Bottom Up and Top Down Assembly Design in BricsCAD Mechanical Explained
Quick answer: Assembly modeling represents a designed product’s structure using mechanical blocks to group geometric entities into hierarchies, common in MCAD applications and useful for simplifying complex products containing thousands or millions of entities. An assembly is a mechanical block containing inserts of other mechanical blocks; a subassembly is a separate assembly inserted into another; a part is a mechanical block with no inserts of its own. An assembly with inserted blocks (which can themselves be assembled with further inserts) forms a tree called the model’s mechanical structure, displayed in the Mechanical Browser panel, confirming the tree display covered on this hub’s Mechanical Browser page.
Getting Started
Use BMNEW to create a new document for a new mechanical block, BMMECH to convert existing blocks and external references into mechanical blocks, and BMUNMECH to reverse that conversion.
Bottom-Up Design
Start by detailing a low-level mechanical block, then group those blocks into higher-level ones, continuing until the topmost block represents the product itself. The main command here is BMINSERT, which inserts an existing mechanical block into the current drawing.
Top-Down Design
Start by defining (without detailing) a product’s first-level mechanical blocks, then refine each one by defining its sub-blocks, continuing until every low-level block is fully defined and detailed. Use BMFORM to create a new sub-mechanical block for an existing one.
Adding Mechanical Structure to Existing Geometry
Add mechanical block hierarchy to any existing drawing, whether created in BricsCAD or imported from another CAD format. Use BMFORM to convert a selected set of geometric entities into a sub-mechanical block of the current drawing.
Changing Mechanical Structure
If a mechanical block needs to move up or down the hierarchy during design, use BMFORM to form a new sub-mechanical block containing a selected set from an existing block, or BMDISSOLVE to delete a mechanical block entirely, moving all its sub-blocks up to the level the dissolved block occupied.
Using Standard Parts
Common standard parts include screws, nuts, washers, bearings, and holes. BricsCAD provides a library of more than 30,000 standard part details in various sizes, significantly boosting design performance when used. Insert them into your parts and assemblies via the Library panel.
Assembly Constraints
Assembly constraints are ordinary 3D Constraints that link faces and edges of bodies to represent an assembly’s mechanical blocks, linkable both at the top level and within sub-mechanical blocks.
Mechanical Block Visibility
A mechanical block within a complex assembly can end up effectively invisible, surrounded by other blocks, a piston hidden inside a cylinder block, for example. Hide the surrounding blocks to reveal it: hide any sub-mechanical block at any level of nesting, or make one transparent or display it as a wireframe instead. Use BMHIDE to hide a specific sub-mechanical block, and BMSHOW to make a hidden one visible again.
Frequently Asked Questions
What’s the difference between an assembly and a part? An assembly is a mechanical block that contains inserts of other mechanical blocks; a part is a mechanical block that doesn’t contain any inserts of its own.
Which command should I use to start a bottom-up design? BMINSERT, to insert existing mechanical blocks and group them into higher-level ones.
How do I fix a mechanical block that ended up at the wrong level of the hierarchy? Use BMFORM to form a new sub-block containing the selected set, or BMDISSOLVE to remove a block entirely and promote its sub-blocks to its former level.
Where do I find BricsCAD’s library of standard parts like screws and bearings? The Library panel, which provides access to more than 30,000 standard part details in various sizes.