# Top Secret Notes On…

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# Fingerboard Geometry Part 2: Scoop vs. Radius

As seen in our survey results, scoop gaps along string paths and the cross profile radius were regarded as highest priority.

The model I am using to study their interaction is a grid of 3×3 points. Think of it as 3 smaller radii crosswise (usually around 42mm), and 3 giant arcs lengthwise (representing scoop gaps under G, E, and down the centerline).

Each of the 9 points are connected to a cross radius and the lengthwise scoop radii, so trying to force definitions on all 6 radii doesn’t always work. That’s too many constraints, like saying, “I want a right triangle with sides of 2, 3, and 4.”

Here, I’ll define 4 of the radii completely, and show the relationship between the remaining two. All solutions along the graphed lines are geometrically compatible with the constraining dimensions specified in the subtitles (which are based on median survey results).

## Middle crosswise radius vs Center-line scoop gap

Option 1 (one radius all the way) from our survey is the only one where all crosswise radii are defined. But one of the three scoop gaps can’t be set to our target without changing the middle radius. In this case, with G scoop set to 0.75mm and E set to 0.25mm, the center-line gap comes out to 0.68mm, or 36% over our target of 0.50mm.

If we instead prioritize our scoop gaps, setting them to 0.75mm, 0.50mm, and 0.25mm, we can calculate the middle radius out to 38mm, which matches our survey’s Option 3 (tighter radius in the middle).

Some survey participants have observed this phenomenon:

I’ve made fingerboards for respected [players] that prefer an even radius along the whole board, but there will wind up being more scoop on the center strings, which makes the string heights higher than the g string in the middle of the board.

Note that our target scoop gaps decrease linearly from G to E. Some makers intentionally skew the gradation of scoop gaps, reasoning that players may not perceive the larger gap due to similar tensions and diameters of the G and D strings. It is difficult to draw conclusions about scoop gap skewing trends from the survey, because responses were given to the nearest 0.25mm.

Below are a few other gap size permutations from our survey.

Note that in every case with nut & bridge radii set to 42mm, for the centerline scoop size to be halfway between G and E, the middle radius always comes out 38mm. As the middle radius flattens from 38mm, the centerline gap skews toward G. The tighter the radius (<38mm), the more it skews toward E.

## Crosswise radius of the nut vs middle

Those who selected Option 4 (tighter radius at the nut) describe it as a section of a cone, with radius changing proportionally to the width of the fingerboard.

Option 2 (flatter radius at the nut) respondents did not give specific numbers about how much flatter. It turns out that gradually increasing the radius toward the nut as I had described in the survey does not produce a plausible solution for our given gap sizes. In general, for a flatter nut radius, the middle radius will still tighten relative to the bridge end.

As shown in Fig 2b, when a centerline gap size is halfway between G and E, the relationships between the crosswise radii are practically identical. Here is another way of looking at that:

## tl;dr

Here are some geometrically plausible dimensions:

The differences we are talking about are tiny (though maybe not to a player). Here’s what they’d look like under your 42mm radius template:

Have fun!!!!

### Coming soon…

I will tell you how to go all Traité de Luthérie so you can resolve your own favorite dimensions! Or if you’re too lazy, I’ll give you an Onshape CAD file to play with.

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# Fingerboard Geometry Part 1: Survey dimensions

Here are the results of the survey, with 67 responses collected between 12/19 – 12/25.

Participants were asked pick answers that best described their mental model of a typical violin fingerboard. If their concept of fingerboard dimensions were not well-described by the given answers, they were asked to pick the closest answer and elaborate about discrepancies in the comments section.

## Fingerboard radius

Remarks from participants who selected…

Option one (one radius all the way):

I initially plane a consistent radius but this is slightly altered in the middle portion as a result of asymmetrical scooping. –Mitch McCarthy

Essentially an adjustment to the position relative to the centerline in combination with an adjustment to its height of said radius along the length gives the resultant change in width and edge heights.

Option two (flatter curve at the nut end):

Roundness at the bridge end keeps the middle strings from being in a hole in the upper positions (the string doesn’t need to be depressed as far) making double stopping and bowing easier. Flatter at the nut means the left hand doesn’t have to climb over the strings and chords are easier to play cleanly.

… more flat closer to the nut to give a “security sensation” to the musician, I meant for their finger to press the string, I think that my base of my concept the relation with the bridge radius. –Martin Cruz Aragon

Option three (one radius at nut & bridge ends, tighter curve in between):

I’ve made fingerboards for respected [players] that prefer an even radius along the whole board, but there will wind up being more scoop on the center strings, which makes the string heights higher than the g string in the middle of the board.

I actually make the board 42 at the bridge end slightly tighter in the middle and slightly flatter at the nut. –Nathan Slobodkin

Option four (narrower curve at nut end):

The radius is tighter at the nut, and then flattens out as it widens toward the bridge, allowing dead purchase for double stops and such in high positions. –Chris Jacoby

I am limiting this analysis to arcs, but some participants noted that their cross-sectional profiles were not arcs (parabolic, “egg-shaped,” etc).

## String scoop gap size

Many players commented that string scoop was dependent upon various factors: string heights, string diameter and properties, player preference & style of playing, etc.

## Edge scoop gap size

The treble and bass scoop gaps were symmetrical for the majority of responses.

For asymmetrical edge scoops, bass side was deeper than treble for all but one participant, who explains that:

… reaching over the strings comes from the treble side of the board so I put a little more scoop on this side to facilitate this action.

## Apparent edge thickness

One participant remarks:

I make the edge thickness the same instead of measuring side scoop.

Those who use edge scoop to achieve uniform edge thickness would need a deeper edge scoop for deeper string scoops, explaining why most asymmetrical responses have heavier gaps on the bass side.

This is also observable in general by correlating string scoop and edge scoop:

Lifting the floor of the bridge end of the fingerboard is another method for achieving a more uniform edge thickness.

## String scoop symmetry

Remarks:

I try to get a consistent radius curve throughout the scooping. That means that, for me, scoop centering is irrelevant.

… generally centered on fb, if anything slightly towards nut with the thought that the further away from the bridge the fingering point is, the smaller the angle between the string and fingerboard, requiring more clearance. –Mitch McCarthy

With a bit of thought, it’s apparent that scoop is only important close to nut. –Jim Biggs

I did not receive comments about scoop centering from participants who bias the depth toward the bridge.

## Priority rankings

Many participants cited player style and player preference as the main determining factors for the dimensions used. The overall highest ranked factors are string scoop size and radius fidelity.

## Coming soon…

The geometric interactions between scoop gap and the radius at nut, middle, and end will be discussed in greater depth in my next post.

Thanks to everyone involved in this survey.

# Fingerboard Geometry: Project Overview

I am investigating how the fingerboard’s radius, scoop depth along string paths, and scoop along edge interact with one another. In the first phase, this project will use Google Forms to survey makers to find the ranges and averages of dimensions needed to generate the models.

I am studying this because, in my personal experience of shaping fingerboards, I have run into issues reconciling various sets of dimensions and scoop requirements, leading me to wonder how the surface of the fingerboard is modeled theoretically. In arc-based construction, some target dimensions will conflict with one another, and most people end up “fudging” or compromising to achieve their desired balance of playability and aesthetic. My goal here is to solve for a few quantities on the relationships between dimensions that luthiers do actively pay attention to while shaping. I am aiming for results to look something like, “If I put priority on a perfect constant radius and G & E scoop, the D & A scoop will be off target by ___ mm or %.”

## Part 1: Survey dimensions

12/26/2017: The results of the survey are up.

## Part 2: Case study models

1/13/2018: Analysis of Scoop vs. Radius relationships is up.

I’ll make models following the most frequent orders of priority rankings, and show how lower priority dimensions are sacrificed for precision of higher priority ones per case.

Model building details (sorry, I didn’t ask for these in the survey!):

• 24mm nut end width
• 42mm bridge end width
• 16mm nut string spacing
• 30.73mm string spacing at bridge end (projected from a 34mm spacing on a bridge  330mm away from nut)
• 42mm baseline radius (will calculate exactly what “tighter” and “flatter” are case by case)
• 270mm length
• 5mm thick at bridge end (as there will surely be twists in many of the models).

For simplicity (and because many makers use templates of specific radii), I am limiting this study to arcs. All arcs will be drawn from 3 points (i.e. G, middle, and E). I will give you beautiful illustrations of cute 10x squished fingerboards so you can really see what’s going on with those scoops and twists!

The image above is from Rhino, but I actually plan to use OnShape (thanks to recommendation by my sister, Dr. Maxine Fontaine) to generate the models for everyone’s viewing pleasure. I will also calculate relationships where possible (as in, have my brother-in-law, Dr. Nick Fontaine, do it with Python for me).

## Post script

The point of this study is NOT to tell you what dimensions are right or wrong! (I’m pretty sure that’s your client’s job, haha!) It is just to quantify how the dimensional factors interact.

# Baby Plane Making, part 1

A trustworthy new brass fingerplane will cost you some \$50-\$70. You need a few different sizes, times two, for flat-bottom and round-bottom. That’s a lot of money.

So make your own out of wood! I love wooden fingerplanes – they are lighter, wooden tools feel nicer in your hand, and they sound a little different from the brass ones.

Here’s what I’ve learned from making my 3 wooden planes. This is a general post, and I’ll go into more detail over the next couple posts.

The type of plane we’re making here is based on the sandwiched Krenov style / crosspin + wedge / whatever you want to call it.

## Ingredients

1. BLADE. This is your constant. You will build your plane based on the width and thickness of your blade. The sizes we’re working with at school are 3/8″, 1/2″, and 1″ wide, all O2 untempered steel stock that we have to cut, shape, harden and temper ourselves.
2. BODY WOOD. Something stable and not too soft. Since we are violin makers, maple is plentiful and will do fine (but maybe don’t use that pink streaky low-density stuff we get for our first instrument). Grain orientation for the body is not crucial for tiny planes, but I’ll go into further detail on that in a future post.
3. SOLE WOOD. If you can manage it, get some quartersawn very hard wood for the sole. Some folks are using bubinga, ipe, ironwood, ebony, rosewood, etc. Most of the dense, tight-grained tropicals are supposed to work well for this, though I did make one out of purpleheart and it turned out to be surprisingly/disappointingly mushy! Keep a little extra of this sole wood for the wedge. Straight grain will make life easier but if it’s interlocking, you’ll live.
4. CROSS PIN. Use a metal rod of some sort. Make sure you have an appropriate drill bit for it.

## Design Considerations

Plane making is really not that difficult. If you haphazardly slap some wood together, chisel out an opening, pin it, and stick a wedge and blade in it, you’ll probably have a functional plane. But if you plan ahead, these little points could make the difference between a plane that functions and a plane you loooove.

On any plane, the area right in front of the throat takes the most wear.

You need to find a balance between supporting that area, and having space to relieve wood shavings. On a small plane, the space for chip relief fills up very very fast, especially if you tend to stick your finger in it.

As you redress your plane, the throat will get larger and larger, so consider ways to minimize that. In violin making, finger planes are used for arching, but they do not produce the finished surface, so a tiny throat (which is supposed to reduce tear-out) is not as important as it would be on, say, a smoothing plane. Still, a giant throat on any plane disturbs me… For a small finger plane, if you keep your plane sole reasonably thick, this configuration is my happy medium for achieving a roomy chip receptacle and well-supported throat that will stay somewhat small with each redressing. My finger planes are mostly for arching and/or some edge work, so I’m just sticking with the traditional 45° bed angle, blade bevel down. Low angle is supposedly better for cutting endgrain, high angle is supposedly better for figure.

Cross pin placement should not be mindlessly done. Factoring in blade thickness and wedge shape, you’ll end up a line of acceptable cross pin placement. Again, you’ll have to strike a balance. The lower you place the pin, the better the support near the cutting edge, which should help reduce blade chatter. Don’t go too close though, or your wedge will have less room to do its job without getting in the way of chip flushing.

The wedge pushes up on the pin pretty forcefully, so consider your final shape and make sure you have enough meat above the pin.

Consider how you tend to grip your finger plane. I hold really really tiny tiny planes a little differently from just kinda tiny planes. Consider how the final shape of your plane and your grip on it could affect some of the other design points.

Next post will be on actual construction.

I’d like to give a little shout out to my friend David Finck, who I met at Oberlin. He is the author of Making & Mastering Wood Planes, which would make an excellent gift for any of your woodworking or luthier plane nerd friends.

# Cello Arch Reading

I’m shaping the arch on my cello. As with drawing, I have a little problem where I have my face is right up against the workpiece. It’s always good to step back and get a macro reading of the shadows to judge where to scrape next. On cello this is harder to do at your desk unless you have extendable arms (which I unfortunately do not).

Fortunately there is a very bright light (the sun) and a mirror (our front door) outside. Everyone passing by on Oakton at 5:30pm today would have seen me holding my cello plate up to the school, Simba-style.

I love the hard shadows from the sun.

Keep the centerline of the plate at the same angle of the sun for a very unforgiving reading of arch symmetry (looks like I still have a little work to do):