A PCD combination drill-reamer can look perfect on tool-change math—until a deep blind bottom packs chips against the finishing edges and the scrap mode turns into “the combo.”
This page is the deep + blind risk layer only. Decide whether to evaluate combo at all with Drill Then Ream vs PCD Combination: Decision Tree. For the general failure gate when you already wanted combo, see When a PCD Combination Is the Wrong Choice—link it; do not reprint that matrix here.
What deep + blind changes vs through-holes
Through-holes can send chips ahead of the tool. Blind holes return chips only through flute space. Add depth and the finishing section sits farther into a volume where chips and coolant already struggle.
Deep-hole create strategy (peck / L/D) for drills: Deep-Hole Carbide Drilling—use it for create tactics, not as a promise that combo geometry survives the same depth blind.
Chip and coolant dead-zone risks unique to blind combo
| Risk | Why combo feels it harder | First evidence |
|---|---|---|
| No through exit | Chips must pass finish edges | Packed flutes on retract |
| Tip coolant dead zone | Flood never reaches bottom finish | Burnt chips, sticky bottom |
| Cross-hole interrupt | Chip shape changes mid-pass | Marks starting at interruption |
Symptom checklist for packing (create-focused): Carbide Drill Chip Packing Checklist. On a combo body, the same packing hits finishing edges harder.
Finish-edge survival at the blind bottom
Finishing edges near the bottom work where chips collect and coolant is weakest. Recut chips, dragged walls, and edge damage show up as size/finish scrap that looks like “reamer geometry” but started as evacuation.
If the only “fix” is slowing the whole path into a mediocre middle for both sections, prefer split: controlled drill + separate PCD reamer. Product concept still available when cleared: PCD Combination Drill-Reamer. Multi-diameter create without PCD finish demand: Step Carbide vs PCD Combination.
When depth forces split even if the tree said “maybe”
Return to split tools when:
- Blind depth leaves no practical flute volume past the finish section
- Coolant cannot be shown to reach the finishing zone
- Create and finish cannot keep speed/feed/peck each in a safe band on one path
- Sample scrap is bottom packing / finish-edge damage, not tool-change count
That is a depth-blind override—not a rewrite of the whole wrong-choice article.
Validation → Cost → RFQ
Before freezing a combo drawing for deep blind work, require: documented chip exit plan, measured depth, coolant route, chip photos, and scrap mode. Cost per accepted part still beats tool-count fashion. Packet → Custom Cutting Tool RFQ.
Frequently Asked Questions
Is every blind hole banned for combo?
No. Depth + chip/coolant evidence decide; shallow open blinds can still clear.
Is this the same as the wrong-choice article?
No. Wrong-choice is the general failure gate; this page is deep-blind failure modes only.
What L/D number bans combo forever?
None published here—send measured depth and chip evidence instead.
What to send XRZ?
Drawing, depth, blind features, coolant, chip photos, scrap mode, current tools → RFQ.