Fence
Is a CNC fence a workholding device or a datum device?
It is both: it provides a repeatable reference datum and also contributes to workholding by resisting lateral movement and helping square the blank.
Why use a fence instead of only vacuum hold-down?
Vacuum prevents vertical lift but does not inherently establish a repeatable edge relationship to the machine axes; a fence gives a fixed mechanical reference for squaring and part-to-part consistency.
What should be measured first when building a custom fence?
The actual plate/spoilboard thickness, fence face offset, and cutter diameter, because those dimensions control clearance, cut depth, and final part location.
A CNC fence is a rigid reference edge or stop mounted to the machine table or wasteboard that squares, locates, and restrains workpieces for consistent registration to machine axes. It serves as both a datum device for repeatable positioning and a workholding element that resists lateral forces during machining.
On a CNC router or in millwork assembly, a fence is typically bolted, screwed, or clamped to the spoilboard to establish a repeatable datum edge for blanks, sheet goods, or profiles. The operator pushes the part against the fence to maintain squareness to the machine, reduce setup time, and keep repeated cuts aligned across multiple parts. In custom fixture work, the fence may be CNC-cut to match a part profile, with internal corner relief for cutter clearance and clamp pockets to avoid collisions. In vertical setups, the fence registers hole patterns and clamp locations, with exact offsets measured from the fence face to preserve repeatability. Small fence errors translate directly into cumulative dimensional error and off-square assemblies, making precision critical for production quality.
Fence not square to axes: Even slight angular error transfers to every part, causing trapezoid cuts and twisted assemblies, critical for symmetric joinery like cabinet doors.
Fence/toolpath interference: Too tall, overhanging, or unrelieved fences cause bit strikes, resulting in tool breakage and damaged fixtures. Insufficient corner relief leaves uncut material.
Bad datum control and clamping: Referencing different edges, ignoring fence offset, or uneven clamping shifts parts, causing location drift and out-of-tolerance holes. Mis-set offsets also compromise fit-up.