Proccess journal-MYP PP

This is something that I received a long time ago, and is an interesting technical post about how a laser cutter works.

One of the things that we thought would be cool for this project is to have the Stratford Hall logo on one of the end sections of the marimba. there are a number of ways to do this, but the best (in terms of time and practicality) that we could think of was to have the logo laser cut and then attach it to the end somehow. with this in mind, we mailed a sheet of walnut veneer to my uncle, who is a laser physicist and owns a small laser cutter. the following is what he sent back:

Steps in laser cutting of logo for Stratford Hall:

The first step was to convert the logo for Stratford Hall into a digital vector format in the Program Corel Draw. I was supplied both JPEG and Postscript files. My first try was to open the JPEG scans in Corel Photo. This showed the files as a low density bit map, I then used the draw function to draw a perimeter on each of the figures; see figure 1. This turned out to be slow and time consuming, and subject to interpretation, so I searched for alternatives. After some trial and effort, I discovered that if I "imported" the post script version of the logo directly into Corel Draw, the program would display the figures as a vector perimeter filled with bit map. One simply had to delete the fill to be left with simply the vector perimeter which was needed to direct the laser cutter.

The logo as currently used is in four different colours. I can't do four colours, I only have the one piece of wood. I was thinking that I would do the pieces with four different orientations of the symbol with respect to the grain in the wood. So the close-to-final configuration is as shown in Figure 2. In reality, this was printed four times, with each print being only one of the four symbols.

The figures were cut on a Versa-Laser Model VLS2.30 made by Universal Laser Systems Inc in Scottsdale Arizona. The laser engraver’s software is set up so that it behaves just like a printer. That is, the pattern in Corel Draw was printed, and the laser device was selected as the printer.

I was supplied with 1/32" thick walnut veneer. This was cut into 16" lengths to fit into the laser engraver. The veneer was not totally flat but had some undulations. This meant that the laser beam would not be accurately focused on the veneer at all points in the cycle. To achieve a cut when the beam was not accurately focused, the power was enhanced over that which was necessary. The power enhancement was achieved by programming into the pattern a material thickness in excess of that actual thickness (2.5 mm thick for the "crest" symbol). To do the cut, the laser was operated at the maximum power (nominally 25 watts), and pulsed at 400 pulses per second. The motion is slow enough that the individual pulses would overlap on the workpiece, appearing like a continuous beam. The speed of motion was 3.8% of the system capability (but nowhere in the manual or technical information provided is this maximum speed given). The crest symbol took 1 minute 17 seconds to cut, and the book symbol took 2 minutes 16 seconds to cut.

The laser cut letters and symbols are long, stringy, and delicate. My preference would be to glue them to the end supports of the marimba, because then they will be no longer so delicate, and then coat the marimba supports including the symbols with clear laquer or poly-urethane. The coat should round off the edges of the cut veneer, adding to the integrity of the glue.

Lasers of this type usually have a divergence angle somewhat greater than a milliradian. The beam diameter at the focus is given approximately by the product of the divergence and the focal length of the lens (2.5 inches). Guessing a divergence of 1.5 milliradians would imply the beam is focused to a beam diameter at the focus of 95 micrometers (I’ve switched from English to metric units). The twenty five watt beam would result in a power density, or intensity, at the focus of 3.5 kilowatts per square millimeter. The focused beam behaves like a cone ... it converges towards the minimum value. After passing through the minimum value the beam diverges in a cone shape if there is no workpiece present to intercept the beam. The minimum beam size is not a point because of diffraction, but is found by the expression given above. Because our veneer was not completely flat, some portions of the veneer were at the accurate focus, where the beam had its minimum width, but other parts intercepted the beam when it was in the cone away from the narrowest part. In this case, the cut is a bit wider than at the position of the narrowest focus, so there is a bit of deviation in the cut part away from the programmed value.

There is a jet of dry compressed air co-linear with the beam. The air jet blows the smoke and fumes away from the lens, since a dirty lens would overheat and be damaged. About half of the heat required for the cutting comes from the laser beam, and the other half comes from the chemical reaction with the oxygen in the air jet.

V.E. Merchant