Which part of a planer functions to hold down the stock and help prevent chatter?

The pressure bar is an essential component of a planer designed to maintain stability and prevent unwanted movement during the planing process. It exerts downward force on the stock as it passes through the machine, which helps to hold the material securely against the bed of the planer. This downward pressure minimizes chatter, which is an undesirable vibration or movement that can lead to an uneven finish on the workpiece.

In contrast, while components such as the chipbreaker and sectional infeed roll play critical roles in the overall functioning of the planer, they primarily focus on different aspects of the planing operation. The chipbreaker helps to control chip flow and reduce tear-out, ensuring a smoother cut, but does not specifically address the issue of holding down the stock. The sectional infeed roll assists in moving the material through the machine, but again, its role is not primarily about preventing chatter.

The bed roll is also involved in the feeding process, but similarly, it does not apply pressure to secure the material against the planer bed and manage vibrations effectively. Thus, the pressure bar is the correct answer as it specifically addresses chatter prevention by maintaining the appropriate downward force on the workpiece during planing.