The customer was a supplier of surgical and medical components, machining parts in 316L stainless steel. The job was a Ø5mm hole, 200mm deep, in a small implantable device component. Nothing exotic on paper.
In practice, it was a nightmare. Every gun drill lasted about four holes before the edge gave out. When the edge went, the surface finish went with it — rough, torn, inconsistent. So they were replacing drills constantly, re-running rejected holes, and watching scrap pile up. A job that should have taken minutes per part was eating hours.
The frustrating part was that the machine was not the problem. It was a good machine, running on a good setup. The problem was the process — specifically, a set of assumptions about how to cut stainless that were exactly backwards.
Why 316L Is a Different Animal
Stainless steel is not steel. Anyone who has machined 316L knows it does not behave like 4140 or P20. Three properties make it genuinely hard to drill:
- It work-hardens. The moment the cutting edge passes, the surface underneath hardens. The next cut has to chew through a harder material than the one before it.
- It holds heat. Stainless has roughly one-third the thermal conductivity of carbon steel. The heat that would normally leave through the workpiece stays in the cutting edge instead.
- Its chips are sticky. 316L chips weld to the cutting edge and build up into a ball — machinists call it built-up edge, or BUE. Once BUE forms, the effective cutting geometry is ruined, and the edge starts tearing instead of cutting.
Any one of these is manageable. All three together, on a 200mm-deep hole where the cutting edge is invisible and unreachable, is a different proposition.
The Diagnosis: Reading the Failed Tool
When our engineers looked at the customer’s used drills, the failure pattern was clear. The edge showed a rounded, galled deposit of welded material — classic built-up edge. The wear was concentrated at the outer corner, where heat concentrates. The flute showed signs of chips packing.
Three questions uncovered the causes:
- What feed rate were you running? — Very low. The operator had slowed the feed “to be safe” with a difficult material.
- What coolant pressure? — Standard pressure, well below what deep hole drilling needs.
- What tool geometry? — A standard off-the-shelf gun drill, not one ground for stainless.
All three are the mistakes we see most often with this material.
The Fix That Feels Wrong: Go Faster, Not Slower
The instinct with a hard material is to slow down and take it easy. In stainless, that instinct is wrong — and it is the single most important thing to understand about this problem.
Feed too slow is actively harmful. Here is why: stainless work-hardens in the cut. If your feed is so low that the cutting edge shaves just below the surface, the edge keeps cutting into the work-hardened layer it just created. The edge dulls immediately, which makes it cut even less efficiently, which generates more heat and more hardening. It is a death spiral.
The fix was to increase the feed rate enough that the cutting edge bites beneath the work-hardened layer on every pass. Counterintuitively, a more aggressive feed gave longer tool life, not shorter.
Two more changes completed the setup:
- Tool geometry ground for stainless. The gun drill was reground with a sharper, polished flute and a strengthened edge hone. The polished flute prevents chips from welding to it; the edge hone resists the micro-chipping that starts on sharp edges in hard materials.
- Coolant pressure raised, through the tool. Deep hole drilling at this depth lives or dies on coolant. The pressure was raised so the coolant reached the cutting edge with enough force to flush chips before they could pack — and with the right concentration of cutting oil, it also reduced edge temperature and chip stickiness.
The Result
With geometry, coolant, and feed all corrected, the change was dramatic and repeatable:
- Tool life went from 4 holes to over 80 holes per drill — a 20× improvement on the same machine, the same part, the same operator.
- Cycle time dropped by about a third, because the machine was no longer creeping along at a destructive low feed.
- Surface finish stabilized to a consistent Ra, eliminating the rejected holes.
- Scrap all but disappeared, along with the emergency tool ordering.
The customer now runs the job with predictable tooling costs, and the operators know not to “play it safe” with feed rate anymore.
The Takeaway: When You’re Fighting a Material, Talk to Someone Who Fights It Daily
If you recognize this pattern in your own shop — short tool life, rough finish, built-up edge, the urge to slow down and take it easy — the problem is almost never the machine. It is a process that the material is actively fighting. Stainless is not the worst of it — titanium fights the same way, but silently.
Before you order another box of drills, talk to a tooling and process engineer who works with these materials daily. The fix is often a geometry change, a coolant change, and a feed rate that feels wrong at first — and it costs a fraction of the tooling you are burning through.
And if the answer to your stainless problem is a setup you are not sure about, ask us — this is the kind of problem we diagnose every week.