#Joints

A joint is a rule between two parts: not “put it here” but “hold like this”.

Key J starts a joint: point at a place on one part, then on the other.

#The anchor is inferred under the cursor

You do not declare in advance what you are about to pick — the anchor follows from the geometry under the cursor:

So “a bolt into a hole” is simply a click on the rim of the hole. The separate “by origins” mode in the tool bar is kept for rough assembly: there the anchor is the part's own origin, and the click is only a way to point at which part you mean.

#Kinds and what they leave free

#One list of mates

Everything that holds parts together sits in one list: joints, constraints (group, width, tangent) and relations between degrees of freedom. Each row has its own icon, name and state:

• a plain row — the item is healthy;
• “faulty” — it has nothing to hold with: a lost anchor, a degree of the wrong kind. Such a row is to be repaired, not deleted: re-pick the anchor;
• “conflict” — the item is sound, but the solver could not hold it because another one argues with it.

The difference matters: in the first case the item itself is at fault, in the second a pair of items demand incompatible things.

#The solver tells the truth

Parts are placed so that all joints hold at once, not one after another. That lets the solver say two things that “placed and forgot” cannot:

• how many degrees of freedom the assembly has left — that is, what you have not defined yet;
• which constraints are redundant — they repeat what others already said. A redundant constraint is not an error, but it hides the intent: two constraints hold one thing and it is unclear which one is the real one.

#The joint panel

A finished joint is edited in place, right at the geometry: pick it in the list or click its glyph in the viewport. The panel offers:

• Swap roles — the two sides are not equal, the part of the second anchor is the one that moves. Swapping changes which part stays and which one travels.
• Flip side — the part faces the wrong way: this turns it half a turn about the joint axis.
• Limits — the bounds of travel. When a degree runs into a bound, its row reads “at limit” and “put at limit” appears next to it: the degree is set exactly on the bound.

#When there is nothing to hold on to

Two arrangements look workable, yet the assembly will not settle with them. The program names both.

“Anchor on a moving part”. The anchor is declared on the assembly but takes its geometry from a part that travels inside it. Such an anchor moves away every time that part moves, and the assembly cannot settle: every recompute shifts it again. The row is marked “faulty”.

What to do: re-pick the anchor on the still part — or attach the joint straight to the moving part if that is what you meant to hold on to. A new anchor like this is refused right as you pick it.

“Grounded inside a moving unit”. The part is grounded but sits inside a subassembly driven by a joint: it stays put relative to its neighbours and travels along with the unit. The assembly panel lists such parts in a separate row.

What to do: if you need it still in the world, ground the unit itself; if the unit is meant to travel, remove the grounding from the part — it holds nothing anyway.

#Ground

One part in an assembly must be grounded — otherwise the whole structure has nothing to measure its position from and floats as a whole. Usually the housing or the frame is grounded.

#See also

• The second lesson: an assembly — the same joints, step by step.
• Joint limits and drives — so a mechanism moves within bounds.