Posted on June 13, 2024May 15, 2025 by Dale Phillips Fretboard Slotting with a CNC Router Fretboard Slotting with a CNC Router by John Svizzero and R.M. Mottola Originally published in American Lutherie #98, 2009 Lutherie shops with CNC machines use them for a number of applications, some of which are discussed in the article “Desktop CNC Machines” in AL#77. We (authors Svizzero and Mottola) met at meetings of the New England Luthiers and began comparing notes on our shop-built CNCs. Svizzero’s full-sized machine uses Porter-Cable routers as spindles, and Mottola’s desktop machine uses the Proxxon grinder. We have each been using CNC routers in our own shops for slotting fretboards. By CNC routing standards, fretboard slotting is a slow operation due to the small diameter end mills used (typically .022" to .025"), but in our small shops we have found slotting in this manner to be both cost effective in terms of time and to offer some advantages that other slotting schemes do not. With a suitable CNC machine already in the shop, it is possible to experiment with CNC fretboard slotting with a small investment in tooling and time. Our experiences may help others get over some of the startup problems. Become A Member to Continue Reading This Article This article is part of our premium web content offered to Guild members. To view this and other web articles, join the Guild of American Luthiers. Members also receive 4 annual issues of American Lutherie and get discounts on products. For details, visit the membership page. If you are already a member, login for access or contact us to setup your account.
Posted on June 6, 2024May 27, 2025 by Dale Phillips Questions: Guitar Rib Depth Questions: Guitar Rib Depth by R.M. Mottola Originally published in American Lutherie #85, 2006 Mark Korsten of Hastings-on-Hudson, NY asks: The article concerning how the rib depth of guitars should be adjusted when plates are domed was clearly written and illustrated (AL#84). I truly appreciate the insights offered by author R.M. Mottola. Being a neophyte luthier, however, I have what is probably a naive question. How does changing the depth of the guitar’s ribs modify the manner in which the bindings are fitted to the binding ledge? It’s a fairly easy operation to bend the binding when the plates are flat. However, when the depth of the ribs are varied, doesn’t that introduce another plane to the geometry? Do you simply use more flexible, thinner bindings or stronger binding tape to keep things tightly apposed in the ledge? R.M. Mottola responds: Although the doming of the plates means that the side depth of the ribs must be varied so the ribs and plates can be glued, this type of construction has little effect on binding. The ledges can be routed using a router resting on the plate or inverted in a router table. In either case the doming is so slight that the routing can be done just as if the plates were flat. And although the bindings must be bent vertically to accommodate the varying rib depth, the amount is so small as to be inconsequential. ◆
Posted on June 6, 2024May 23, 2025 by Dale Phillips Questions: Jig for Oval Rosettes Questions: Jig for Oval Rosettes by R.M. Mottola Originally published in American Lutherie #97, 2009 Greg Pacetti of Fairbanks, Alaska asks: I’m in pursuit of some kind of jig for oval rosettes. The Questions column editor responds: Probably the single best source of info on cutting oval rosette channels appeared in the American Lutherie article “Making Oval Mandolin Rosettes” by Jonathon Peterson, et al. (AL#41 p. 34, BRBAL4 p. 140). In it, seven builders describe their methods. ◆
Posted on June 6, 2024May 27, 2025 by Dale Phillips Questions: 17″ Scale Length String Questions: 17" Scale Length String by R.M. Mottola Originally published in American Lutherie #87, 2006 Rob E. of Lexington, MA asks: Does anyone know of a string that can be put on a 17" scale length instrument, with an after length to the tailpiece of about 6", that is strong enough to reach a mandolin E tuning without breaking? “Questions” editor R.M. Mottola answers: A string with a small enough diameter to be tuned to this pitch at this scale length will have a breaking tension that is too close to its static tension to be of much practical use. When specifying the scale length for a new type of instrument, use a length similar to that used by existing designs with the same pitch range. For high-pitched instruments with long scales (such as the instrument in question) the issue is finding a string with high enough breaking tension to be useful. Unfortunately there are no simple solutions short of having strings custom-made of high-tensile-strength materials. For low-pitched instruments with short scale lengths the issues are tone and, for fretted instruments, intonation. Short, fat bass strings don’t behave much like ideal strings and their partials tend to skew way sharp as they are further shortened by fretting. There is a mathematical relationship among scale length, pitch, tension, and mass per unit length of a vibrating string. Download the paper entitled “Catalog Supplement / String Tension Specifications” from the D’Addario website. The paper (in PDF format) can be found at www.daddariostrings.com/Resources/JDCDAD/images/tension_chart.pdf. It contains comprehensive info on the string formulae as well as mass-per-unit-length (unit weight) data for every string D’Addario makes. Unfortunately, breaking strength data is not available in this paper as the issue of specifying the tension at which a real string will break is more complicated than it might appear. Tensile strength data is available from wire manufacturers but the best bet is to design high-pitched instruments using short enough scale lengths.
Posted on June 6, 2024May 23, 2025 by Dale Phillips Questions: Scale Length Questions: Scale Length by R.M. Mottola and Tim Olsen Originally published in American Lutherie #93, 2008 Byron from the Internet asks: Is there any way to determine the scale length of an instrument from a piece of the fingerboard? I have the fingerboard from an instrument that is currently being restored. The nut end had been trimmed, I think to accommodate some kind of odd nut. The Questions column editor responds: All inter-fret distances are related to scale length, so measuring, say, the distance between the 1st and 2nd fret and then looking that up on a fret scale chart will tell you the scale length. Or measure the distance between the 7th and 19th fret (assuming the instrument has a 19th fret of course) and multiply by three. The result will be within a few hundredths of the actual scale length, close enough to identify it. American Lutherie editor Tim Olsen responds: Imagine you put a capo on the 1st fret of a functional guitar. Measure from the 1st fret (now functioning as the nut) to the 13th (now functioning as the 12th), and double that to find the mathematical bridge point. If the fretboard is still on the guitar, you can find that point near the saddle and mark it. If the fretboard is not on the guitar, you can clamp it to something, like a 2×4, and mark the bridge point on that. Now you could easily find the nut point by measuring from the newly marked bridge point back to the 12th and doubling it. Confirm it by calculating the scale and checking the position of the individual frets. ◆
