by Bob Beranek

The last step in fabricating an auto glass piece is finishing the edges. There are four edges that are used in auto glass fabrication, the edge number one, edge two, edge three and edge four. Let’s take a look at each finishing edge and how they are accomplished by the cutter.

Edge #4 (90 degrees)

Edge number four is sometimes called a clean-cut edge. Its distinguishing feature is that the edges are 90 degrees to the surface of the glass. This edge is used when the glass edge is not exposed or when the glass is to be bonded or fitted into a channel.

Once the glass is cut to size, the cutter will use a belt sander or vertical wet sander to create a flat ground edge.

 Bob Finishing Edges 1

Edge #3 (seamed)

Edge number three is sometimes called the seamed edge because the sharper edges are seamed or dulled off. This type of edge is the easiest way to prep the glass for installation into a gasket or weather strip. The dull edge does not bite into the rubber of the gasket that can hinder the ease of installation.

It is accomplished by first attaining edge four and then using the belt sander at a 45-degree angle to seam the sharp edge. If a vertical wet sander is used, simply lift the glass to a 45 degree to the belt and run it the length of the glass piece.

Bob Finishing Edges 2

Edge #2 (rounded)

Edge number two is a rounded edge sometimes called a pencil edge. This edge is used when the glass edge is exposed and possible injury or damage can result. This unpolished edge is used in vehicles that have a high risk of frequent breakage but require the edge to be exposed. An example of this might be the door glass for a delivery vehicle.

The “two” edge is accomplished by first attaining the “four” edge and working the belt sander the length of the piece starting at the flat edge and slowly changing your angle until a rounded edge is achieved. Then turn the glass over and repeat on the other layer of the glass. The edge will appear dull in finish.

If a vertical wet sander is used, start the glass on the flat edge and run the glass the length of the edge. Then lift the glass slightly and repeat. Continue to lift and run until the top layer of glass is rounded. Then turn the glass over and repeat on the other layer of glass.

Bob Finishing Edges 3

Edge #1 (polished)

The number one edge is a polished edge used on exposed edges where a person’s arm may come in contact, such as in a door glass or vent glass. This type of edge may be used for collector vehicles or for vehicles that require a higher degree of detail.

It is accomplished by first attaining the edge three and then polishing the rounded edge with a cork or fine diamond grinding belt. Use the same tool action as described in the edge three procedure.

There you have it, the conclusion of the lost arts of an auto glass professional. I hope you enjoyed the step back in history and the possibilities of profit enhancement. I have no idea what to write about next week so please send in your ideas.

Many of the glass parts fabricated by an auto glass shop include radius corners. Experienced glass cutters can create their own radiuses through geometric procedures. However, there are radius templates available for purchase, which makes this job a lot easier.

To make your own radius corners you will need a tape measure, compass and marker. Measure in from the two edges the amount of the radius. For example, if you want a 2-inch radius corner, measure in two inches from the horizontal edge and make a mark then measure in two inches from the vertical edge and make a mark. The two marks should cross.



Place the point of the compass on the crossed mark and create a curve from the horizontal edge to the vertical edge. This creates a 2-inch radius.


Do the same procedure for any radius desired.

Once the radius corner pattern markings are drawn on the glass, the next step is to score and break the glass. This is where the rubber meets the road, so to speak. If the corners are mis-scored or broken with too much pressure, the entire part can be lost to a runner stretching into the main part. There are two ways to score and break the glass; the straight and grind or the curved and break.

The straight and grind is the safest method when starting out cutting. It is simply scoring the radius at its highest point and then grinding and rounding off the remaining edges.


The curved and break method is scoring the radius directly on the curved pattern markings and then using a drop-jaw pliers to gently break the score. I usually started on the score that leaves the glass edge and break it to the height of the radius. Just gently squeeze the handles and don’t bend the glass down. This will cause pressure at the edge points and cause an unwanted fracture.


I also scored and broke one side before I scored and broke the other. I feel that the solid underside gives me better control. Once both sides are scored and broke, I then pull the scrap outward to stretch the lamination and cut it with my tape measure cutter. Do not bend down on the scrap piece or the edges can chip the main part and cause a fracture.

The part is now at the size and shape needed to install but the part is not finished yet.  Next week we will talk about the finishing edges.