Woodworking Machinery: Press Stroke and Feed Positioning

Displacement measurement on woodworking machinery concentrates on two lines: the press line (platen stroke on hot presses, cold presses and laminators, which determines panel thickness and pressing quality) and the processing line (feed and positioning on CNC panel saws, multi-spindle drills and edgebanders, which determine finished dimensions). Both lines share an environment of airborne wood dust, glue splash and, at some stations, hot-press heat. Dust is the factor most often underestimated here — it builds up on guideways, brackets and the magnet face, gradually changing the mechanical fit rather than causing an immediate electrical fault.

Press line: platen stroke and thickness consistency

The finished thickness of a hot-pressed panel is determined by the platen closing position (except on machines that use thickness gauges as mechanical stops). Displacement feedback serves three purposes: closing-position control, multi-point platen parallelism monitoring, and hold-position monitoring during the press cycle. Multi-point monitoring is the key point — if the positions at different points on a large-format press are inconsistent, panel thickness will vary by zone. Multi-channel sensors should be the same model and the same measuring range, with attention paid to channel-to-channel consistency rather than the figures of a single unit; the rationale is given in resolution, repeatability and non-linearity. Hot-press stations also require a check of the ambient temperature; keep the electronics head away from the hot platen and, where necessary, specify a remote-electronics construction.

Processing line: feed and panel-cut positioning

The pusher on a CNC panel saw, the transverse positioning of a multi-spindle drill and the fence adjustment of an edgebander are all, in essence, linear positioning. These mechanisms are typically driven by a servo ballscrew or a rack-and-pinion, with native feedback from the motor encoder. Fitting a magnetostrictive sensor that measures the actuator-end position directly eliminates backlash and transmission error and provides absolute position — after a power loss the machine can continue without homing (see absolute vs. incremental position). For positioning applications a 5-20 μm resolution grade is recommended, and the refresh rate should be checked against the control cycle (see matching response time to the control cycle).

Typical measuring-point layout on woodworking machinery

Installation points in a dusty environment

Wood dust is copious, dry, prone to build-up and combustible. It affects displacement measurement in three ways: first, build-up between the magnet and the rod changes the gap and, over time, can cause unstable readings; second, a coating on the rod surface impairs heat dissipation; third, dust can enter connectors. Countermeasures: mount the rod below or to the side of the mechanism rather than directly above it (to reduce direct fall-out), fit a simple dust cover, and specify connectors to IP67 or better with the cable routed downwards (for the meaning of each rating see IP protection ratings). For profile-type products see Series 18 profile type; for wet areas or stations that require washdown, specify Series 13 mobile hydraulic. Installation practice is covered in installing and commissioning in practice.

Interfaces and line integration

Woodworking CNC equipment is typically built around a servo system. Position feedback may be SSI (deterministic timing, suited to the position loop) or a fieldbus type (convenient for centralised multi-axis management); for SSI models see Series 192 SSI digital interface. The selection framework for hydraulic circuits on the press line is given in why hydraulic cylinder position control uses magnetostrictive sensing. Similar pressing and tilting positioning requirements exist on glass machinery; see glass machinery: pressing and tilting positioning.