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7/25

Laser cutting

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

 this is an image of the laser cutting of the letters.

this is a (slightly blurry) image of the logo on the finished endpiece.

well that's just great... (old post)

this is the resonators being spray painted with primer. the great thing about this is how they're all standing upright. like an accident waiting to happen, right? well, it all ready did. domino style.

this is the resonators hanging from the support struts, without the lower supports attached.

testing plugs.

a jumbled mess

sorting out the aforementioned jumbled mess

a pile of offcuts from the curved resonator bottoms.

this image shows the finished resonators in place in the marimba. the only thing that remains to be done is to tune them, and then the marimba should be done!

A series of slideshows

Here is a series of slideshows, organised according to topic:

Mallets:



Keys: (warning, there may be some repeats)




I will upload the ones for the frame and resonators as soon as I get home tomorrow.

Finished Bars & Power Tools

No.

Still alive

Though the amount of activity on this blog recently may not show it, I am still alive and still working. Expect an influx of posts over the next few days, as I finish several posts that I started long ago.

Sanding and Varnishing...

This

More or less sums it up.

Also

before I forget, Thank you to mr. summers for lending me his chromatic tuner when I left mine at home.

Anatomy of a resonator plug V2.0

so it turns out that just foam inside the resonator plugs is not such a great idea... the main problem is that it does not seal, and it compresses way to much to be very useful to me. here is the inside of version 2.0:

this consists of a small loop of latex tubing, which is supported by one of the small foam disks that we cut. it is sandwiched between two of the wooden disks, as in the original design. it is then compressed using the same bolt system as the original resonators. 

fully disassembled. as you may or may not be able to see in this image, the tubing is cut at a slight angle to make it much easier to seal the resonator. if there are any leaks, the effectiveness of the resonator will be severely reduced.

in other news, yesterday we finished drilling the holes through the aluminum to hold up the resonators. the deadline is approaching, but I am fairly confident that we can have this project finished soon. maybe not by the deadline, but soon.

Jig!

this post is really just another photo dump. This time, however, it is mostly just a couple of jigs that I have used for various purposes during the project:

this is the jig that was used to make it much easier to drill the holes in the frame, by allowing me to position the drill in exactly the right spot and angle every time.

this is the jig that we used to hold the pipes for the resonators in place while we were cutting them with the hole saw.

thats all for this post. 

I'm working on another one, it should be up in a day or 2.

Information dump incoming!

over the last few days, I've completed quite a bit. In a nutshell:

finished cutting the arcs out of the resonators

cut the first four metal strips to length, and drilled holes in them

build this fancy jig thing for finding the nodes of the bars; this image shows how it starts, and after beating the key for a while, the salt that is sprinkled on it tends to do this:

the salt has collected at the points on the key which vibrate least, which is where we want to drill the holes through the keys. 

if we look back a few days earlier than this, we will find me cutting the final few resonators...

using a saw that I have officially christened "Medieval Torture Device A" to cut one of the resonators

 this is a looooooong pipe.

a slightly more effective view of the pipe

and a view from the other side. 

thats all for now! more to come in the coming days...

Almost-Failure...

this is another title that deserves a little explanation...

after travelling to the island this weekend, I quickly realized that I had forgotten to bring the tuner. this meant that the entire trip was more or less worthless. luckily, we were able to borrow one from a local piano store. this meant that we were able to proceed as planned, though we did waste an hour or so trying to find a functional tuner...

Travelling!

This is just a quick update to say that I am going to the island to recut the four lowest keys. They are all both out of tune, and have problems with the arch. When I was cutting them originally, I did not stop to consider where the keys would be resting on the frame; because of this, the arch overflows onto both support points, which means that the keys are in effect useless because it is impossible to drill holes in them without damaging the arcs or affecting their tuning significantly. Because of this, I need to recut and retune these keys. this is a bit of a hassle. but worth it to get keys that sound good...

Has Anyone...

... ever done this before?

this is the third way we've broken the drill press so far today. not a good sign...

a slideshow of pictures

this is the basics of what happened today:

Back to work- for real this time!

once again, I make a blog with the title "Back to work".

this time, however, it actually means something. earlier today I started working on the plugs for the resonators which will allow the aforementioned resonators to be tuned relatively quickly and easily.  creating them, however, is easier said than done:

these are the makeshift devices that were used for levering finished disks out of the hole saw. until these were created, it was very difficult to extract the disks due to an unfortunate design flaw of the hole saw.

this is the sheet of MDF/perfboard/whatever its called that I was drilling the plugs out of. You can also see the hole saw & drill press ready for the next hole to be drilled.

these are the nut/bolt/washer combinations that I was planning to use for the plugs. unfortunately, the bolts are too short...

a few ready-cut disks

the first plug. it consists of a foam disk, contained by two wooden disks. the foam looks (and is) rough, and that is one of the first problems than needs to be solved. we are thinking (after a conversation with the same uncle that laser cut the letters) that it might be best to send the foam off to be laser cut again, as this would be a quick and easy solution that would also be very precise.

the first of the larger disks.

last but not least, a comparison of the two disks. Though they don't look that much different from one another,  it is easy to tell the difference in sizes by the size of the bolts.

thats all for now! more to come in later days...