Aerospace Fastener Holes — Fit Is the Spec, and the Burr Is the Failure
Four failure modes, and only one of them shows on a calliper
- Interfacial burr. The burr at the exit of one layer sits between the sheets and prevents them closing. This is the classic fatigue initiator in a bolted or riveted joint, and it is invisible after assembly. It is controlled at the exit, not by deburring afterwards.
- Composite exit damage. Fibre pull-out and delamination at the exit of a CFRP layer are driven by the support behind the exit and by the feed as the drill breaks through. An unsupported exit can pass a diameter check and still be rejectable on a micrograph.
- Taper and out-of-round in an interference-fit hole. An interference fit assumes a defined hole and a defined interference. A tapered or lobed hole gives a contact pattern the joint was not designed for, and the fastener installation force starts to scatter.
- Stack spring-back. The hole that measured correctly under clamp does not measure the same after the stack is released. If the process is verified on a clamped part only, the released part is unverified.
If the part has no fit requirement and no stack — a simple through hole in a single plate — the general drill-then-ream path is the right reference and this page is more than you need.
What drives tooling on a fastener hole
| Feature | What usually decides the tool | HENTRY entry |
|---|---|---|
| Entry on a curved or inclined surface | Whether the drill can start without walking; a spot or flat-bottom entry is often the difference between a centred hole and a scrapped part | Carbide Spot / Center Drill · Flat Bottom Carbide Drill |
| Through hole across a stack | Total depth, layer count and whether coolant can reach the exit interface | Through-Coolant Carbide Drills |
| Hole brought to fit size | Allowance left by the pre-hole, and whether the material will smear or shear | Carbide Reamer · PCD Reamer |
| Countersink or head seat | Concentricity to the hole axis, and whether the seat is cut after the hole is finished | Countersink · PCD Chamfer / Spotface |
| Exit edge at the far face | Support behind the exit and the feed at breakthrough — geometry, not speed | Solid Carbide Drills |
Which layer decides the route
| Stack or material | Route | Why |
|---|---|---|
| 2024 / 7050 / 7075 aluminum | Carbide drill, then carbide or PCD ream | Shears cleanly; PCD earns its cost only on long runs or where built-up edge is the limit |
| Ti-6Al-4V | Carbide, with heat pushed into the chip | Low thermal conductivity keeps heat at the edge; PCD is not the answer here |
| CFRP | Carbide geometry chosen to shear fibre rather than push it | A metal drill tears the surface ply and delaminates at exit |
| CFRP / Ti stack | One drill sized for the pair, or a defined two-step sequence | Two chip mechanisms in one axis; the metal exit burr is usually the controlling defect |
| 15-5PH / 17-4PH stainless | Carbide with edge prep against work hardening | A dull edge hardens the surface and the next hole starts oversize |
Boundaries, so you do not read the wrong page
- The alloy physics itself — heat, chip form and exit burr in titanium or nickel, independent of any joint → Tough-Alloy Holemaking.
- Plain drill-then-ream logic for brackets and covers with a locating hole → Standard Drill-then-Ream Paths.
- Bores in casings and rings where concentricity, not fit, is the criterion → Aerospace Engine & Actuation Bores.
- What the tool change costs in paperwork and records → Aerospace Tool Traceability.
What makes a fastener-hole quote reviewable
- Stack definition in order, with each layer thickness and material
- Hole schedule: Ø, tolerance class, perpendicularity, countersink angle and depth
- Fit type — clearance, transition or interference — and the installation method
- Which face is the exit, and whether it is open or backed
- Machine, holder, stickout and measured runout at the tool tip
- The defect you are trying to remove, described as you see it on the part
Quick answers before you RFQ
Can one drill handle a CFRP/Ti stack?
Sometimes, and it is a compromise. A single drill has to survive abrasive composite at the entry and heat-generating titanium at the exit, so geometry and coating are pulled in two directions. Where the hole quality class is tight, a defined two-step sequence usually gives a more predictable result than one compromise tool.
How much allowance should the pre-hole leave for reaming?
It depends on the material and on the pre-hole quality, which is why we ask for the current pre-hole method rather than a single number. The useful rule is that the reamer must remove enough to correct the pre-hole and not so much that it has to cut a taper. Send the measured pre-hole and the allowance follows from it.
Does PCD make sense on aluminum fastener holes?
On long runs where built-up edge or tool life is the limit, yes. On short runs or where the geometry is the constraint, a carbide reamer is cheaper and just as capable. The material grade decides, not the fact that the part is aluminum.
Where do I go for the bore and casing work?
To Aerospace Engine & Actuation Bores. Fastener holes are a fit problem in a stack; casing and seal bores are a concentricity and thin-wall problem, and the two are quoted differently.
Send the stack layout and the exit face marked
Primary tooling entries for fastener holes: solid carbide drills, through-coolant drills, carbide and PCD reamers, and countersink geometry. Stack order matters more than the hole count.