Why did my arc or curve change shape after I trimmed or cut it? (Trimming converts arcs to Beziers)

Why it happens

Experienced users on the software maker's forum report that when a true arc is trimmed with the Interactive Vector Trim (scissors) tool, the remaining piece is rebuilt as a Bezier curve instead of staying an arc. Usually the drift is tiny: one member measured about two ten-thousandths of an inch after converting a trimmed piece back to an arc. Other veterans report that on some shapes the deviation reaches thousandths, enough to spoil parts that must mate, such as an arch split across several sheets. A curve that was never a true arc (an "arch" drawn as a Bezier) has no single radius to preserve in the first place.

What to do

1. Rule out the machine first. Run the simulation, look straight down at it, and turn vector visibility on. If the preview matches the vectors and the cut part does not, the machine or the work holding is the cause, not the file.
2. Split arcs in Node Editing instead of trimming. Press N, right-click the node where the split should be and use the cut command from the pop-up menu. Users report this keeps arc spans as arcs.
3. Repair a piece that already became a Bezier: select it, open Fit Curves To Vectors, check Circular Arcs and set a small Tolerance. The official page states the new spans stay within plus or minus that tolerance of the original.
4. For critical radii, redraw the arc with the Create Arc tool at the exact radius and delete the trimmed copy.
5. Open vectors cut on the wrong side are a separate problem: Outside/Right versus Inside/Left depends on the vector direction and start point. Reverse the direction from the node-edit right-click menu if needed.

Pro tips

• Keep the original arc on a hidden layer so you can measure the trimmed piece against it with the Measure tool.
• Check Keep Sharp Corners in Fit Curves when the piece has a corner that must survive.
• Imported polylines never were arcs; fit arcs to them before trimming.