ACTE Presentation: Intro to Top-Down Assembly Modeling
John Ciprian, Miami Valley Career Technology Center, Fab Lab Teacher
6800 Hoke Road, Englewood, OH 45315
Who can benefit: Teachers and students who have some experience with creating individual 3d models, or beginners who would like to get started with this form of 3d modeling by using the provided parts.
Applications of this method:
Engineering projects, PLTW Engineering, Robotics, Machining, Skills USA contests, STEM projects, Fab Lab projects
Purpose: Provide methods to create a matching 3d model or a complete assembly of 3d model parts given one part as a starting point. The matching parts can then be produced on prototyping machines (laser engravers, CNC routers/mills/lathes or 3d printers).
Tolerance Considerations: This method creates parts that fit exactly with each other, which is fine as is for prototyping with subtractive processes (cutting parts on a laser engraver or CNC router). For an additive process (3d printer), the material tends to expand slightly with heating and so dimensions on one pieces of a pair of matching parts will probably need to be lengthened slightly to accommodate. This can be done in the 3d modeling software or by scaling in the 3d printer setup. If you are modifying the 3d model, it is common to create a difference of 0.006” (0.15 mm) between matching parts in a static assembly, though this may vary for different 3d printers and filament types. For an assembly it works best to print the parts individually rather than attempt to print the assembly itself, since printing the assembly is likely to weld the parts together and make it impossible to separate. If you are attempting to create a tolerance by adjusting the scale factor, keep in mind that all edges and faces on the scaled part will be affected. You will need to calculate the percentage of scale reduction (or increase) by dividing the desired amount of tolerance by the total dimension (such as width) in the same axis.
For example, if a part had a width of 3 inches, and you would like to reduce the width of a tab by 0.012”, then: .012/3.0 x100 = 0.4% reduction in scale.
3d Modeling Software available:
Free for Educational usage: Autodesk Inventor Professional (Windows), Autodesk Fusion 360 (Windows and Mac).
Solidworks (Windows, discounts available for educational usage)
Rhino 3d (Windows and Mac, discounts available for educational usage)
Birdhouse (parts cut on laser engraver), Container and Lid (created on CNC router), Battle Bots—motor and mounting plate (wheel slots cut on CNC milling machine), pinion gear with matching embossed shaft.