Drilling Inconel and Nickel Alloys: What Makes Them Difficult and How Shops Survive Them

Inconel, Hastelloy, Waspaloy, Rene alloys — the nickel-based superalloys show up in aerospace components, turbine hardware, chemical processing equipment, and anything that needs to maintain strength at elevated temperatures. They are among the most difficult materials to machine, and drilling them specifically is a discipline that separates experienced aerospace shops from everyone else.

If you've just landed a job that involves drilling Inconel 625, 718, or a similar material and you've never done it before, this guide gives you the framework that experienced shops use. Most of the principles also apply to Hastelloy C-276, Waspaloy, and other precipitation-hardened nickel alloys.

Why Nickel Superalloys Are So Hard on Drills

These materials combine several properties that are individually difficult and collectively punishing:

High hot strength

Superalloys were engineered to maintain strength at temperatures where steel softens. At the cutting zone, where temperatures routinely exceed 1000°F, the workpiece material stays hard and strong while the drill is trying to cut it. This defeats the usual mechanism by which machining becomes easier as heat softens the material.

Work hardening

Like stainless, nickel superalloys work-harden at the cutting zone. The hardening rate is higher than stainless. A brief moment of rubbing instead of cutting creates a surface layer significantly harder than the base material. The next pass of the drill hits that hardened skin first.

Abrasiveness and built-up edge

The microstructure of precipitation-hardened Inconel contains hard carbide and intermetallic particles that abrade the cutting edge rapidly. At the same time, nickel alloys have an affinity for adhering to HSS at the cutting zone, forming built-up edge that degrades geometry and generates additional heat.

Low thermal conductivity

Heat stays at the cutting zone. Inconel conducts heat roughly half as well as stainless, which is already poor. The cutting zone in Inconel drilling reaches temperatures that will destroy standard HSS geometry in seconds and challenge cobalt significantly.

Drill Selection for Nickel Superalloys

Cobalt HSS as the floor

M42 cobalt HSS (8% cobalt) is the minimum acceptable drill material for nickel superalloys in a job shop context. Standard HSS will fail too rapidly to be economical. Cobalt's higher red hardness extends tool life meaningfully, though it still wears faster in Inconel than it does in stainless.

Carbide for production

For production quantities — more than a few dozen holes in the same part — solid carbide drills with TiAlN or AlTiN coatings are the correct choice. The wear resistance advantage over cobalt is dramatic in superalloys. The tradeoff is rigidity requirements: carbide drills in Inconel require a rigid setup, short overhangs, and a machine with minimal spindle runout. Any vibration will snap the bit.

For job shop one-off work, cobalt with careful technique is more practical than carbide. The cost of snapping a carbide drill mid-hole in Inconel — and then having to extract it from a very hard material — often exceeds the cost of running cobalt with more frequent resharpening.

Geometry considerations

In nickel alloys, a split-point or web-thinned geometry is strongly preferred. The standard chisel edge exerts enormous thrust in these materials — more than most drill press setups can handle cleanly, and enough to accelerate work hardening at the entry. Split points reduce thrust by 40-60% and allow the drill to start cutting immediately. Shorter flute length (stub-length drills) increases rigidity and reduces deflection.

Speeds and Feeds: Go Slow, Feed Hard

The counterintuitive rule in nickel superalloy drilling is that you need to feed hard and run slow. Most machining intuition runs the other direction — slow down when something gets hard. But in Inconel, inadequate feed is more damaging than adequate feed.

SFM

For HSS cobalt in Inconel: 10–20 SFM. For solid carbide: 30–60 SFM depending on diameter and alloy. These speeds feel painfully slow. They are correct. Running above these values generates enough heat to destroy tooling rapidly.

Feed rate

Feed per revolution in cobalt for Inconel: 0.001–0.003" per revolution depending on diameter. Keep the feed in the cutting range — if the bit is rubbing rather than cutting, you are creating work-hardening ahead of the tool and destroying both the drill and the hole surface.

No dwelling

Once the spindle is turning and the drill is in contact with the workpiece, the feed must keep moving. Any pause in feed while the spindle turns is a rubbing event. Program out any dwell at the bottom of peck cycles. On a manual machine, don't hesitate — if you need to peck, retract cleanly and re-enter with immediate feed engagement.

Cutting Fluid

Straight sulfurized cutting oil at high concentration, delivered to the cutting zone. Water-soluble coolant alone is insufficient in nickel superalloys — the lubrication component matters as much as the cooling. Through-coolant tooling (coolant delivered through the drill body to the tip) is the preferred method for production. For job shop work, flood or directed oil delivery is the practical alternative.

The Tooling Discipline

Shops that successfully machine Inconel consistently share one practice: they replace or resharpen tooling before it's obviously worn, not after it fails. A drill that's cutting Inconel at 80% efficiency is generating more heat and more work hardening than a fresh drill. The marginal extra holes you get from a slightly worn bit are paid for in harder material ahead of the next tool, increased cycle time from lower cutting efficiency, and accelerated wear on the replacement.

Set a hole count limit per drill and stick to it. What that limit is depends on your specific alloy, drill diameter, and parameters — but it exists and should be determined empirically, not by running until failure.

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Cobalt drills used in Inconel and nickel alloys are resharpeable. MachinistPost resharpens HSS and cobalt drill bits by mail, restoring split-point geometry, lip relief, and chisel edge. Mail us your worn superalloy-work drills and we'll have them back sharp and ready.