Treatment of Deep Holes Wire Parts of Special Materials

• 1
  What Counts as a Deep Hole in Wire Parts
• 2
  Why Deep Holes Wire Parts Fail
• 3
  Pre-Hole Diameter and Thread Contact Rate
• 4
  Tap Geometry and Coating for Tough Alloys
• 5
  Speeds, Feeds, and Coolant for Deep Holes Wire Parts
• 6
  Which Treatment Fits Which Deep Hole

A hole is deep when its depth passes three times its nominal diameter. A Ø6 mm tapped hole 20 mm deep is already past that line. In wire parts the hole is usually small, the wall is thin, and the hole is often drilled from one side only, so the drill and the tap both work blind.

Two things make these holes different from a normal tapped hole. First, chips and cutting fluid cannot leave the hole on their own; the flutes have to carry them up against gravity. Second, the tap rubs the same wall for a long distance, so heat builds along the full thread instead of at the tip.

Special materials raise the cost of every mistake. Titanium alloys such as TC4 (Ti-6Al-4V) hold heat at the cutting edge and spring back onto the flank. Inconel work-hardens the moment the edge rubs instead of cuts. Magnesium AZ31B and AZ91D cut fast but catch fire if chips pile up dry.

So the treatment plan for deep holes wire parts is really a chip plan and a heat plan. Everything below follows from those two.

The first failure is chip packing. A spiral flute tap pushes chips ahead of it or back up the flutes. In a hole deeper than three diameters there is nowhere for them to go, so they are re-cut. Re-cutting doubles the torque and dulls the edge in a few holes.

The second failure is heat. Cutting speed at the thread crest stays the same, but the contact length is three to five times longer, so more heat enters a small volume of material. In titanium that heat cannot leave through the chip fast enough, and the edge softens at temperatures above roughly 550 °C.

The third failure is tap breakage. Torque rises as the flutes load, and a small tap in a hard material has little margin. When a tap snaps inside a finished wire part, the part is usually scrap, and the removal work costs more than the hole.

The fourth failure is thread geometry. In a deep hole the tap drifts, so the pitch diameter at the bottom is smaller than at the top. A gauge that enters the first five threads may not enter the last five.

Tapping torque falls quickly when the pre-hole is opened up. A slightly larger drilled hole lowers the radial engagement of the tap, cuts heat, and lets the flutes clear chips. The trade-off is thread contact: less material is left on the wall to carry load.

For deep holes in small special-material parts, a common working limit is about 50% of the full thread height on the wall. A normal 75% thread is not required to hold a screw in a ductile alloy, and the extra 25% is what breaks taps.

Pick the drill from the thread contact rate you need, not from a chart alone. For a 1/4-20 UNC thread at 50% contact, the pre-hole lands near 5.4 mm (0.213 in) instead of the 5.1 mm (0.201 in) used for a 75% thread.

Check the minor diameter after drilling, not before. A drill that runs 0.05 mm oversize is normal in titanium, and that shift changes the contact rate more than the chart suggests.

Use a spiral flute tap for blind holes and a spiral point tap only when chips can exit the far side. In deep holes wire parts, the spiral flute is the safer default because it pulls chips back out of the hole.

For titanium and Inconel, choose a tap with a small rake angle and a strong core. A bright finish works for aluminum, but TiAlN or AlCrN coatings last longer in titanium and nickel alloys because they slow diffusion wear at the edge.

Thread forming taps are worth a look in ductile materials. They produce no chips, which removes the packing problem, and the rolled thread is stronger in fatigue. They need a larger pre-hole and more torque, so they suit softer alloys more than hardened steel.

Replace the tap on a count, not on feel. In Ti-6Al-4V, logging 20–30 holes per tap and swapping early costs less than one broken tap.

Run slower than you would in a shallow hole. A starting surface speed of 4–6 m/min (13–20 sfpm) for titanium and 3–5 m/min for Inconel keeps edge temperature in a range where the coating survives. Aluminum can run at 60–100 m/min.

Feed per revolution should match the thread pitch exactly. On a rigid machine with a synchronous tapping head, use rigid tapping; on a long tap, add a small dwell at the bottom before reversal so the tool does not drag.

Coolant has to reach the cutting zone. Through-tool or spiral-flute high-pressure coolant clears chips and drops temperature. Flood coolant alone often misses the bottom of a deep hole, especially when the part is held vertically.

Peck if the material is gummy. A 2 mm peck on a deep aluminum hole clears chips without a full retract. On titanium, a full retract every 3–5 mm is safer than a peck that rubs.