rwilson504 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-<what>.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.
- The part in profile, drawn recognisably — not an abstract box.
- The tool, shown in contact, with its measuring faces emphasised. Users align on the tool's faces, not on your arrows.
- The dimension line with arrowheads at both ends, and the quantity it represents stated in words next to it.
- The trap, called out explicitly. Every measurement has one dominant failure mode — name it and show the wrong version.
- 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 xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1000 740"
width="1000" height="740"
font-family="Segoe UI, Arial, Helvetica, sans-serif">
<defs>
<!-- auto-start-reverse gives a double-headed dimension line from ONE marker -->
<marker id="dim" markerWidth="9" markerHeight="9" refX="9" refY="4.5"
orient="auto-start-reverse">
<path d="M0,0 L9,4.5 L0,9 Z" fill="#c0392b"/>
</marker>
</defs>
<rect width="1000" height="740" fill="#ffffff"/>
<!-- ... -->
<line x1="140" y1="378" x2="740" y2="378" stroke="#c0392b" stroke-width="2"
marker-start="url(#dim)" marker-end="url(#dim)"/>
</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:
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:
& "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.