Sunday, 12 February 2012

Cutting Glass

Techniques and Safety

Use of the Glass Cutter 

When cutting glass your are first scoring the surface to weaken the glass and then second, breaking along the score line. The glass will always follow the path of least resistance. It is important to keep this in mind when “cutting” glass as it has significant implications for scoring and breaking.

Use the cutter by moving it away from you, so you can see the cartoon lines as you score. When using a straight edge, you can pull the cutter toward yourself or push it away, whichever suits you.

Grasp of the cutter
The classic or traditional grasp is for use with a pencil cutter. The cutter is placed between the first/index and second /ring fingers with the thumb at the back of the cutter. This initially is awkward. Its advantages are that it transfers most of the work to your arm rather than fingers and wrist, and it restricts the movement of your wrist, leading to smoother curves.

Basic cutter in traditional grasp

Oil filled pencil cutter in traditional grasp

The modified grasp is also for use with pencil cutter. The cutter is placed between the thumb and first/index finger. The second /ring finger is also most often used beside the first/index finger. The fingers should be straight to avoid excessive strain on the fingers and possible carpal tunnel problems later.

Modified grasp with straight fingers

The fist grip for use with pistol grip cutters. The cutter is held similar to a gun, with the first/index finger pointing down the shaft holding the cutter head. This pointing action seems to aid the accuracy of cutting. This applies to cutters with right angle handle attachments also.

The palm grasp is for the small Toyo and other palm cutters. The cutter is placed on the pad beneath the thumb and held with the first/index finger and thumb.

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.

Moving 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.

The cutter should be held so that your dominant eye looks along the cutter to the cut line just in front of the wheel. This ensures you are looking directly to the line and that your cutter is not tilted to one side or the other. Looking down the side of the cutter only ensures that it is tilted with the effects indicated above.

Tilted Cutter Effects*

A tilted glass cutter has the effect of changing the angle of the cutter wheel. It narrows the angle on one side and increases it on the other side. So on the side tilted away from vertical (which is what happens when you look down the side of the cutter) has an sharper angle with the glass. This is likely to produce chips along the cutting line. The side which is tilted toward the glass has a more blunt or shallow angle with the glass. This produces high stress along the line.

The combination of these two effects make for a rough edge when broken and for break failures because of the stresses being at angles to the desired vertical fissure line.

Steering the Cutter

It is important that the work be done from the forearm rather than the fingers or the wrist. The elbow 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. Of course, for long cuts your arm will have to extend from you body in a parallel direction with the score line.

All these instructions about holding the cutter are reasonably straight forward when cutting straight lines, but become more difficult when applied to curves. The temptation is to use your wrist to make the curve. However this both tilts the cutter – giving the results described – and risks the cutter sliding across the glass.

You should attempt to be behind the cutter at all times. This means that the steering action should be from your body. Most times, turning your torso from the waist is sufficient to enable the cutter to follow the curve. On curves which are deep or go through more than 90 degrees, it is best to place the glass at a corner of the work bench and “walk” around the piece by both twisting at the waist and moving your feet around so that you can make a 270 degree cut without moving your forearm from your side.

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 4 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 consistently at the 4 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 4 kg pressure and break the glass. You will see and feel the lesser scoring 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. 4 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.

The pressure needs to be applied consistently throughout the length of the score. Uneven pressure leads to inconsistent breaks.

Scoring Opalescent Glass

Cutting opalescent glass often gives difficulties in getting clean breaks along the score line. You need to remember that the opals do not make much if any sound when cut with the correct pressure. If you are scoring so that you hear the ziiip sound, you probably are pressing too hard. When the score is too hard, the opals do not break easily or truly. Only the same pressure as used on transparents is required. Feel the pressure rather than listen for the sound.


The speed of the cutter needs to be consistent too. If the speed is not nearly constant, different pressures are transmitted to the glass. This also leads to inconsistent breaks.

Where to Start Cuttng the Glass

As a general rule, always make the hardest cut first. Glass tends to run in a straight line. This means inside curves should be done before any other cut is made. This avoids excessive wastage should the break come away from the score line.

Glass placed to make the inside cut at the right first

It will be most efficient to place the glass to be cut with the inside curve facing the raw edge of the glass. If something goes wrong, the glass can be moved and tried again, resulting in less glass and time wasted.

It is also hard to run very thin strips of glass without getting ragged, chipped edges. Allow a 6mm minimum distance from the edge of the glass when placing the glass on the cartoon unless the edge glass is going to be used for the whole edge of the cut piece.

Direct or Trace cutting

Place the glass over the pattern and run the cutter along the cartoon lines you see by looking through the glass. There's no need to draw lines on the glass. For translucent glass you may need a light box.

You should be aiming to cut glass efficiently and accurately. Trace cutting is the most efficient, as it completes in a single operation what other methods – such as drawing on the glass or making templates from the cartoon - take several steps to accomplish.

It is more accurate because each extra step required for other methods increases the possibility for error. The fewer times you copy the original pattern lines, the less likely you are to diverge from the original pattern.

It is very important to keep the cutter at right angles to the glass - as seen from side to side, not vertical. This of course is true of all cutting. It makes the cutting inaccurate, because the light is bent when coming through the glass much like water changes the apparent angle of sight into its depths. Tilted cutters also have undesirable effects when breaking the glass.

Cutting with Patterns or Templates

When scoring around a paper pattern it is necessary to steer (turn) the cutter in the proper direction. The paper will not turn the cutter for you. You should steer the cutter by turning your upper body rather than your fingers, wrist or elbow. Failing to do this may allow the cutter to run over the pattern and so fail to score the glass.

Alternatives to using the paper pattern directly as a guide in cutting glass are to outline the pattern paper on the glass with a pen or to draw the pattern on the glass while it overlays the drawing. You follow the inside edge of the pen line with the cutter. However this results in cutting glass much as when cutting directly over the cartoon and so merely introduces an additional step in cutting glass.

This shows a set of templates in use for repetitive cutting. The template is drawn around for subsequent cutting

Directly cutting the glass over the cartoon avoids the time spent in making patterns, and the difficulties and inaccuracies in multiple transfers of the shape. Often a light source is required under the cartoon to enable the lines to be seen through the glass. There will always be times when the glass is so dark or opalescent that the lines cannot be seen and therefore a pattern is required.

Keep the pattern cutting restricted to the times when nothing else will do. The only times I use patterns for cutting are when the glass is too dense for the cartoon lines to be seen through the glass with light behind or for repeat shapes where a pattern can speed the process.

I draw around the pattern pieces, as that avoids the possibility of the cutter riding up on the card that I use for templates. This comes from several occasions when the cutter did go over the template which prevented the score and so created a bad break. Others do score successfully around the template stuck to the glass.

Pattern Scissors Usage

The purpose of pattern shears/scissors is to cut out the space between pattern pieces equivalent to the came heart or the space needed for foil.

The scissors come in two thicknesses – one for leaded and the thinner for copper foil.

If you must use pattern scissors, use them in short cutting motions. Use only the first 50mm of the blades which are closest to the pivot point. Otherwise the paper jams in between the blades. It remains difficult to cut long straight lines without quickly having an “accordion” of paper blocking the cutting action.

Some suggestions to make things easier:

  • Clean the blades regularly. If you are cutting anything with adhesives, clean the blades after each use with spirits.

  • Often running a little soap along the blades helps to lubricate and smooth the action of the blades.

  • Use stiff high quality paper so you do not catch fibres in the scissors. Waxed paper or stencil card are good materials to use.

Organising pattern pieces.
You have made a second and third copy of the cartoon haven’t you?
 Now that you have a lot of pieces need to decide how to organise them
  • Mark any grain direction before you cut the pieces apart.
  • You need to code the pieces in some way. 
Numbering with reference to the main cartoon is most common. 

  • It is a good idea to colour code the pieces and if the surface will take it, a shading of the colour makes a quick visual reference.

Keeping the pieces together
Envelopes are easy to write on for colours, or areas such as borders, background, etc.
  • Freezer bags that are transparent and have a band to write on are very good, as you can see the pieces without opening the bag.
  • You need a labelled bag or container to keep all the envelopes together.

Alternatives to pattern scissors
For copper foil, you can use normal scissors, by cutting to the inside of the pencil or inked line. You can also use a scalpel or craft knife to cut to the insides of the marked lines.

For leaded glass you can use a felt tip pen (a bullet point is almost exactly the right width when new). Cut with scissors or craft knife at the sides of the line.

Alternative to pattern pieces

Use the European or trace cutting method as described here.

Breaking glass with your fists 

For scores with significant, but not necessarily equal, amounts of glass on each side of the score this is a quick simple approach to breaking glass. After scoring, raise one edge of the glass and put your fingers under the glass on each side of the score. Curl you fingers into your palm, and put your thumbs on top of the glass. Turn your wrists outward while holding the glass firmly, and the glass will break cleanly.

With practice, the initial part of a curved score can be run by applying light pressure. Then you can turn the glass around and run the score from the other end to the opened score. This avoids lots of tapping and gives clean edges to the cut glass. It is just as simple as using cut running pliers and avoids the flare often associated with using cut running pliers.

This technique works best with glass that has at least 50 mm each side of the score and on gently curved lines. For tight curves and narrow strips other methods need to be used.

Breaking Pieces from Large Sheets  

Breaking a piece of glass from a large sheet is often a frightening prospect. It doesn't have to be. It is better to cut a straight line piece from your larger sheet than it is to try to cut a curve.

Use a cutting square or other non-slip straight edge to guide the cutter. You can push as in normal stained glass cutting, or you can draw the cutter toward you as glaziers do. In either case, the pressure needs to be even and the speed consistent.

When moving large scored sheets, avoid pulling the sheet by one end. The score may run suddenly and not always along the line. Instead, move the sheet with support on both sides of the score.

After the glass is scored, you have choices about how to run the score.

One easy way to break off large pieces is to move the sheet so the scored line is just inside the edge of the bench. The biggest piece will be on the bench and the smaller piece in your hands. Give a quick, sharp downward push with both hands on the overhanging glass. This action will separate the piece from the main sheet. Having the glass score inside the bench edge gives you a place for the broken off piece to rest, rather than pivoting toward the floor.

Or you can slide the straight edge under the glass on one side of the score, and press firmly, but not sharply on each side of the score. The glass will break evenly along the score line. This is a more gentle method of breaking the glass. A variation on this is to place a couple of matchsticks or glass painting brushes at each end of the score and apply the pressure.

If the glass sheet is of a size that you can hold it in both hands with the score between, you can draw it off the bench, let it hang vertically, and bring your knee up briskly to hit the score line, and it will break easily. This is a showman’s way of breaking glass sheets when the score line is approximately centred on the sheet.

Cut running pliers often do not work very well for long straight scores on large sheets of glass. However, if you use this method, tapping at the start and at the end the score line before squeezing the running pliers will help the score to run the way you intend. This is sometimes the only way to achieve the break of the score.

Refining rough cuts and sharp 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 more sustained 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.

*This section has been prepared from information provided by the Fletcher-Terry company:

Other sections:

No comments:

Post a Comment