EPISODE 2: The art of flat glass cutting

Sarah: Welcome back to our second episode about glass processing. My name is Sarah Hummelsberger and I am working at the LiSEC Marketing, and today my expert is Harald, or Harry, who’ll be telling us a bit about himself?

Harald: Yes, my name is Harald Miksch, I’m working for the product management department, and I’m thrilled to learn the questions Sarah is gonna put on the table today as well.

Sarah: And I’m looking forward to learning more about the answers. Today we are once again situated at our headquarters in Austria, and outside we are in the training department, so we will be hearing machines and customers learning how to use them. So we’re sorry for any disturbances. Today, we’re talking about glass, which is a very brittle material. I’m a crafter, so I know it’s very hard to do anything with glass without shattering it or hurting yourself. Even just cutting it is always a hassle. So how do you cut glass, what’s savest and most efficient?

Harald: Well, glass is an amorphous material which means that is does not have a regular cristalline structure and therefore lacks a long-range order in its molecular structure. This lack of structure gives its uniform properties, making it ideal for cutting and shaping.

Sarah: Intriguing! So, which different glass cutting techniques are there and what are their advantages and disadvantages?

Harald: The most obvious cutting technique is to use a saw for instance with diamond cutting disc. Depending on the thickness of the glass, the cutting process can get quite slow and you always lose some of the glass material due to the saw cut. And then you need water cooling to prevent the overheating of the cutting disc and glass.

Another possibility is to use water jet cutting which is also quite slow (especially for thicker glass) but causes only minimal material loss and does not require any cooling and the cuts can have very complicated shapes.

Also laser cutting is an option, especially for thin glass. Laser allows a very high cutting precision and with hardly any material loss and no need for cooling. Further, very complicated shapes can be cut.

But the process, that is most commonly used in the flat glass industry is the scoring and breaking process that is way faster than the mentioned processes, does not require any cooling, causes almost no material loss, but can only be used for straight cuts or simple curves.

Sarah: So how does that work then?

Harald: As the name suggests, the glass pane is not actually cut, but rather scored and then broken along this score line. We use a cutting wheel, usually a small wheel made of a hard material like thungsten carbide or diamond. And as mentioned, it doesn’t actually cut the glass but scores it’s surface by creating a weak line. When then a force is being applied from the opposite side, the glass breaks with a clean 90 degree edge.

Sarah: An what is the cutting oil for?

Harald: The cutting oil fulfills a whole range of tasks, first and foremost lubricating the cutting wheel and the cutting head. Further, the lobrication reduces friction between the flass and the cutting wheel, allowing for a smoother scoring action. Another very important function of the cutting oil is to avoid material healing and to ensure that the score line remains open and distinct. When the glass is scored, microscopic cracks are formed. Without proper lubrication, these cracks can begin to close up due to the inherent surface tension and viscosity of the glass. The oil helps keep the cracks open, maintaining the integrity of the score line.

Sarah: Fascinating! Anyway, the cutting wheel seems to play a key role in that process.

Harald: That’s right, the cutting wheel and how it is being moved across the glass surface is very important in the process.

Cutting wheels come in various diameters, typically ranging from as small as 2 mm to over 6 mm or from a little less than 0,1 inch to more than 0,2 inch. Larger wheels can maintain a longer life and are often used for thicker glass, smaller wheels are preferred for precision and intricate work on thinner glass.

The angle of the wheel is also very important as it affects how the wheel interacts with the glass surface, it ranges from 120 to 155 degrees. A smaller angle, meaning a sharper wheel, is generally used for thinner glass and it creates a finer score. A larger angle (blunt) is suitable for thicker glass, providing a deeper score.

And then there is the speed with which the cutting wheel is moved across the glass and the pressure that is being applied. Speed and pressure are interdependent – a change in one often requires an adjustment in the other to maintain the quality of the score.

So, the faster the wheel is being moved, the lighter the score will be. Too fast, and the score might be too light, leading to an uneven break. Too slow, and it might create a rough, chipped line or overheat the cutting wheel.

Then obviously, the more pressure is applied, the deeper the scoring line goes. Too much pressure can cause unwanted cracks or even breaks during the scoring phase and can reduce the lifetime of a cutting wheel. Too little pressure and the score might be too light for an even break.

Sarah: OK, so the thicker the glass, the blunter the cutting wheel needs to be, the higher the cutting pressure needs to be and the lower the speed at which the cutting wheel is moved across the glass sheet. This seems to be quite a difficult and delicate process that requires a lot of experience, how do the machine operators deal with this?

Harald: Well, the machine usually helps with that. The parameters are already stored for different glass thicknesses and glass types, and therefore the pressure and speed at which a particular cutting wheel must be moved across the glass. All the machine operator has to do is select the type of glass and the machine adjusts itself accordingly - this machine setting - also known as a recipe - is usually provided by the machine manufacturer. But sometimes it may be necessary to find out the parameters for a particular type of glass simply by trial and error.

Sarah: A cutting table also has a cutting bridge. What is that needed for?

Harald: Well, the cutting bridge is the heart of the cutting table. It is essentially a movable arm that spans across the table. Attached to this bridge is the cutting head which holds the cutting wheel. The primary role of the cutting bridge is to guide the cutting wheel precicely over the glass surface with the right pressure and speed, according to the recipe. The glass sheet lies horizontally as it is scored by the cutting wheel, hence the name cutting table.

Sarah: I keep hearing about these X, Y and Z cuts. What are they actually all about?

Harald: Well, in most cases you don't need very large panes of glass, but lots of smaller ones - that's the purpose of the cutting table, to get lots of smaller panes. If you want to cut many, let's say 20 or more panes out of a large pane, then there are quite a few variations on how you can arrange them and how the individual cuts have to be made. In any case, with cutting tables, a cut must always go across the entire width - a so-called edge-to-edge or guillotine cut. This means that it is not always possible to cut all 20 or more panes out of the large sheet of glass with cuts in one direction only, but that you have to turn a section to cut it further. And that is exactly what is meant by the X, Y and Z cuts. The first cuts all go in the same direction, these are the X-cuts. If you then have to rotate a part by 90 degrees, you have to make Y cuts according to the coordinate system. If you take another part of it and have to rotate it by 90 degrees again, then this is a Z-cut - this then goes in the same direction as the X-cut, but could not have been made before the Y-cut without cutting into another part.

Of course, how many cuts you need in total and whether you need Z-cuts at all depends very much on the optimization. However, there are also cases where further cuts are necessary after the Z-cuts, these are the W-, V- and U-cuts, but this goes into great detail.

Sarah: So a very crucial part of the cutting process is actually the optimization beforehand?

Harald: There are already very sophisticated software algorithms that manage to optimize a large number of glass panes over several jumbo panes so that as little waste as possible is produced or so that as few cuts as possible are required or so that the individual parts have to be turned as little as possible - i.e. Z-cuts can be avoided altogether.

Optimization can mean that the glass panes from many customer orders can be cut out of a single jumbo pane, thus reducing waste to an absolute minimum. This can pose a major challenge for downstream logistics but can greatly optimize the cutting process.

Sarah: You mentioned that you can only make straight cuts with cutting tables, i.e. not diagonal or curved cuts, so how does that work?

Harald: It is also possible to cut diagonally and curves are also possible, but there are certain limits set by the phyisc and so-called special shapes are also a particular challenge for optimization. These are often broken manually, whereby straight cuts are no problem. Positive radii are also less of a problem with curves, but negative radii or acute internal angles often pose a problem, which is why other cutting technologies are then used.

Sarah: The fact is that there is not only float glass, but also other types of glass, such as laminated glass or coated glass. Can this score -and-break approach be used there as well or do you need different cutting technologies for these?

Harald: In principle, the score-and-break process can be used not only for single glass, but also for laminated or coated glass. However, the process must be adapted for this.

Laminated glass consists of two panes of glass that have been joined together by an intermediate layer. This intermediate layer can have different functions, usually it is a foil for sound insulation or to provide protection if one glass pane breaks. In any case, when cutting laminated glass, care must be taken not to damage this interlayer.

If you want to cut laminated glass, you practically have to score both panes of glass and break them in the same way as single panes, but twice. The intermediate layer, which normally remains undamaged when the glass is broken, must then be cut with a knife - which is all done fully automatically on cutting tables for laminated glass.

Glass can be coated in different ways to provide additional properties, such as improved thermal insulation and therefore greater energy efficiency. One or more layers of precious metals are applied, up to 15 layers are possible, but they are not visible. When coated glass is cut, this coating is removed from the edge area.

Different or special cutting tables or cutting bridges are required for the different processes for single glass, laminated glass or coated glass.

Sarah: Very interesting – so the question remains, why the coating is removed in the cutting process – well the reason lies not in the cutting process but rather in what happens after the glass is cut and when insulating glass is made out of it. But that is a topic for another episode of Listen Lisec.