1. The CNC milling table used in PCB prototyping serves as a positioning plate, which is typically made of aluminum alloy and is secured in place by pins and screws.

2. Each CNC milling spindle is equipped with a hole-slot positioning system on the worktable beneath it.

3. The milling pad functions as an intermediate positioning fixture, often referred to as “soft positioning.” It must ensure reliable and quick board positioning and unloading, which helps to reduce auxiliary time and improve production efficiency.

4. Before milling the outline, a groove matching the PCB profile dimensions is pre-milled into the milling pad. Typically, the width of the groove is the actual cutting diameter of the milling cutter plus 0.5 mm, and the groove depth is 2.5 mm.

5. During the machining process, this groove serves as the path for the milling cutter’s movement. A vacuum system removes dust, generating airflow within the groove that helps expel chips, ensures a smoother machining process, and prevents clogging of the cutter’s chip groove. This also reduces edge wear.

6. When processing, the milling cutter should be extended into the groove by 1.5 to 2 mm. This helps avoid excessive wear at the board’s edge due to continuous cutter engagement, which could reduce the cutter diameter and lead to processing inaccuracies in the PCB dimensions caused by the reduced cutter end diameter.


Before each batch production, the milling backing plate is installed onto the CNC milling table, and the new nylon thread plug is screwed in place. Drill holes in the threaded plug and install the positioning pin for use. The chip groove on the milling pad is deeper and wider, which enhances smooth airflow, chip removal, and a more uniform surface for machining. However, this design weakens the supporting surface, especially when the chip flute is near the positioning pin, potentially leading to instability in positioning.

Most milling pads used in PCB design are made of non-metallic laminates, which are relatively soft. As a result, the positioning holes can wear and enlarge over time due to the repeated loading and unloading of the pins. Semi-specific and consumable milling pads are particularly prone to this condition.

Typically, the pin is press-fitted into the milling pad with an interference of 0.005 to 0.01mm. For special milling pads or high-density fiberboard, a tighter fit is preferred. However, for semi-specific or consumable milling pads, an interference greater than 0.007mm can lead to part of the base material being cut away in the pin hole, creating deep grooves or gaps during the press-fit. With repeated loading and unloading, the pin holes may experience delamination or chipping. During milling of the printed board, most of the pin-cutting force is absorbed by the positioning pins. This lateral pressure can cause defects in the pin holes, leading to loosening or misalignment of the pins, which in turn affects the overall dimensions of the printed board and compromises tight tolerance control.

The smaller the diameter of the positioning pin, the greater the relative deflection. Therefore, using larger diameter holes for positioning is recommended whenever possible. The diameter of the locating pin and the amount of deflection directly impact productivity. For example, a stack of four pieces, originally intended to be milled in one pass, may need to be milled in three passes due to the small diameter of the pin and the resulting deflection, thus reducing efficiency by 25%.

During the PCB prototyping process, the positioning pins should be closely matched to ensure reliable alignment of the workpiece. This eliminates the need for adhesives or tapes that require time to bond and cure. A tight fit also ensures strict tolerances, which is crucial when milling multi-layer or high-quality double-sided boards to maintain accuracy.

Typically, the drill bit diameter has a lower deviation, generally between 0 to 0.005mm, and in some cases, 0 to 0.01mm. Most non-metallic laminates undergo slight shrinkage after drilling, usually around 0.005 to 0.007mm. A hole with a lower deviation of 0.005 to 0.01mm, when paired with a standard pin, can create an interference of 0.005mm. This eliminates the need for special processing techniques or the need to measure the drill bit diameter individually. Using a smaller drill bit to meet the press-fit requirements is an efficient way to save time.