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To mill a double-sided PCB accurately, keep the blank flat, register both sides to the same mechanical reference, and mirror the bottom-side toolpath around that reference. Mill the first side’s traces, flip and realign the board, then cut the mirrored traces, holes, and outline. The method is useful for prototypes, but a milled bare board has no plated-through vias, so connections between its copper faces need manual wiring and soldering.
What makes double-sided PCB milling work?
The critical challenge is not simply cutting copper; it is keeping the second-side artwork in the correct position after the board is turned over. The stock must sit flat, the flip must return it to a repeatable reference, and the bottom artwork must be mirrored around the same axis used by the fixture. If those references disagree, traces and holes can land out of alignment.
The workflow below combines machine-specific guidance from Bantam Tools’ double-sided-board instructions with FlatCAM’s 2-side PCB documentation. Fixture details and settings depend on the machine and software; use the instructions for your own setup rather than treating one manufacturer’s values as universal.
Choose stock and prepare the machine
For the Bantam Tools workflow, use double-sided FR-1 PCB blank stock: copper on both faces over a phenolic resin core. More generally, confirm that the blank is copper-clad on both sides and approved for the machine you will use. Material rules are local: Aalto Fablab’s guidance for its Bungard CCD milling machine specifies FR-1 or FR-2 and warns against milling FR-3 or FR-4 because of harmful dust. Follow your lab’s material and dust-control policy; do not assume those rules apply identically to every machine. See Aalto Fablab’s PCB-milling guidance.
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- Clear chips and debris from the spoilboard, bracket, and the surfaces that locate the blank. Debris caught at a reference edge can shift the board.
- Secure the blank using the fixture and fastening method specified for your machine. If you use tape, avoid overlaps or trapped debris that could create a gap or alter the board’s position.
- Measure stock thickness at several points. Enter the measured stock thickness and, where the software requests it, the tape thickness.
- Before cutting, confirm that the board remains flat and cannot move during machining.
Bantam’s procedure uses an alignment bracket and a defined board corner. Its electronics workflow lists a 7 × 9 × 3.5 inch working volume for the Bantam Desktop CNC Milling Machine; that is a specification for that model, not a general minimum for PCB milling.
Plan the flip, mirror axis, and toolpaths
Before machining, decide exactly how the board will turn and what feature will locate it on each side. In FlatCAM, set the bottom layer, mirror axis, and axis location, then create paired alignment holes symmetric about that axis. Mirror the bottom layer around the same axis and generate the alignment-hole, top-side, and mirrored bottom-side jobs before cutting. FlatCAM describes using the holes with pins in the board and sacrificial bed to restore registration after the flip.
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On the Bantam bracket workflow, install and locate the bracket, place the board against the specified corner for the first side, and run only the trace toolpath. Mark or otherwise keep track of the intended flip direction so the physical orientation matches the CAM setup.
Mill the first side
Preview the job before cutting and check that the chosen tool can reach the features. PCB engraving bits isolate traces; flat end mills are used in the Bantam workflow for holes and outlines. A tool that is too large for a clearance or detail may leave copper connected where the design expects isolation.
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Bantam’s electronics workflow gives 0.003 inch as a typical trace depth, while noting that it varies with the design. Treat that figure as machine-specific starting guidance, not a universal setting or a substitute for the manufacturer’s instructions. Set trace depth and clearance for the stock, tool, machine, and design you are actually using.
Flip and register the blank
- Remove the blank carefully and clean away chips from its surfaces and the fixture.
- Turn it in the direction planned in CAM. In the Bantam bracket method, align the corresponding board corner against the bracket’s opposite reference corner, so the same corner is located on both sides.
- In a pin-based setup, fit the same alignment holes over the pins in the board and sacrificial bed.
- Check that the board seats flat against the reference and that no chip, tape edge, or gap is preventing full contact.
Do not compensate for a poor physical fit by casually shifting the bottom toolpath: that can make the artwork appear aligned in one place while moving other features out of registration.
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Mill the second side and inspect the result
Run the mirrored bottom-side job only after confirming that its flip axis and physical orientation match the setup. In the documented Bantam sequence, the second-side pass includes the bottom traces, holes, and outer profile. Inspect the finished board for alignment, complete trace isolation, and clean cuts before separating it from the stock. If the preview shows a feature the installed tool cannot reach, revise the toolpath or tool choice before machining.
Plan for unplated vias and prototype limitations
A milled PCB is a bare board, not the equivalent of a board-house board with plated-through holes. Bantam states: “Unlike double-sided PCBs produced by a board house, PCBs made on your Bantam Tools Milling Machine are bare, meaning they do not have plated vias.” For connections between copper faces, its guidance describes passing wire through a via, soldering it on both sides, and trimming the excess. Because each such connection adds hand assembly, designs with fewer vias are generally more practical to build this way.
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Double-sided CNC milling is therefore best considered a prototype fabrication approach, not an automatic substitute for professionally fabricated multilayer boards. Account for the extra manual work wherever the circuit requires electrical connections between faces.
What to compare when choosing a CNC setup
There is no universal machine winner established by these workflow sources. For a particular machine and shop, check the practical requirements that determine whether the process is repeatable and safe:
Quick Recap
- Work area: confirm the usable area accommodates the blank and fixture.
- Registration: check whether accurate probing or a mechanical bracket-and-corner or hole-and-pin setup is supported.
- CAM support: confirm the software can mirror a bottom layer about a defined axis and create suitable jobs for traces, holes, and outlines.
- Tool compatibility: verify that the spindle and tool-holding system accept the small engraving bits and flat end mills needed for the intended features.
- Approved materials: ask the machine shop which PCB substrates are permitted and what dust controls are required.
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