Why Does Tapping Titanium Need a Different Pecking Strategy Than Mild Steel at the Same Depth-to-Diameter Ratio?

Titanium work-hardens fast and generates heat that doesn't dissipate through
the chip the way it does in steel, so the same peck depth that clears chips
fine in mild steel packs heat and pressure into a titanium tap flute long
before you hit the same depth-to-diameter ratio. You end up pecking shorter
and more often, not because the hole geometry is different but because the
material behaves differently under the tap.

Where the comparison breaks down

Depth-to-diameter ratio is a useful shorthand for "how much chip has to clear
before it jams," but it assumes chip formation and heat transfer are roughly
similar between materials. Mild steel forms a chip that carries heat away
with it and shears relatively cleanly. Titanium alloys, Grade 5 included,
have low thermal conductivity, so the heat generated at the cutting edge
stays concentrated right at the tap rather than moving into the bulk chip and
away from the tool. That heat buildup accelerates work hardening in the
surrounding material. A tap cutting into work-hardened titanium needs more
torque to keep advancing, and that's the failure mode a pecking strategy is
trying to avoid: galling followed by a broken tap.

What that means for peck depth

In practical terms, a peck depth that would be fine for a 3xD hole in mild
steel needs to shrink meaningfully in titanium. Say a steel job pecks every
one or two diameters comfortably. In titanium the flute is doing double duty:
clearing chips and giving the heat somewhere to go before the next cut.
Shorter pecks with a full retract to clear chips and let coolant reach the
hole matter more here than in steel. Skipping the full retract and backing
off only partway defeats the point, since the heat has nowhere to go during
that partial retract.

Coolant delivery matters more here than the peck schedule alone

Through-tool coolant changes this more than it would in steel, since it puts
fluid right where the heat is concentrating instead of relying on flow down
the flutes from the top of the hole. Flood coolant alone doesn't reliably
reach the bottom of a blind hole. If that's what you've got, compensate with
shorter pecks and more frequent retracts than the depth-to-diameter number
alone would suggest.

The practical rule

Don't scale peck depth off a steel chart by ratio alone.

Start conservative, with shorter pecks than you'd use in steel at the same
D/d. Back off the peck count only once the tap is cutting quietly and chips
are clearing clean. If you hear the tap start to load up or feel the spindle
torque climb mid-hole, that's the heat buildup talking, and the fix is
shorter pecks, not more coolant pressure on its own.

Titanium punishes a pecking strategy borrowed wholesale from steel. The
depth-to-diameter number was never the whole story.