{"course":{"slug":"vector-and-print","title":"Vectors, Logos and Things That Get Printed","url":"https://addaly.com/learn/vector-and-print","level":"beginner","track":"create"},"lesson":{"slug":"cnc-and-built-up-letters","title":"CNC routing, tool radius, and the letters on a shop board","url":"https://addaly.com/learn/vector-and-print/cnc-and-built-up-letters","position":70,"of":73,"minutes":8,"answer":"A router bit of finite diameter cannot cut an internal corner sharper than its own radius.","keyIdea":"A router bit of finite diameter cannot cut an internal corner sharper than its own radius, so ask the bit diameter before drawing and add dog-bone fillets where a square corner is needed. And built-up or channel letters impose a minimum stroke width and make every counter a separate piece."},"body":{"format":"markdown","text":"## A router is a spinning cylinder\n\nWhere 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.\n\n## The internal corner problem\n\nA 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.\n\nThree responses:\n\n1. **Design the radius in.** If the corner is going to be round, draw it round so the result is what you intended.\n2. **Use a smaller bit** for the detail, in a second pass. Slower, and a small bit in hard material breaks.\n3. **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.\n\nThe 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.\n\n## Tabs, and the part that flies across the workshop\n\nA 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.\n\nTab 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.\n\n## Climb and conventional, briefly\n\nThe 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.\n\n## Materials for signage\n\n- **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.\n- **Acrylic**: for illuminated letters and panels. Cast acrylic routs better than extruded.\n- **MDF**: interior only; it swells with any moisture.\n- **Foam PVC (Forex, Sintra)**: light, cheap, interior and sheltered exterior.\n- **Aluminium and brass**: for engraved plates and premium letters.\n\n## Built-up and channel letters\n\nA shop-board name in three-dimensional letters is built rather than cut flat.\n\n- **Flat-cut letters**: routed from sheet, mounted on studs standing a few millimetres off the wall so they cast a shadow.\n- **Built-up letters**: a routed face, a returned side (the \"return\"), and a back. Effectively a shallow box in the letter's shape.\n- **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.\n\nWhat each demands of your artwork:\n\n- **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.\n- **Stroke width sets the return depth's feasibility.** A stroke thinner than about 25 mm cannot hold LEDs and a return.\n- **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.\n- **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.\n\n## What to send\n\nDXF is the lingua franca of CNC, and most shops also accept AI, EPS or PDF. Whatever the format:\n\n- 1:1, in millimetres, one closed path per cut.\n- Layers named for the operation: `CUT`, `POCKET`, `ENGRAVE`, `DRILL`.\n- Strokes outlined, text curved, overlaps welded, duplicates removed.\n- A separate reference PDF showing the assembly, the colours and the mounting.\n- The material and thickness in the filename and in the note.\n\n## The conversation\n\n> What bit diameter, what material and thickness, and will these be flat-cut, built-up or channel letters?\n\nThree answers, and the design constraints follow from them. Asking after the design is finished is how a mark gets redrawn for free."},"check":{"question":"A logotype with sharp internal angles is routed from ACP with a 6 mm bit. The fabricator returns a sample where every inside corner is visibly rounded. Whose problem is it, and what fixes it?","options":[{"text":"The fabricator's: a finishing pass with a smaller bit is entirely standard practice and should have been included in the quotation.","correct":false,"tempting":"A second pass with a small bit is possible but slow, fragile and chargeable; it is a negotiated solution, not an omission on the fabricator's part."},{"text":"The fabricator's: the toolpath should have been offset inwards to compensate for the bit radius.","correct":false,"tempting":"Offsetting moves the cut but cannot make a round tool produce a corner sharper than its own radius; the geometry is not recoverable by offsetting."},{"text":"Neither: it is inherent to the process, and the artwork should carry the radius or dog-bone fillets.","correct":true},{"text":"The designer's: the file should have been supplied as DXF rather than PDF, since PDF does not preserve corner geometry.","correct":false,"tempting":"PDF preserves geometry exactly. The rounding is a property of the cutting tool, not of the interchange format."}]},"prev":{"slug":"laser-cutting-and-engraving","title":"Laser cutting, kerf, and the material you must never use","url":"https://addaly.com/learn/vector-and-print/laser-cutting-and-engraving"},"next":{"slug":"screen-print-separations","title":"Screen printing, separations and the underbase","url":"https://addaly.com/learn/vector-and-print/screen-print-separations"},"authorship":"Founder and series editor: Dr Mustafa Mun. Written by AI models to a written standard. Exam keys checked blind by a second model: 3,665 of 3,666 agreed. Academic review invited.","review":{"status":"Draft, open for academic review"},"cite":"Addaly (2026) 'CNC routing, tool radius, and the letters on a shop board', in Vectors, Logos and Things That Get Printed. Edited by M. Mun. Addaly. Available at: https://addaly.com/learn/vector-and-print/cnc-and-built-up-letters (Accessed: 13 October 2026).","licence":{"name":"CC BY-NC-SA 4.0","url":"https://creativecommons.org/licenses/by-nc-sa/4.0/"},"reader":{"schema":1,"title":"CNC routing, tool radius, and the letters on a shop board","courseTitle":"Vectors, Logos and Things That Get Printed","module":{"index":8,"slug":"machines-that-make-things","title":"Machines that cut, stitch and burn"},"answer":null,"keyIdea":"A router bit of finite diameter cannot cut an internal corner sharper than its own radius, so ask the bit diameter before drawing and add dog-bone fillets where a square corner is needed. And built-up or channel letters impose a minimum stroke width and make every counter a separate piece.","hero":{"kind":"plate","image":null,"alt":null,"caption":null},"blocks":[{"t":"h2","id":"a-router-is-a-spinning-cylinder","runs":[{"text":"A router is a spinning cylinder"}]},{"t":"p","runs":[{"text":"Where a laser has a beam of near-zero width, a CNC router has a "},{"text":"bit of finite diameter","b":true},{"text":": commonly 3 mm or 6 mm for sign work. Everything that follows comes from that one fact."}]},{"t":"h2","id":"the-internal-corner-problem","runs":[{"text":"The internal corner problem"}]},{"t":"p","runs":[{"text":"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."}]},{"t":"p","runs":[{"text":"Three responses:"}]},{"t":"ol","start":1,"items":[[{"text":"Design the radius in.","b":true},{"text":" If the corner is going to be round, draw it round so the result is what you intended."}],[{"text":"Use a smaller bit","b":true},{"text":" for the detail, in a second pass. Slower, and a small bit in hard material breaks."}],[{"text":"Dog-bone or T-bone fillets.","b":true},{"text":" 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."}]]},{"t":"p","runs":[{"text":"The same limit governs letterforms: a serif’s inner angle, the junction of a stem and a bowl, the point of a "},{"text":"V","code":true},{"text":". "},{"text":"Ask the fabricator the bit diameter before you draw","b":true},{"text":", and check every internal angle in the mark against it."}]},{"t":"h2","id":"tabs-and-the-part-that-flies-across-the-workshop","runs":[{"text":"Tabs, and the part that flies across the workshop"}]},{"t":"p","runs":[{"text":"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 "},{"text":"tabs","b":true},{"text":" (uncut bridges of 3 to 6 mm) holding the part in place, which are cut through by hand afterwards and sanded."}]},{"t":"p","runs":[{"text":"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."}]},{"t":"h2","id":"climb-and-conventional-briefly","runs":[{"text":"Climb and conventional, briefly"}]},{"t":"p","runs":[{"text":"The bit can rotate into the material ("},{"text":"conventional","b":true},{"text":") or with the direction of travel ("},{"text":"climb","b":true},{"text":"). 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."}]},{"t":"h2","id":"materials-for-signage","runs":[{"text":"Materials for signage"}]},{"t":"ul","items":[[{"text":"ACP / ACM (aluminium composite panel)","b":true},{"text":": 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."}],[{"text":"Acrylic","b":true},{"text":": for illuminated letters and panels. Cast acrylic routs better than extruded."}],[{"text":"MDF","b":true},{"text":": interior only; it swells with any moisture."}],[{"text":"Foam PVC (Forex, Sintra)","b":true},{"text":": light, cheap, interior and sheltered exterior."}],[{"text":"Aluminium and brass","b":true},{"text":": for engraved plates and premium letters."}]]},{"t":"h2","id":"built-up-and-channel-letters","runs":[{"text":"Built-up and channel letters"}]},{"t":"p","runs":[{"text":"A shop-board name in three-dimensional letters is built rather than cut flat."}]},{"t":"ul","items":[[{"text":"Flat-cut letters","b":true},{"text":": routed from sheet, mounted on studs standing a few millimetres off the wall so they cast a shadow."}],[{"text":"Built-up letters","b":true},{"text":": a routed face, a returned side (the “return”), and a back. Effectively a shallow box in the letter’s shape."}],[{"text":"Channel letters","b":true},{"text":": built-up letters with LEDs inside and a translucent acrylic face. "},{"text":"Face-lit","b":true},{"text":" glows from the front; "},{"text":"halo-lit","b":true},{"text":" or reverse-lit has an opaque face and throws light onto the wall behind, which is the expensive, good-looking one."}]]},{"t":"p","runs":[{"text":"What each demands of your artwork:"}]},{"t":"ul","items":[[{"text":"Counters need connections.","b":true},{"text":" The middle of an "},{"text":"O","code":true},{"text":" or an "},{"text":"A","code":true},{"text":" 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."}],[{"text":"Stroke width sets the return depth’s feasibility.","b":true},{"text":" A stroke thinner than about 25 mm cannot hold LEDs and a return."}],[{"text":"Every letter is a separate piece.","b":true},{"text":" 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."}],[{"text":"Mounting.","b":true},{"text":" Studs, a hidden rail, or a raceway. Ask which, because a raceway is a visible box behind the letters and it changes the design."}]]},{"t":"h2","id":"what-to-send","runs":[{"text":"What to send"}]},{"t":"p","runs":[{"text":"DXF is the lingua franca of CNC, and most shops also accept AI, EPS or PDF. Whatever the format:"}]},{"t":"ul","items":[[{"text":"1:1, in millimetres, one closed path per cut."}],[{"text":"Layers named for the operation: "},{"text":"CUT","code":true},{"text":", "},{"text":"POCKET","code":true},{"text":", "},{"text":"ENGRAVE","code":true},{"text":", "},{"text":"DRILL","code":true},{"text":"."}],[{"text":"Strokes outlined, text curved, overlaps welded, duplicates removed."}],[{"text":"A separate reference PDF showing the assembly, the colours and the mounting."}],[{"text":"The material and thickness in the filename and in the note."}]]},{"t":"h2","id":"the-conversation","runs":[{"text":"The conversation"}]},{"t":"quote","runs":[{"text":"What bit diameter, what material and thickness, and will these be flat-cut, built-up or channel letters?"}]},{"t":"p","runs":[{"text":"Three answers, and the design constraints follow from them. Asking after the design is finished is how a mark gets redrawn for free."}]}],"check":{"question":[{"text":"A logotype with sharp internal angles is routed from ACP with a 6 mm bit. The fabricator returns a sample where every inside corner is visibly rounded. Whose problem is it, and what fixes it?"}],"options":[{"text":[{"text":"The fabricator’s: a finishing pass with a smaller bit is entirely standard practice and should have been included in the quotation."}],"correct":false,"tempting":[{"text":"A second pass with a small bit is possible but slow, fragile and chargeable; it is a negotiated solution, not an omission on the fabricator’s part."}]},{"text":[{"text":"The fabricator’s: the toolpath should have been offset inwards to compensate for the bit radius."}],"correct":false,"tempting":[{"text":"Offsetting moves the cut but cannot make a round tool produce a corner sharper than its own radius; the geometry is not recoverable by offsetting."}]},{"text":[{"text":"Neither: it is inherent to the process, and the artwork should carry the radius or dog-bone fillets."}],"correct":true,"tempting":null},{"text":[{"text":"The designer’s: the file should have been supplied as DXF rather than PDF, since PDF does not preserve corner geometry."}],"correct":false,"tempting":[{"text":"PDF preserves geometry exactly. The rounding is a property of the cutting tool, not of the interchange format."}]}],"order":[0,3,1,2],"why":null},"prev":{"slug":"laser-cutting-and-engraving","title":"Laser cutting, kerf, and the material you must never use","minutes":8},"next":{"slug":"screen-print-separations","title":"Screen printing, separations and the underbase","minutes":9}}}