Laser-Cut Tap Holes: Cut to Size or Drill a Pilot?

Laser-cut tap holes split opinion, and the cut edge explains why.

Should Tap Holes Be Laser Cut to Final Size, or Cut as a Pilot and Drilled Before Tapping?

Many shops tap laser-cut tap holes cut to tap-drill size in mild steel, aluminum, and austenitic stainless. For hardenable steels, or where cut edges have broken tools, cut a pilot and drill it out.

Both approaches work, which is why forum answers contradict each other. Some fab shops tap laser-cut holes all day; others have broken drills and taps on laser-cut edges.

Why a laser-cut edge can be hard

The laser melts and re-solidifies a very thin skin along the kerf, and the surrounding metal cools it almost instantly. That self-quench is the main reason some edges come out hard:

• Carbon content. Steels with enough carbon to harden (forum users cite roughly 0.35% and up), plus martensitic stainless, can form a hard layer at the edge. Low-carbon mild steel, austenitic stainless, and aluminum are much more forgiving.
• Process conditions. Some fabricators also blame nitrogen or air assist gas, but that explanation is anecdotal.

Forum posters report the affected layer as only a few thousandths of an inch to around 0.010 in thick. These are anecdotal figures, not test data. Even a thin layer sits right where a tap cuts.

When laser-cut tap holes can go straight to the tap

• Mild steel, aluminum, and common austenitic stainless at modest thicknesses
• Thread sizes where the tap-drill hole is still a clean, round laser feature
• Form (roll) taps, which displace material instead of cutting it. They're less bothered by a thin skin but are sensitive to the hole diameter.

When to cut a pilot and drill

• Hardenable steels, or any time you've had tool breakage before
• Holes that need a tighter diameter or a straighter wall than the laser gives, since thicker plate cuts with more taper
• Tapped holes that must stay perpendicular through thick plate

Keep the pilot clearly undersize, about 1 mm undersize is a reasonable starting point, so the drill removes the whole heat-affected wall instead of skimming it.

Is the sheet thick enough to tap at all?

One sheet-cutting supplier cites a handbook rule of about one bolt diameter of engagement in steel and two in aluminum, or at least three full threads. Thin sheet rarely meets that, which is where self-clinching (PEM) hardware usually makes more sense. See PEM hardware vs tapping.

Related: why laser cutting leaves a tapered edge. Source discussion: Hobby-Machinist on the laser heat-affected zone.

At DigiForge

Our sheet metal service covers laser cutting, press-brake forming, and PEM insertion in Cold Rolled Steel, Galvanized Steel, Stainless 304 and 316, and Aluminum 5052 and 6061 sheet. We list a ±0.13 mm cut tolerance. For tapped holes, call out thread size and depth rather than a hole diameter, and ask us when you quote how the hole will be produced.