Drilling stainless has a reputation that is only partly deserved. The metal is not unusually hard in the annealed condition, and a sharp drill will cut it cleanly. What makes it difficult is a pair of properties that work against the tool at the same time: it work hardens quickly under pressure, and it conducts heat badly.
Those two properties explain almost every failure a shop sees. The drill rubs instead of cutting, the surface under the edge hardens, the drill dulls faster, the operator pushes harder, and the heat that cannot escape into the workpiece stays at the cutting edge. The hole gets harder, the drill gets hotter, and the process runs away in a direction nobody intended.
Stainless of the austenitic kind hardens as it deforms. A cut that shears the metal cleanly produces a thin hardened layer that the next pass removes. A cut that rubs produces a thicker hardened layer without removing metal, and that layer is what the next revolution has to get through.
The distinction matters because it means the problem is usually created rather than found. A drill that is allowed to dwell, or that is fed too lightly so it polishes the surface, hardens the very surface it needs to enter. Once that has happened, a sharp drill will struggle and a dull one will not cut at all. The surface may also harden deeper than the first attempt can reach, so the second attempt is worse than the first.
This is the reason a hole that has been started badly cannot usually be rescued by pressing harder. The hardened skin has to be cut through with a new, sharp edge and a firm, continuous feed, or removed by another method entirely.
In aluminium and mild steel, a large share of the heat generated at the cutting edge leaves with the chip and the rest spreads into the workpiece, which conducts it away. Stainless conducts heat poorly, so the heat concentrates at the edge and in the chip. The drill tip runs hotter than it would in other metals, and the margin between working and softening the tool is narrower.
That is why coolant matters here more than in general drilling, and why the coolant has to reach the tip rather than flood the outside of the hole. A drill running dry in stainless will fail at the point, and a failed point is a rubbing point.
The useful mental model is that the drill has to be permanently engaged. Every moment the edge is in contact but not removing material is a moment of work hardening. That argues for a feed that keeps the edge biting and a speed that does not let the tip overheat.
In practice the failure mode looks like a scream and a shower of fine powder rather than chips. Chips are the sign that the cut is working; powder means the edge is rubbing. A machinist who watches chip form rather than the spindle will know within a second whether the hole is going to be trouble.
Pecking is a compromise. Withdrawing the drill clears the chips and lets coolant reach the tip, which helps in a deep hole. Done too often on a shallow hole it interrupts the cut repeatedly and can harden the surface at each re-entry. The useful version is a peck deep enough to cut properly between withdrawals.
The standard point on a general-purpose drill has a chisel edge at the centre that does not cut, it scrapes. On stainless that scraping is exactly the rubbing that starts work hardening, and it is worst at the moment of entry, when the drill is trying to find its position.
A split point removes most of that dead centre and gives the drill a cutting edge all the way across. The result is a drill that enters without skating and without needing a pilot hole in many cases. Cobalt tooling holds its edge at higher temperature than ordinary high-speed steel, which extends life in a metal that runs hot. Coating helps, but a coating on a badly ground point does not fix the geometry.
In thin sheet the drill breaks through before it has engaged fully, and the moment it breaks through it grabs. The helix pulls the drill into the work, the sheet climbs the flutes, and the hole becomes a torn triangle with a burr on the far side. On a formed panel the grab can also distort the panel.
The usual answers are to clamp the work firmly on both sides of the hole, to back the exit with a sacrificial piece, and to reduce the feed at the moment of breakthrough. A step drill behaves better in thin sheet because its conical form does not screw itself in, and a hole saw or a punch avoids the problem altogether.
A hole drilled through a weld or beside one is being cut in material that has already been through a thermal cycle. The heat-affected zone near a stainless weld can be harder and more abrasive than the parent metal, and the transition from parent to weld is where the drill will tend to wander.
Aluminium weldments present a different version of the same issue, because the filler and the parent metal machine differently and the edge can load with soft metal. Titanium is the one to treat with most caution: a drill that rubs there can produce a hardened, damaged surface that is difficult to detect and difficult to remove.
Where the drawing allows it, drilling before welding is the better sequence. It gives a clean, uniform material to cut, and the hole then acts as a locator during assembly rather than as a repair afterwards. When the hole must come after welding, a pilot through both parts and a firm, continuous feed is the safest route.
Chips in stainless are stringy and they do not break cleanly. They pack into the flutes, and a packed flute stops the coolant reaching the tip and adds friction along the drill body. Clearing them is part of the operation rather than an interruption of it.
Cutting fluid should be directed at the point of cut, and a deep hole benefits from through-tool coolant if it is available or from a pecking routine that lets fluid down the flutes. When the drill starts to squeal, the useful response is to stop, clear the flutes and check the edge, not to add pressure. A squealing drill is already rubbing.
Holes in welded and formed parts are usually specified by diameter and position. The rest of what decides whether they can be made well is often left to the shop.
Small machined and drilled parts sit outside our tube and sheet ranges and are grouped under other parts, while the welded assemblies the holes pass through appear under TIG stainless steel welding and titanium fabrication components. The tube side is under exhaust pipe and TIG titanium welding parts, and the route from drawing or sample is on our customization page.
