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4.6.1 Selecting an oval thumb: starting bit, oval width, oval degree, and taper

4.6.1   oval

An oval thumb hole needs four inputs beyond the standard thumb measurements. Together they define the shape, orientation, and profile of the finished oval so Spectre Cloud can generate the drilling sequence. The calculation mechanics live in the Oval Calculator (Book 05) — this page covers what the four inputs are and where they go on the spec sheet.

🔵 The Four Oval Inputs

  • ✅ Starting Bit — the diameter of the round pilot hole drilled before any oval cuts. It fits within the oval's narrowest dimension so the oval passes open the hole outward from it.
  • ✅ Oval Width — the finished width of the thumb hole (the larger of the two oval dimensions), including the appropriate fit clearance.
  • ✅ Oval Degree — the orientation of the oval's width axis, set as an angle around the hole. It matches the oval to the thumb's natural resting angle.
  • ✅ Taper — how much larger the top of the hole is than the bottom, matching the thumb's profile so it seats fully without binding at the base or feeling loose at the tip.

🛠️ Entering the Oval Inputs

  1. In the thumb section, set the shape to Oval (see 4.5.1).
  2. Enter the starting bit, oval width, oval degree, and taper.
  3. Review all four before proceeding — an error in any one changes the drilling sequence.

⚠️ Verify with your Spectre team: confirm the degree convention, the unit taper is entered in, and whether an oval depth field is required in addition to width.

  • 4.5.1 — Selecting the thumb hole shape (Round or Oval)
  • 4.6.2 — Entering the oval inputs (step-by-step)
  • 4.6.3 — How the system calculates the oval cut
  • 05 — Oval Calculator (full mechanics and derivation)

Tip: Oval degree is the most judgement-dependent of the four inputs — observe the bowler's thumb in a round hole before committing to an angle, and record your observation in Notes for future visits.

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