ARCVTX-bgol (Mark Imaginary Vertex of a Filleted Arc)

Command Type: AutoLISP routine (works in AutoCAD, BricsCAD, ZWCAD and most AutoLISP-compatible CAD platforms)
Developed by: BGol Community, bgol.in

Inspired by the general concept of fillet-corner / imaginary-vertex reconstruction utilities used in CAD drafting. This is an original, independently written routine, new file name, new command name, new source code, created so the CAD community has a free, editable, open-source alternative.

Overview

When a polyline corner has been filleted (rounded), the original sharp "imaginary" vertex where the two straight segments would have met is no longer explicitly present, it's implied by the arc. AVX reconstructs that point: it draws construction lines from the arc's two tangent points back to the implied vertex, and labels each tangent-to-vertex distance as text.

How to Use

1. Load ARCVTX-bgol.lsp into your CAD session (drag-and-drop the file onto the drawing window, or use the APPLOAD command).
2. Type AVX at the command line and press Enter.
3. At the Select arc: prompt, click the filleted arc segment you want to analyze.
4. Two construction lines are drawn from the arc's tangent points to the reconstructed imaginary vertex, each labeled with its distance. New entities are placed on a dedicated layer, BGOL-AVX (colour magenta).

Why Use It

• Simplicity, removes the need for manual construction-line drawing and trigonometric calculation to find an arc's implied straight-corner vertex.
• Accuracy, automates precise measurement of the tangent-to- imaginary-vertex distance, minimizing calculation errors.
• Efficiency, speeds up analysis or documentation of filleted corners where the original (pre-fillet) corner geometry needs to be understood or verified.

Where to Use It

• Analyzing or documenting the original (un-filleted) corner dimensions of a rounded polyline, such as verifying setback distances or property corner locations that were later rounded for drafting/construction reasons.
• Quality-checking fillet radius and offset consistency across multiple rounded corners in a drawing.
• Any task needing to reconstruct or measure to an arc's implied straight-line vertex without manually drawing extension/construction lines.

Notes

• Uses standard fillet-corner geometry: the imaginary vertex lies at distance radius * tan(included_angle / 2) from each tangent point, measured along that point's tangent direction.
• Only true ARC entities are supported; selecting anything else reports that nothing was marked.
• Text label height automatically scales with the arc's radius (with a sensible minimum), so labels stay legible on both small and large fillets.
• Source code is fully open, feel free to study, modify, and redistribute it under the same open, free-for-all spirit.

Credits

Developed and shared for the CAD community by BGol Community. Website: https://bgol.in/ Free and open-source, use it, modify it, share it, for personal or commercial work alike.