CNC Isolation Milling
Cutting a board with a V-bit: cut width, depth, feeds, leveling, drilling and what a mill can and cannot do.
What isolation milling is
A CNC machine can make a board without chemicals by isolation milling: a small pointed bit cuts a thin groove through the copper around every pad and track, leaving each conductor isolated from its neighbors. The rest of the copper stays on the board (as a ground fill, if you wish). A second tool drills the holes and a third cuts the board out. The result is a single- or double-sided board in under an hour, without etchant (but with dust: see Chemicals, Dust, Tools and Safety).
The V-bit: cut width and depth
The tool is a conical V-bit (engraving bit) with a small flat tip. The groove it cuts is as wide as the bit at the depth of the cut, so the width depends on how deep the tip goes: width = tip + 2 × depth × tan(half angle).
Worked example: depth and width of the groove
- The copper is 0.035 mm thick. To cut through it reliably, set the depth to about 0.055 mm (copper plus 0.02 mm margin).
- For a 30° bit with a 0.1 mm tip: width = 0.1 + 2 × 0.055 × tan 15° = 0.129 mm. For a 60° bit: 0.164 mm.
- A groove of 0.13 mm wide isolates tracks that are 0.13 mm apart: a 0.4 mm gap needs 4 passes with 30 % overlap. The finer the tip and the narrower the angle, the tighter the spacing.
| Depth | Cut width (30° bit) | Result |
|---|---|---|
| 0.020 mm | 0.111 mm | copper not cut through: shorts remain |
| 0.035 mm | 0.119 mm | good |
| 0.055 mm | 0.129 mm | good |
| 0.100 mm | 0.154 mm | too deep: wide gap, bit wear, breaks tracks |
| 0.150 mm | 0.180 mm | too deep: wide gap, bit wear, breaks tracks |
| Bit angle | Width at 0.055 mm | Width at 0.105 mm (0.05 mm too deep) | Change |
|---|---|---|---|
| 20° | 0.119 mm | 0.137 mm | 0.018 mm |
| 30° | 0.129 mm | 0.156 mm | 0.027 mm |
| 45° | 0.146 mm | 0.187 mm | 0.041 mm |
| 60° | 0.164 mm | 0.221 mm | 0.058 mm |
| 90° | 0.210 mm | 0.310 mm | 0.100 mm |
A narrow bit is more sensitive to depth errors in the cut-through direction (copper left behind), and wider bits change less with depth. A 20 or 30° bit with a 0.1 mm tip is the usual choice for 0.2 to 0.3 mm gaps.
The machine and tools
| Item | Choice | Notes |
|---|---|---|
| Machine | A desktop PCB mill, or a hobby CNC router (such as a 3018) with a high-speed spindle | Rigid, repeatable Z; 0.01 mm resolution; at least 10 000 rpm, preferably 24 000 or more |
| V-bit | 20 to 30°, tip 0.1 mm, carbide | The consumable; keep spares |
| Drill bits | 0.8, 1.0 mm (and 0.6 mm), carbide, 3.175 mm shank (or collets) | Breakable; PCB drills have thin shanks |
| End mill | 1.0 mm, 2-flute, carbide (a “router”) | Cuts the outline in several depth passes |
| Work holding | Double-sided tape on a sacrificial board, or clamps | The board must be flat |
| Dust control | Vacuum shoe and a HEPA vacuum; enclosure | FR-4 dust is harmful (see Chemicals, Dust, Tools and Safety) |
| Probe | Z-probe or auto-leveling contact on the copper | Measures the board surface |
From design to G-code
- Export Gerber and drill files from your layout tool (see Manufacturing Rules, Files and Design Review), with rules suited to the mill: 0.3 mm tracks, 0.3 mm gaps, 1.8 mm pads.
- Convert to G-code with a CAM program for PCBs (FlatCAM, CopperCAM, pcb2gcode or the one that comes with your machine): choose the bit, depth, number of passes and overlap; it writes the toolpaths for isolation, drilling and cutting out (see CAM and Toolpaths for the ideas).
- Preview the paths: check that the isolation does not remove thin tracks and that the drill and outline files are in the same origin.
- Send the code with a sender (Candle, bCNC, UGS or the machine’s own), and run the toolpaths in order: isolation, drill, outline.
- Change tools between jobs and re-measure the Z-zero each time.
Flatness and leveling
Copper-clad board is never perfectly flat, and a 0.05 mm height error is as large as the whole cutting depth. If the groove is too shallow the copper is not cut through; too deep and it is wide and wears the bit. The cure is to measure the height of the surface at a grid of points (auto-leveling) and let the software vary the depth to follow the board.
Worked example: why you must level
- Nominal depth 0.055 mm. If the board bows by 0.10 mm over its length, the depth varies by ±0.050 mm along it: from 0.005 mm to 0.105 mm.
- At 0.005 mm the tip is at less than the copper thickness: the copper is not cut through, and a track stays shorted; at 0.105 mm the groove is 0.156 mm wide (against 0.129 mm nominal).
- With leveling the depth stays within ±0.01 mm and the groove is uniform.
- Use flat stock and flat tape: a warped or dusty board needs more leveling points.
- Level just before cutting and do not move the board.
- Probe on the copper with the V-bit or a probe wire and a clip to the board (check the continuity circuit).
Speeds, feeds and bit life
Worked example: feeds for the V-bit and the end mill
- Spindle at 24 000 rpm. For the V-bit at a feed of 600 mm/min with 2 flutes, the chip load is feed ÷ (rpm × flutes) = 600 ÷ (24 000 × 2) = 0.013 mm per tooth: a light cut in copper.
- The V-bit works at a tiny effective diameter (0.13 mm at the groove top), so its surface speed is only 10 m/min: low, which is why high rpm helps.
- The 1 mm end mill at 200 mm/min: chip load 0.004 mm per tooth, surface speed 75 m/min. Cut FR-4 in passes of 0.5 mm depth.
- These values are typical. See Speeds and Feeds for the ideas, and use the bit maker’s advice first.
Worked example: bit cost per board
- A carbide V-bit lasts about 25 m of cutting in FR-4 (typical). The board in the example needs 1 840 mm of isolation path, so one bit makes about 14 boards.
- At $2.00 a bit, that is about $0.15 per board for the V-bit; drill bits cost more (see Home Etching: Toner Transfer and Photoresist).
How long it takes
Worked example: a 40 × 25 mm board
- Isolation: 40 pads of 8 mm perimeter plus 300 mm of track with two edges is 920 mm; two passes make 1 840 mm; at 600 mm/min that is 3.1 min.
- Drilling: 60 holes at 3 s each: 3.0 min.
- Outline: perimeter 138 mm, 3 depth passes at 200 mm/min: 2.1 min.
- Total machine time 8 min; with 15 min for tool changes, leveling and clean-up the job takes about 23 minutes.
What a mill cannot do
- No plated holes: vias need wires, rivets or tubes; use through-hole parts and jumpers.
- No solder mask or silkscreen: you can apply solder-mask lacquer by hand, but fine-pitch parts bridge easily.
- Limited fine pitch: tracks of 0.2 mm and gaps of 0.23 mm are about the limit; a 0.5 mm QFP gap of 0.2 mm is not reliable (see Choosing a Route: Breadboard to Production).
- Double-sided boards need alignment pins and a flip: accuracy ±0.05 mm leaves a ring of 0.35 mm on a 1.6 mm pad with a 0.8 mm hole: thin.
- Dust and noise: a mill makes a mess: enclose it and vacuum at the tool.
Troubleshooting
| Symptom | Likely cause | What to do |
|---|---|---|
| Copper remains between tracks (shorts) | Cut too shallow; board not leveled | Level; increase the depth by 0.01 mm; clean burrs with a knife |
| Tracks cut away or too thin | Cut too deep or the bit too wide | Reduce the depth; use a narrower bit; re-level |
| Burrs and copper fringes | Dull bit; too slow a feed | New bit; faster feed; light sanding |
| Bit breaks | Plunged into the board, too fast, wrong Z-zero | Slower plunge; probe again; check the spindle |
| Holes off-center | Drill wander; board moved; origin different | Center-punch pads (a milled mark); same origin; secure the board |
| Board lifts during cutting | Weak tape or clamps | Better holding; tabs on the outline |
Related
- PCB Prototyping: the overview of this guide.
- Home Etching: Toner Transfer and Photoresist: the chemical alternative.
- Chemicals, Dust, Tools and Safety: dust control.
- CNC Machining: machines, workholding and G-code.
FAQ
What bit do I need for PCB milling?
A 20 or 30° V-bit with a 0.1 mm tip for isolation, a 0.8 mm drill for holes and a 1 mm end mill for the outline.
How accurate is a hobby CNC for PCBs?
The Z-height matters more than XY: with a probe and leveling you can get 0.2 mm tracks and gaps; without it, 0.5 mm at best.
Can I mill a two-sided board?
Yes, with pins to register the second side, but you have no plated holes, so vias must be wires or rivets.
Why does my milled board have shorts?
Usually the cut did not go through the copper where the board is high; level the board, use a deeper pass and remove burrs.
Is milling cheaper than a fabrication service?
For one board, yes; for five or more a service is usually cheaper and gives plated holes and a mask (see Cost, Quantity and Lead Time).
References
- Machine and bit manufacturers’ data (engraving bits, PCB mills); FlatCAM and pcb2gcode documentation.
- Coombs, C. F. (ed.), Printed Circuits Handbook, McGraw-Hill; IPC-A-600.
- Smid, P., CNC Programming Handbook, Industrial Press (G-code).
Last reviewed: October 2026. Cut widths follow from geometry; speeds, bit life and times are typical.