Customizable Scotch Yoke Desktop Model - 10, 15 and 20 mm Crank Radius Presets (Educational)

CUSTOMIZABLE educational desktop model: compare 10, 15 and 20 mm crank radius Scotch yokes with a hand-turned knob. For students and teachers to study how rotation becomes straight-line, sinusoidal motion. Stroke = 2 x crank radius: 20, 30 and 40 mm.

UNTESTED DESIGN STUDY. This model has not been printed, assembled or physically operated. It is not a load-bearing machine part or a safety device. The assembled STLs are layout references, NOT validated print-in-place files. Separate the moving components, inspect/repair the meshes in a slicer, and test-fit before printing a classroom set.

How it is intended to work

Turn the knob on the crank wheel. The pin on the wheel rides in the slot of the yoke, and the yoke slides in a straight line between two rails. Yoke position follows x = r cos(theta). Actual fit, friction and smooth running need a physical test print.

Presets

Crank radius 10 mm: stroke 20 mm, wheel radius 22 mm.

Crank radius 15 mm: stroke 30 mm, wheel radius 27 mm.

Crank radius 20 mm: stroke 40 mm, wheel radius 32 mm.

All presets: 8 mm pin in a 9 mm wide slot, assembled size 170 x 120 x 64 mm, Y up.

What is included

Three assembled native STL references, three full native Playground recipe TXT files (19 chunks each), and a five-page Playground/customization guide PDF.

Mesh checks and limits

Each raw native STL has 11,784 triangles, 0 boundary edges, 296 zero-area facets and 24 non-manifold edges. These remain in the files. Fixed parts overlap as separate shells, un-unioned, and no external mesh repair was used. This is not a printability certification.

Clearance evidence

Hand calculation plus a numeric sweep every 0.25 degrees through a full turn (1440 steps). Pin-to-slot side gap 0.5 mm (constant), pin-to-slot end gap 2.0 mm minimum, yoke-to-rail gap 1.0 mm, wheel-to-rail gap 3.0 mm, knob-to-yoke gap 2 mm in Z, axle cap to yoke 0.5 mm in Z. No native /intersect result is claimed. These are nominal CAD gaps; printer tolerance, friction and warping can remove them. Dynamics are untested.

Customize in TrueGeometry Playground

Pick one complete 10/15/20 mm recipe. Start a fresh scene, open Script, paste ONE numbered chunk, use plain Run, wait, then continue. Keep creation chunks separate. Other radii need regenerated wheel, slot and rail coordinates and a new clearance check. Editing a comment does not change the geometry.

Full Playground commands, tables and all build images:

https://blog.truegeometry.com/0_0_0_0/blogs/post-54e90a6a.html

The commands are also in the three attached recipe files. The guide shows where to paste them.

Native technique: /sm_hole adds real bores to /sm_part profiles without boolean cuts. The C-shaped yoke is closed by a separate top bar. All construction uses native Playground commands. No external CAD geometry was substituted.

Community test prints welcome

If you print and assemble a revision, please post a Make and tell us about the yoke slide on the rails, pin fit in the slot and knob clearance. CAD renders are shown, not photographs of printed parts.

AI-assisted design. License: CC BY-NC 4.0. Share and adapt with attribution to TrueGeometry for non-commercial use. Commercial use requires consent from TrueGeometry. Educational use remains subject to the license.

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