Cutting circles from sheet glass

There are a number of ways in which this can be done, including;

• Using a straight shaft circle cutter,

• Using a gooseneck circle cutter,

• Freehand cutting using a cartoon, or,

• Cutting using a template.

Using a straight shaft circle cutter

With a common size of this tool it is possible to cut circles up to 800mm diameter. Tools with longer shafts are available to cut even larger circles.

The tools

Fig 1

A full set of tools used for cutting circles are shown in fig 1 above.
A.    is the adjustable circle cutter
B.    Is a fibre and plastic faced mallet
C.    Is a crack running tool
D.    Is a small brush for applying lubrication to the cut path
E.    Is a pair of running pliers
F.    Is a swivel head self lubricating glass cutter.

Fig 2
The circle cutter comprises a lever lock suction cup assembly around which can rotate a square brass scale rod. This rod carries a cutting head assembly which can be slid along it and locked in any desired position by a knurled screw. The rod is calibrated on its top face with a scale in millimetres against which the inside face of the cutting head is locked to cut circles of the desired diameter.

A free swivelling knob on top of the locking screw allows proper force to be applied whilst the arm and cutter is rolled around the glass. The knob is carried on the top of the cutter locking screw. Sometimes the knob may bind on its shaft, so it is best to turn the cutter in an anti clockwise direction to avoid the risk of the cutter locking screw being loosened and the cut circle not being true.

Preparing to make the score

As was seen when making straight cuts, it is easier to make the break when there is a significant width of glass either side of the break line. So also with cutting circles, there should preferably be a significant width of glass outside the circle to be cut. This is not essential, but when the scrap is narrow there is more chance of the crack running wild and breaking inward instead of going where it should.

• Place the glass on a firm base and wipe clean of dust.
• Set the cutter to the correct diameter.

• Position the suction cup at approximately the centre of the glass and lock in position by moving the locking lever from the vertical to the horizontal position.

• Lift the cutter clear of the glass and swing it to all four sides to make sure that it is correctly positioned. Release suction cup and re-position if margin is too narrow on any side. 

Fig 3

Holding the cutter slightly clear of the glass, take the oil brush and place it in line with the cutter and run an oil track right round the circle where the cut will be made.

Making the score

• Place one hand on top of the locked down suction cup and with the other hand lightly grasp the knob on the cutter. Apply pressure to the cutter and score the full circle. In Fig 4 the experienced operator is not only holding the knob but also making sure the locking nut does not come loose. Obviously a lesson learned in the school of lost glass.

Fig 4

At the end, try to not track over the start of the score. If the score is hard to see, it’s a good idea to use a felt pen to mark the start point so this won’t happen.

• Remove the cutter from the glass. Wipe up excess cutting oil

Cracking the score

• Turn glass over on table so that score is on the bottom.

Fig 5

• Take crack running tool, place it centrally over the score and tap with the mallet until a crack appears. 

• A crack running tool can be made from any soft wood with dimensions roughly approximating those shown in Fig Fig 6.
   

Fig 7

• Move tool to near one end of the crack and tap until crack travels further.

• Continue to chase the crack until it is completely round the circle.

Fig 8

• Turn the glass over again and tap gently all the way round the break line with the mallet to make sure the break is complete. With thin glass there is a chance that both the circle and the outer ring can be retained, but the likelihood gets less the thicker the glass.

 Removing the dump or waste glass

Fig 9

• Cut the outer dump free by making cuts on the four sides from the circle break line to the edge of the glass.

Fig 10

• Using the running pliers, crack the cuts just made and pull the dump away from the circle.

Cutting a circular hole

Sometimes it is the outer which is to be kept in preference to the inner circle.

• To do this, proceed as above to the stage where the crack has been fully cleared as at Fig 8.

• It is then a matter of removing the inner, if necessary by cutting it in small pieces and breaking them out a piece at a time. It is important that minimum force be exerted on the outer.

One technique is to cut a series of smaller circles inside the main one and then make numerous radial cuts from the centre to the edge of the wanted piece. The waste can then be broken out a piece at a time starting at the centre.
Undoubtedly there are other techniques and any one having to do this task will soon find the one which suits best.

Using a Goose-neck cutting machine

Fig 11.

Another type of circle cutting machine is shown in fig 11. Sometimes called a goose-neck cutter. It can cut circles from xxmm to zzmm in diameter and is ideal for cutting large numbers of similar size circles.

The cutting head is carried on the end of an adjustable radial arm. The frame has a vertical spring loaded shaft with a crank on its top and a clamp to accept the radial arm on its bottom. Putting downward pressure on the crank lowers the cutting head onto the glass.

The cutting head is self lubricating, so there is no need to spread lubricating oil with a brush. However, because there is no way to successfully stop the oil flowing out of the reservoir when the cutter is not in use, many people use a brush to avoid the mess of oil leaking out when making only a small number of cuts. In this image the operator is using an oil brush to run an oil track.

Making the score

• Make the score as with the other cutter, again marking the start of the cut with a felt pen to avoid running over the previous cut and blunting the cutter wheel.

Breaking out the circle

Fig 12

Unlike the previous circle which was cut from thick glass, this one has been cut from thin 3mm sheet. In this case the crack can often be run simply by placing in on the felt cutting board and applying pressure with the thumb directly beside the score; repeating the pressure at the end of the crack as many times as necessary to completely break the circle.
 Often, thin circles can be broken out simply by continuing to apply pressure in this way after the crack has been run all the way round, so that both the disc and the rim are retained. Whether or not this is successful will depend on the quality of the score and whether the sides of the crack are square with the surface of the glass. Cracks which are uneven, with sides which vary from the perpendicular to lying on an angle will be difficult to break out without cutting the rim as previously shown.

NOTE:

It has been noticed that the vertical shaft of this type cutter is not always absolutely perpendicular to the base board. This means that, unless the central shaft can rise and fall as it rotates it will not allow even pressure to be applied at all points of the score. This can possibly be adjusted by loosening the screws holding the arch to the board and inserting paper packing in the appropriate place. It will require very little packing, but need patience to get it right. 

Lubricating the central shaft where it passes through the body of the arch frame may allow it to move up and down more freely and minimise variation in the cut. Place oil in the top with the cutter in its normal position, but it must be inverted to apply oil at the bottom as there is a flange on the shaft inside the housing which stops the oil running down from the top.

Freehand cutting

• Of course, circles can be cut without a circle cutter, in a manner similar to the way leadlighters cut most of their glass. As was mentioned in 11. Straight cutting of sheet glass, leadlighters cutting freehand usually cut away from themselves so they can see the line ahead of the cutter.

Fig 13

• A circle of the correct diameter is drawn in ink or pencil on a piece paper (cartoon), the glass is placed on top and a self lubricating glass cutter is used to score along the image line seen through the glass.

If the glass is opaque there are a number of techniques which can be used to mark the circle directly onto the top surface of the glass.

• Use a drawing compass with a fine felt tip pen in place of the pencil to draw the circle. Use a piece of Bluetak or putty to hold the pointed leg at the centre.
• Paint the cut area with diluted water based paint and use a pair of dividers to scribe the cut line as soon as the paint has set. It will wash off easily if not left too long.

Fig 14

• The score is cracked and run as previously, choosing the method most appropriate to the thickness of the glass and the quality of the score.

Of course, cutting is likely to be somewhat more uneven than would be the case if using a circle cutter, so there may be need for some trimming using grozing pliers or  a diamond grinder.

Cutting using a template

Especially convenient for cutting multiple pieces of an irregular shape is the use of a female template of the shape, around the inside of which is run the cutter. If the shape is reasonably circular or comprises gentle curves an ordinary glass cutter will follow the shape reasonably well.

Of course, the template must be made larger than the piece to be cut, to allow for the width of the cutter.

Where the shape involves sharp corners or angles and the cutter head cannot swivel, then a diamond tipped wheel dressing tool can be used. This is a tool used for dressing the face of abrasive grinding wheels and consists of an industrial diamond fixed into the end of a piece of steel rod. The tool must be held vertical and is not as easy to use as conventional glass cutters.

Compressed wood sheet such as Masonite or thin MDF makes good templates as they are thick enough to ensure that the body of the cutter will contact them securely.