"Getting an Edge"

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Process

As mentioned on the previous page, you start with a coarse grit, like 60 or 80, and progress to finer grits until you get the finished edge you want. A reasonable succession of grits would be 80-120-220-400.  The coarsest grit actually removes glass and shapes the edge, while the others are used to smooth and polish.

So, let’s walk though the steps of coldworking the edge of a rounded piece. In this situation, after the piece was fired, there was significant irregularity to the contour of the piece due to the method of layup.

First, I make a cardboard template of the desired shape of the finished project, lay it on the glass, and draw a line with permanent marker in a contrasting color to mark the periphery. I then cover this line with some sort of wax (furniture wax, Chapstick, Carmex etc) to prevent the line from disappearing as it is washed by the water spray from the WBS. I usually mark the side of the glass that will be the most visible surface of the finished project in case the bottom side becomes scratched during the coldworking process.

In this project as much as ½” of glass needs to be removed in some areas, while in others only about 1/16”. One could trim the wide areas with a tile saw (if available) to minimize time at the WBS and wear on the belts. Nonetheless, whichever way you do this, never do any grinding without having your desired edge clearly marked – or you will invariably stray from your desired shape. If your line disappears as you’re working, I’d recommend drying the project off, removing the wax, and redrawing the lines.

In this project the piece was ground with the 70 grit belt first to bring the edge down to the line that had been  drawn with the template. As seen in the photo below right, the glass rests on the wheels or platen, depending on your machine, and intersects the belt as it rolls over the lower wheel.

As you are grinding, it is important to keep the glass moving at all times to maintain a rounded edge and avoid overheating of the piece (and possibly breaking it by thermal shock). It is also important to keep the plane of the glass consistently parallel to the floor or you will end up with a multi-planar edge. You do not have to press too hard on the glass, but rather let the abrasive on the belts do the work.

After the initial shaping, this piece  was progressively smoothed with 120-220 and 400 grit silicon carbide belts. The concept is that each belt should remove the deeper scratches made by the previous belt. It is hard to accurately judge the progress you’ve made on wet glass, so it is necessary to keep drying the edge of the piece in order to see how you’re doing. Keep in mind that as the belts become worn they behave like a higher grit than they began. In this case, my somewhat worn 400 grit belt gave more of a polish than would a new 400 belt. You may want to keep an older 400 grit belt to use as a final polish. 

I would also recommend putting a slight bevel on the edges of the glass piece. A sharp 90 degree edge is very fragile and prone to chipping. The bevel prevents chipping from trivial trauma and the bevel actually makes the edge look more finished and smoother. Lightly pressing the edge of the glass against the belt at a 45 degree angle and rotating the glass will give a nice beveled edge. I usually use the 220 belt to put a bevel on the back edge, and 400 grit on the front edge. But, you can use a more aggressive grit for either if you want more of a bevel. Next Page