Sunday, 5 February 2012

Cutting Glass

Tools and Materials

Glass Cutters

There are a large number of styles and types of cutters available on the market.  It is best to get the feel of one before buying.  You can try various styles out in classes, and usually shops will have examples for demonstration. 
Simple steel wheel cutters


The cutters above are inexpensive and that is the only advantage.  The one on the left with a turret of wheels is unsuitable, as the need to have a number of spare wheels might indicate.  It also is unsuitable as its design obscures the cutting line in use.  The cutter on the right is a traditional plain steel cutter with notches for grozing different thicknesses of glass.  It is better, but because of its large wheel, requires more pressure to create the score.


Oil filled pencil type cutters
These cutters are examples of high quality oil filled cutters.  The choice between plastic and brass barrels is one of preference and often is related to the way in which you hold the cutter.  This picture illustrates the narrow and broad heads for cutters.  The broad head is most useful in cutting straight lines with a straight edge to rest beside.  The narrow head cutters are most useful in cutting curves from a cartoon or pattern




Toyo thumb grip and a generic pistol grip cutter
These are two examples of the various shapes in which cutters are produced.  In both these cases the pressure is applied with the palm of the hand rather than the fingers.  The Toyo is relatively easy to steer around curves, while the pistol grip is a bit more difficult.  Both are expensive in relation to the pencil type.


Glass Cutter Wheels

There are many different types of glass cutters on the market today. Choosing the proper cutter is the most important decision you must make, as it will be a large element of how you enjoy scoring and breaking glass. There are some elements that you should consider in addition to the style of the holder.

Steel Wheel Cutters are inexpensive but usually not long-lasting. They must be replaced frequently as they easily become damaged.

Tungsten Carbide Cutters are more expensive than steel wheel cutters, but far outlast them in durability. Self-lubricating cutters eliminate the need to constantly lubricate your cutter between scores.

Wheel Angles. There are steel wheel cutters which are good for general purpose cutting, production cutting, or pattern cutting. Some are good for cutting thinner glass, others for thicker glass. The angle of the bevel on the wheel determines this and differs from model to model, although some offer wheels of different angles. Pistol grip cutters produce more pressure than others.


Cutting Wheel Angles*

These are the wheel angles recommended by The Fletcher-Terry Company for various glasses:

114 to 134 degrees – 2mm float glass
130 to 140 degrees – 4mm float glass
134 to 140 degrees – 3mm to 6mm float glass
148 to 154 degrees – 12mm to 25mm float glass

This suggests that wheel angles of 134 to 140 degrees are suitable for stained glass work.

Wheel angles of 88 to 114 degrees are suitable for borosilicate glass.



The Effects of Wheel Angles on Glass Cutting*

Measurement of angle
The wheel of a glass cutter does not “cut” the glass. The objective is to create a crack or "fissure" along which we expect the glass to break when we bend it. The idea is to produce a fissure which is continuous, and of uniform depth, without creating a flaky score line full of loose glass chips. While the wheel angle is only one of several variables which influence the quality of the fissure, it is the best place to start. The other main variables are wheel diameter and cutting pressure. The angle is identified as the included angle to which the apex is honed. This means it is measured from one beveled face of the wheel around through the wheel to the other face. Thus the angle between the wheel and the glass on a 150° wheel will be 15° on each side.

Cutting Pressure
When downward pressure is exerted on the wheel rolling along the glass, forces are created which radiate down and slightly to the side trying to shear or separate the glass along the surface. If these forces are great enough to overcome the inherent compressive conditions near the surface, a crack will be generated along the path of the wheel.The direction of these shearing forces is determined by the wheel angle.

A wheel with a large or blunt angle produces shearing forces that tend to be directed downward more than to the side. It would require a great deal more cutter pressure to create enough lateral force to overcome the compression in glass. This explains why a cutter requires more pressure as it gets older. The apex tends to flatten so its effective angle becomes greater.

With a very sharp wheel angle, the shear forces are directed more parallel to the surface of the glass. This suggests it is easier to produce a fissure with a sharp wheel than a dull one. The shear forces are directly opposing the compressive condition near the surface of the glass therefore, requiring less downward pressure to make a crack.

A sharp wheel tends to cause chips and a flaky score. If the shear forces run close to the surface of the glass they are more likely to cause a lateral crack which then breaks out to the surface, creating a chip. You can see these chips leap out of the glass a minute or so after scoring. Again, the compressive condition of glass near the surface literally squeezes the fissure closed, spitting out loose chips. They can be seen lying on top of the glass.


Effect of wheel angles on the cut edges of glass

Another factor to consider in selecting the proper wheel angle is the "edge". The objective of good glass cutting is to produce an edge which is flat and relatively free of irregularities such as "shark teeth".

Shark teeth are the occasional deep spikes in the edge and are accompanied with flakes or tiny chips on the surface. A three mm thick glass scored with a sharp wheel (114°) will produce this effect. This edge irregularity may lead to failure during the life of a window.

A three mm thick glass scored with a proper angle (134°) of wheel, will produce a fissure that is made up of individual "hackles" which overlap one another. They have a unique semi-circular shape and indicate the direction of the cutting wheel. With proper pressure the edge will be relatively free of irregularities and without shark teeth.

The effect of glass thickness on cutting

Most of the thicker glass being used today is produced by the "float" method. In this process the glass travels horizontally from the furnace, through a molten tin bath, through annealing lehrs, then continues on rollers where it is inspected, scored and broken into the sizes required. The thickness generally dictates how fast the ribbon of glass moves. The thicker the glass, the slower it is processed and the more effective the annealing. This applies to thicker art glass too.

The key to subsequent cutting is the annealing cycle. Thicker glass tends to have less compression at the surface and tension in the interior. As a result, the glass cutting wheel encounters less resistance to producing a fissure with the shearing forces. However, this means the glass surface will chip more readily. Therefore, a larger wheel angle is required to prevent chipping. It is also common practice to use a larger diameter wheel and larger angle so the fissure can be driven deeper without chipping.

Cutting Oil*

No matter how good a fissure is when scored dry, it is better if scored with a liquid between the wheel and the glass. Several good things happen with an "oil" cut and only one undesirable thing.

The bad thing is you have to wash the glass afterwards, but in many cases washing is required anyway.

A good thing is the fluid reduces the effect of healing - the compressive strength overcoming the fracture caused by scoring. It is probable that the liquid seeps into the fissure contaminating it enough to prevent atomic reattachment of the molecules.

Cutting oil reduces chipping and prevents a flaky score line. The oil tends to provide a hydraulic cushion between the glass and the wheel. This allows more uniform transmission of the shearing forces into the glass at an angle dictated by the wheel, not by particles of crushed glass.

It is important to check the cutter wheel is moving freely, since a wheel not rolling freely may skid. Skidding causes abnormal wear to the wheel and subsequently it becomes a skipper.
You should not use kerosene or white spirit by itself because it removes whatever oil is on the axle.





Breaking Pliers

Description

The breaking plier is a special stained glass tool that has smooth jaws that meet at the tip of the pliers. This enables the tool to reach over the top and bottom of the glass with only the tip coming into contact with the glass exactly against the score line.
Large breaking pliers

Use

The plier handles are held at a right angle to the score line. The edge of the glass needs to be close enough to the score line in order to use this tool, as the tip of the jaw needs to be against the score line. It is used in lieu of your hands when the piece being broken off is too narrow to be comfortably grasped by hand. When bending the glass, the top jaw comes down flat against the surface of the glass (that's the reason for the smooth jaw) and as the bending pressure is applied, lateral/pulling pressure is applied at the same time. This tool can also be used to groze the glass by carefully nibbling away the edge.

Note
If the score line is further than 20mm away from the edge, cut running pliers are usually the appropriate tool to use.





Combination grozing and breaking pliers

Description
The jaws of grozers are serrated and used to gently remove small pieces of glass which remain after the glass has been scored and broken. They are normally supplied with one straight jaw and one curved making them combination grozing and breaking pliers.


Narrow and standard width combination pliers

Use as breaking pliers
When used as breaking pliers the flat side should be up and the nose of the jaws almost touching the score line. The breaking pressure should be down and slightly to the side to bend and pull the piece away from the main piece of glass.

Use as grozers
When used a grozers – to gently wear away some small part of glass - the curved jaw should be up. The plier jaws should be used slightly open, and a downward motion of the plier does a “rasping” of the glass edges, taking away small bits of glass. With experience, a lot of glass can be removed quickly shaping the glass with a fair degree of precision. The edge can then be tidied by a grinder if required.


Cut running pliers 

These are intended to assist with the running of long straight or gently curving scores.

Description
These are pliers with curved jaws. When looking at the pliers from the nose toward the handles you will see the curve like a down turned mouth. These assist the breaking of the score by putting tension on the glass.


Steel cut running pliers



Cut running pliers almost always are supplied with covered jaws. This are normally two pockets of flexible plastic that fit snugly onto the slightly flared, curved jaws. Keep these on. When worn they can be replaced by buying more, or by wrapping the jaws with tape, using "liquid plastic", or sometimes, even using them bare.

There is a spacing screw on top which allows adjustment for different thickness of glass. It is intended that the jaws should not close completely – which can place excessive pressure on the glass – but be adjusted so that there is about 1mm less opening than the thickness of the glass. So when cutting 3mm glass there would be a 2 mm gap. On 4 mm glass the gap would be 3 mm, and so on. experience will show what relationship is best for you. The principle is that the pliers perform a gentle tensioning of the glass. Using your hands exerts more tension on the glass and cannot be so delicate.

Use
Align the centre of the top of the jaws - often the pliers have a centre line on the top to assist – along the direction of the score. Squeeze gently and the score will begin to run.

I have found my best success when using cut running pliers is to avoid trying to run the whole score from one end. With a bit of practice you get the feel for how much pressure you should be applying and so when to stop. Then turn the glass around and start the run from the other end. Usually the open ends of the score will meet and the break is completed. Occasionally the two runs will not meet. Then it depends on how complicated the curve is as to whether you use your hands to complete the break or start tapping the score line to finish the break.


Pattern Scissors

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.



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) to draw the cartoon or template. Then cut with scissors or a craft knife at the sides of the line.




Glass Cutting Surfaces

Make sure you are putting the glass to be cut on a smooth and level surface with no glass shards. If the surface is uneven, it will give difficulties in scoring and breaking. The tell-tale squeaks as you move the glass indicate there is other glass under the sheet. These shards and any other small almost invisible things under your glass can promote unwanted breaks. Also, if there is glass or other grit on the surface, it may scratch the glass. So make sure you brush the cutting surface clean frequently, and ensure there are no bumps.

Many people have a slightly cushioned cutting surface. Some use short pile carpets or rugs, others use thin rubber or foam sheets, others use dining table protectors. All these are useful for cutting large pieces and have advantages and disadvantages.

Carpets and foam can trap shards of glass, so have to be cleaned very carefully to avoid retaining sharp glass within the pile or foam.
Smooth, wipe-able surfaces avoid trapping glass, but can be slippery. Choose one with a non-slip surface.

It is better to cut small pieces on smooth hard surfaces, as the flexible surface will not provide overall support, and so allow breaks, especially on long thin pieces.

Clean the glass, at least along the cut line, as this makes the action of the cutter smoother. The grit on the glass actually interrupts the action of the wheel, so you get a staccato effect in the score line.



*This section has been prepared from information provided by the Fletcher-Terry company: http://www.fletcherviscom.com/home.shtml

1 comment:

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