Cutting Glass with Accuracy

From the Beginning
By Eloyne

About the Author

Eloyne Erickson is the Owner/Operator of Grand Central Stained Glass,  serving the Tampa Bay area for over nine years. She has been involved in the craft for over thirty years, and serves as Principal Designer, Project Manager and  fabricator of all custom pieces.

Being able to accurately cut glass is an obvious prerequisite to successful glasswork. There are many different ways to cut the many types of glass, glass shapes and glass applications. Let’s discuss some of the common ways to cut 1/8” art glass we stained glass, kiln glass artists or mosaic artists generally use.

There are a few simple tools for cutting glass.

  • a wheeled cutter with a carbide wheel – Pencil or Barrel/Pistol/Thomas grip
  • cutter lubricant
  • straight edge guides
  • a smooth cutting surface
  • running pliers, breaking & grozing pliers
  • a glass grinder
  • a waffle grid

Scoring Glass
To cut the various shapes of glass for our design, we will create a “score” in the glass. The carbide wheel of a glass cutter creates a line of microfractures into the glass.  The glass is stressed along the score line making it possible to break away the wanted glass shape with a little pressure.
The basic concept is to hold the cutter perpendicular to the glass, apply even pressure, and roll the cutter along your proposed score line – starting at one edge of the glass and ending at the other. Apply even pressure all along the score line – especially at the beginning and at the end. It doesn’t matter if you run the cutter toward yourself, or away. If you are following a pattern line on the glass, score in the direction you can see the line and follow it precisely. You may find that the type of cutter you use and the length of the score line changes your mind about what is comfortable in a particular situation. The glass should be clean before scoring.  It is easiest to cut glass on the smoothest side because the cutter head will have equal contact with the surface of the glass – but it technically can be scored on either side.
It is almost impossible to actually know how much pressure to apply to the cutter when scoring glass. Applying too much or too little pressure will adversely affect the ability of the glass to break cleanly and accurately–practice helps. Different brands and even different colors of glass will behave differently. When learning to score glass listen for the “zipping’ sound that the cutter makes. The sound should be smooth, not jagged or crunchy. The sound will vary from glass sheet to glass sheet—it should only be a consistent sound. Practice the speed with which you run the score line. You will be more successful with a slow to medium, controlled score.

Breaking Glass
After scoring the glass—a straight line or simple curve–the next step is to break it along the score line. (Circles, ovals, thicker glass, and tight inside curves have their own techniques. We will address these in a separate discussion.) The general concept of breaking along the score line is to apply upward pressure underneath the score, and downward pressure on the rest of the glass.
Use a running pliers with center of the pliers placed directly along the score line. These pliers are designed with convex and concave jaws that apply upward pressure on the score line and downward pressure on the adjacent glass. For a long score or slightly curved line, squeeze the pliers very lightly at one end until you hear a faint click, then turn the glass around and use the pliers at the other end to complete the break.
Use a breaking/grozing pliers placed with the pliers jaws edge parallel to the score line. The motion to break the glass is up with the jaws and down with the handle.
Turn the glass scored-side down onto a soft surface and press on the unscored side along the score line.
Align the straight score line with the edge of your work table and apply downward pressure on the edge of the glass overhanging the edge. It is usually takes a “pop” down onto the surface to get the break.
Thin strips of glass can be broken crosswise into shorter pieces by simply holding each end and snapping them apart. Your thumbs are under the glass to either side of the scoreline pressing upward; your remaining fingers are on the top pressing downward.
A cord or string can be placed on your worksurface under the score line. Pressing on both sides of the glass will cause it to crack on either side of the cord.
Use your hands to break the glass: grasp the glass on either side of the score line, thumbs on top and curled fingers underneath, and applying force down and out with your thumbs, and up with your curled fingers. Be sure to position your thumb and fingers so that they will not be cut when the glass separates.
For narrow glass breaks, use two pairs of breaking pliers, one on each side of the score line, with the jaws across from one another and on either side of the score line. Apply downward pressure to open up the break along the score line.

Technique Tips

  • Make certain that your cutter wheel is perpendicular to the glass in all directions. An angled cut will create razor-like edges.
  • Vary your pressure, using either greater or lesser to see which works best for that particular sheet of glass.
  • Keep even pressure on the cutter at both the beginning and the end of your score to avoid “flairs”.
  • If as you are scoring the glass and it just doesn’t seem like you did it right, flip the glass over and try the other side. Do NOT go over a score line again on the same side of the glass.
  • The glass does not know which score line to follow and you will possibly damage your cutter.
  • Try a different tool or method to breaking the glass.
  • Use a cork-backed straightedge tool for straight cuts.
  • If you are using glass with a rolled edge, it is best to cut that edge off first.
  • Use strip cutters that facilitate cutting multiple straight and angle cuts. Use a circle cutter for scoring accurate circles.
  • Check your cutter wheel for debris in the wheel slot that may impede the cutter from spinning freely.
  • Check your cutter wheel for damage by looking carefully at a test score line for any skips. Purchase a new cutting head when necessary.
  • Treat uneven, handrolled, heavy-textured or expensive glass with a bit more respect. Use the method of glass scoring, sawing or breaking method that you feel most confident with.
  • Score on a clean, flat worksurface. Chips or other debris under the glass can scratch the glass or break it when you apply pressure to make a score.
  • Check your lubricant supply in the cutter. I typically place approximately 1” in the cutter. When that is gone, I slip the cutter head off and clean the mechanism. It’s just simple maintenance.
  • Use a glass grinder to fine tune miscuts.

Safety Issues

  • Consider using eye protection when breaking glass. Chips go everywhere!
  • Always wear shoes with covered toes when working with glass.
  • Use gloves when reaching into your glass stash or a glass case.
  • If glass is going to fall – let it. Don’t try to catch it.