2.2.5 - Skill Check - Level 3

🛠️ Skill Check: Precision Hub & Axle

Objective: Demonstrate precision turning on the lathe, square milling on the Bridgeport, and accurate hole placement to create a concentric, mated assembly.

🚦 Phase 1: The Axle (Lathe Operations) Material: 1/2” Hex Shaft

Cut to Length: Use the Milwaukee Bandsaw to cut a ~3” section of hex shaft.
- Reasoning: Always leave “extra” material (stock) so the lathe can face the ends perfectly flat and square.
Face the End: Mount the shaft in the lathe chuck and “face” the end until it is perfectly flat.
Turn the Diameter: Use a turning tool to reduce the hex to a 0.500” Outer Diameter (O.D.) for exactly 1.000” of length.
- Tip: Use Calipers frequently. As you approach 0.510”, take smaller passes (e.g., 0.002”) to avoid “undershooting” the fit.

🏗️ Phase 2: The Hub & Cross-Drill (Milling Operations) Material: 2” x 2” x 1/2” Aluminum Square

Square the Block: Mount the plate in the Milling Machine vise with parallel standoffs. Use an endmill to “square” the edges so they are 90° to one another.
Find Center & Bore: Use an Edge Finder to locate the exact center of the 2”x2” face. Drill a pilot hole, then use a 1/2” Drill Bit to create the center hole.
Transition: Cross-Drilling the Shaft: You must now switch the Hex Shaft from the lathe to the milling machine to drill the locking hole through the side.
- Setup: Place the hex shaft in the mill vise using vise parallels to keep it level.
- Location: Use the edge finder to locate the end of the shaft. Move the table to locate the hole exactly 0.250” (1/4”) from the lathed end.
- Execution: Drill a #21 clearance hole completely through the 0.500” diameter section.
- Reasoning: Drilling this on the mill ensures the hole is perfectly centered and perpendicular, which is nearly impossible to do by hand.

🔩 Phase 3: Assembly & Lock

The Mate: Slide the lathed end of the shaft into the aluminum hub.
The Stand-off: Use 10-32 Washers on the backside of the block to ensure the nut does not hit the aluminum block directly.
The Lock: Align the side hole of the shaft with the hub and secure the assembly with a 10-32 bolt.

📊 Evaluation Rubric (Mentor Review)

Category	Emerging (1 pt)	Proficient (2 pts)	Mastery (3 pts)
Lathe Precision	Diameter is ±0.010”; surface finish is rough.	Diameter is within ±0.003” of 0.500”.	Diameter is 0.500” (+0.000 / -0.001”); mirror finish.
Mill Accuracy	Center hole is visibly off-center.	Center hole is within ±0.010” of center.	Center hole is within ±0.002” of true center.
Side Hole Location	Hole is more than 1/16” off the 0.250” mark.	Hole is within ±0.010” of the 0.250” mark.	Hole is exactly 0.250” from the end; perfectly centered.
The Fitment	Shaft is too loose or requires a press.	Shaft slides into the block with minor friction.	Precision Slip Fit: Smooth entry with zero slop.
Mechanical Sympathy	Metal chips left in assembly; washers missing.	Assembly is clean; correct hardware used.	Deburred edges; all tool marks removed; perfect torque.

Total Score: _____ / 15 (12+ required for Level 2 Machining Sign-off)