by Bob Beranek

Is the old cold knife disappearing? Will the wire/cord crimping tool overtake other auto glass removal tools? Are violent power removal tools going to be replaced with more passive tools?

Recently I thought about who invented the cold knife and when it was first offered? It certainly had a good run. When I started in the industry, in the mid-70s, the cold knife was a major tool in my toolbox along with a hook tool and a utility knife. Wire and handles were also present, but the cold knife took the lead when it came to glued-in glass removal tools.

I figured the cold knife would be easy to research, so I began probing. There was no shortage of sites offering “auto glass removal tools” for sale, but the history of the tool was seriously lacking. The many names associated to the tool over the years caused a major problem. Cold knife, cable knife, windshield removal tool were all used interchangeably, which makes it difficult to track down.

The first cold knife

I called my friend and colleague, Bob Nilsson from Ultra Wiz who steered me to Jeff Cothery. Jeff told me that his father, Harry R Cothery, invented the cold knife based on PPG’s suggestion in the early 1960’s. He went on to invent and patent the “Hot Knife” as well, patent number 3,448,517. Unfortunately the hot knife never really took off like the cold knife did, and due to the existence of similar tools, it was unable to be patented. Jeff turned out to be a wealth of information and I want to thank him for satisfying my curiosity.

The first hot knife

Now to address the questions above.

Is the cold knife disappearing? I believe current technicians will never let it disappear completely. The cold knife took them a while to master and once they did there was never a tool that could remove the glass faster and smoother than a well sharpened cold knife.

Will the wire/cord crimping tool overcome other auto glass removal tools? With the potential for vehicle damage minimized, no physical exertion required, and providing even an inexperienced tech the ability for a safe efficient removal, they provide a lot of benefits. As people get used to seeing wire-out tools and trying them, they will increase in popularity.

Will the power removal tools be replaced with more passive removal tools? I don’t think so. Power tools are the fix when all else fails. If power tools are used incorrectly they can cause serious damage to the vehicle, but for experience techs there will always be a place in the tool box for the ultimate tool that will “get-er done”.

You have to say a few things about Harry’s cold knife invention, the long learning curve it takes to master and the effort expended using it, the cold knife, when used properly still works great. Don’t throw your cold knife away yet.

You may be thinking cold knives? Why is Bob writing about cold knives when there are power tools and wire-out tools that are taking over the automotive glass removal process? The answer is because it hasn’t happened yet, as much as tool companies and safety advocates would like to make that case. The cold knife is still the most popular tool for glass removal.

The productive use of a cold knife can be nearly as difficult to learn as hitting a golf ball straight, or hitting a baseball on every pitch. There are angles to find, leverage to master, and muscles to develop before a rookie technician can be proficient.

It takes some training. I call it the “sweet spot.” If a technician is able to find that sweet spot, he (or she) is able to cut through urethane like butter. It is amazing the first time a tech gets it just right. Only experience allows you to find it every time.

In one of my classes I taught a 14-year-old girl (no more than 70 pounds soaking wet) to pull a cold knife in two days. However, that isn’t always the case. I had a former employee who took six months to finally learn the technique. The problem with my ex-assistant is that he was a body builder in his spare time. He had a lot of muscle, so he thought that should be the secret to cutting out glass. Because he didn’t want to consider angles, leverage and the use of body weight, it took him a lot longer to learn than it could have.

Another component involved with the efficient use of a cold knife is the blades we choose. One of the most popular blades is the pointed and tapered sided blades invented and manufactured by UltraWiz.


Their blades have been the choice of many new technicians country wide if not globally. They are easy to insert into a hardened urethane bead. The company offers super thin steel construction and the coated blades, pictured above. This is the blade style of choice by many. However, one thing I noticed as a trainer is that the tapered design does pose an inherent problem for the new and learning technicians. As the blade is pulled or pushed, it has a tendency to move out from under the glass and move towards the wall of the pinchweld. This wasn’t a problem when we had reveal mouldings to cover any mishaps in paint scratches but the modern “exposed-edge” glass mounting requires that the technician pay attention to the blade making contact with the exposed wall of the pinchweld.

When I started as an installer, we had the standard style of cold knife blade with parallel sides.


This blade was substantially more difficult to insert and to pull. In fact, we needed to regularly use the shortest to the longest blade progression to comfortably cut out the glass. Even then, the blade itself was not always easy to remove from under the glass.

If you are relatively new to the industry and you use the tapered blade in your knife, please be aware of the action of your tool. Tweak the vertical handle in at the base to keep it under and against the glass edge during the cutout. Have the vertical handle of the knife with a slight backward angle while pulling and a slight forward angle when pushing the tool. Positioning the tool in this way will allow for the most efficient use of the cold knife without damage to the wall of the pinchweld.

Please let me know if you have other suggestions for mastering the cold knife. I am always interested in hearing your experiences.