A craft knife — X-Acto and similar — is a precision blade holder used in puppet building for card, template stock, mouth plates and fine trimming.
It is genuinely essential, and it is also the tool beginners most often reach for on jobs it is wrong for.
Where it excels
Cutting mouth plates from poster board, chipboard or thin PVC. Trimming felt flush after the cement has cured. Scoring a fold line. Cutting paper patterns. Anything where the cut needs to be exactly on a drawn line.
Where it fails: foam
A short straight blade sawing through foam block compresses the material ahead of itself, and the foam springs back after the blade passes. The result is a wavy, uneven cut on what looked like a straight stroke. The tool for foam blocks is a long serrated blade — a bread knife or an electric carving knife — drawn in one continuous motion. The teeth cut rather than crush, and the length lets you finish the stroke without stopping.
Change blades far more often than you want to
A dull craft knife blade is the leading cause of injury with this tool, because a blade that will not cut requires force, and force is what makes it slip. New blades are cheap. Change them at the first sign of dragging.
Always cut on a mat
, always away from your body, and never toward the hand steadying the work.
Snap-off knives
are a good alternative for longer straight cuts in card, since a fresh section of edge is always a snap away.
The hack: keep a jar for used blades on the bench. Loose blades in a bin are how somebody gets hurt emptying it.