CNC Machining for PCB Milling: Complete Guide to Circuit Boards

PCB milling is one of the most practical and satisfying applications for a desktop CNC machine as covered in our Feeds and Speeds Guide. Instead of waiting weeks for a manufactured circuit board you can mill a board in 15 to 30 minutes on your CNC router. The process is straightforward once you understand the right tools, techniques, and feeds and speeds.

This guide covers everything you need to know to mill PCBs on a desktop CNC machine including tool selection, material preparation, CAM software setup, and troubleshooting common problems.

How PCB Milling Works

Isolation Routing

PCB milling uses a CNC router to cut copper traces on a copper-clad board. The tool removes the copper around the traces leaving the trace pattern isolated on the board. This process is called isolation routing.

The copper-clad board consists of a non-conductive substrate material typically FR4 fiberglass or phenolic resin with a thin layer of copper foil bonded to one or both sides of the substrate. The copper layer is typically 1 ounce per square foot which is 0.0014 inches thick or 2 ounce which is 0.0028 inches thick.

The CNC tool follows the trace pattern generated by the CAM software. The tool cuts a narrow groove around each trace removing the copper in that area. The remaining copper forms the circuit traces connecting the component pads.

The PCB Milling Process

After the trace milling is complete, the PCB board needs through holes drilled for component leads and vias between layers. The same CNC machine that milled the copper traces can also drill the component holes using a small diameter carbide drill bit. The same CNC machine that milled the copper traces can also drill the component holes using a small diameter carbide drill bit. The drilled holes are then either plated with copper for through-hole connections or component leads are soldered directly into the holes.

Tool Selection for PCB Milling

V-Bits and End Mills

The most important tool for PCB milling is the engraving bit (V-bit or carbide burr). These bits have a narrow tip that creates fine traces. The tip diameter determines the minimum trace width and the gap between traces.

A 30-degree V-bit with a 0.005 inch tip diameter is suitable for most standard PCB milling work. The 30-degree angle on the V-bit provides good side clearance for chip evacuation while maintaining adequate tip strength for cutting. The 0.005 inch tip produces traces as narrow as 0.008 inches which is adequate for most circuit designs.

A 10-degree V-bit with a 0.002 inch tip produces finer traces for dense circuit boards. The narrower tip allows tighter trace spacing but is more fragile and breaks more easily. Use the 10-degree bit only for boards that require very fine traces.

Drill Bits

Use solid carbide drill bits in sizes matching component leads. Common sizes: 0.8mm, 1.0mm, and 1.2mm. Use a drill speed of 10,000 to 15,000 RPM with a light pecking feed. Standard V-bit sizes and their capabilities:

V-Bit Angle	Tip Diameter	Min Trace Width	Best For
30-degree	0.005“	0.008“	Most PCB work
10-degree	0.002“	0.005“	Dense boards
45-degree	0.008“	0.012“	Beginner, robust

Straight carbide end mills can also be used for PCB milling. A 1/64 inch or 0.4mm end mill cuts clean traces and provides consistent trace width. Common drill sizes are 0.8mm, 1.0mm, and 1.2mm. Use a drill speed of 10,000 to 15,000 RPM with a light pecking feed.

Material Preparation

PCB Substrate Types

Copper-clad PCB material is available in different substrates. FR4 fiberglass is the most common — rigid, stable, and resists soldering heat. Phenolic (FR2) is less expensive and easier to cut but less heat-resistant. It is rigid, stable, and resists soldering heat. FR4 is more abrasive than phenolic and wears tools faster but produces better quality boards.

Phenolic resin board also called FR2 is less expensive and easier to cut than FR4. Phenolic board is suitable for prototype work where the board does not need to withstand high temperatures. The material cuts more easily and extends tool life.

Surface Preparation and Mounting

Clean the copper surface thoroughly before milling. Any oil or residue affects copper removal quality. Clean with isopropyl alcohol and a lint-free cloth. Mount the PCB material on a flat spoilboard using double-sided tape or vacuum hold-down. Ensure the board is flat and level — any height variation causes inconsistent cut depth. Clean the board with isopropyl alcohol and a lint-free cloth before mounting it on the machine.

Mount the PCB material on a flat spoilboard. Double-sided tape or vacuum hold-down works well for holding thin PCB material. Ensure the board is flat and level across the entire surface. Any variation in height causes the cut depth to vary which results in incomplete copper removal or excessive cutting into the substrate.

CAM Software Setup

Software Options

PCB milling requires CAM software that supports isolation routing. FlatCam and CopperCAM are excellent free options designed specifically for PCB milling from Gerber files. FlatCam generates isolation toolpaths from Gerber files. It is open source and runs on Windows, Mac, and Linux.

Fusion 360 has PCB milling capabilities in the CAM workspace. It supports isolation routing, contour cutting, and drilling from standard PCB design files. The PCB workspace in Fusion 360 is available with a standard subscription.

CAM Parameters

The CAM setup for PCB milling uses the top surface of the copper as Z zero. The cut depth should be deep enough to remove the copper completely without cutting too deep into the substrate. For 1 ounce copper set the cut depth to 0.002 to 0.004 inches. For 2 ounce copper set it to 0.004 to 0.006 inches.

The toolpath strategy for PCB milling typically uses a single pass at the cut depth. The tool follows the trace outline and removes the copper around the traces. Multiple passes are not needed because the copper layer is thin.

Feeds and Speeds

Recommended Parameters

The recommended spindle speed for PCB milling with a carbide V-bit is 12,000 to 18,000 RPM. The feed rate should be 10 to 20 IPM depending on bit size and material. The chipload is very low: 0.0005 to 0.001 inches per tooth.

The chipload for PCB milling is very low compared to metal cutting. The chipload is typically 0.0005 to 0.001 inches per tooth. A single-flute V-bit has one cutting edge so the feed rate is RPM times chipload.

Contour Cutting Parameters

For contour cutting that separates the board from the panel, use a 1/32 inch carbide end mill at 12,000 RPM and 15-20 IPM.

The stepover for isolation routing is effectively zero because the cutting tool follows the trace outline in a single pass along the path. The trace width is determined by the tip diameter and the cut depth. A deeper cut produces wider traces with a V-bit.

For contour cutting that separates the board from the panel use a 1/32 inch carbide end mill at 12,000 RPM and 15 to 20 IPM feed rate. Make two passes for thick boards. Cut entirely through the substrate to separate the board from the panel.

Drilling PCB Holes

Drill Sizes and Parameters

Drilling is the second major operation in PCB milling after the trace routing. The drill holes must be positioned accurately at the component pad centers. Most PCB CAM software generates the drill file automatically.

Use carbide PCB drill bits with a 1/8 inch shank. Standard sizes: 0.8mm for most through-hole components, 1.0mm for larger leads, and 0.4mm or 0.6mm for vias.

Use carbide PCB drill bits which have a shank diameter of 1/8 inch and a cutting diameter that matches the hole size. Standard drill sizes for PCB work include 0.8mm for most through-hole components, 1.0mm for larger component leads, and 0.4mm or 0.6mm for vias between layers.

Drilling Parameters

The drilling feed rate should be 5 to 10 IPM at 10,000 to 15,000 RPM. Peck drilling is not required for standard PCB thickness (0.062 inches). Drill through in one pass with a slight dwell at the bottom to clear the chip.

Troubleshooting Common Problems

Most Common Issues

Incomplete copper removal is the most common PCB milling problem. The tool did not cut deep enough. Increase cut depth by 0.001 inches and test again. Check that the board surface is flat and Z zero is correct.

Broken traces occur when the tool cuts through a trace that should remain. This is caused by misalignment between the PCB design and the actual board position. Ensure the board is positioned correctly and the origin matches between the CAM software and the machine.

Additional Problems

Tool breakage is common with small V-bits. Broken traces occur when the tool cuts through a trace that should remain. Burnt copper indicates excessive heat — reduce spindle speed or increase feed. Poor hole quality is caused by worn drill bits — replace regularly.

Here is a sample PCB isolation routing program:

; PCB isolation routing — single trace
G90 G94 G17 G54
G21
M03 S15000
G00 X0 Y0 Z0.1
G01 Z-0.003 F5    ; Cut depth for 1oz copper
; Trace contour follows here (CAM-generated)
G00 Z0.1
M05
M30

Tool breakage is common with small V-bits. The 10-degree bits with 0.002 inch tips break easily if the feed rate is too high or the board surface is not flat. Reduce the feed rate and ensure the board surface is level. Use a 30-degree bit for most work and switch to 10-degree only when necessary.

Burnt copper or FR4 indicates excessive heat from the cutting action. Reduce the spindle speed or increase the feed rate to reduce the amount of heat generated during the cut. Ensure adequate chip clearance during cutting so the fine copper and FR4 chips do not clog the cutting area and cause heat buildup.

Poor hole quality from drilling is caused by worn drill bits or incorrect feeds and speeds. Replace drill bits regularly and use the recommended speeds and feeds. Support the back side of the PCB board during drilling with a sacrificial backing plate to prevent breakout and hole edge chipping.

PCB Design Considerations for Milling

Designing a PCB specifically for milling improves the success rate and reduces problems. The minimum trace width for milled PCBs is 0.008 inches for 30-degree V-bits and 0.005 inches for 10-degree bits. Design your circuit traces wider than the minimum whenever possible for more reliable machining results. Wider traces are easier to mill and less likely to break.

The minimum gap between traces is determined by the V-bit angle and depth of cut. A wider gap reduces the risk of bridging between traces. Design with 0.010 inch gaps or larger for reliable milling. Larger gaps also reduce the risk of incomplete copper removal between closely spaced traces.

Use rounded corners on pads and trace ends. Sharp corners concentrate stress and can lift during milling. Most PCB design software supports rounded pad shapes. Use the largest practical pad size for component holes because larger pads hold better during soldering.

Add fiducial marks to the PCB design for alignment. Fiducial marks are copper pads that the CNC machine can probe to verify position. They ensure the drill holes align with the milled traces. Two fiducial marks on opposite corners of the board provide accurate alignment reference for drilling operations after the traces are milled.

Through-Hole Plating Alternative

Milled PCBs do not have plated through holes like manufactured boards unless you add post-processing. Without plating, the component leads must be soldered on both sides of the board for connections. For single-sided boards this is not an issue because all components mount on the copper side.

For double-sided PCB boards you need a reliable way to connect traces on the top and bottom copper layers together. The simplest method is to use component leads that pass through the board and are soldered on both sides. Small gauge wire can be inserted into the via holes and soldered on both sides to create connections between layers.

Another option is to use eyelets or rivets that are pressed into the holes and soldered. Eyelet setting tools are available from electronics suppliers and are easy to use for prototyping work. The eyelet provides a conductive path through the board without requiring plating.

Surface Finish Options

Bare copper traces oxidize quickly and become difficult to solder. Apply a surface finish within an hour of milling to protect the copper and improve solderability. The simplest surface finish option is liquid flux applied to the copper traces which prevents oxidation temporarily.

HASL hot air solder leveling is the most common finish for milled PCBs. Apply solder paste to the board and heat it with a hot air tool or reflow oven. The solder coats the copper traces and provides a solderable surface. HASL leaves an uneven surface that can cause thickness variations.

Immersion tin or immersion silver finishes provide a flat solderable surface. These chemical processes deposit a thin layer of tin or silver on the copper. They are available from PCB finishing suppliers and provide excellent solderability.

Board Population and Soldering

Soldering to a milled PCB is similar to a manufactured board. The main difference is copper pads may be less uniform. Use a fine tip soldering iron at 650-700°F with flux to improve wetting.

Use a fine tip soldering iron with temperature control for surface mount component soldering. Set the soldering iron temperature to 650 to 700 degrees Fahrenheit for leaded solder wire. Apply soldering flux to the copper pads before soldering to improve wetting and heat transfer. Clean the finished PCB board with isopropyl alcohol and a clean brush after soldering to remove flux residue.

For through-hole components insert the component leads through the board and solder them on the copper side of the board. Trim the excess component leads flush with the solder joint using cutters. For double-sided boards solder both sides of the lead to ensure connection to both copper layers.

Comparing Milled PCBs to Manufactured PCBs

Milled PCBs have speed as their main advantage — produced in under an hour vs days or weeks. The main disadvantage is lower resolution and less consistent quality than manufactured boards. Trace widths below 0.008 inches are difficult to produce reliably. Milled boards are ideal for prototyping; manufactured boards are better for production.

The main disadvantage is that milled boards have lower resolution and less consistent quality than manufactured boards. Trace widths below 0.008 inches are difficult to produce reliably with desktop CNC machines. The copper adhesion to the substrate is weaker because the mechanical cutting can stress the copper-substrate bond. Manufactured boards use chemical etching processes which produce cleaner trace edges and better adhesion.

Milled boards do not have solder mask or silkscreen which are standard on manufactured boards. Solder mask protects the copper from oxidation and prevents solder bridges between closely spaced traces. Silkscreen helps identify component locations during assembly.

For most hobby and prototype circuit board work the significant speed advantage of milled PCBs far outweighs the quality limitations compared to manufactured boards. For production or high-reliability boards manufactured PCBs are the better choice.

PCB milling is a valuable skill for any electronics enthusiast or small shop. The ability to produce custom circuit boards on demand speeds up prototyping and allows design iteration without waiting for manufactured boards. While milled PCBs cannot replace manufactured boards for production they are ideal for prototypes, one-off projects, and educational use. PCB milling requires patience and attention to detail. The small tools and thin copper layer leave little room for error. Setting up the Z zero correctly, leveling the board surface, and using the correct feeds and speeds are the keys to success. With practice you can produce reliable circuit boards in under an hour from design completion. PCB milling is a skill that improves with practice. Start with simple single-sided boards with wide traces and work up to more complex designs. The ability to produce your own custom circuit boards quickly in your shop is one of the most useful capabilities you can add to your CNC machining skills. For more CNC machining guides see our CNC Router Bits Guide and our Feeds and Speeds Guide.