Alright, long day, but finally found the chance to sit and write for a minute. Here's a breakdown of my goals for what I would consider an ideal caul set.
The traditional approach is of course to say if you are fretting a 12" radius board, use a 12" radius caul. My experience however, places this practice to be one of blind faith in numbers and specs, with shortcomings easily overlooked for convenience. By and large, for the majority of fret work seen in real life, these specs rarely end up so perfect. From the thickness of the wire, to the glueing, leveling, and sanding of the board, it is quite rare for a 12" radius to be a true 12.0" radius, even when it comes time to install frets on new construction.
For anyone who does refrets or uses compound radius boards, the schism between spec and reality become even more obvious.
So my approach is a preference to abandon the false comfort of tidy round spec numbers, and instead shift toward a series of narrow increments. This means the radii these will be cut and marked as will not be a nice neat series of 10, 12, 14, 16, etc. Instead I would prefer a series of consistent increments spanning whatever range may be typically encountered.
I write this long preface because at first glance the odd numbers they will be spaced to could no doubt be received as discomforting when they do not line up with the precise (but rarely real) numbers we like to list as specs. In practice however, I find the neat round numbers to only be of false comfort, and when pressed to the board don't necessarily provide the best palette of choices to work with.
Instead I've come up with a series ranging from around 6" to 24", with the focus on incremental change in the sagitta, or the height at the center of the radius. The geometry gets a bit complicated, but the current layout is arranged to set increments ranging from approximately .002" in the shallower range (slightly less when viewed from perspective toward the nut, slightly more as it widens toward the extension), up toward .0025"-.003" as you approach the shallowest radius of 6".
The end result will be that if you find yourself pressing against any radius not perfectly matching the caul (which in reality pretty much amounts to every guitar or caul set you'll ever work with), you will use the next smaller radius. The difference in the center is then automatically compensated for by the slight bit of slop that exists in just about every pressing system. At the widest gaps between radii, the caul need tilt no more than 1.5° max to press the center fully down that last ~.002" once the ends are seated.
Those who have used the StewMac presses certainly witness this on a regular basis. When you are 10" caul to press in to an 11" region on a compound board, the head tilts slightly to better conform, and you can press quite neatly with this method.
The issue I've had with our current arrays of cauls is that the increments don't provide the smoothest transition between the neat tidy whole numbers they come in. After years of using the StewMac and Watkins sets, as well as a few customized cauls, I find there to be a number of gaps which are too large, and push the "auto-regulating" limits of sight tilt in the caul holder or clamp. I actually busted the tip on one of our older Jaws clamps from trying to push the tilt too far on an old D'Angelico that had a 6-7ish radius.
So, after years of using existing sets of cauls, and a number of custom ones, this is going to be the arrangement I've always wanted, set not to a series of comfortingly round numbers, but rather laid out in a more comprehensive and practical spread from one end to the next, without all the gaps which have frustrated me in the past.
Can't wait to finally have a full set!
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