# Cad Measurement Diagrams > Generate self-guided SVG instruction diagrams whenever you ask a human to take a physical measurement (caliper placement, thread pitch spans, datum choice, across-flats vs across-corners). USE FOR: "how do I measure this", "show me where to put the calipers", asking a user for dimensions, reverse-engineering a physical part, thread pitch measurement, any bench procedure you are describing in words. - Skill: `rwilson504/cad-measurement-diagrams` (Agent Skill) - Install (CLI): `npx skillmds@latest add rwilson504/cad-measurement-diagrams` - Raw SKILL.md: https://api.skillmd.com/api/skills/rwilson504/cad-measurement-diagrams/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - License: MIT-0 - Author: rwilson504 (https://skillmd.com/u/rwilson504) - Updated: 2026-09-10 - Page: https://skillmd.com/skills/rwilson504/cad-measurement-diagrams --- # CAD Measurement Diagrams > **Prerequisite:** none. This skill is standalone and is loaded by any > agent that asks a human to go measure something. ## Purpose When you ask a user for a physical measurement, **words are not enough**. "Span 20 pitches" reads as unambiguous to you and produces three different caliper placements in practice. This skill produces a **self-guided SVG diagram** — a picture the user can follow at the bench without re-reading the chat — and defines when generating one is mandatory. Outputs land in the project's `reference/` folder as `howto-.svg` (plus a `.png` sibling for phone viewing at the bench). ## The rule **Any time you ask a human to perform a physical measurement or a manual procedure whose outcome depends on *where* or *how* they place a tool, generate a diagram in the same response.** Do not ask first; just draw it. Trigger examples: | You are about to write… | Draw a diagram | |---|---| | "measure across 20 thread crests" | ✅ off-by-one crest/gap trap | | "caliper the plain shank next to the thread" | ✅ which shank, which axis | | "measure the bore depth" | ✅ from which datum face | | "measure the hex across flats" | ✅ flats vs corners | | "measure the wall thickness" | ✅ where — walls are rarely uniform | | "how long is the part overall?" | ❌ genuinely unambiguous, skip | | "what colour is it?" | ❌ not a measurement | The test: *could a reasonable person place the tool somewhere other than where I mean?* If yes, draw it. ## Anatomy of a good measurement diagram A diagram that works unsupervised has all five of these. Missing any one sends the user back to the chat to ask a question. 1. **The part in profile**, drawn recognisably — not an abstract box. 2. **The tool**, shown in contact, with its *measuring faces* emphasised. Users align on the tool's faces, not on your arrows. 3. **The dimension line** with arrowheads at both ends, and the quantity it represents stated in words next to it. 4. **The trap**, called out explicitly. Every measurement has one dominant failure mode — name it and show the wrong version. 5. **The arithmetic**, printed on the image. `pitch = reading ÷ 20`. The user should not have to remember what to do with the number. Add a **predicted value** when you have one ("expected ≈ 24.0 mm"). It lets the user catch a gross error immediately, and it makes your hypothesis falsifiable rather than something you quietly retrofit later. ## SVG scaffold Light background — these get printed and viewed in bright garages, not in a dark IDE. ```svg ``` ### Palette | Role | Colour | |------|--------| | Part body / metal | `#c9c9c9` fill, `#4a4a4a` stroke | | Tool (calipers, jaws) | `#5b83bd` fill, `#2f4d7a` stroke | | Measuring face (emphasis) | `#16233a`, stroke-width 5 | | Dimension lines + the trap | `#c0392b` | | Correct / confirmed | `#2e7d32` | | Warning panel | `#fff8f0` fill, `#e0a96d` stroke | ### Thread / serration profile helper Hand-computing a 49-point zigzag is where these diagrams get wrong. Generate the points: ```python def zigzag(x0, n, pitch, y_valley, y_peak): """Points for a triangular thread profile; peaks land exactly on x0 + k*pitch.""" pts = [(x0 - pitch / 2, y_valley)] for k in range(n + 1): x = x0 + k * pitch pts += [(x, y_peak), (x + pitch / 2, y_valley)] return " ".join(f"{x:g},{y:g}" for x, y in pts) ``` Mirror the same x-values with swapped y to get the opposite flank. ## MANDATORY: rasterize and look at it **Never ship an SVG you have not rendered and viewed.** Coordinate arithmetic errors (overlapping labels, arrows pointing at the wrong crest, text running off-canvas) are invisible in source and obvious in the rendered image — and a wrong diagram is worse than no diagram, because the user will trust it. Windows, no Python imaging deps required: ```powershell & "C:\Program Files\Google\Chrome\Application\chrome.exe" ` --headless=new --disable-gpu --force-device-scale-factor=1 ` --window-size=1000,740 --screenshot="out.png" "file:///D:/path/to/diagram.svg" ``` Then view `out.png` and **count the features you annotated** — if the diagram claims 20 gaps, count 20 gaps. Gotchas found the hard way: | Symptom | Cause | Fix | |---|---|---| | `--screenshot` produces no file | Old `--headless` mode on Edge | Use Chrome with `--headless=new` | | `import cairosvg` fails | Not installed; needs native cairo on Windows | Don't fight it — use the browser | | Single-headed dimension line | `orient="auto"` on `marker-start` | Use `orient="auto-start-reverse"` | | Blurry raster | HiDPI scaling | `--force-device-scale-factor=1` | Keep the PNG next to the SVG — the SVG is for the repo, the PNG is for the phone at the bench. ## Known measurement traps worth a diagram | Measurement | The trap | What the diagram must show | |---|---|---| | Thread pitch over N pitches | Counting **crests** instead of **gaps** — N crests is N−1 pitches | Numbered crests, numbered gaps, jaws on crest 1 and crest N+1 | | Thread major Ø | Jaws seating in a valley instead of on crests | Flat jaw faces bridging two crests | | Thread minor Ø | Jaw tips can't reach the root | Offer the plain-shank proxy instead | | Hex / nut size | Across flats vs across corners (ratio 1.155) | Both, with the wrong one struck through | | Bore depth | Datum ambiguity — from the rim or from a shoulder? | The datum face highlighted | | Wall thickness | Walls are rarely uniform | The exact station to measure at | | Hole spacing | Centre-to-centre vs edge-to-edge | Both, with centrelines drawn | | Sheet / shim thickness | Caliper jaws compress soft material | "Do not squeeze" callout | ## Quick reference | Symptom | Likely cause | Fix | |---|---|---| | User returns a number that disagrees with your estimate by ~5–11% | Off-by-one on a repeated feature count | Re-ask with a diagram; span twice as many features | | Two readings of the same thing differ | Tool seating, not part variation | Diagram the seating; average and widen your error bar | | User asks a clarifying question about your instruction | The instruction needed a diagram | Draw it now, and note the trap in this skill's table | | Error that compounds along the part (pitch, spacing) | Measured over too short a span | Double the span; absolute error stays, relative error halves | ## See Also - `cad-reverse-engineer-stl` — measuring a *digital* mesh; this skill covers measuring the *physical* original. - `electronics-circuit-schematics` *(separate plugin)* — the same hand-authored SVG approach applied to circuit symbols.