Cutting edge quality

The wrong choice can cost you £thousands, while the right one will put them straight onto your bottom line. Dennis Kampmann, director of Industrial Glass Cutting, Bohle, asks: where should you start when choosing industrial glass cutting wheels?

It’s about physics and chemistry – and probably just a little alchemy. Get your cutting table set up with the right cutting wheel, with the right parameters and you’re heading for perfect edge quality in glass processing and IGU manufacture. Get it wrong and the implications for your bottom line can be dire.

In a way the problem is that there is too much choice; there is always a perfect choice for every cut, but you have to find it.

The application of the right cutting wheels can save glass processors £thousands, improving product quality, reducing wastage, and saving time on edge-grinding.

We have some customers who are very disciplined. They will change cutting wheels during production runs according to what they’re processing. Yes, there’s a little downtime involved, but the edge – and therefore product quality and strength – is that much greater.

They’ve worked out that the positive impact changing to the right cutting wheel has is worth a little downtime and the change of wheels.

There are other glass processors and IGU manufacturers who don’t. They lose more product, and their edge quality isn’t as good.

An industrial glass cutting wheel has a very small individual unit price but its impact on your productivity can be far bigger. It’s completely disproportionate.

While Bohle has grown since the 1920s to offer an extensive range of glass processing consumables and hardware, its origins are firmly rooted in glass cutting. This is founded on the iconic Silberschnitt cutting wheel range, which has evolved over almost a century from simple hand-held tools to high-tech automatic cutting wheel designs.

In a way you could say we’re perhaps part of the problem: we manufacture so many different industrial cutting wheels. There are 10 different cutting wheel types, 10 to 15 different cutting wheel angles on each, and three different finish types. Each is designed to process glass slightly differently, but the solution is always there.

When you score glass with a cutting wheel you effectively transmit tension into it. This creates a fault line in the glass which, as you bend it, triggers a break along that line.

This may seem simple enough in theory but the science that underpins a clean break is complex, defined by multiple factors including: the individual characteristics of the glass; the shape of the cut; the diameter of the cutting wheel; the angle of the cut; cutting force; cutting speed; and the cutting fluid used.

This is also where the laws of physics come into play: the thinner the glass, the sharper the cutting angle should be; the thicker the glass the blunter the angle.

The angle of the cutting wheel also has an effect on the force required to create an effective cut; obtuse angles, the blunter ones, require a higher cutting force because more of the cutting wheel is in contact with the glass.

Bohle’s ‘principles’ of industrial glass cutting:

The thinner the glass: the sharper the cutting angle, eg 135°-90°; the smoother the grind of the cutting wheel; the lower the cutting force, eg 60N-10N.

The thicker the glass: the blunter the cutting angle eg 135°-165°; the coarser the grind of the cutting wheel; the greater the cutting force, eg 60N-250N.

It’s very difficult to see the difference between scores through a visual check in a production environment. If you’re looking at it from above, two scores made by completely different cutting wheels might look fairly similar depending upon parameters being used.

The way that the tension is transmitted into the glass is, however, going to be very, very different and that has an equally large impact on how cleanly the glass breaks, the crack depth, lateral cracks, the size of the shatter area, and chipping.

If you don’t get your cut right, you significantly undermine edge quality and with it the structural integrity and strength of the glass.

Sharing the floor with me at Bohle’s recent industrial glass cutting master class, Dr Michael Emonds, senior chemist at Chemetall, said: “The strength of glass is defined by its surface condition. The largest deepest defect in the surface of the glass is the breaking point or the cut edge. In cutting the glass, you’re increasing tension and putting more surface defects into it.

“That’s why you grind glass. You’re taking out some of those defects, although it’s important to remember that the grinding process can influence edge stress positively and negatively.”

Standard float glass has a surface strength of around 210 megapascals (MPa) or one million newtons/m2. In a good quality cut edge, this however drops to between 50MPa and 70MPa, while in a poor edge it drops to as little as 20MPa, significantly compromising edge strength and increasing risk of failure.

These principles apply equally to manual cutting of glass, but the implications of getting it wrong are far more significant in industrial processes because of the volume of glass being processed.

Bohle has developed products in partnership with glass machinery companies including Glaston Bavelloni, Bottero, Bystronic, Benteler, CMS Brembana, Grenzebach, Hegla, Intermac, Macotec and Lisec. With its own research laboratories and a rigorous research and development programme, it’s this partnership approach that has allowed Bohle to deliver a step-change in cutting-wheel technologies.

This includes the supply of an extensive range of tungsten carbide and PCD (polycrystalline diamond) cutting wheels and their holders.

These are available with colour coded plastic and as metal cutting wheel holders, including quick change systems (style 432). Colour coding of plastic cutting wheel holders is something our customers like because it makes it very clear which angle of cut you’re using but metal cutting wheel holders will give you greater precision and far more stability in high pressure cutting – ultimately delivering better value for money.

We have also recently added a laser marked inscription citing the angle of most of our cutting wheels, making it far easier to identify right option when using metal cutting wheel holders.

Bohle’s premium cutting wheels are the Cutmaster Gold and Cutmaster Platinum.

Cutmaster Gold is a highly engineered tungsten carbide cutting wheel ideally suited to cutting of float line and laminated glass. Its service life is around eight to 10 times longer than a standard cutting wheel but, more importantly, it maintains a consistently high cutting quality from the first to the last cut, using a specially developed coating to capture and ‘freeze’ ideal cutting conditions.

And because it’s held in this state right up until the end of its service life, you find that you don’t have to adjust the cutting pressure that often.

Cutmaster Platinum represents a further evolution of cutting wheel technology. This uses a micro-structure that forms a permanently sharp cutting edge. Rather than forming a continuous cut, it creates a series of microscopic dots along the score line creating a far more precise line.

That gives you a very high edge quality because it requires less cutting force, and there are fewer lateral cracks and side wings, so less chipping.

Its design means that Cutmaster Platinum is suitable for cutting float glass from 0.5mm up to 19mm (straight cutting) and ultra-thin glass of as little as 0.05mm. There are going to be more and more applications requiring ultra-thin glass, for example, foldable mobile phones, and we have customers using Cutmaster Platinum to cut glass as thin as 0.05mm, because it allows you to cut at a very low pressure reducing the risk of breaking the glass in an uncontrolled way while cutting it.

Bohle also offers dedicated solutions for cutting mirror and TPF cutting, which was an increasing requirement as glass technologies and their protection developed.

The use of the right cutting fluid also assumes significant importance in driving up edge quality. In the same way that there is a mind-boggling range of cutting wheels available, Bohle also supplies an extensive range of cutting fluids, dedicated to specific types of processing and glass including soft coats.

This includes washable cutting fluids for use in cutting of thick glass (10mm and up), shape cutting, and laminate glass cutting. Bohle also supplies a range of evaporating cutting fluids designed for cutting insulated glass and, among others, low-e glass.

The benefits they deliver are, however, clear.

For example, during four-point edge testing a dry cut sheet of 4mm glass will break at a bending force of roughly 130 Newtons. Using Acecut 5929 evaporating cutting fluid, the same sheet of glass won’t break until its exposed to 170N. Acecut 5503, also an evaporating cutting fluid, pushes this on again to 175N, and Acecut 6000 washable cutting fluid to 225N.

The cutting fluids are doing several things: they lubricate the grinding wheel, minimising friction and reducing the cutting pressure by on average 10%-15%. This means you get fewer chips and create a better scoreline with fewer branches.

It also acts as a ‘chemical wedge’ penetrating the cut and increasing the depth of the median crack, supporting a more effective soft break-out. This again reduces the amount of energy required to create the cut and the speed of the cut, also reducing branch formation.

This creates a major advantage, particularly in the processing of thicker glass, reducing the risk of run-out of the cut.

Cutting under reduced pressure and effective lubrication of the cutting wheel will prolong the lifetime of your cutting wheels by up to 25%. This is, however, just the tip of the iceberg of potential savings, which includes better edge quality and fewer remakes but also less time grinding.

For example, modelling reveals that by using kerosene as a lubricant in the cutting of 19mm glass meant that it was necessary to grind the edge twice. This compared to Acecut 6000, which delivered a near perfect break first time around, requiring only minimal edge grinding.

The negative impact of the former on service life and labour meant that although nominally more expensive to purchase, the use of Acecut delivered significant savings in labour and in wear of edging tools.

There are a lot of reasons to invest in time and a little bit of possible downtime in choosing the right cutting wheel and fluid because it delivers far better edge quality and, with it, time savings downstream. You don’t lose so much time on edgework or product in toughening. It protects your product quality and it protects your reputation.

The maths adds up.