OneCAD/docs/book/src/guide/assemblies.md

Components and joints

A design is a tree of components. New Component (component.create) adds one, and Create Components from Bodies (component.from_bodies) turns bodies into components. Each component has its own origin, bodies, sketches and timeline entries. The active component (the radio button in the browser) receives new sketches and features; the others are drawn faded.

A component is placed by its occurrences. Paste creates another instance of the same component, and Paste New creates an independent copy. Move an occurrence with the move triad, then Capture Position to keep it or Revert to put it back. SolidCraft asks before the next command if a move wasn't captured. Ground fixes an occurrence in place.

Paste New copies nested component definitions, sketches, feature histories and generated dimension parameters. Repeated child instances remain shared within the new copy. Materials, appearances and wholly internal joints/motion records receive copied references. Design-wide user parameters remain shared; retained external links and omitted cross-boundary motion records are reported. Unsupported destructive links refuse the entire copy. See individual-instance and copy details.

Joints (joint.create, J) connect two components by joint origins that snap to faces, edges, circles and vertices: rigid, revolute, slider, cylindrical, pin-slot, planar and ball, with offsets, flip and limits. As-built Joint keeps the current position, Rigid Group locks components together, Drive Joints moves a joint through its values, and Motion Link couples two joints. Links can be chained (one joint drives a second, which drives a third), and a value is carried through a chain of up to 64 links in whatever order they were made. Contact sets stop motion where bodies would pass through each other, Motion Study keys joint values along a timeline, and Interference shows overlapping volumes in red.

Gear, Rack and Pinion and Screw (on the ASSEMBLE panel) are motion links whose ratio comes from the parts rather than a typed number:

• Gear (joint.gear {a, b, ra, rb}) couples two joints that turn. Joint 2 turns ra/rb as far as joint 1, the other way round; Internal gear (one of them is a ring gear) turns both the same way. Pick the round face of each gear, such as its pitch cylinder, or type the radii. Tooth counts work as well as radii, since only their proportion counts. Picking a gear's face also chooses the joint that gear turns on.
• Rack and Pinion (joint.rack_pinion {a, b, radius}) couples a joint that turns with one that slides: the rack travels the pinion's pitch radius times its turn in radians (r·θ). Either joint may drive the other. In joint.list the ratio is given per degree, as for any link between an angle and a distance.
• Screw (joint.screw {joint, pitch}, or {a, b, pitch} for a revolute joint and a slider) advances pitch per turn. A cylindrical joint is a screw on its own: its turn drives its travel. The thread is right-handed; Reverse makes it left-handed.

A radius or pitch may be an expression of the design's parameters, and a picked face is found again whenever the design changes. Change the parameter or the gear's diameter, and the ratio follows. The new link leaves the driven joint where it is, so teeth modelled in mesh stay in mesh. Reverse turns the driven joint the other way, for joints whose axes point opposite ways. These couplings are motion links, so they work in closed loops, with contact sets and in motion studies: a rack can push the piston of a slider-crank, or a pinion can steer through a rack and its tie rods. Drive the joint that drives the coupling (joint 1, the pinion, or the screw's turn), not the driven one. A joint follows one link at most, and a coupling that would drive its own driver is refused. joint.list lists each coupling's definition under links with an error when it can't be worked out (the picked face is gone); the link then keeps its last ratio.

A joint moves one component against another, so its two picks must be on different components. A STEP file with several parts often comes in as bodies of one component, and picks on two of those bodies make no joint. The Joint and As-built Joint dialogs say so under the picks and offer Create Components from Bodies: each picked body gets a component of its own and stays where it is, the picks are kept, and the joint previews. joint.create refuses such origins with the same advice.

Joints may close a loop, as in a four-bar linkage or a slider-crank. When one joint of the loop is driven, the other joints in the loop follow so that every joint still meets, and the mechanism stays in the assembly mode it started in (an open four-bar does not flip to the crossed one). If the motion would take a loop past the point where it can still close, for example driving a rocker beyond its swing, the motion stops at the last position where the loop closes, and the drive reports stopped_by_loop. A loop that cannot close at all is listed under conflicts and nothing in it moves. joint.list reports how many degrees of freedom each part still has with its loops closed: the moving parts of a four-bar have one between them, and a loop locked by a rigid joint has none. The Drive Joints dialog says when a drive stops short and why. While it previews, a row under the values gives the value the joint reached and the reason (the loop that can't close further, or the two parts a contact set keeps apart), and after OK the status line repeats it. The value box keeps what you typed. The dialog opens on the joint picked in the browser, or on the first joint that moves when none is picked.

Drag a part that joints place and it moves as far as its joints allow: the point you grabbed follows the pointer in the plane through it that faces the view, the joints on the part's chain take the values that bring it nearest, within their limits, through motion links, and with closed loops kept closed, and a four-bar's rocker stops at the end of its swing. The whole drag is one undo step. A grounded part, or one no joint places, is not moved; a drag on it draws a selection box as before. Each frame runs joint.reach, which scripts and agents use directly to put a part's point somewhere: joint.reach {occurrence, point} moves the joints on its chain from where they are to the nearest pose (the arm keeps its elbow up or down, and a linkage stays on its branch). at picks the part's point (the centre of its bodies by default), z with axis also turns one of its axes, and joints names the only joints to move (the other joints of a closed loop they are in follow). The answer says whether it reached the point and how close it got (error_mm, error_deg), and when it falls short, the limits it rests on (stopped_by_limit) or the loop that stopped it (stopped_by_loop). Contact sets are not checked by a reach; drive the joints for that.

The solver places what it reaches from the root and from grounded components. A chain of joints that is joined to nothing grounded is never placed, so driving one of its joints moves nothing. The drive says so: joint.drive answers moved: false with a reason and the unconnected part ("Pedestal:1 is not connected to anything grounded"), the Drive Joints dialog shows the same row, motion.play lists such joints under unmoved, and joint.list names every such chain under floating. Ground a part of the chain, or join it to a grounded part (a rigid as-built joint keeps it where it is), and it moves.

joint.drive answers the joint's values as joint.list gives them, in degrees and millimetres with their units and limits, and moved says whether any part moved. A joint's origins are mated by default: B's z axis is turned against A's, as two faces meet. flip: true lines them up instead, so two origins given at one point with the same z stay as they are. For scripts that drive many steps, quiet: true leaves the per-part degrees of freedom out of joint answers, and motion.play with quiet: true gives the poses of only the parts that moved.

Insert Design (component.insert_design {path, translate?}, on the ASSEMBLE panel and in the File menu) adds a component linked to another saved design, so each part can live in its own document. A STEP or mesh file picked there is inserted as Insert STEP or Insert Mesh would. The part's solid bodies come in as they sit in that design, and they are kept in the assembly, so it opens even without the part file. The link is stored relative to the assembly's folder once the assembly is saved. After the part file changes, Update Links (component.update_links) reads it again. Break Link (component.break_link) keeps the bodies and forgets the file, and component.links lists the links and whether their files are found. A linked component holds the part's bodies only: its timeline, parameters and sheet-metal data (flat pattern, bend table) stay in the part's own design.

STEP export writes the component tree as an assembly.

Right-click a component in the Browser and choose Properties to inspect material-aware mass, volume, surface area, centre of mass and world bounds. Descendants and repeated instances contribute to assembly totals; pending moves are included. A component uses its first placement; occurrence.properties selects a specific instance. Root Properties measures the complete assembly. Volumes are summed, including overlapping bodies, rather than measuring their union. The same workflow is available through component.properties and occurrence.properties; see placement and measurement details.