CNC routing, tool radius, and the letters on a shop board

A router is a spinning cylinder

Where a laser has a beam of near-zero width, a CNC router has a bit of finite diameter: commonly 3 mm or 6 mm for sign work. Everything that follows comes from that one fact.

The internal corner problem

A router bit cannot cut an internal corner sharper than its own radius. Draw a square hole and cut it with a 6 mm bit and you get a square hole with 3 mm radiused corners.

Three responses:

1. Design the radius in. If the corner is going to be round, draw it round so the result is what you intended.
2. Use a smaller bit for the detail, in a second pass. Slower, and a small bit in hard material breaks.
3. Dog-bone or T-bone fillets. Add a small circular overcut at each internal corner, so a square tab still fits. This is standard practice in CNC joinery and looks deliberate once you know what it is.

The same limit governs letterforms: a serif’s inner angle, the junction of a stem and a bowl, the point of a V. Ask the fabricator the bit diameter before you draw, and check every internal angle in the mark against it.

Tabs, and the part that flies across the workshop

A part cut fully free in the middle of a sheet will move once it is loose, and the bit will hit it. So the toolpath leaves small tabs (uncut bridges of 3 to 6 mm) holding the part in place, which are cut through by hand afterwards and sanded.

Tab placement is a decision: put them on edges that will not be seen, and never on a visible curve where the witness mark will show.

Climb and conventional, briefly

The bit can rotate into the material (conventional) or with the direction of travel (climb). Climb milling gives a better surface finish on most sign materials and pulls the workpiece, so it needs good hold-down. This is the operator’s choice; you need it only to understand why they may ask which face is the visible one.

Materials for signage

• ACP / ACM (aluminium composite panel): two thin aluminium skins on a plastic core. The standard shop-board material: light, flat, weatherproof, cuts and folds well. Brand names like Alucobond and Aludecor.
• Acrylic: for illuminated letters and panels. Cast acrylic routs better than extruded.
• MDF: interior only; it swells with any moisture.
• Foam PVC (Forex, Sintra): light, cheap, interior and sheltered exterior.
• Aluminium and brass: for engraved plates and premium letters.

Built-up and channel letters

A shop-board name in three-dimensional letters is built rather than cut flat.

• Flat-cut letters: routed from sheet, mounted on studs standing a few millimetres off the wall so they cast a shadow.
• Built-up letters: a routed face, a returned side (the “return”), and a back. Effectively a shallow box in the letter’s shape.
• Channel letters: built-up letters with LEDs inside and a translucent acrylic face. Face-lit glows from the front; halo-lit or reverse-lit has an opaque face and throws light onto the wall behind, which is the expensive, good-looking one.

What each demands of your artwork:

• Counters need connections. The middle of an O or an A is a separate piece. For a flat-cut letter it is mounted separately; for a channel letter it must either be its own small channel or be bridged.
• Stroke width sets the return depth’s feasibility. A stroke thinner than about 25 mm cannot hold LEDs and a return.
• Every letter is a separate piece. A script with connected letters must be split at sensible points, and the split lines are a design decision you should make rather than the fabricator.
• Mounting. Studs, a hidden rail, or a raceway. Ask which, because a raceway is a visible box behind the letters and it changes the design.

What to send

DXF is the lingua franca of CNC, and most shops also accept AI, EPS or PDF. Whatever the format:

• 1:1, in millimetres, one closed path per cut.
• Layers named for the operation: CUT, POCKET, ENGRAVE, DRILL.
• Strokes outlined, text curved, overlaps welded, duplicates removed.
• A separate reference PDF showing the assembly, the colours and the mounting.
• The material and thickness in the filename and in the note.

The conversation

What bit diameter, what material and thickness, and will these be flat-cut, built-up or channel letters?

Three answers, and the design constraints follow from them. Asking after the design is finished is how a mark gets redrawn for free.