass would be good enough, so I use a granite flat reference. Doesn't bend when I set a reasonably heavy machine piece on it to blue up.

For you, might be fine.

[The Artful Bodger]

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.