A Homemade Surface Plate from Plate Glass

I am wanting to make the Gingery Lathe. The problem I encountered was finding a flat surface to reference against. In Dave's book he suggested using the plate steel that was to be the ways as a test standard. The problem with my piece was that it had a slight bow to it. Nothing critical and would be pulled out when bolted to the bed, but in my opinion it would not due as a standard.

In another one of Dave's books (the second or third) he addresses the issue by suggesting the use of normal plate glass as a surface plate. He informs us that plate glass as it stands is close to being flat. And that the smaller imperfections can be fixed by lapping two plates against each other. (Although I am leaning towards three separate plates to do this job more accurately) I decided I would give it a go.

I called my local glass supplier and ordered two pieces of 1/4" x 8" x 30" plate glass, then stopped by a local automotive supply store for a couple tubes of valve grinding compound. I brought the stuff home and laid the glass out on a piece of wood that came with my tool chest to protect the top. Next I coated one piece of glass with a thin coat of grinding compound and then the fun began. (NOTE - You can click on the images for a bit of a larger view)

Homemade Surface PlateYou could tell the plates were fairly flat, they stuck together like the dickens. As I tried to lap the pieces together, the bottom glass slid all over and the top plate would not budge. My solution if you look at the picture to the left, was to take a piece of rubber shelving material like you would find in the kitchen area at your Walmart store and lay the bottom glass on top of it. My favorite helper (Melissa) held the board still so that I could lap the plates together. I would like to tell you that it was a simple process for me to do but it was work. Even though the bottom glass stays put the plates adhere to each other and did not slide well. Dave instructs us to lap the plates together until they are uniformly frosted (about 200 strokes). Attention to the photo shows the two plates stacked together and the grey stuff between them is the grinding compound. After about 50 strokes (circular movements of the plates) I thought I would slide them apart a bit and see how progress was coming. I smeared some of the compound from the bottom glass and it looked as pristine as when I started. Tonight after going to a function, I will lap some more. Hope my helper holds out! :-)


My plates got knocked to the concrete floor, shattering them,  before I had them finished.  Needless to say I was rather discouraged.  :-(  But I did find a free shipping code for a $50 or more order from a company so just bought a granite surface plate.