Fastened joints
Choose screws, nuts, spacers, and access so parts clamp together without crushing or binding.
A joint has to keep its parts in place while the moves, takes impacts, and carries loads. Draw the and their contact surfaces, not only the holes. Then check how the joint will be tightened and taken apart.
Start with the two plates and spacers from Assemblies. That model defines their position; this page adds the decisions needed for a physical fastening arrangement.
Read the screw description
M4 × 0.7 × 25 describes a metric thread with a nominal 4 mm diameter, 0.7 mm pitch, and 25 mm length. is the distance between adjacent threads. Check the head and drive style separately.
Most projecting-head screws measure length from underneath the head. Countersunk screws generally include the head in their stated length. Confirm the supplier drawing when length or head seating is important.
A screw needs compatible mating threads. Similar diameters do not make metric and imperial threads interchangeable. List the thread type and size in the so a builder can choose the right screw without trying a nearly matching one in the or nut.
Clearance or thread
A hole lets a screw pass through without engaging its threads. A contains a matching internal thread. A joint often has clearance holes through the clamped parts and a nut or threaded component at the far end.
For the practice plate's 4.5 mm holes, an M4 screw has nominal clearance. Choose the actual hole fit for the process and alignment requirement. A printed 4.5 mm opening may need a different nominal value or finishing operation to achieve that fit.
If a screw passes through two independently located plates, both patterns must line up. Locate the holes from shared references and leave suitable clearance for the screws. If the parts need more precise alignment, design features to locate them rather than relying on two tight screw holes.
Work out the length
The is the stack clamped between the screw head and nut or threaded component. Add every plate, spacer, and washer in that stack.
For an illustrative joint based on the plate :
| Item | Thickness |
|---|---|
| Lower plate | 3 mm |
| Spacer | 12 mm |
| Upper plate | 3 mm |
| Two washers | 1 mm each |
| Clamped stack | 20 mm |
If a selected ordinary nut is 3.2 mm high, a 25 mm screw would extend 25 − 20 − 3.2 = 1.8 mm beyond it. With a 0.7 mm pitch, that is roughly 2.6 thread pitches. Check the actual nut, screw tip, washers, and required engagement before ordering. A locking nut may have a different height and engagement requirement.
For a blind tapped hole, calculate how far the screw enters after passing through the outside parts. Compare that with both usable thread depth and the maximum permitted insertion. A screw can bottom out while leaving the bracket loose.
Make the clamp path clear
The clamping force should pass through surfaces intended to carry it. In the plate assembly, each spacer supports the gap around its screw. Without the spacer, tightening the screw would bend the plates toward each other.
Thin-walled tube can be crushed by a through-bolt if the joint lacks appropriate support. A suitable internal spacer or crush block can carry the clamp force, but its length and contact surfaces must match the tube and load.
Printed plastic can also deform under sustained clamp load. Use appropriate washers, inserts, spacers, geometry, and material. Check the joint after the robot has been used. Plastic can slowly deform under pressure, leaving the joint less firmly clamped even if the screw has not turned.
is the tension created in a tightened fastener. Follow the hardware and component guidance for tightening; there is no single setting for every M4 joint in every material.
Keep pivots moving
A pivot has a different job from a rigid clamped joint. The fastener or must stay in place while the moving part turns freely.
Use a shoulder, spacer, , or bushing arrangement that allows the stationary stack to be tightened without squeezing the moving link. Check which each spacer contacts. A collar rubbing a stationary housing can add friction even when the shaft fits its bore.
Model the installed washers and heads. A joint can have enough clearance at its center while a large washer touches the neighboring link during rotation. Use Shaft supports for a complete rotating stack.
Captive nuts and inserts
A captive nut sits in geometry that prevents it from turning or falling out. A threaded insert provides metal threads in another material. Both need installation access and sufficient supporting material.
For a simple nut-pocket exercise, model a 20 × 20 × 8 mm test block. On one face, cut a hexagonal pocket 7.2 mm across flats and 3.4 mm deep, with a centered 4.5 mm through hole. The screw enters from the opposite face and pulls the nut against the pocket floor.
Those are trial for a nominal 7 mm-across-flats, 3.2 mm-high nut. Measure your actual nut and test the pocket with the intended printer or machining process. The open side must remain reachable for insertion, and the surrounding wall must resist the tightening load.
For a heat-installed insert, use the insert manufacturer's hole and installation guidance. Its outside shape and installation method determine the pocket; copying the screw's thread diameter will not define it correctly.
Check tool access
Reserve the space needed by the driver, hex key, socket, or wrench in the installed assembly. A nut may need to be held while the screw is turned. Check the path before surrounding it with another plate.
Model the assembly order. If the nut must be inserted before a cover closes, show that in the instructions. If replacing a motor requires cutting a trapped cable tie or removing a structural axle, decide whether the mount can be made easier to service.
Add each fastener, washer, spacer, nut, and insert to the BOM. Then ask another student to assemble the practice joint from the drawing and parts list, noting any missing dimension or screw they cannot reach.