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Cutting Circles Makes Me Crazy

How to easily cut a circle in glass

Ever watch an old movie starring the hunky cat burglar that walks up to a plate glass window and scores a perfect circle?  Then he pulls out a suction cup and pops it out easily and he’s in. Only in Hollywood. When glass breaks, it will often have a slight flare, which would be nearly impossible for a circle to drop out without releasing the glass around it. But there is a way to cut a perfect circle!

Let’s start with a the circle cutter. I prefer the circle cutters that have a suction cup on the pivot point. This will help hold the cutter in place while you are scoring the circle. Set your cutter for the diameter of circle you want to cut. Remember, we are dealing with the diameter of a circle, so if you want a 4″ circle set your wheel at 2″. If your circle cutter’s ruler is in metric, you can place a regular ruler on the center of the suction cup and slide the cutting head on top of the ruler to get the size you need.

Circle glass cutter trial on paper
Testing glass circle size on paper

A trick I use to check my measurement before I cut the actual glass is to grab a piece of paper or cardboard and practice your circle cutting. You can see what size you will end up without actually cutting the glass.  When you place the cutter on the glass, make sure you leave about 1/2″ from the edges of the glass. This will allow you to break your circle cleanly. If it is too close to the edge, you will end up with a pointy nub that will need grinding. Go ahead and waste a little glass, it will save time grinding later.

Your score is the most important part of this process. Do a dry run, not actually score the circle, to make sure you will be able to smoothly execute the score well. It never fails, you are in the middle of a score and the circle cutter bar hits something near the glass and interrupts your score. Once you feel that you are cleared for take off, it is time to start your score. Do one full rotation with the cutter, careful not to go past where you started. You can usually feel the cutter fall into the score where you began. Stop there. It is important to have consistent pressure, not too hard, not too light. You can use clear window glass to practice scoring your circles so that you can get the pressure down. Practice makes perfect.

Morten Safety Break System and Glass Circle Cutter
Morten Safety Break System and Glass Circle Cutter

Now, here is the real secret to breaking out the perfect circle, providing you have the perfect score. Use the Morton Safety Break System to run your score around the circle. Glass is like a balloon over filled with air, full of energy. When you score the surface, you want to try to release that energy that will best work for you. The Safety Break System allows you to break the score along the circle in a controlled manner.  It actually has two breaking systems. The Push Block and Button works similar to your breaking pliers only more powerful. The Button will put pressure upwards on the bottom of the glass, while you use the Push Block to apply pressure downward on the top of the score. It allows you to gently manipulate your break around the circle. The Running Tool is more versatile and doesn’t need the button to break out the score. You can simply hold your glass while you apply pressure on the score with the Running Tool. Either tool will do the job and you get to decide what you are comfortable with.

Once you have the circle free of the it’s surrounding glass then you can score lines to finally release your perfect circle. I usually make score lines off at an angle to the circle, so I don’t accidentally run my cutter into the circle, which could start a new run.  Break these lines out and release your perfect circle. If all the stars are aligned and the technique is executed well, there will be no need to grind.


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