Practice: Telescope

Anchor it in the real world first. Show a climber telescope from a recent game (e.g. 2024 Crescendo chain climb). Ask: where have you seen something slide inside something else? Antennas, camera tripods, drawer slides. The key mechanical idea: total extension = sum of all stage strokes, but retracted it packs to roughly one stage length. That's why teams love it for endgame — long reach, small stowed size.

This is the Fusion equivalent of Onshape Variables. Demo creating a User Parameter with an EXPRESSION — stroke depends on the other two. Common mistake: students type raw numbers into every sketch, then can't change the design later. Teach them that overlap is non-negotiable — too little overlap and the telescope racks (binds) under load. A good rule: keep at least 1x the tube width engaged at full extension.

Make a COMPONENT, not just a body — this matters for joints later. Use the stageLength parameter in the extrude field (type 'stageLength', not 20). For real FRC, 2x1x.100 is the workhorse extrusion; the .058 wall is lighter but flexier. Shell instead of sketching the inner rectangle — fewer dimensions to manage. Remind them to model it hollow because the inner stage has to fit inside.

Clearance is the lesson here. Tube-on-tube never slides well in real life — that's why bearing blocks exist (next step). In CAD, give ~0.030-0.060in gap per side so it's not interfering. Common mistake: modeling the inner tube the exact ID of the outer — it'll bind or, in CAD, show interference. Use Inspect > Interference later to check. Assign different appearances so stages are easy to tell apart.

Explain WHY: without guide blocks the inner stage rattles side-to-side and racks under load. Real teams use UHMW/delrin slide blocks or roller bearings on the tube corners. You can insert a STEP from REV or WCP (File > Insert > Insert Derive/Insert McMaster), or just model simple low-friction pads. Keep it simple for beginners — two pads top and bottom that sandwich the inner tube. This is the 'bearing' in bearing block.

This is the centerpiece. Onshape calls this a Slider mate; in Fusion it's a Slider JOINT (Assemble menu, press J). Pick the two component origins or edges that share the slide axis. After creating it, grab the inner tube and drag — it should slide along the tube and nowhere else. Common mistake: using a Rigid joint (won't move) or not grounding the outer tube first (whole thing floats). Right-click OuterTube > Ground before jointing.

Joint limits ARE your safety/acceptance check. Set Maximum to the 'stroke' parameter so it can never extend so far the stages disengage. This mirrors real hardstops on a robot. Demo dragging to the limit — it stops cleanly. If you didn't set overlap correctly in Step 1, you'll see the stages nearly separate here — good teachable moment. Always test both ends: full retract and full extend.

Keep actuation conceptual for beginners — a real cable is flexible and hard to fully simulate in Fusion. Model the spool and a sketch line representing the cable so the intent is clear. Explain the real physics: a NEO or Kraken spins the spool, cable wraps, telescope rises; a constant-force spring or surgical tubing usually retracts it. For multi-stage, a continuous (rigged) cable makes all stages move together — mention but don't require it.