A peg that rolls on its precision partner slowly gains speed

What is wrong

A precision pair (a peg and the block it fits, precision_partners on their cards) is made for the fit: the peg in its bore. A peg that misses the bore, tips over and rolls across the top of the block speeds up a little by itself instead of slowing down. The world's health stays OK: the energy check flags a gain of 10 % of the scene's energy scale, and this one stays under it over a few seconds.

The shipped demo_peg_d12 no longer shows it: since 2026-11-10 every world of the demo starts inside the fit's clearance and the peg seats. Any scene where a part rolls or slides for long on its precision partner can.

Number

demo_peg_d12 with its former start spread (the peg up to 0.5 mm off the axis, against 0.1 mm of radial clearance), profile precision, one L4, in an energy sweep of 53 scenes: - worlds where the peg missed the bore and rolled on the block: the peg's speed rose about 10 % in 1.7 s, and the world's energy rose by 1.1 to 5.8 % of the scene's energy scale; - worlds where the peg seated in the bore: 0 %; - the other scenes of the sweep that were measured: 0 %.

It has not been measured for other parts (a bolt rolling on its nut, a plug on its receptacle) or on the CPU.

Why

Not found yet. The contacts of a precision pair come from the precision layer, built from the two features' exact shapes, and the pair's plain contact is switched off. A contact point that travels round a rolling peg is the case those rows were not written for: the fit's laws (clearance, lean, insertion) all hold the peg nearly still in its bore.

Workaround

• Start a precision fit inside its clearance, so the part seats (the demo does; a peg dropped 0.15 mm or more off the axis of a bore without a chamfer lands on the rim).
• For a part that is meant to roll or slide on the other, do not use the precision pair: place a copy of the card without precision_partners, and the two collide as plain shapes (a bored block then collides as its convex hull: its bore is closed).
• In a long run, compare the part's speed over time; do not rely on sim.check() alone for this.

Planned fix

After the release candidate: reproduce it with a peg laid on the block, find the row that does the work, and add the law that a rolling peg's energy never rises.