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Guides / Process

Invar Drilling and Tapping: Hole Quality and Thread Practices

September 20, 2026

A drilled and tapped hole is one of the simplest features on a drawing, and one of the more common places for a first-time Invar job to go wrong. The hole itself can look fine and the thread can still bind, gall or strip, because the wall the tap has to cut through is not the same material it was before the drill passed through it.

Why drilling Invar is different

Invar 36 shares the work hardening tendency covered in why Invar work hardens, and drilling concentrates the conditions that trigger it: a small diameter cutting edge, high local pressure, and heat that has nowhere to go because the tool is surrounded by material on every side except the flutes. A drill that is dull, run too fast, or fed too lightly rubs more than it cuts, and the hole wall comes out harder than the surrounding material.

That hardened wall is invisible until the next operation tries to work through it. A reamer following a poorly drilled hole struggles more than it should. A tap sees a wall it was not sized for. Neither problem points back to the drilling step unless someone is specifically looking for it.

Practices that keep the hole wall soft

  • Sharp drills, changed on a schedule. As with turning and milling, waiting for visible wear costs more in Invar than in steel, since wear accelerates once an edge starts to dull.
  • Adequate feed. A feed rate too light for the drill diameter causes rubbing rather than shearing, the same principle covered for turning and milling parameters in Invar 36 machining parameters.
  • Peck drilling on deeper holes. Clearing chips regularly prevents them from recutting and building up heat in the hole, rather than drilling through in one continuous pass.
  • Coolant delivered into the hole, through spindle coolant where available, rather than only flooding the surface around the part.
  • Pilot holes sized correctly for the finish drill or tap, since an undersized pilot forces the next tool to remove more material under higher load than it was designed for.

Tapping Invar

Once a hole is drilled correctly, tapping still needs its own attention. A cutting tap, rather than a forming tap, is the more common choice, since forming taps rely on plastically displacing material around the thread form, which adds mechanical work exactly where the risk of hardening is already highest. Generous cutting lubricant, a tap sized appropriately for the hole diameter, and a controlled, steady feed reduce the torque the tap sees.

Tapping by hand versus on the machine matters less than consistency: whichever method a shop uses, a tap that binds partway through a hole is a sign the hole itself, not just the tap, may be the problem.

Thread inserts as an alternative

For threaded holes that will see repeated assembly and disassembly, or where a stripped thread would be costly to fix, some designs specify a helical thread insert rather than tapping directly into the Invar. This is a design decision more than a machining necessity, but it is worth considering on features that will be unscrewed and rescrewed many times over a part’s service life, since it moves the wear point to a replaceable insert instead of the base material.

What to specify on your drawing

  • Thread callouts with standard sizes where possible, since odd or metric-imperial mismatched threads add tooling cost
  • Thread depth and any blind hole bottom clearance needed for the tap
  • Whether a hole is functional (bolted, load bearing) or just a clearance hole, since functional holes deserve more attention to hole wall condition
  • Any features where a broken tap would be especially costly to recover, since a shop may adjust its process specifically for those holes

Getting it right on the first article

Drilled and tapped holes are a common place for a first article to reveal an issue, since the problem often only shows up under torque, not under visual or dimensional inspection. A shop that machines Invar regularly checks thread engagement and torque on critical holes as part of first article inspection, not just thread gauge acceptance.

Send your drawing through our RFQ form with your thread callouts and any critical holes flagged, and we match you with shops that already account for this in their drilling and tapping process.

Frequently asked questions

Why do taps break more often in Invar than in mild steel?

A hole drilled with a dull bit or the wrong speed work hardens the wall, and a tap then has to cut through that hardened layer instead of the base material. The tap sees higher torque than the hole size would suggest, and that is usually what breaks it, not the alloy itself.

Should threaded holes in Invar use thread forming taps or cutting taps?

Cutting taps are the more common choice, since forming taps rely on the material flowing plastically around the tap, which adds even more work hardening in an alloy that already hardens readily. Shops that tap Invar regularly usually default to a sharp cutting tap with generous lubrication.

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