How to Prevent Chip Packing at the Bottom of a Hole?

Chip packing—also called chip crowding or bird-nesting—is the silent killer of taps. It happens when cut chips have nowhere to go: they compress at the bottom of a blind hole, jam the flutes, spike torque, and snap the tool. In high-volume production, a single packed hole can scrap a part, destroy a tap, and stall a CNC cell for hours.

The good news? Chip packing is almost entirely preventable. Below are the six most effective strategies, backed by data and real shop examples.

1. Choose the Right Tap Geometry

The fastest win is selecting a tap designed to move chips out​ of the hole.

For blind holes, a spiral flute tap​ is the standard solution. The helical flute acts like an auger, lifting chips upward as the tap rotates. A 40° spiral is aggressive; 30° is a balanced choice for steel and stainless.

Real case: A shop threading M10 blind holes in 1045 steel used straight-flute taps. Breakage every 40–60 holes. Switching to a 35° spiral flute HSS-E tap with TiCN coating extended life to 520 holes. Chip packing dropped to near zero.

2. Use High-Pressure Coolant Through the Tap

If your machine supports coolant-through tooling, this is the gold standard. Fluid (80+ bar / 1,160+ psi) blasts chips out of the flutes and off the cutting face.

Real case: In a 3×D blind hole in 4140 alloy steel, a standard spiral flute tap lasted 180 holes. The same tap with 100-bar through-coolant lasted 1,100+ holes. The fluid didn’t just evacuate chips—it cooled the deformation zone and reduced work hardening.

3. Peck-Tapping (Intermittent Retraction)

Peck-tapping means the tap advances partway, then reverses and retracts​ to clear chips before continuing. For blind holes deeper than 2×D, this is essential.

Real case: A customer tapping M16 × 2.0 blind holes at 4×D in 304 stainless saw taps break every 25 holes. Adding a peck cycle (retract every 1.5×D) increased tool life to 140 holes. The extra 4 seconds per hole saved $3,200/month in scrapped parts and replacement taps.

4. Optimize Spindle Speed and Feed

Running too fast is a common mistake. High RPM generates long, stringy chips​ that are hard to evacuate. Slowing down slightly produces short, manageable chips​ (type 6 or 7 on the machining chip chart).

• Aluminum: 60–80 m/min (Vc), peck if >2.5×D
• Steel: 15–25 m/min, peck if >2×D
• Stainless: 8–15 m/min, peck if >1.5×D

Real case: An operator pushed an M8 tap in 316 stainless to 120 RPM. Chips wrapped the tap like wire. Dropping to 80 RPM with a 30° spiral flute produced short C-shaped chips. Tool life jumped from 30 to 200 holes.

5. Verify Pilot Hole Depth and Diameter

In a blind hole, chips need air space​ at the bottom. The drilled hole should be at least 1.5–2 thread pitches deeper​ than the required thread depth. If the drill is exactly at the thread depth, displaced and cut material has nowhere to go—pressure locks the tap.

Also, ensure the drill diameter is correct. An undersized hole increases cutting force and chip volume, making packing inevitable.

Real case: A drawing called for 12 mm thread depth. The programmer drilled exactly 12 mm. The tap broke every time. Drilling to 14.5 mm (adding 2.5 mm) gave chips a compression zone. Breakage stopped completely.

6. Machine Rigidity and Tool Holding

A floating or worn holder causes the tap to enter at an angle. The flutes cut unevenly, producing oversized chips that jam. Use a tension-compression tap holder​ or a rigid synchro holder​ with runout <0.005 mm.

Real case: An older CNC with a standard ER collet holder suffered random tap breaks in blind holes. Switching to a tension-compression holder with 0.003 mm runout eliminated the issue. The tap lasted 3× longer because chips were cut evenly and evacuated cleanly.

Summary: Your Anti-Chip-Packing Checklist

Chip packing is a process problem, not a tool problem. Choose the right geometry, control your speeds, give chips room to escape, and you’ll turn a frustrating cell into a reliable, high-volume threading operation.