4.5.4 Entering thumb oval span and pitch information

Entering thumb oval span and pitch information

4.5.4   oval

For most spec sheets, span is measured and entered the same way regardless of thumb hole shape. Oval thumb holes introduce one specific scenario where the entered span requires an adjustment — when the driller chooses to make oval cuts in both directions from the pilot hole center. Understanding when this adjustment applies, and how to account for it, ensures the finished span matches the bowler's fitting measurement after all cuts are complete.

🔄 Two Approaches to Making Oval Cuts

When executing the x/y offset cuts calculated by Spectre Cloud, the driller has a choice about where to place the material removal:

Option 1 — Cuts made at the bottom of the hole only

All oval material is removed from the side of the hole away from the finger holes. The top edge of the thumb hole — the edge closest to the finger holes — remains at the original pilot hole boundary and does not move.

• ✅ Span is unaffected — the top of the thumb hole stays exactly where it was drilled, so the distance to the finger holes remains consistent with the entered span measurement.
• ✅ No span adjustment is needed in Spectre Cloud when using this approach.

Option 2 — Cuts made in both directions (bidirectional)

Oval material is removed from both sides of the pilot hole — including the side facing toward the finger holes. For example, at 45° oval degrees, cuts are made at both the 4:30 position and the 10:30 position on the clock face of the hole.

• ✅ Produces a more symmetrically centred oval — some drillers prefer this approach for the finished appearance and feel.
• ❌ The cut at the top of the hole (10:30 in the example above) moves the top edge of the thumb hole toward the finger holes — effectively shortening the span if no adjustment is made.
• ❌ If the finger holes were drilled first and no span allowance was added, the finished span will be shorter than the bowler's measurement requires.

📐 The Span Allowance for Bidirectional Cuts

When bidirectional oval cuts are planned and the finger holes have already been drilled, the span entered in Spectre Cloud must include an allowance equal to the distance the top-of-hole cut will travel toward the finger holes. This ensures that after the cut is made, the effective span — from the finished top edge of the thumb hole to the finger holes — matches the bowler's measured span.

The allowance is determined by the y-axis component of the oval cut at the top of the hole — specifically, how much of the bidirectional cut travel moves in the direction of the finger holes.

• ✅ At 0° oval degrees (cuts at 12:00 and 6:00), the top cut moves directly toward the finger holes — the full y-axis travel distance becomes the span allowance.
• ✅ At 90° oval degrees (cuts at 3:00 and 9:00), neither cut moves toward or away from the finger holes — no span allowance is needed even with bidirectional cuts.
• ✅ At 45° oval degrees (cuts at 1:30 and 7:30) or 135° (cuts at 4:30 and 10:30), only the y-axis component of the top cut contributes to span encroachment — the allowance is the y-axis portion of the offset travel, not the full cut distance.
• ✅ Spectre Cloud's calculated x/y output gives you the y-axis value directly — use this as the basis for the span allowance when bidirectional cuts include a top-of-hole pass with a y-axis component toward the fingers.

Verify with Spectre team: confirm whether Spectre Cloud prompts the operator to specify bidirectional vs. bottom-only cuts and calculates the span allowance automatically, or whether the operator is expected to calculate and apply the allowance manually before entering the span value.

🛠️ Entering Span for an Oval Thumb in Spectre Cloud

1. Determine which oval cut approach the driller will use — bottom-only or bidirectional. Confirm this before entering the span, not after drilling.
2. If using bottom-only cuts: enter the span as measured — no adjustment needed.
3. If using bidirectional cuts:
   1. Review the y-axis value from the Spectre Cloud oval cut output for the top-of-hole pass.
   2. Add this y-axis value to the measured span before entry — this is the span allowance.
   3. Enter the adjusted span value in the span field.
4. Note in the Notes field which cut approach was used and whether a span allowance was applied — e.g. Bidirectional oval cuts. Span adjusted +3/32" for top cut y-axis encroachment.

Important: The span allowance only applies when the finger holes are drilled first. If the thumb hole is drilled first, the finger holes are positioned relative to the finished thumb hole edge after the oval cuts are complete — no pre-adjustment is needed. Confirm the drilling order before deciding whether to apply the allowance. Verify with Spectre team: confirm whether Spectre Cloud tracks or prompts for drilling order (thumb first vs. fingers first) as part of the spec sheet workflow.

📊 Span Adjustment — Quick Reference

✨ Tips for Oval Thumb Span Entry

• ✅ Decide on the cut approach — bottom-only or bidirectional — as part of the fitting conversation, before the bowler leaves the counter. Changing approach after the finger holes are drilled is too late to adjust the span.
• ✅ Document the cut approach and any span allowance in the Notes field every time. On a future visit when a ball is being cloned, this note prevents the next driller from applying or omitting an allowance incorrectly.
• ✅ If you are unsure which approach a previous driller used on a cloned spec sheet with no notes, measure the finished thumb hole on the existing ball — a symmetrical oval indicates bidirectional cuts; an oval that extends only to one side of the pilot center indicates bottom-only.
• ❌ Do not apply a span allowance when drilling thumb-first — the allowance only compensates for encroachment on already-drilled finger holes. Applying it unnecessarily will push the finger holes further from the thumb than the bowler's measurement requires.

Related Sections

• 4.5.2 — Entering starting bit, oval width, oval degrees and taper
• 4.5.3 — How the system calculates the oval cuts from your inputs
• 4.5.1 — Selecting "Oval" thumb hole on the spec sheet
• 4.4.3 — Entering bowler's span (Full and Cut to Cut)
• 4.3.2 — Entering span measurements (Full Span and Cut to Cut)
• Book 06 — Drilling Your First Ball

Tip: The span allowance for bidirectional oval cuts is a small detail that is easy to overlook — but a bowler who comes back reporting their ball feels slightly cramped after an oval thumb drilling is almost always experiencing the result of a missing allowance. Making it standard practice to note the cut approach on every oval spec sheet eliminates this class of complaint entirely.

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