Straight cutting of sheet glass

Cutting sheet glass involves the making of a score on one side and then cracking it along the score.

A good score should have clean sharp edges free of chipping.

Doing this is not difficult, but it is greatly simplified if one uses the correct tools and follows the proper procedure.

The cutting operation is simplified if done on a proper cutting surface. It should be flat and preferably covered with felt, to prevent the glass being scratched and to give a firm grip to it.

One can be easily made from a piece of chipboard covered in felt which is stretched tight and secured on the underside with wooden strips. It can be packed away in a plastic bag when not in use.

The size can vary, depending on the size of glass sheet you plan to buy and cut down.

Tools

Essential tools include a 90 degree square, a glass cutter, a measuring rule or tape and some cutting oil.

Glass cutting squares have a small insert on each side of the base leg which registers against the edge of the glass: allowing it to be used for both left and right hand cuts. Engineers squares are not suitable.

They are available in a range of sizes allowing one to have a selection at reasonable cost: from a small square for cutting small pieces of glass to one large enough to break down the largest sheet to be handled.

One with a blade about 600mm long is a good compromise for general use if only one square is needed.

Fig 11-01

The next essential item is a good quality glass cutter. There’s a wide variety of styles and shapes available but three versions of the most common style are shown in Fig 1. They consist of a cutting head, a body and a weighted screw cap. (label parts)

To minimise chipping the glass, and to extend the life of the cutter wheel, the wheel or the cut area should be lubricated. The bodies of the illustrated cutters double as a lubricant reservoir. The lubricant can be mineral turps or a good quality glass cutting oil.

Inside the head is a fibrous wick which conducts the lubricant from the reservoir onto the cutting wheel.

To prepare for cutting, remove the screw cap plug and put some lubricant in the reservoir. Leave the cap loose to allow the oil to flow to the wick.  Be sure to tighten the cap when not in use to stop the oil running out.

Fig 11-02

Let’s have a closer look at the cutting head. 

The head can swivel on the body, ensuring that the head remains in contact with the edge of the square or cutting guide and parallel to the cutting line: unaffected by minor twisting of the body itself.

The line through the wheel is offset from that through the body so that, when the score is made in the correct direction, the wheel trails behind the body: enhancing the tracking action.

When the body is held at the correct angle with respect to the glass, the downward pressure on the body ensures optimum tracking and clean cutting.

The cutters in 11-02 are designed to be drawn toward the operator.

There are other swivelling head cutters designed to be pushed away from the operator, as shown in Fig 3.  In these the relative position of body and wheel axis has been reversed.

Fig 11-03

They are used for freehand cutting of irregular shapes, typically by leadlighters who wish to follow curves or wavy lines, and are not normally used when making straight cuts with straightedge or square.

Cheaper cutters without the swivelling head are shown in Fig 4.

While these cutters are effective tools, and cheaper than the swivelling head type, they require more skill to use effectively and also require the cut line to be hand lubricated before making the score. For this purpose you can use a piece of cotton wool dipped in mineral turps or glass cutting oil.

Fig. 11-05

Other than holding the cutter with the bottom face square to the glass as in Fig 2, and not drunk like in Fig 5, there are no right or wrong ways to hold the glass cutter. They can be held in either hand. Some people are ambidextrous, some right or left handed only, so the way the cutter and square are held, and how the glass is laid out for cutting, will depend on the individual. Practice with scrap glass and find the way that suits you best.

Diamond as a glass cutter

Reference is made elsewhere in this site to the use of a diamond as a long life glass cutter. It should be noted that all the cutters shown here use a sharp edged wheel so that it can roll over the glass and make a clean edged score. This makes for the cleanest cut. 

As a diamond cannot roll it can produce a more jagged edged score than a wheel cutter. A diamond embedded centrally in the end of a steel rod has the advantage of being able to be moved in any direction and can follow a complex template without having to worry about the uneven shape of the head. It is not recommended for general cutting. Such a tool is used in engineering as a grinding wheel dressing tool and is available from major engineering supply houses at reasonable cost.

Making the score

The score should be made with the correct amount of pressure. Too little and the crack may not follow the score line, too much and the edges of the score can be chipped instead of being clean and sharp.

Make the score line with one smooth even stroke along the full length. Try not to halt anywhere along the run. If you must stop, try and keep the cutter in place and start again without lifting the cutter from the glass.

If you must lift the cutter and start again, do not go back over the previous cut. Apart from being the most certain way of mucking up a good cutter, the score will be a mess and can crack anywhere. Better to leave an uncut gap of a millimetre of so between the old and the new scores. Not too much, but just enough.

Preparing to cut the glass

 Fig 11-06

First check that the glass is square. If it isn't you will need to trim it to make it square.

It is wise to first check that the edge of the glass that is being used as the ‘base’ edge is straight, and doesn’t have a curve or unevenness, (a possibility if during a previous cut the straightedge slipped).  

 Fig 11-07

Position the set square ready for the first cut. Remember that the cutting head is 5mm wide, so half this amount must be added to or subtracted from the glass measurement required when positioning the square. For convenience, this allowance is assumed to be 2mm. See Fig 2C.

Here, the cut is being made on the 'wanted piece' side of the square, and the square is resting on the waste piece, so 2mm must be added to the desired measurement when positioning the square. If not, the glass will be 2mm too small. 

Make sure that the lip on the base of the set square is pressed firmly against the bottom edge of the glass. Put firm pressure on the square with one hand to ensure that it won’t move.

Fig 11-08

Make the score. Starting as close to the top edge of the glass as possible, apply even pressure and move the cutter smoothly to the bottom of the sheet. Note the way the middle finger presses the cutting head against the straight edge.

Make sure that the cutter is held at the correct angle of slope toward you and that it leans neither to right nor left.

 Fig 11-09

If the square was on the wanted piece, as in Fig 9, the 2mm cutting head allowance would be subtracted from the required glass measurement when positioning the square. 

Cracking the glass

There are a number of ways of cracking glass, depending on the thickness of the sheet, the distance of the score from the edge and on ones experience and confidence. They all work on the same principle; of supporting or putting upward pressure under the score line and downward pressure either side of the score.

Here are some of them:

Fig 11-10  

Using thumbs to crack across the knuckles, as in Fig 10.

 Fig 11-11

 Fig 11-12 

Cracking on edge of bench, as in Figs 11 & 12.

Position the glass so that the score line is just back from the edge of the bench with the wanted piece well supported and the offcut overhanging the bench. Apply downward pressure on the offcut. This method works best when both pieces are of substantial size.

 Fig 11-13

There are a number of types of pliers used for glass breaking and trimming, as shown in Fig 13.

The ‘running pliers’ shown have soft padded jaws and are used in a manner similar to the thumbs and knuckles in fig 10. Hold the pliers so that the jaws describe an inverted Vee, and grip the glass so that the score is in the centre of the Vee; apply pressure.

 Fig 11-14

 Fig 11-15

 Fig 11-16

 Fig 11-17

When the offcut is narrow, or when one wants a narrow strip of glass, the break can be ‘run’ using ‘breaking’ pliers as shown in the series figs 14 to 17. These pliers have smooth parallel jaws.

Starting right at one end, the crack is started by gripping the strip in the pliers and applying a gentle but firm downward pressure to the pliers; halting it immediately a crack appears.

Advance the pliers along the strip to near the end of the crack and repeat as above: as many times as is necessary to remove the sliver in one piece.

In Fig 14, note the way the glass is gripped in the right hand; tightly between the thumb and the knuckle of the first finger. The finger tip is close to the score where upward pressure can be applied; but not under it. To put it there is asking for it to be cut.

Fig 11-18 

Sometimes the strip won’t break cleanly and jagged pieces will remain. These pieces can be removed using the grozing pliers shown in Fig 18. They have serrated jaws and one curved and one straight jaw.

Cutting circles

Cutting textured glass

The break will run best if the score is even and continuous.This can be difficult with some of the textured rolled glasses. Where one has the choice, make the score on the smoother side.

Edge treatment

 Fig 11-19

It is good practice to clean up the edges of the glass, not only to reduce the risk of cutting oneself, but also to improve the quality of the fired edges. This can be done in a number of ways, including:

• the use of an oilstone of the type used for sharpening chisels or axes,
• abrasive papers or cloths of the type used for sanding belts or sanding down paintwork, either laid flat or wrapped around a sanding block,
• special holders of abrasive stones of the type shown in use in  Fig 20 which grind both edges of the sheet at the same time.

 Fig 11-20

For more on abrasive materials see Grinding & polishing glass.