Reverse engineering violins

June 21, 2014

So what do you do when you have about 20 violins from the 1730’s show up in town? Attempt to find out why these particular instruments sound so much better than others of course. Over the years and currently, many are still trying to find that answer as to why some sound so much better than others when the apparent construction is virtually the same.

Many factors come into play, but probably the most obvious is the shape. Several other things contribute, like the type of wood used, wood grain and density, humidity at the time of construction etc. Now back to the shape, getting that data isn’t the easiest. One thing is for certain, you don’t want to be touching these with any type of mechanical instrument that could damage the finish of these $2 million dollar and up violins.

So how do you do it? Laser scan it of course with a Portable Coordinate Measuring Machine (PCMM) with a Laser Line Probe (LLP) attached to it. This way you can get a point cloud of the exterior shape good to about .003” without having to touch the instrument at all. The total time it took to scan the violins was about 45 minutes each. Most of that time was spent inspecting the cloud of points for holes in the data and then going back to fill them in. The actual time scanning the majority of the outside was about 10 to 15 minutes. A small bit of time was also taken to delete extra data collected from the jig the violins sat on. The LLP is a line of sight instrument, so anything scanned with the laser will be picked up by the cameras and added to the cloud of points. Once we were satisfied with the data, it was saved to a file.
That takes care of the outside data collection, but just as important for future construction, is what this violin looks like on the inside. Cutting it open is obviously not an option, and neither is virtually anything else. Thankfully downtown at the local hospital is an MRI that can give point cloud data back much like the LLP does on the outside. The stated accuracy is not as good as the LLP, but for this application it was more than adequate. The violins were sat on a soft cushion on top of the table normally used for humans, and in it went. About 10 minutes later the MRI had totally mapped what the inside of the violin looked like. This data was also exported to a file for future use.

Now that the data was collected for both the inside and outside of the violin, it was now time to bring the data together so that a complete model could be made. First, the LLP data was opened in the software that collected the LLP information, and then via .igs, the MRI data file was imported as well. The MRI data also had information collected on the outer shape, so that is what was used to start the alignment process between the two files. First a point pairs style alignment was done to get close, then we allowed the computer to do a best fit analysis and really dial in the two clouds of data. Now that the data was aligned, it was saved as one dataset. From this, the point cloud was analyzed and manipulated in to what was eventually a 3D cad model usable in virtually any cad system.

Without focusing on too many details, with this cad file, they could now send info off to CAM software to write tool paths for CNC machines to cut both the top and bottom surfaces exactly the way the old violins were. The sides were fairly standard if I recall, so most of the focus was the top and bottom surface. Once all the pieces were made, the builder would then assemble the violin and hopefully it would sound as good as the violin they were trying to replicate in the first place.


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