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To CNC mill a double-sided PCB, use copper-clad stock approved for your machine, register both faces to the same physical reference, and mirror the bottom-side toolpath around that reference. Mill the first side’s traces, flip and re-align the board, then cut the mirrored traces and—if your workflow calls for it—the holes and outline. The key is registration: a small shift during the flip can misalign traces and pads on opposite faces.
What you need to plan before milling
Choose stock your machine permits
For the Bantam Tools workflow, use double-sided FR-1 PCB blank stock: copper on both faces over a phenolic resin core. Do not assume that another CNC mill accepts the same material. Aalto Fablab’s guidance for its Bungard CCD machine specifies FR-1 or FR-2 and warns against milling FR-3 or FR-4 because the dust is harmful. That is lab-specific guidance, not a universal material rule; check the machine’s approved-material policy and dust-control requirements before cutting.
Prepare a stable reference
The board must stay flat, and its position must be repeatable after it is turned over. The Bantam method uses an alignment bracket and references the same board corner against opposite corners of the bracket on the two sides. FlatCAM’s documented alternative uses paired alignment holes and pins arranged symmetrically around the mirror axis. In either setup, chips, debris, tape overlap, or a gap at the reference surface can shift the board and spoil registration.
Clear the spoilboard and fixture before mounting the blank. Measure the stock thickness at several points; enter the measured board and tape thickness wherever your software requests them. If you use tape, keep it flat and clear of the alignment interface.
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Plan the mirrored toolpaths and flip
Choose the mirror axis first
Before machining, decide which axis the board will be flipped around and use that same axis for the bottom artwork. In FlatCAM’s version 8 documentation, the two-sided workflow defines the bottom layer, mirror axis and its location, and paired alignment holes. The holes are symmetric about the axis, and the bottom layer is mirrored around that same axis. Generate the alignment-hole, top-side, and mirrored bottom-side jobs before cutting so the registration method and toolpaths agree.
Keep the first pass to traces
In Bantam Tools’ documented sequence, install and locate the alignment bracket, place the stock against the specified first-side corner, and mill the first-side traces only. Reserve the holes and outer profile for the second-side pass. This keeps the board’s reference geometry intact while you remove it and flip it.
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Mill the board in sequence
- Mount and measure: clean the spoilboard and bracket, secure the blank flat, and enter measured stock and tape thicknesses if the software asks for them.
- Preview each job: verify the first-side traces, alignment features, and mirrored bottom-side job. Check whether the selected tool can reach every feature before starting.
- Cut the first-side traces: use the trace-isolation toolpath only in the Bantam sequence. The Bantam guide cites 0.003 inch as a typical trace depth for its workflow and says depth varies by design; treat that as machine-specific starting guidance, not a universal setting.
- Remove and clean the board: lift it carefully, clear chips from the board and fixture, and avoid damaging the reference corner or alignment holes.
- Flip and re-register: in the bracket setup, seat the corresponding board corner against the opposite reference corner. In a pin setup, fit the same alignment holes over the pins in the board and sacrificial bed. Make sure the board is flat and no debris, tape, or gap interferes with the reference.
- Cut the second side: run the mirrored bottom-side toolpath. In the cited Bantam sequence, the second pass includes bottom traces, holes, and the outline.
- Inspect before separating: check that the two faces are registered and traces are isolated before cutting the finished board free from the stock.
Match the cutter to the operation
Small PCB engraving bits are used to isolate traces; flat end mills are used for holes and outlines in the Bantam guide. Tool choice, machine setup, and board design determine whether a feature is machinable, so inspect the preview for unreachable details rather than assuming one bit size fits every job. Bantam’s electronics workflow includes trace-depth and trace-clearance controls; the cited 0.003-inch depth is a typical value in that guide, not a setting to copy blindly to another mill.
Account for vias and the limits of a milled board
A CNC-milled board is bare: unlike a board-house PCB, it has no plated-through vias. Bantam’s documented hand method is to pass wire through a via, solder it on both faces, and trim the excess. Because each connection adds assembly work, design to keep the via count manageable where practical. A successfully milled prototype is not automatically a substitute for a professionally fabricated multilayer board.
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What to compare when choosing a CNC mill
The cited documentation describes particular workflows, not a market-wide test or a universal best machine. Compare a mill against the actual demands of your board and workspace:
Quick Recap
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- Usable work area: confirm the bed can accommodate the blank, fixture, and any alignment method you plan to use. Bantam lists a 7 × 9 × 3.5-inch working volume for its Desktop CNC Milling Machine; that is one model’s published specification, not a general minimum.
- Registration capability: look for accurate probing or a fixture that can reproduce the board position after a flip.
- CAM support: confirm the software can create mirrored bottom layers and separate toolpaths for traces, holes, and outlines.
- Tool compatibility: check support for the small engraving bits and flat end mills required by your design.
- Approved materials and dust controls: verify local stock availability and the machine shop’s rules for PCB substrates and dust extraction.
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