Finish lines and margins

Chamfer, shoulder, knife-edge. Then tracing the margin without lying to the mill.

Objectives

Match a finish line to a material.
Trace a continuous closed margin on a prep.
Name three ways a margin trace goes wrong.

Mandibular left first molar

#19

Finish line

Auto-trace is a guess. Drag a point onto the finish line you actually marked. Click the tooth to add one.

0 control points

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Three finish lines

Chamfer: a curved slope, 0.3–0.5 mm, the workhorse for metal and zirconia. Shoulder: a butt joint, ~1 mm, for porcelain that needs a ledge (PFM facial, many pressed ceramics). Knife-edge: conservative, metal only, easy to over-extend subgingivally.

Tracing

The margin is a closed curve on the finish line, not a suggestion. Gaps, spikes, and hops onto the gingiva or onto the axial wall are the most common D2 CAD errors. Zoom in. Orbit. Trace in small bites.

Cement gap

Inside the margin, CAD applies a spacer (often 0.04–0.08 mm) so the crown seats. The marginal band itself is kept tight. If you apply spacer to the margin, you designed a leak.

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