Saturday, 18 February 2012

Cutting Glass

Cutting Shapes

Outside Curves

Outside curves are usually the easiest curve to break out. You do not have to worry about breaking the piece, as the break - if it goes off the score line - will be away from the piece rather than into it. Outside curves normally 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 that would be the last, as it is the most straightforward cut.

Deepest concave curve taken out

Curve on right that has both concave and convex curves is taken out second

Score started from both ends and finally tapped out

Simple curve on the left taken out

Finally the almost straight line is cut and removed

Cutting Circles

Fixing the suction cup at an appropriate place on the glass sheet

Start score at limit of movement of arm

Hold the cutter nob firmly and rotate until you hear the click of the wheel meeting the start of the score

First, score the circle, making sure that you start and stop the score line at the same point. Circles can be scored freehand or with a circle cutter.  This example shows the use of a circle cutter with a graduated arm.

Looking closely shows the scored circle

Turn the glass over - the scored line shows more clearly

On a firm, but not hard surface begin to run the score by pressing over the score  with your thumbs

Turn the glass over onto a piece of corrugated cardboard, or other surface with some give, with the score line face down. With your thumbs, press along the score line until you see the score line "run" progressively and completely around the circle. This prevents the relief scores you are going to make from running through the circle.

Continue running the score completely around the circle

Turn glass back to scored side and mate relieving scores at right angles to the run score

Turn the glass back over to the side on which you scored it. Score several lines perpendicular to the circle to the corners of the piece of glass. Gently open these scores by tapping with the ball of your cutter, or with your hands, pliers, or other tools.

Tap the relieving scores to run them to the circle

Break the pieces away from the circle without tapping it to give a cleanly cut piece of glass

 The the pieces should fall cleanly from the circle leaving you with no rough or jagged edges.

Cutting Circles from Opalescent Glass

Score as normal.

Be careful about putting too much pressure when scoring as, in general, opalescent glass does not make as much sound when being scored as transparent glasses do.

The difficulty with opalescent glass is seeing where the score is when you turn the glass over.

If you are using a piece of glass not much larger than the circle you are cutting, you can place the fingers of your hand over the score line and your thumb on the back as you lift the glass to turn it over. This gives you the location to begin the pressure to run the score. As the first part of the score runs, you will be able to follow the leading edge of the opening score around the circle.

Positioning the Circle Cutter

If you have a suction cup on the circle cutter, it will be easier to hold in place. But a three legged circle cutter is possible to keep in place too.

In both cases, one hand holds down the centre and the other operates the cutter. Make a test circle with no pressure to ensure before you start that you know the cutting bar will not bump into anything else on the bench. This also ensures that you have the circle to be cut placed appropriately on the glass.

To make the score start with the bar under your supporting arm and swing around to the other side of your arm until you hear the click or scratch indicating that you have come back to the start.

Cutting Small Diameter Circles

It is possible to cut regular, small diameter circles without buying a lens cutter. It can be done with the assistance of a Lazy Susan or cake decorating turntable.

Cake decorating turntable with circle inscribed

Draw the circle of appropriate diameter on the turntable with a compass. Place the glass on top of the turntable, and position your cutter above the drawn circle. Press on the cutter with one hand and turn the glass with the other.

Glass scored and tapped to run the score 

Steady your hand with the cutter by keeping your elbow tight against your side. This enables you to make a very good, if not perfect, circle without buying an expensive small circle cutter.

Relieving scores tapped oug

You will not be able to run the score by turning the glass upside down and pressing as you can with larger circles. You will need to make a number of relieving cuts to the tangent of the circle and break them away one by one. Yes, this does leave a rough edge at various places around the circle, so grozing or grinding will be necessary.

Excess glass  removed

Circle removed.  Note very sharp shard at 10 o'clock which along with other irregularities need to be grozed or ground off

If the glass is too dark or opalescent to see the line, make a template and put it onto the glass. Cut beside the template or use the template to mark the glass. Then place the marked glass on to the turntable and cut as with transparent glass.

Cutting ovals

Cartoon with oval placed on turntable ready for scoring
 This example uses a small oval, but the same process can be used for large ovals too.

Scored and run with one relieving score placed
When scoring large ovals, the same procedure as for glass circles can be used.  Simply turn the glass over and run the score with your thumbs as for the circle.  No tapping will be necessary.

Small oval broken out from the glass

Cutting concave curves

There are several methods that can be used to break out extreme inside curves. In all the cases you should retain a significant amount of glass around the edges of the curve. You should make this most difficult cut the first on the piece. If it fails, you may be able to move the glass a little and score again, without loosing too much glass.

Glass scored along the concave line on the right

Relieving scores made and the first piece taken out

Further crescents of glass taken out

Next to last crescent taken out

Last crescent taken out in several pieces

To accomplish inside cuts by using the hand breaking method and/or pliers method, you must first score according to the cartoon line. Then you can make a series of concentric scores. Gently run the primary score line so any break does not run beyond this. Remove the graduated concentric scores in sequence.

You can also accomplish this type of cut by using the criss-cross pattern of score lines instead of concentric scores. First you must run the score of the curve to avoid the criss-cross lines from running beyond the curve. Then you begin to take out the little pieces from the waste area.

Another method is to score and run the curve, and then score a number of small crescents in the waste area, looking like fish scales or the fan type of paving seen in some European cities. Pull out each small crescent working toward the main curve.

Deep inside cuts can be assisted by using a lazy susan – a turntable affair, similar to a cake decorating turntable.

The first question you have to ask yourself is whether you should make such deep inside cuts or redesign the piece to avoid creating such fragile shapes.

OK. You have decided to go ahead with your plan in spite of good advice. Put your cartoon onto the turntable and the glass over it. If the glass is too dark or opalescent, make a template and mark the glass. Adjust the starting point, put one hand on the glass and cartoon, and turn the glass instead of yourself to get round the score with ease.

Then you can begin the task of breaking out the glass from the score line as described above.

Cutting thin strips

Cutting thin strips of glass such as used in Mission Style patterns and precision fusing projects requires skill and persistance. For transparent and translucent glass you can arrange a right angle guide on a board and tape a piece of lined notepaper to the jig. Use a cutting square and move it right along the lines on the note paper making four or six scores at a time and then breaking on the last score first and then every other score, and then each one in half.

Another method is to use the edge of the bench as a guide. With a small adjustable carpenter’s square, you hammer in nails at the predetermined width (plus half the thickness of the cutter head). Align the glass to the edge of the bench between the nails. Place a straight edge against the nails and score. This gives strips of the same width every time, but works best with strips of 10mm (3/8”) or more. This is illustrated in the processes section. 

The thinner the strips are to be cut, the more important it is to make the scores and then divide the scored sheet in half - the two halves in half each - the 4 quarters in to halves, etc, until you are down to the piece that only needs to be divided in two.

Breaking Apart the Last Two Thin Strips

For multiple thin strips of even widths, score all the strips first. Then break all the scored strips off the remaining sheet as one piece. Start the breaking process by breaking the scored sheet in the middle, then in the middle again, until there are only two to break apart.

Strips scored and broken from main sheet

Breaking pairs off the scored sheet

Cut running pliers are most useful until the last two thin strips are to be divided.

At that point use two breaking pliers to hold each side of the two pieces of glass. The noses of the pliers should almost touch on either side of the score line. Apply pressure in a downward pivoting motion to break the pieces apart.

Breaking Tapering Pieces

Breaking thin pieces of glass can be tricky, but there are a few things you can do to help direct the break the way you want it to go.

Tapering piece scored

Broken from main sheet
Relieving scores made alongside curved and tapering pieces make the breaking more certain. A relieving score is one that is in addition to the primary score. This additional score will allow you to break the thin or tapering piece from the larger sheet safely, and then go on to break out the delicate piece.

Relieving score broken away

The object is to always be breaking away less glass than is retained. The use of two breaking/grozing pliers, one on each side of the narrow pieces gives more even pressure than fingers or cut running pliers with wide jaws.

Score run started from narrow end

Running score from blunt end

Completed break

Rehearsing Special Cuts

It is important to remember the basic tips as they become even more important with difficult cuts:
  • Keep an even/constant speed during the scoring
  • Make sure the cutter is vertical – eye the cutter from top to wheel to cut line
  • Stand behind the direction of the cut line
  • Use your body to turn, do not use wrist or arm

For difficult cuts you can increase your confidence by rehearsing the score with a feather light movement of the cutter on the glass along the score line. Make any adjustments to the placing of the glass or yourself shown to be necessary by this rehearsal before beginning the score.

Start with the most difficult score first. Any break-outs or mistakes will not waste much work or glass.

Break out each score as you make it. You can store up trouble by making multiple scores before starting to break. The score lines can run across the main piece when breaking off the scored glass. Any inaccuracies will also be magnified by making all the scores before breaking.

Cutting Bottles

Cutting bottles seems to have a fascination for many people. There seem to be three methods – thermal shock, scoring, sawing.

There are various ways to apply thermal shock to assist with breaking the bottles.

- A string tied around the bottle and soaked in a flammable liquid is a common way to apply heat. As soon as the flame has gone out, you immerse the bottle in cold water; the temperature differential should crack the glass where the string was.

- Filling the bottle with water to the level where the break is wanted and then applying gentle heat with a torch flame at that level should promote a crack.

- Alternatively, the bottle can be scored and put into the freezer for a while and then into hot water.

Scoring is the common method to start a crack.

- This is followed by tapping from inside the bottle with tools from a purchased kit or home-made tappers – a metal ball on the end of a curved piece of metal.

- The score line can also be the preliminary step in the application of heat or cold.

These provide the cleanest edges to the cuts. However there is quite a high failure rate using these methods.

Sawing is method that provides a higher success rate, but is wet, and leaves rough edges to the cut, requiring further cold work.

- Band saws designed for glass can be used, but usually do not have a high enough throat to allow the thickness of the bottle to pass through.

- Most tile saws cut from underneath, so rotating the bottle can lead to a cut completely around. This requires a lot of skill to do free hand, so you need a jig to keep the bottle at right angles to the blade and the bottom the same distance from the blade while rotating the bottle all the way around.

Cutting Flashed Glass

Some recommend cutting flashed glass on the clear or non-flashed side. This is based on the idea that the flash is only laminated to the main body of glass. My view is that flashed glass has proved to be very stable over many centuries, and so is firmly a part of the whole sheet.

What is more important is to observe that flashed glass often has a bow. If you place the glass on the bench, you may find that it rocks or sits up from the bench. If you cut the glass on the convex side, that is the side which is not resting on the bench except at the edges, you may find that you break the glass during the scoring, unless you are using the lightest of pressures. It is more certain to get a good break if you score the glass on the concave side - that is where the edges are slightly raised from the bench. So the important element in deciding which side to cut is to score the concave side whether that has the flashed colour or not.

This does not occur with all flashed glasses, and is more important on large sheets than small ones. On the small ones, the curvature is so small as to be immaterial.

Preventing Chipping When Using a Tile Saw to Cut Glass

One of the most common problems in using a tile saw to cut glass is the tendency for the saw to chip the edge of the glass as it completes the cut. This occurs when the blade of the saw has less glass to cut through. Excessive and uneven pressure and the lack of support cause this break-out.

It's possible to improve the quality of the cut by slowing down and pushing the glass through the blade more gently, but this seldom solves the problem completely. Pushing equally on both sides of the cut is also important to minimise the break-out.

One solution that does work is to provide support for the end of the bar. This adopts a woodworking method for preventing splintering at the ends of cuts.

Use a scrap length of pattern bar or other thick glass. Place it against the glass being cut. As the blade emerges from the glass being cut, hold the two pieces firmly together and continue cutting. The blade should immediately engage the second piece of glass. Once the saw blade entirely clears the first piece, you can turn off the saw and remove a chip-free slice from the pattern bar.

You'll need to trim off the ends of the scrap piece from time to time, but you can use the scrap over and over until it becomes too small to do the job.

This works best with a tile saw where the blade is below the cutting surface. When you use an overhead saw, the breakout is much rarer.

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