On a drawing, an intersecting hole looks trivial. A line crosses another line, and the tolerance block says the intersection must be within a few tenths of a millimeter. It is one symbol among hundreds.
In the real part, that symbol is one of the hardest features in machining. The hole is drilled at an angle into a hardened surface, it runs deep into the component, and its destination — the cross-hole it must meet — is inside the part, invisible to the eye and unreachable by any measuring tool while the cut is happening.
Miss it, and the consequences are not a rework. They are a scrapped part.
The Part: Crankshaft Oil Galleries
The project was a crankshaft for a next-generation engine platform. Oil holes are drilled at an angle — typically 15° to 30° — into the crank pin journal surface, and they must intersect the pre-machined oil galleries running through the crankshaft.
- Hole diameter: Ø10–25mm, depending on the location.
- Hole depth: up to around 50mm — short by deep hole standards, but drilled in high quantity across every journal and pin.
- Material: 4140 steel, hardened to 45–52 HRC.
- The critical feature: the intersection point of each oil hole with its gallery.
That intersection is how oil gets from the crankshaft’s internal gallery to the bearing surface. If it is even slightly misplaced, the oil flow is restricted or blocked — and in a high-revving engine, that is not a cosmetic issue. It is an engine failure. The tolerance on the intersection is so tight that a positional error of half a millimeter can be the difference between a passing part and scrap.
Why the Intersection Is Hard to Hold
Three forces work against you:
1. The drill slides on a slope. The oil hole enters at an angle onto a curved journal surface. A drill starting on a slope tends to walk downhill — it drifts along the angled surface before it bites, and the first few millimeters of the hole are already off-line before the drill has depth. That early deviation grows through the hole. It is the same drift mechanism that sends a 600mm-deep hole off-axis — here it happens in the first few millimeters.
2. The intersection is invisible. The drill cannot see its target. No operator can watch a cross-hole being met inside solid steel. The only way the intersection is correct is if every upstream input — the position, the angle, the drill’s line, the material’s response — is correct, in the same part, at the same time.
3. Error accumulates from the datum. The oil hole is positioned from previously machined features. If the datum feature is slightly off — a journal that was ground a couple of hundredths out of position — the hole inherits that error, and the intersection moves with it. The problem is rarely one big error; it is several small ones adding up.
How the Intersection Is Actually Held
None of this is solved by luck or by a better feed override. It is solved by a process engineered so that the intersection is produced the same way every time:
Control the start. The drift-on-a-slope problem is eliminated at the source. A guide bushing locates the drill precisely at the entry, and the entry surface is prepared so the drill starts on a flat, not a slope. The first few millimeters — where the hole’s direction is decided — are no longer left to chance.
Reference the real part, not the drawing. Instead of positioning the hole from theoretical coordinates, we measure the actual machined features and compensate. If the datum journal is a few hundredths off, the program shifts to match reality. Small errors are cancelled before they can accumulate.
Drill it all in one setup. Repositioning the part between operations reintroduces error. By completing the angled oil holes in a single setup, the reference is stable for every hole, and there is no “second clamping” to undo the first.
Prove the first part. Before the process is released, the first part is verified by cutting and inspecting the actual intersection — checking that the oil hole meets its gallery within tolerance. Once the first part proves the process, the rest of the batch follows the same proven path. The parameters, the tool, and the setup are locked in as the standard.
Respect the material. At 45–52 HRC, this is hard, abrasive steel. Tool life and hole quality live and die on the cutting parameters and the coolant delivery. Get the process right for the hardened material, and the holes are consistent; get it wrong, and the tool wear shows up as intersection error before anyone notices the drill.
The Result
- Intersections held within tolerance, part after part — the oil holes met their galleries reliably across the full quantity of journals and pins.
- Scrap from intersection failure effectively eliminated — instead of a steady trickle of parts rejected at final inspection.
- A process that transferred — when the customer added a new crankshaft variant, the same methodology (controlled entry, measured datum, single setup, first-part proof) applied to the new geometry.
The Takeaway: The Intersection Is a Process, Not a Luck
The most misleading thing about an intersecting hole is that it looks simple. It is one line on a drawing, and the dimension is just a number. But that number represents an entire chain of control: the start of the hole, the reference it is positioned from, the stiffness of the setup, and the material behavior — all of it has to be right in the same part, every time.
For the automotive industry, where intersecting oil holes are everywhere and a failure is catastrophic, this is not a nicety. It is the difference between a process and a gamble. The crankshaft project behind this article is documented in our crankshaft oil hole case study.
If your part has an intersecting hole that keeps costing you parts, send us the drawing and your current numbers. We will tell you honestly where the process is losing the intersection — and how to hold it.