Orientation of Cutter Heads

In addition to the alignment of the cutter, you need to be sure you are moving the cutter forward in the way it was designed. If you look at the cutter from the side you will see two things. The most obvious one is that the angles of the cutaway on the head are shallower at the front and steeper at the back. In addition if you look carefully, the axel for the wheel is slightly forward of the centre line of the whole cutter.

Cutters aligned to score to the left

So it does not matter where the fixing screw on the head is. It may be at the front or at the back. What matters is the cutaway at the back which allows the cutter to be used with a shallower angle, than if reversed. This is most obvious on wide cutter heads.

Cutters arranged to score to the right

Scoring Glass

Cutting glass is done by “scoring” the surface of the glass with a glass cutter, then breaking it along the score line. The break you make will always follow the path of least resistance, so you want to be sure that the score you make becomes that easy path and glass breaks the way you want it to.

Holding the Cutter
Generally, you use the cutter by moving it away from you, so you can see the cartoon lines as you score. When using a straight edge such as a cork-backed ruler to guide your cutter, you can pull the cutter toward you, or push it away as suits you. The cutter should always be held at a 90 degree angle (left to right). You can determine this by looking down the cutter to the wheel and to the cartoon line below.

It is important that the work be done from the forearm rather than the fingers or the wrist. The forearm should be held closely to the body. This reduces the freedom of movement, giving clean flowing score lines. It also reduces the actions that can lead to repetitive stress injuries. Any turning required by tight curves can be done by turning the body from the hips or shuffling around the bench with the glass at a corner.  Of course, for long cuts your arm will have to extend from you body in a parallel direction with the score line.

Scoring Pressure
The second and very important element in scoring glass is the amount of pressure used.  Very little pressure is required.  You should hear no more than a quiet hiss on transparent glass and almost no sound on opalescent glass.  However some manufacturer's transparent glass has almost no sound either.  So the important element is the pressure, not the sound.   Most people start with applying far too much pressure. Tests have shown that only about 2 kg of pressure is required for a clean score.

You can test the effect of this amount of pressure on a bathroom scale.  Place a piece of clear glass on the scale and without touching the glass with your other hand, score it noticing how much weight is being recorded.  Keep trying until you are at the 2 kg area of pressure.  Try breaking the glass.  Score a curve with the original amount of pressure and break the glass.  Then using the same curve score the glass with the 2 kg pressure and break the glass.  You will see and feel the lesser pressure provides a clean break.

Excessive pressure leads to breaks showing significant stress marks on the edge of the glass.  Too little pressure has no effect on the glass, making it impossible to break along the score line.  The correct pressure (ca. 2 kg.) leads to almost vertical stresses being put into the glass which assists the breaking along the score line.  Too heavy pressure creates stress marks which are at increasingly large angles with the increasing pressure.  This will still break cleanly on straight lines, but when working around curves the glass can follow one of the lateral stress marks away from the score line.  Excessive pressure is often the cause of glass breaking away from the score line on a curve, especially a tight one.

Refining Edges

You can make the freshly cut glass safer to handle by gently wiping the edges of the cut piece with the waste piece. This removes the sharpest edges without chipping the glass.

After the glass is scored and broken, you can remove small, unwanted chips with grozing pliers. The serrated jaws of these pliers are used to gently nibble away at the jagged edges.

Rough edges can also be smoothed with a carborundum stone. You rub the stone along each edge, upper and lower, to remove any sharp edges. You can remove more glass with the stone if you wish by a little more aggressive grinding action or just a longer light rubbing of the stone against the edges.

A diamond smoothing pad removes glass in much the same way as a carborundum stone, but does it more quickly with the coarser grades. You can use a number of grades to get an almost bright polish to the edges. These pads must be used with water.

A glass grinder is used by many people. Many models of grinders are available. The grinding surface of the bit is covered with fine diamonds, which grind away unwanted glass very quickly without chipping the edges. In addition, they are water-fed which keeps the glass from cracking due to heat, prolonging the life of the diamond bit, and preventing the powdery ground glass form flying around.

A glass grinder is not a substitute for accurate cutting.

Outside Curves

Outside curves are usually the easiest curve to break out. You do not have to worry about breaking the piece you wan to keep.  The break, if it goes off the score line, will be away from the piece rather than into it. 

Outside curves often have complimentary inside curves. So, on the principle of making the most difficult break first and the easiest last, the outside curve will be broken out last. Unless, of course, there is a straight line on the other side of the curve when the straight line would be the last, as it is the easiest score to break out.