by Bob Beranek
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The most difficult part of training a technician to be productive is teaching the mastery of the tools unique to the auto glass industry. Unlike the common tools at your local hardware store, auto glass tools may need practice to acquire the skill to use them properly. In the next couple of blog posts, I plan to make a number of observations, hints and ideas that will assist in the use and care of auto glass tools. I urge my friends in the tool business to contribute and add their thoughts to the discussion. The comments that I will be sharing are my opinions based on experience with tools and observations of problems I’ve witnessed over the years. I will be happy to share my thoughts on any tool you wish to discuss, so, please don’t hesitate to contact me with questions or comments.

Let’s begin with cut out tools and with the most difficult tool to master, the cold knife. Before there was a cold knife there was piano wire with two pieces of wood for handles. There will be a whole separate discussion of the new wire-out tools in the near future.  But for now, I would like to discuss the most commonly used cut out tool in the industry which followed the piano wire as a new innovative tool that would change the industry for decades.

According to Reid manufacturing, the cold knife was invented in the early 1960s by Harry Cothery. When it was invented it was thought to be the savior of vehicle paint and damaged interior trim, as well as a quick way to cut out the new glued-in glass parts. It certainly proved its claim. Our industry embraced the new tool and improved the tool and the blades over the years so that now it is still the tool of choice for many techs.

The introduction of urethane as the adhesive of choice by vehicle manufacturers led to a challenge when it came to the use of the cold knife. Those out there that remember the first time they tried to pull a cold knife through urethane knows what I mean. We swore the manufacturers used liquid concrete to adhere the glass to the vehicle.  However, over the years we perfected the method by which the cold knife is now used and have developed different blade designs introduced to ease the cut out.

Today, I want to talk about the proper use of the tool. The various styles of cold knives and its wide variety of blade styles are designed to separate the adhesive from the glass at the glass surface. The rigidity of the glass and the sharpness of the blade are used in a scissor type action to cut the adhesive from the glass.  This is the difficult part to master because the user must find the place where the tool separates the glue from the glass without breaking through the glass and without pulling the blade through the strength of the urethane. This is what I call the “sweet spot.” It is like hitting the baseball right on the meat of the bat, hitting the golf ball in the center of the driver and scratching the itch in the center of your back. Once you find it you know it, but to find it is the problem. It may take you 20-50 uses to find it and another 20-30 pulls to perfect it.

It starts with the insertion of the blade into the adhesive. Typically, it is easier to pull the cold knife instead of pushing it because you can use your body weight more efficiently. There are times, however, when both methods must be used due to access to the glass edge. Insertion is usually in the top or bottom center and pulled to the corners. When you first insert the tool the blade is positioned in the adhesive about midway through so the goal is to get the blade up to the surface of the glass.

To accomplish this, it is necessary to use the blade configuration and the tool’s vertical handle (steering handle) to position the blade for smooth and easy cut out. The blade configuration is the configuration of most cut out blades used in the auto glass industry, flat on the glass side and tapered on the other. When the handle is slightly angled back while pulling and pushing, the blade’s natural travel is up towards the glass part’s underside surface. When it reaches the glass surface the handle’s angle is then positioned more vertical.

As you pull the tool with the cabled handle (power handle) toward the corners, it is best to use your entire body weight to create the power of the cut. Using just your upper body strength will work, but long term muscle fatigue and injury is a possibility. As you pull, pay attention to the glass curvature and position your body behind the curve. If you pull the knife while in one stationary position, the blade has a tendency to break through the glass. In other words, go with the curve. The same goes for going down the “A” pillar. It is best to be below the tool and not stand above it unless you’re pulling upwards from the bottom. Standing creates the same result as not going with the curve—the tool breaking out from under the glass.

If you notice the design of the cold knife, the pull handle is mounted to the tool under the horizontal part of the blade. The most efficient use of the tool is based on the proper placement of your hands in relation to the handles and the side you’re currently working on. The proper use of the knife requires that you will be pulling with your right hand on the bottom driver side and the top and side of the passenger side, and you will be pulling with your left hand on the remaining sides. This causes you to use both of your hands for a power source and that is uncomfortable for most installers. However, if you use only your predominate hand for power, the tool will be harder to learn because it will be much more difficult to maneuver. The discomfort will go away once you master the tool.

Next week will talk about the evolution of the cold knife and its blades and their various designs

Comments (3)

  1. Ron Tremayne said on 30-08-2012

    I remember the introduction of the Hot Knife.
    It was replacement for the cold knife. I retired in 92 a
    fter 35 years. Still have the t

    ool box with my rope and cotter key tool aka my hook.

  2. Jeff Cothery said on 31-08-2012


    Nice article, I still remember my Mother telling my Dad that he couldn’t keep making cold knives and windshield sticks in their basement and to go find a somewhere else to make the mess! Just over 50 years ago.


  3. Glasseye said on 02-09-2012

    To keep the blade in peak condition for cutting, use a sharpening stone or small flat file to regularly hone the cutting edges.

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