Diagnosis of Cutting

If your scoring and breaking of your glass is not going well, you need to diagnose the reasons.  There are always a lot of suggestions that warming the glass will solve the problem. Yes, warming glass may help. A discussion of the effect is here. But it will not overcome any faults in the basic skills of scoring.

A lot of images, shown on the internet, of straight line scores failing to break along the score, indicate some possible elements in scoring that lead to these unwanted break-outs. 

One possibility is you are using too much pressure. A discussion of the amount of pressure required is here.  You should be scoring to the pressure required, rather than any sound that may come from scoring.  This is emphasised when cutting opalescent glass.  The correct scoring pressure makes almost no sound or only a gentle rumble as it cutter moves over the undulations of the glass.  The most frequent reason for more difficulty in breaking opalescent glass is excessive pressure while attempting to get the same sound as from transparent glass.  There are even a few transparent glasses that make little or no sound when being scored with the correct pressure.

Another common problem in scoring is keeping an even pressure throughout the score.  It can be difficult to keep the pressure even on complicated cuts.  When the cartoon has multiple curves or deep concave lines, it can be difficult to keep the pressure even as you move your body around to follow the line.  One piece of advice I received early on in my learning was to rehearse the score allowing the cutter wheel to move along the score line with virtually no pressure.  This shows how the piece of glass needs to be oriented to ease your movement around the glass to make the score.

Slowing the cutting speed can help to keep the pressure evenly distributed along the score.  Straight lines are often scored quickly.  But, even on straight lines, slowing the speed can make the pressure more even throughout the score.  It can also avoid variable speed during the scoring, which leads to different forces being placed on the glass.  The pressure may be consistent, but the effective pressure is greater when slow than when fast scoring is used.  If the speed is variable, the effective pressure differs along the score line.

A fourth thing that may be happening on straight lines is that the cutter wheel is at an oblique angle to the direction of the score.  This will often be heard as a scratching sound as you move along the score line.  This can be overcome by a gentle pressure against the straight edge you are using to align your score.  Of course, the straight edge needs to be held firmly to avoid having it move.  Allowing the head of the cutter to have a little freedom of movement also helps keep it parallel to the straight edge.

All this is merely speculation about your scoring practice.

You need to get someone to observe you scoring.  They do not need to be experts, nor other glass artists.  They just need to be observant. Tell them what you are looking for in each of the four elements of scoring and have them observe only one thing at a time.

First get scales that you can zero when you have a small piece of glass on it. Score without touching the glass. Have the observer tell you if the pressure was consistent throughout the score, and if you are in USA, whether the pressure was above 7 pounds or below 4 pounds. (For the rest of the world 3kg to 1.8kg). Practice until you can score consistently at about 2.2kg (ca. 5 pounds).

Second, have the observer stand a little distance from you. Score toward the observer. They need to observe whether your cutter is perpendicular to the glass while scoring and if there is any variation.

Next, they need to tell you if your head was directly above the cutter all the way through the score. They will be able to see whether your eye is directly above the cutter

Is your body behind the cutter, or do you use your arm to direct the cutter?  The observer will be able to tell that when you are scoring curves. The most consistent speed and pressure is delivered when the cutter is steered from your torso, rather than your arm and wrist.  It slows the scoring action, gives smoother curves, and more even pressure.

The last element, you can do yourself.  Once you are doing all the things above, you will be able to hear any scratching noise, rather than the gentle creaking noise of an even score with adequate pressure.  If the scratching noise is intermittent or only at one point, the likelihood is that you are twisting the cutter head, so the wheel is not in line with the score line.

Tapping Glass Scores

Many people tap the underside of the glass after scoring.  The purpose of this is to run the score.

However, this tapping is often unnecessary.  Running the score can be done in a variety of ways, some more suitable for one kind of score line than another.

Straight score lines can be run in several ways.

•       Move the score line to the edge of the bench or cutting surface and use a controlled downward force on the glass off the edge while holding the remainder firm.  Works best if at least a third is being broken off.
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•      You can place a small object, such as the end of your cutter or a match stick, directly under the score and place your hands on either side and press firmly, but not sharply, down on each side at the same time.  This is good for breaking pieces off from half to a quarter of the full sheet.
•      Make your hands into fists with the thumbs on top of the glass and the fingers below.  Turn your wrists outwards to run the score. Works best if the glass is approximately half to be kept and half to be broken off.
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•      Take the glass off the cutting surface, hold in front of your knee at about 45 degrees and raise you knee quickly to the glass.  This will break the glass cleanly, but is only useful for moderate sized sheets and where you are breaking off about half of the sheet.
•      Use cut running pliers to run the score.  Be sure the jaws are adjusted for the thickness of the glass, and do not apply excessive pressure.  If the score does not run all the way, turn the glass around and run the score from the opposite end. Best where there are approximately equal thin parts to be broken away from each other and when the score line is no less than an oblique angle to the edge. It does not work very well for thin pieces or acute angles.
  
  
  
•      Use two grozing pliers nose to nose and flat side up at the score line and bend them down and away.  This works best on thin and or pointed pieces.
•      Breaking pliers can be used at intervals along the score. This is most useful on long thin pieces.

Curved score lines, of course require a bit more care but generally employ the same methods.

•     Gentle curves can be dealt with as though they are straight lines, although the breaking at the edge of the cutting surface is a bit risky. This means the two-fist, running pliers, two grozing pliers and breaking plier methods can be used.
•     Lines with multiple curves usually require cut running pliers to start the run at each end of the score.
•     Deep curved scores may require the running pliers whose angle can be adjusted to be at right angles to the score.  The ones I know are Silberschnitt, made by Bohle, although the ring pliers by Glastar work in the same way. This usually requires that the edge of the glass is not more than 5 cm from the score.  This blog gives information on a variety of cut running pliers. 

Tapping

After trying all these methods to run the score, sometimes the score is so complicated or deep into the glass that you cannot simply run the score.  Tapping may then be required, but it is a last resort.

Tapping, to be effective, must be accurately directed to places directly under the score line.  The tapping cannot be at random places under the glass. Each tap must be controlled – to be direct and to be firm. 

The impact needs to be directly under the score. 

•     Taps that are either side of the line will either not be effective, or will promote breakage other than along the score line. 
•     Tapping to either side of the score also promotes shells to either side of the score line.  These are not only dangerous when handling, but also require further work to remove these ledges of glass.

The impact also needs to be firm. Random impacts to the glass promotes breakage other than along the score line.

•     The taps need to be firm – neither light nor hard.
•     Each tap should be at the end of the run begun by the previous one.  This promotes a smoother run of the score with less opportunity to start a run off the score line. 
•     To avoid the incomplete running of the score that leaves parts of the score untouched you need care. As the glass begins to break along the score line, place the next impact at the end of that start to continue the run. 

Tapping the glass under the score should be a last rather than first resort in running a score.

Cutter Wheel Angles, 1

The Effects of Wheel Angles on Glass Cutting

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 of a wheel 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.
When downward pressure is exerted on the wheel rolling along the glass, forces are created which radiate down and to the side trying to shear or separate the glass along the surface. These forces are in a downward direction with little angle to the side when an appropriate angled wheel is used. If these forces are great enough to overcome the inherent compressive conditions near the surface, a crack or fissure 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 might suggest 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. But a sharp wheel tends to cause chips and a flaky score. Also, when 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 short time 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.

Part 2

Based on information from the Fletcher Terry Company.

See also wheel angles

Revised 23.12.24

Cutter Wheel Angles, 3

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 float glass 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.

Part 1

Part 2

Prepared from information provided by the Fletcher-Terry company.

Cutter Wheel Angles, 2

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.

Part 3
Part 1

Prepared from information supplied by the Fletcher-Terry company

revised 23.12.24

Testing your Scoring pressure

 Most often people are asked to listen to the sound of scoring.  Unfortunately, different glass styles make different sounds. Float glass makes a particular sound, transparent stained glass makes a slightly different one, and opalescent glass makes almost no sound. Consistent pressure of the right amount is important to the clean breaking of glass. Therefore, we must learn to cut with the same consistent pressure on all types of glass, rather than listening for sound.

It is easy to tell when the scoring is too heavy.  A white line shows along the score.

The heavy score line near the break shows the white line and the irregular break

It is not so easy to tell if the score is too light or just right.

A heavy score in the distance and a lighter score nearer

Pressure

The general recommendations for the pressure to use during scoring is 4.5 – 7 Lbs or 2 – 3 Kg. This is difficult to judge. I found that I needed a means of letting people know for themselves the pressure they were exerting. It is not enough to watch and say that was too hard, that was too soft, etc.

My method of teaching novices how to judge the pressure they are using is to use a digital kitchen scale that can have the scale set to zero. Place a piece of glass no larger than the platform on top of the scales. Zero the scale display.  Have someone watch the scale display while you score in your usual way. Of course, you must not touch the glass with your other hand. Have them tell you the maximum and minimum weights displayed. Keep repeating until you can consistently use that 4.5 – 6.5 pounds (2 - 3Kg) pressure.

The testing setup showing a heavy score on the right and the start of a 1.9kg score on the left.

Consistency

The other important element of scoring is to keep the pressure consistent throughout the score. This test will also show how evenly you apply the pressure during the score. The objective of scoring is to use the correct pressure throughout the length of the score. If your pressure varies significantly during the score, it will be difficult to get the glass to break consistently along the score line. Because the amount of weakness in the surface created by the score is variable.

Your observer can tell you when the pressure is less than optimum or more than desired.  If the pressure variation has a reasonably consistent place in scoring - such as at the beginning, or on a curve - you can fix it. Concentrate on correcting the fall off in pressure. For example, most people start off with a lighter pressure than further into the score.  Getting the feel of the correct pressure will enable you to apply it right from the start of the score. Sometimes, people increase the scoring pressure when they come to curves. This test will show if that is true for you.

This curve was scored with 4.3kg pressure showing that heavy pressure can result in break outs from the score line

This testing can take quite a while. But it is worth the time spent in getting the scoring pressure right to reduce the number of unwanted breaks. However, it is not a one-time test. When I begin to have difficulties in breaking glass, I go back to this test to check whether I am scoring too heavily. In my scoring practice, I find that my best ones are those with 1.8 to 2.5kg (4.0 to 5.5 pounds) with the cutter I use.  This is less than many, but it has worked well for me for years.

There are, of course, other elements that go to making a good score and break. But the most important thing in scoring and breaking opalescent glass is to avoid too heavy a score by listening for a sound. Cut to a consistent pressure whatever sound is heard.

 

Circles – Some Methods of Scoring and Breaking Out

 Scoring the circle

Setting out the centre point and radius of the circle

Set up a centre point. You can do it by estimating. But it is more accurate to measure a centre point that is at least 1cm from the edges. Use that crossing point to measure out the radius of the circle.

 

Setting out the circle measurements

Place the pivot point of the circle cutter on the cross hairs. Lock it into place.

 

Locking the pivot into place on the cross hairs

Adjust the cutting wheel along the length of the circle cutter bar to the marked radius. The measure marks on the bar are not always accurate and anyway wear away quickly.

 

Adjusting the scoring wheel to the correct length

 

Tightening the set screw at the top of the turret

Tighten the set screw.

 

Add a touch of oil to show the score line. Make a test circle by pushing the bar around with no pressure. This shows up any obstructions around the end of the bar. 

Preparing to test the sweep of the cutter arm

Score in an anti-clockwise direction. This avoids accidental loosening of the set screw if it is under the button or handle. 

 

Start with the scoring head under the arm with which you are securing the centre pivot. This allows you to do the whole score in one motion. The pressure you use should be no more than in your normal scoring.

 

Oil trace of the score can be seen in the upper left quadrant

 

Running the score 

Running the score is a glazier’s term to indicate the way in which a scored line gradually separates under gently applied breaking pressure. This can be seen when gently applying pressure with cut running pliers. The score line is made progressively visible by the gradual separation of the glass. This results from the light passing through in a different way than when it is not separated from top to bottom.

 

The glazier’s method to get a clean break of a circle is to score on one side with a trace of oil. Then turn the glass over and press with your thumbs on the score line. Running the score from the back requires a little skill and a certain amount of courage or determination.

The object is to bend the glass just enough to crack it along the score without breaking it anywhere else. The best surface is a short pile carpet square, a rubber mat or a single layer of towel or an unfolded newspaper. These provide a surface with a little “give” to allow the glass to deform.

But if you have too soft a surface, it is easy to break through the circle. A too soft surface is given by a household carpet, several layers of towel, or any other surface with a lot of “give” to it.

You may need both thumbs to start the run depending on the pressure you can exert. Try one thumb first. If that is not enough use both thumbs.

 

At the start of the second pressure point

If you place your thumb opposite a corner, you have greater leverage to start the run of the score. This leverage makes the running of the score easier as less pressure is required. You will hear a loud click at the opening of the first part of the score. 

Score has been run completely around the circle

Continue around the circle by pressing at the end of the opened score, until the whole score has been run. You may hear quiet clicks as the score opens. Other times there will be no sound, but you will see the score run away from the pressure point.

Once you have run the score from the back, turn the glass over to have the scored surface up. This makes it ready for the relief scores. It is much more successful if all the scores – circle and relief - are made from the same side. Unless you are scoring float or other glass that is smooth on both sides, this flipping back will be the easiest anyway.

 

Make the relief scores on the front.. Sometimes only one relief score is required to release the circle from the surrounding glass.

 

Another method is to cut the corners off so that you have an octagon around the circle. This gives you the opportunity to run the score from the top with breaking pliers. Ease the score open progressively around the circle.

Opening the score with pliers

This method is easiest for opalescent glass where you cannot see through to the score. By opening the score from the top, you do not have to estimate where the score line is. You can see the oil trace indicating where the score is and where to place your breaking pliers. 

You should start at a place where there is only a centimetre or two between the edge of the glass and the score. This may mean that you have to move around the broader areas of the rim so that the score runs both back to the first opened score and forward. It is in effect, opening a new score four times. But with gentle persuasion the scores will run toward one another. Do not use heavy pressure in griping the pliers, or in bending the glass. Gradually increase the bending pressure until you hear the click of the glass separating.

These two methods of running the score give the cleanest break-out of circles.

  

Relief scores

There are multiple ways to create the relief scores.

Perpendicular

Score by drawing the cutter from the circle out to the narrow edge, leaving a small gap between the opened score of the circle and the start of the score.

 

You can also score a longer line to the corner. Again, leave a space between the circle and start of the relief score.

 

Tangents

 A third way is to score tangents from the edge to the opened score of the circle. 

Tangent scored from both sides of the circle

Tangent broken from both sides of the circle

Alternatively, score a first tangent and run the score

Further tangents scored and broken away

All the tangents run and broken away 

This reduces the risk of breaking through the centre of the circle. But it does leave little nubs of glass at the point of each tangential score. And for that reason alone, is the least satisfactory of the methods of breaking out circles.    

In each of these methods of running a relief score, you need to tap under the relief score to run it to the edge of the circle. Normally, the quarters or other fractions will drop out one by one. Occasionally the rim will drop away after the scores of the first two quarters are run.

 

Breaking out without running circle first

There are at least two ways to cut a circle without running the circle score first.

The first is to cut the corners off the glass to create an octagon, but do not run the score.

First corner scored and removed

 

The four corners removed

Starting to run the circle with breaking pliers

Then use breaking pliers to run the scored circle. Once the score is run, make a single or multiple relief scores, and carefully run it. the circle can then be removed from the octagonal rim. This provides a clean cut.

 

corners and rim removed 

It does not risk breaking through the scored circle to the interior when tapping the relief score.

Many people run scores at a tangent to the circle without having opened the score of the circle. These are then run, in turn, to and along the edge of the scored circle.

They can then be broken off in turn, if they don’t simply fall off when they all are run.

 

tangent removal 

Finally, some people tap under the scored circle to run the score, as in the first of the tangent removal methods. This leaves shells – or ledges – on the glass circle. These must be removed by grozing or grinding. Grinding leaves a rough surface which takes copper foil tape less well than clean cuts. It also increases the possibility of devitrification when fusing.

Tapping leads to shells as in the centre left of this picture

The least satisfactory method of running the score of a circle is to tap under the score before breaking the circle out.

 

In conclusion, running the scored circle first and without tapping will provide you with the cleanest cut circle. This will be so however your make your relief scores. But, making relief scores before running the circle score risks breaking through the circle.

Finally, it should be noted that cutting out larger circles is easier than cutting small ones. Better, cleaner results are obtained because the curves are less tight.

 

Accurate cutting

Cutting glass pieces accurately is a matter of practice whether done by hand or by person-guided machines.  Computer guided machines rely on accurate instructions being given to it. This is mainly about human-guided cutting with some information on saws.

Hand Cutting

The right approach to cutting is necessary to accuracy. 

• ·        You need clean flat surfaces with only a little flexibility, with no glass shards, bumps or warps that would make scoring the glass more difficult. 
• ·        You need to work at a comfortable height – usually about 10cm below your elbow. 
• ·        You should be moving your torso and sometimes your whole body to score, following the cutline.
• ·        Your elbow should be at your side and your wrist should be fixed. 
• ·        You need to work at a corner of the workbench to be able to move your body around as you follow a curve.
• ·        Your cutter should be at right angles to the glass (side to side)
• ·        The cutter should be angled back toward you slightly, so you can see down the barrel to the scoring wheel.  Also, so any unevenness in the glass will not cause the cutter to stall.
• ·        The pressure should consistently be about 2.5kg or 7 pounds. Do not listen for the sound, as it varies between opalescent and transparent glasses.  Heavy pressure on any glass causes unwanted break outs. Score to the pressure, not the sound.
• ·        Keep a consistent speed. Variability in speed also translates to uneven pressure. Even when you could go fast in scoring, keep to a steady pace.
• ·        Score and then break each line before going on to the next.  Do not score all around the piece in one go. It does not allow for any adjustment, if the cut is not accurate.

Oil filled cutter in classic hold

These approaches to hand scoring apply to all the ways of assisting accurate cutting.

There are at least three methods to assist accuracy in addition to the basic requirements, which apply to any method you use. 

• ·        My preference is to score the glass directly over the cartoon. This involves no extra pattern making.
• ·        Drawing the lines from the cartoon on top of the glass and then moving the glass to a separate cutting surface is the preference of many.
• ·        Making pattern pieces with the space for the foil and tolerance already cut out is a preference of many copper foilers.

Each of these assistance methods are acceptable, although the more steps involved in translating the cartoon to the glass, the more chance of introducing inaccuracies.  The accuracy of the final piece depends upon the skill with which you can manoeuvre the cutter around the glass.  This requires practice. With practice, accuracy will improve.

Machines

Glass saws of several types are available to assist in getting accurate shapes.  Mostly they are narrow thin diamond coated blades or diamond coated wire.  The wire-based saw can cut tighter and more intricate shapes than the band saw can.  But you should think about the nature of glass before you get too intricate.  The breakage rate of intricate pieces in stained glass is relatively high, even in the early life of a piece.  In fused glass it is not so difficult as they normally are supported by a base piece of glass.

Accuracy in using these saws relates to:

• ·        Manipulation of the glass to follow the line exactly.
• ·        Permanency of the guiding lines drawn onto the glass.
• ·        Accuracy of the drawn lines.
• ·        Accuracy of the pattern pieces.
• ·        Allowing the machine to cut rather than forcing the glass into the blade.
• ·        Maintenance of the machine.
• ·        Maintenance of the blade or wire.

Your accuracy will increase in the use of hand scoring or machine sawing of the glass with experience.  But, the degree of your critical appreciation of your cutting results is directly related to the accuracy of cutting.  The more you say “that is good enough” the less quickly you will improve your accuracy.