Waterjetting – Growing cracks and glass cutting

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

High pressure abrasive waterjets (AWJ) are able to cut glass with considerable precision, and maintain the accuracy of cut through thick material.

Glass Eagle

Figure 1. Cutting an Eagle from glass using AWJ (courtesy of KMT)

Because of this precision, and because the glass can be cut to leave very delicate webs between adjacent cuts, AWJ glass-cutting has been used to create art objects for a number of years. It is not as easy as it might at first seem, and Dr. Vanessa Cutler, an international leader in advanced cutting, has used the tool to create significant works of art. She has also written on the problems that can arise in cutting what often seems to be a simple, consistent material. (Noting, in passing that through the combination of computer control and memory it is easier at times to re-create works that break than would be the case with other tools for artistic creation). For the more mundane cutting world that comprises the rest of us, cutting glass is more likely restricted to simple activities such as cutting the parts for the windows of wood furnaces.

When the cuts are this simple, time can be saved by stacking two or more sheets of glass, one on top of the other, and cutting all of them at the same time. As one learns the parameters, thicker and greater numbers of plates can be stacked, and still successfully cut.

Cutting through four sheets of glass

Figure 2. Cutting through four sheets of glass simultaneously (courtesy of KMT)

However, if one gets too ambitious, and stacks too many plates then the lower plates may start to crack, often after the cut has started into the plate. As Dr. Cutler has noted in her new book “New Technologies in Glass”, cracks can also create problems for the unwary in dealing with internal stresses in the structure of the glass.

There can be several reasons for this, but it primarily goes back to the point I made in the introduction, about water pressure causing existing cracks and weakness planes to grow, as a way of removing material. There are two sorts of cracks that exist in glass, those created by the impact of the abrasive particles themselves, and those that were already present in the glass.

Micro-Photo - Cracks in Glass

Figure 3. Micro-photograph showing cracks growing out from the point where two abrasive particles struck a piece of glass. (This was adjacent to the main cutting path).

Micro-Photo cutting surface - Glass

Figure 4. Micro-photograph of the edge of the main cut by an AWJ on glass, showing that it is made up of the intersection of adjacent cracks created by the abrasive impact.

I’ll write about the mechanics of cutting glass in a later post or two, but for the moment I would like to write about the basics of crack growth from the point of view of cracks that already exist in the material. In large part this won’t be using waterjets alone to cut glass. Rather there are lots of other materials, particularly soil and rock, which have much higher crack densities, and longer cracks which make it easier to cut and remove material.

So, for the next four KMT Waterjet Blogs, I will focus the issues of cracks in materials including cutting glass, cutting granite and much more.