Assemblies, Mates and Tolerances
Bring parts together in an assembly, connect them with mates that model real motion, and learn how to size holes and fits so parts actually go together in the workshop.
In this lesson you will
- Insert part instances into an assembly
- Choose fastened, revolute and slider mates to match real motion
- Explain the difference between clearance, tight and interference fits
- Apply general tolerance guidelines for screws, bearings and 3D-printed parts
Lesson
A Part Studio tells you what a part looks like. An assembly tells you how parts work together: where they sit, which ones move and whether anything collides. For an FTC robot, the assembly is where you discover that a wheel rubs on the frame or that an arm cannot fold down far enough.
Instances
When you insert a part into an assembly, Onshape creates an instance, which is a copy that points back to the original part. If your drivetrain uses four identical wheels, you insert the wheel four times. If you later change the wheel in its Part Studio, all four instances update. Fix one main part, such as the chassis, in place so the rest of the assembly has something solid to attach to.
Mates
A mate connects two parts and decides how they can move relative to each other. Mates are placed between mate connectors, which are small coordinate markers that snap to points such as hole centres and face centres. Pick the mate that matches the real joint on your robot.
| Mate | Allowed motion | FTC example |
|---|---|---|
| Fastened | None, parts are locked together | A bracket screwed to a channel |
| Revolute | Rotation around one axis | An arm pivot or a wheel on its axle |
| Slider | Movement along one straight line | A linear slide stage |
| Cylindrical | Rotation and sliding along one axis | A shaft passing through a bushing |
Most FTC kit vendors publish CAD models of their parts. Using them in your assembly, instead of guessing their shape, means your custom parts are designed against the real geometry of channels, motors and plates.
Tolerances: real parts are not perfect
A 5 mm hole in CAD is exactly 5 mm. A real hole drilled by hand, cut by a laser or printed on a 3D printer will be a little bigger or smaller. Tolerance is the amount of variation you allow. A fit describes how two parts go together once that variation is included.
| Fit type | What happens | Typical FTC use |
|---|---|---|
| Clearance | Parts slide together freely with a small gap | Screws through holes, shafts that must spin freely |
| Tight (transition) | Parts go together snugly, sometimes with light pressure | Locating pins, parts that must not wobble |
| Interference (press) | Inner part is larger, so force is needed | Bearings pressed into a housing, used carefully |
- Screw holes: model a clearance hole slightly larger than the screw. Standard clearance tables and the Hole feature help you choose a sensible size.
- Bearings: check the bearing's datasheet or vendor page for the recommended housing size, and test before relying on a press fit.
- 3D printing: holes often print slightly smaller and outer shapes slightly larger than modelled. Many teams add a few tenths of a millimetre of clearance as a starting point, but this varies by printer, material and settings.
- Kit hole patterns: take spacing and hole sizes from your kit's documentation or the vendor's CAD files, never from a ruler guess.
- 1Model a test couponMake a small plate with a row of holes in small size steps around your target size.
- 2Label itAdd small engraved numbers or note the order so you know which hole is which.
- 3Make itPrint or cut it with the same settings you will use for the real part.
- 4Test and recordTry your screw, shaft or bearing in each hole and record which size gives the fit you want.
Key terms
- Instance
- A copy of a part inside an assembly that stays linked to the original part.
- Mate
- A connection between two parts in an assembly that defines how they can move relative to each other.
- Mate connector
- A small coordinate marker on a part used to position and connect mates.
- Tolerance
- The allowed variation between a designed dimension and the real manufactured size.
- Clearance fit
- A fit with a small gap so parts slide or spin freely.
- Interference fit
- A fit where the inner part is larger than the hole, so it must be pressed in.
Practical exercise
Assemble your plate on a kit part
- 1Import or insert a CAD model of a kit channel or plate from your vendor into an Assembly.
- 2Insert the mounting plate you designed in the previous lesson and fix the kit part in place.
- 3Use mate connectors on matching holes and add a fastened mate between the plate and the kit part.
- 4Add a second simple part, such as a short arm, and connect it with a revolute mate, then drag it through its motion.
- 5Check that your plate holes line up with the kit holes and write down the clearance you chose for the screws.
Knowledge check
Answer every question, then check your answers.