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Activity 002: Slot Cutting and Step Milling with End Mill

Activity ID: U8M3-ACT-002 Duration: 45 minutes Objective: Cut a precision slot and a step (shoulder) on an aluminum block using end mills, applying edge finding, depth control, and feed rate calculations.

Overview

Students use end mills to cut a slot across the top of an aluminum block and mill a step (shoulder) on one edge. This combines multiple skills: edge finding for positioning, Z-axis touch-off for depth, and coordinated X/Y feed for slot and shoulder geometry.

Materials & Equipment Needed

  • Vertical milling machine with indicated vise
  • 6061 aluminum block (face-milled from Activity 001, or pre-machined 2" × 3" × 1")
  • 3/8" 2-flute center-cutting HSS end mill (for slot)
  • 1/2" 4-flute HSS or carbide end mill (for step)
  • Edge finder
  • R8 collets
  • Digital calipers, depth micrometer (optional)
  • Cutting fluid
  • Safety glasses

Instructions & Procedure

Phase 1: Planning and Setup (8 minutes)

  1. Review the part drawing:
  2. Slot: 3/8" wide × 0.200" deep, centered on the 2" width, running the full 3" length
  3. Step: 0.250" deep × 0.500" wide, along one 3" edge
  4. Calculate parameters:
  5. Slot (3/8" HSS end mill, aluminum, 250 SFM): RPM = (250 × 3.82) / 0.375 = 2547 RPM
  6. Feed: 2 flutes × 0.004 IPT × 2547 = 20.4 IPM (hand feed approximation)
  7. Set up the workpiece in the vise on parallels. Face-milled surface up.
  8. Edge-find on two perpendicular edges (front and right). Record zero positions.

Phase 2: Slot Cutting (18 minutes)

  1. Install the 3/8" 2-flute end mill. Tighten drawbar.
  2. Position the spindle at the slot centerline: from the right edge zero, move X to 1.000" (center of the 2" width).
  3. Position Y so the end mill is just beyond the front edge of the workpiece (clear).
  4. Touch off Z on the workpiece top surface. Zero Z-axis.
  5. Raise the cutter. Move to the starting position (Y beyond the front edge).
  6. Lower Z by 0.100" (first depth pass). Lock Z and X.
  7. Feed Y along the full 3" length. Cut the slot in one Y pass.
  8. Return to the start. Lower Z by another 0.100" (total 0.200").
  9. Feed Y again for the second pass.
  10. Stop the spindle. Measure the slot: width (should be 0.375" ± the end mill tolerance), depth (0.200" ± 0.005").
  11. Clean chips from the slot.

Phase 3: Step (Shoulder) Milling (12 minutes)

  1. Install the 1/2" 4-flute end mill. Tighten drawbar.
  2. Calculate RPM: (250 × 3.82) / 0.500 = 1910 RPM (HSS). Set speed.
  3. Position the spindle: from the right edge (X=0), move X so the end mill hangs 0.500" over the edge (the 1/2" end mill center is at X = -0.250").
  4. Touch off Z on the top surface. Zero Z.
  5. Lower Z by 0.125" (first depth pass). Lock Z and X.
  6. Feed Y along the full 3" length.
  7. Return. Lower Z by another 0.125" (total 0.250").
  8. Feed Y for the second pass.
  9. Check: the step should be 0.500" wide and 0.250" deep.
  10. If the step width needs adjustment, move X and take an additional finishing pass.

Phase 4: Inspection (7 minutes)

  1. Remove the workpiece. Clean thoroughly.
  2. Measure and record:
  3. Slot width at three points along its length
  4. Slot depth at two points
  5. Step width at three points
  6. Step depth at two points
  7. Slot position relative to the workpiece centerline
  8. Deburr all edges and slot corners.
  9. Compare measurements to the drawing tolerances.

Discussion Points

  • Why is a 2-flute end mill preferred for slot cutting?
  • How did the surface finish differ between the slot walls and the step surface?
  • What would you change if cutting steel instead of aluminum?
  • How did chip evacuation affect the cut quality?

Expected Outcomes

  • Slot centered on the workpiece within ±0.005"
  • Slot depth within ±0.005" of target
  • Step dimensions within ±0.005" of target
  • Clean cutting with appropriate chip formation

Assessment Rubric

Criteria Excellent (4) Proficient (3) Developing (2) Beginning (1)
Slot Position Centered within ±0.002" Within ±0.005" Within ±0.010" Significantly off center
Slot Dimensions Width and depth within ±0.003" Within ±0.005" Within ±0.010" Outside tolerance
Step Dimensions Width and depth within ±0.003" Within ±0.005" Within ±0.010" Outside tolerance
Process Correct tool selection, parameters, and sequence Minor parameter adjustment needed Required guidance on sequence Cannot complete operations

Safety Considerations

  • Verify drawbar tightness before each tool change
  • Lock unused axes during all cuts
  • Cutting fluid is essential for aluminum end milling—prevents chip welding
  • Clear chips frequently from the slot to prevent re-cutting (degrades surface finish)
  • End mills can pull the workpiece if climb milling—use conventional milling on manual mills with backlash
  • Safety glasses mandatory throughout

Last Updated: 2026-03-19