CNC Fret Slotting and Fretboard Work for Luthiers: Precision Small-Scale Machining on a Desktop Router

Cutting a guitar body on a CNC router — covered elsewhere on this site — is forgiving work: tolerances are measured in fractions of a millimeter and errors sand out. Fret slotting is the opposite kind of machining entirely. Fret positions follow an exact mathematical scale, slot width has to match fret tang thickness to a few thousandths of an inch, and slot depth has to be consistent to a degree hand tools struggle with. This is precision small-scale work, and it's exactly the kind of job a desktop CNC router is well suited for once you understand the specific requirements.

Why Fret Slotting Is a Different Problem Than Body Routing

Positional accuracy compounds. Fret spacing follows the equal-tempered 12th-root-of-2 scale rule, and every fret's position is calculated from the nut, not from the previous fret — a small error on fret 3 doesn't just affect fret 3, it throws off intonation on every fret above it if you're calculating positions cumulatively instead of from a fixed reference.
Slot width tolerance is tight. Standard fret wire tang widths run around 0.023"–0.025" depending on the wire (narrow vintage-style tangs run tighter, jumbo wire tangs run wider) — a slot cut a few thousandths too wide won't hold the fret with enough friction fit, and one too narrow will crack the fretboard when you press the fret in.
Slot depth must be consistent across the board. Most fretboards aren't perfectly flat — many are radiused (7.25", 9.5", 12", 16" compound radius are all common) — so a fixed Z-depth toolpath cut on a flat spoilboard will cut inconsistent depth once the board is radiused, unless the fretboard is slotted before radiusing, which is the standard order of operations for a reason.

Calculating Fret Positions

Fret position from the nut is calculated with the standard equal-tempered formula:

Distance from nut to fret n = Scale Length − (Scale Length / 2^(n/12))

For a standard 25.5" scale length (Fender-style), fret 12 — the octave — lands at exactly half the scale length, 12.75", which is a useful sanity check on any calculated fret table before you cut anything. Rather than hand-calculating each fret, use a dedicated fret position calculator (StewMac's online calculator is the standard reference) to generate the full table for your chosen scale length, then import those positions as a DXF or SVG with slot centerlines at each calculated distance — this becomes your toolpath geometry.

Scale LengthCommon UseFret 12 Position 24.75"Gibson-style, warmer feel, easier bends12.375" 25.5"Fender-style, brighter tone, more tension12.75" 24.6"–25"PRS and multi-scale/fanned-fret designs (varies by string)Varies per string

Tooling: Slitting Saw vs. End Mill

Two approaches dominate CNC fret slotting, and they produce meaningfully different results:

A slitting saw (jeweler's slitting saw arbor) mounted in the spindle cuts a true, uniform-width kerf the way a fret saw does by hand, and is the closer analog to traditional lutherie technique. This requires the saw's kerf width to already match your fret tang — you're choosing a saw blade by thickness the same way you'd choose a fret saw, not cutting an arbitrary width.
A small-diameter end mill (0.023"–0.025" or matched to your fret tang) run as a slotting toolpath is more flexible since slot width becomes a toolpath parameter rather than a fixed tool dimension, but sub-1mm end mills are fragile, run at very high RPM relative to their diameter, and break easily on a router that isn't dialed in — this is a case where feeds and speeds discipline matters far more than on a roughing operation, since there's essentially no room for chatter or deflection at this scale.

Depth and Order of Operations

Slot depth should be slightly deeper than the fret tang length (typically 0.010"–0.020" of clearance) so the fret's shoulders seat flush against the board without bottoming out in the slot first.
Slot the fretboard while it's still flat, before radiusing — cutting a consistent-depth slot into an already-radiused board means the toolpath Z-height would need to follow the radius curve exactly, which is solvable but adds unnecessary complexity when slotting-then-radiusing avoids the problem entirely.
Cut a registration/fence edge on the CNC first, and reference all fret slot positions from that same edge — any inconsistency between your workholding reference and your CAD reference edge shows up as fret positions that are individually accurate but collectively skewed relative to the neck's centerline.
Consider cutting slightly proud of full depth on a first pass, then a light finishing pass — this reduces tear-out on the exit side of thin, brittle fretboard woods like ebony and rosewood, which chip more readily than the softer woods common elsewhere in CNC work on this site.

Workholding for Long, Thin Stock

A fretboard blank is long, thin, and easy to flex or lift under even light cutting forces — the opposite problem from a rigid sign blank or plywood panel. Full-length double-sided tape along a flat, trued spoilboard, backed up by a few strategically placed clamps or a vacuum spoilboard zone if your machine has one, keeps the board from lifting at the tool's exit point on each slot, which is where most fretboard tear-out actually happens.

Fan-Fret and Multi-Scale Boards

Multi-scale (fanned-fret) designs use a different scale length per string, which means each fret is no longer a single straight slot perpendicular to the centerline — it's an angled slot whose position is calculated independently for the bass and treble edge and connected with a straight or slightly curved line between them. This is genuinely difficult to execute accurately by hand and is one of the strongest arguments for CNC slotting specifically: once the geometry is correctly generated in CAD from the two scale lengths, the router cuts the same operation whether the slot is straight or fanned, where a hand fret saw would need a completely different technique for each.

Fret slotting won't use most of the roughing horsepower a Wolfpawn-class router has — this is finesse work at a scale where rigidity, accurate positioning, and a light touch matter more than raw cutting power. Treat it as its own operation with its own tooling and workholding setup rather than an extension of body-routing technique, and the payoff is fret spacing more consistent than most hand-cut necks ever achieve.

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