Documentation
Assemblies and joints
An assembly is a design with several components. Each component has its own bodies, sketches and features and a position; joints connect components and say how they may move. Select a joint and drag its handle to see the mechanism move.
How do I make components?
- New component makes an empty one, optionally inside another; new sketches and features then go into whichever component is active. Activate a component with the dot in front of its row in the browser.
- Select bodies and choose Make component from the right-click menu to give each body its own component, in place.
- Insert part brings another ProtoJoint design in as a component, with its own components and joints.
- Importing a STEP, .sldprt or .sldasm file makes components too (see Import).
- Copy a component and paste it to get a linked copy (
Holder:1,Holder:2): change one and all change, and ten copies cost no more to compute than one. Paste New makes an independent copy.
Which joints are there?
| Joint | Motion |
|---|---|
| Rigid | none: the parts are fixed together |
| Revolute | turns about one axis |
| Slider | slides along one axis |
| Cylindrical | turns about an axis and slides along it |
| Pin-slot | turns about one axis and slides along another |
| Planar | slides in a plane and turns about its normal |
| Ball | turns about all three axes (pitch, yaw, roll) |
How do I add a joint?
Press J. Click a snap point on the component that should move, then one on the component it attaches to: face centres, circle centres, edge midpoints, vertices and the axes of holes and shafts are offered as you hover, and Ctrl lets you pick any point on a face. The first component jumps into place so you can check it; choose the joint type, flip it, add an angle or offsets, and press Enter. The motion axis is Z of the joint by default and can be X, Y, or any edge, line or cylinder you pick.
To connect parts where they already are, use the as-built joint (Shift+J), or Rigid group to lock several components together at once.
How do I move a joint and limit it?
Select the joint (click its icon in the view or its row in the browser) and drag the ring or arrow that appears, or type a value. In the joint’s panel you can set a minimum, a maximum and a rest position, lock the joint, and animate it. A motion link drives one joint from another at a ratio: a gear pair (360° to 180°, reversed), a rack (360° to 60 mm) or a lead screw (360° to 5 mm).
Several joints on one component, and closed loops such as a four-bar linkage, are solved together. A joint that cannot be satisfied with the others is marked red with the reason.
How do I keep a component from moving?
Right-click it and choose Pin (called Ground in some other programs). A pinned component cannot be dragged or moved; when a joint needs to move it, the pin is released and you are told. After you drag parts around or drive joints, Capture position stores the arrangement on the timeline and Revert position goes back to it.
How do I check for collisions?
Select the bodies or components to check, or nothing to check every pair of components, and run Interference check. Each overlapping pair is listed with its volume and highlighted in its own colour; click a row to see only that pair.
How do I get mass and centre of mass?
Give each component a Physical material: steel, stainless steel, aluminium, brass, copper, titanium, ABS, nylon, PLA, PETG, polycarbonate, acrylic, wood and others, each with its density, or type your own. Mass properties then shows the mass, volume and centre of mass of what is visible, computed from the exact solids. For a bought part you only know by weight, such as a motor, enter its mass directly. A component without a material counts at 1000 kg/m³, the same default other CAD programs use, and the panel says how many those are.
Can an assembly become a robot description?
Yes: components become links and joints become joints. See Robot export.