Machining Steps:

  1. Stock cut with bandsaw down to 9.25"
  2. Faced both ends in Tod shop
  3. Turned down to 6"
    1. 18 thou passed
    2. Feed rate 1.5 in/m
    3. Spindle speed 180rpm
    4. Note it was extremely non-concentric
  4. Machined live center on waterjet
    1. 6" OD
    2. 0.5" ID
  5. Turned down motor casing end OD, base emulator used for slip fit
    1. 600 RPM w/ live center
    2. Centered in chuck using contact between live center plate and faced edge of piece
  6. Bored out motor casing end ID
    1. Please fill in
  7. Turned down mission package OD
    1. Please fill in
  8. Attempted to bore out motor casing ID - see below.

Manufacturing Incident:

Setup:

  • Adjusted boring bar bit to be 6.5" extended beyond tool assembly
  • Charlie centered the piece in the chuck using a dial indicator - final concentricity no worse than +/- 1 thou
  • Initiated boring at 800 RPM, piece and boring tool chattered excessively, lowered to 600 RPM
  • Slow but somewhat variable feed rate used
  • Note the material "felt" different (surface finish) between the area immediately surrounding the cut face (from the bandsaw) and the deeper stock material
  • Began a deep pass into the piece, the desired ID of 4.5" (supposedly the stock ID), zeroed against the end of the piece, which measured to 4.515"
  • The piece continued to notably chatter, but this was judged inevitable due to the cantilevered end

Incident:

  • At a depth of about 4", the bar suddenly caught on the piece
  • The piece was torn from the chuck
  • The boring bar tore several gauges within the interior of the stock
  • The tool stuck was rotated about 40 degrees
  • Spindle off button was immediately triggered (closer than E-stop to my hand)
  • See below for pictures of the damage
  • Note the bar was being moved only along the Z-axis - DRO confirms this after triggering of stop command

Causes:

  • Unknown. A few theories:
    • Resonance between the boring bar and the stock
    • Discontinuous surface of stock, potentially in excess of 15 thou
    • Jerk induced due to lip from previous boring attempt.
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