Presser Foot and Stack Clamping in PCB Drilling

Every drilling machine presses the stack against the tool with a foot that surrounds the spindle. The presser foot flattens the entry material onto the panel, holds the layers in register and limits the vibration that the drill induces. Stack clamping is therefore a process parameter rather than a fixture detail, and it is set deliberately. Its pressure is a process parameter, and it is one of the few that affects the entry burr, the hole position and the tool life at the same time.

The foot is often treated as a fixture rather than as a setting. Its pressure drifts with the air supply, its face wears, and the material under it changes between products. When a drilling defect appears that is not explained by the tool or the parameters, the foot is a good place to look before the machine is re-calibrated.

What the Presser Foot Does

The foot performs three functions. It holds the stack flat, which keeps the entry material in contact with the copper so that the drill cuts a clean edge rather than a frayed one. It holds the layers of a stack together, which keeps the holes in register between the panels. And it damps the vibration of the drill as it enters, which reduces the wander that produces a misplaced hole.

The functions are in tension with one another. More pressure improves the first two and can mark the panel surface, while less pressure avoids the marking and lets the stack move. The correct setting is the lowest pressure that holds the stack flat and keeps the holes in register, and it is found by experiment on the actual stack rather than from a table.

Pressure and Stack Integrity

The pressure is set on the machine and applied through the foot face, and it is usually expressed as a force rather than as a pressure. A typical range for a production stack is measured in tens of newtons, with the upper limit set by the marking that the foot leaves on the entry material and the lower limit set by the movement of the stack.

The stack configuration changes the requirement. A stack of thin panels with entry and backup material on both sides compresses under the foot, and the compression has to be removed before the drill enters or the first panel is drilled at a different depth from the last. The count of panels in the stack and the thickness of the entry material are therefore recorded with the pressure.

Burrs at Entry and Exit

A burr is a raised lip of copper at the edge of a hole. At the entry it is produced when the copper is pushed up rather than cut, and at the exit when the last layer is torn rather than sheared. Entry pressure reduces the entry burr by keeping the entry material in contact with the copper, and the backup material controls the exit burr by supporting the copper as the drill breaks through.

Pressure alone does not remove a burr that the tool generates. A dull drill produces a burr at any pressure, and a mis-set feed produces one as well. The foot, the tool and the parameters are therefore adjusted together, and the burr is measured on a sample hole rather than judged by eye. The entry and exit side of the same problem is described in the notes on entry material and burr.

Entry and Backup Material

The entry material has two jobs: it guides the drill at the moment of contact and it prevents the copper from being pushed up. An aluminium foil entry sheet does the first well and the second less well, while a phenolic or a composite sheet does both and costs more. The choice is made for the hole size and the burr requirement rather than for the machine.

The backup material controls the exit burr and the exit hole quality. A rigid backup supports the copper and reduces the tear, and a soft one lets the drill push the copper outwards. The backup also affects the drill temperature, because it carries heat away from the exit, and it must be flat, because a warped backup lets the last panel move under the foot.

Stack Height and Alignment

Stack height is limited by the drill length and by the accuracy of the machine. A tall stack increases throughput and increases the difference between the first panel and the last, because the drill wanders more as it goes deeper and the chip removal becomes harder. The practical limit is set by the hole position tolerance rather than by the drill length.

Alignment of the layers comes from the tooling system rather than from the foot. Pins or a tooling plate hold the panels in register, and the foot then holds the stack against them. Where holes drift between panels of the same stack, the tooling is the first suspect and the foot pressure the second, and the notes on drill stack height describe the limits of the two.

Machine Setup and Verification

The setup checks are the foot face condition, the pressure setting, the parallelism of the face to the table and the travel of the foot. A face that is not parallel applies more force to one side of the panel, which shows up as a difference in burr across the width. A face with a worn or contaminated surface leaves marks that are mistaken for handling damage.

The pressure is verified with a gauge at the foot rather than at the regulator, because the two are separated by hoses and valves that can leak. The verification is repeated after any maintenance on the spindle or the air supply, and the result is recorded with the machine identification.

Tool Wear and Clamping

A stack that moves or vibrates wears the tool faster, because the cutting edge is loaded in directions it was not designed for. The effect appears as a shorter tool life and a higher rate of chipping at the outer corner, and it can be mistaken for a problem with the tool supplier. Verifying the clamping removes that possibility before a supplier is challenged.

The collet is part of the same system. A worn collet lets the drill run out, and the runout produces an oversized hole at any clamping pressure. The maintenance of the collet and its effect on the hole are described in the notes on collet maintenance, and the counting of hits per tool in the notes on hit count control.

Linking Clamping to Hole Quality

The measurable outputs of correct clamping are a consistent entry burr across the panel, holes in register from the first panel of a stack to the last, and a hole position that matches the program within the tolerance. Each of those is measured on a sample from the stack, and each is compared between the first and the last panel to detect a stack that moves as it is drilled.

Where a defect appears that is not explained by the tool or the parameters, the diagnosis is a short experiment: drill the same stack at two pressures and compare the burr and the position. The result is usually unambiguous, and it costs a few panels rather than a day of investigation.

Process Control and Records

The controls are the foot face condition, the pressure setting, the entry and backup material, the stack configuration and the tooling. Each is recorded per stack, together with the drill parameters and the tool number, and the record is the basis for comparing one shift with another.

The defect data completes the picture. A rise in entry burrs with an unchanged tool points to the foot or the entry material, while a rise in the exit burr points to the backup. Separating the two defects in the inspection record is what makes the difference visible, and it is the cheapest diagnostic available in a drill room.

FAQ

Is more presser foot pressure always better? No. Pressure improves contact and registration up to the point where it marks the entry material or compresses the stack unevenly.

Can the presser foot cause a hole position error? Yes. If the stack moves because the pressure is too low, the drill enters at a position that drifts from the program, and the error grows with depth.

Does the foot affect tool life? A stack that vibrates loads the cutting edge in unintended directions and shortens the life of the tool. Correct clamping is part of the tool life calculation.