A Shop-made Thickness Caliper
By Peter O'Connor
A thickness caliper is virtually essential for measuring the critical thickness of the soundboard and backs of acoustic guitars and similar instruments. Those available commercially are very expensive; here's a shop-made example, with details that can help you to build your own.
I'm not sure why these tools are as expensive as they are. I suppose limited production runs inflate the prices in order for each sale to cover a larger than normal share of tooling and other costs, but even so, these tools are among the most egregious examples of high prices that luthiers are routinely expected to pay for the specialist tools that we require. At the time of writing, a thickness caliper from one of the U.S. luthier outlets is about $190 USD (plus shipping!) and these tools are about $270 here in Canada - ouch! And in the final analysis, they are simply a stable frame supporting a depth gauge, nothing more.
Elsewhere in this issue you might have read a book review of Brian Forbes' excellent "Acoustic Guitar Making - How to Make Tools, Templates, and Jigs". I mention in that review that Brian's book introduced me to the possibilities of making my own tools and jigs. Building my own tools and jigs has not only saved me a lot of money but means I often create a tool that is exactly what I need - thank you Brian!
Before we look at the details, I'll let you know that the total cost of this tool (excluding my time, of course) was about $60 Canadian. The block of aluminum, cut to the size I specified, was purchased from Metal Supermarkets here in London, who also supplied the 0.050" thick aluminum sheet that will be used for the thumb-operated lever to work the gauge, and the feet that allow the tool to stand upright; the two together were about $26. The gauge was purchased from Princess Auto here in London for about $30. And if you're in no rush, I have purchased a similar gauge in the past from either eBay of Amazon for only about $16, from China of course, but the shipping was free!
Most jigs I make feature Baltic Birch plywood, and although a thickness caliper could be made with this, or hardwood, commercial examples are usually made with aluminum, and I preferred the stability and rigidity promised by this material. I knew that I would have no difficulty cutting out the shape with a bandsaw, but having almost no experience with metalworking, I was initially unsure of the best way to drill the holes. These holes are necessary not only to provide secure finger holds but also to reduce the tools' weight without compromising stiffness. My first idea was that a woodworking spade bit would probably work. A quick check on the internet found a helpful video from a metalworker who demonstrated drilling holes in aluminum, confirming my suspicion that this could be done.
The demonstrator advised that while he would recommend using the correct tool for the job, the spade bit could be used in a pinch - he neglected, however, to mention what the "correct tool" actually was, which in my case didn't matter at all since the whole point of building my own caliper was to save money, and I had no intention of purchasing a specialist metalworking item. While the video was helpful in confirming my suspicion, the actual drilling was shown at far greater than
SOLG Newsletter - Issue 2 - September 2020
Page 6