Gun Drill Peck Cycle Optimization: Depth, Retract, Dwell

A manufacturer drills 8 mm diameter holes 320 mm deep (L/D 40:1) in 304 stainless steel hydraulic valve bodies using standard carbide gun drills. The peck cycle uses a fixed 5 mm peck depth with full retraction to the workpiece surface each peck — cycle time 18 minutes per hole, and stringy chips jam in the flute causing 3% scrap. After optimisation using a decreasing peck depth strategy (4 mm → 3 mm → 2 mm), chip-breaking partial retracts (0.5 mm between pecks, full retract every 10 mm), and 0.3 second bottom dwell, cycle time drops to 9.5 minutes (47% reduction) with consistent chip evacuation and zero scrap.

Why Peck Cycles Matter for Gun Drilling ​

Gun drills operate at high coolant pressure (30–120 bar) with coolant delivered through the drill's internal channel. The coolant flushes chips back along the flute and out of the hole. However, in deep holes (L/D above 20:1), chip evacuation becomes the limiting factor:

A well-optimised peck cycle is the difference between reliable production and frequent tool breakage.

Key Parameters ​

Peck Depth ​

The peck depth is the most important parameter — it controls how far the drill advances before retracting to clear chips.

Guidelines by diameter:

Retract Distance ​

The critical insight: full retract after every peck is the most common mistake in gun drilling. It wastes time and can actually worsen chip evacuation by allowing chips to settle back into the hole. A partial chip-breaking retract between most pecks, with full retract only at set intervals, is more effective.

Dwell Time ​

Dwell at the bottom of each peck serves two purposes:

Tip: If the machine control supports a programmable dwell, start with 0.2 seconds at the bottom of each peck. This is almost always beneficial and adds negligible cycle time (0.2 s × 50 pecks = 10 seconds total). Increase to 0.5 s if chip breaking is inconsistent.

Chip Breaking vs Chip Removal ​

There are two fundamental approaches, often confused:

The hybrid approach (recommended for most gun drilling): use chip-breaking partial retracts for most pecks, with a full retract scheduled every N pecks or every M millimetres of accumulated depth.

Decreasing Peck Depth Strategy ​

The German Trade School Method ​

This widely used strategy progressively reduces peck depth as the hole gets deeper:

Peck 1:  3.0 × D
Peck 2:  2.5 × D
Peck 3:  2.0 × D
Peck 4:  1.5 × D
Peck 5:  1.0 × D
Peck 6+: 0.5 × D (maintain)

For an 8 mm drill:

• Peck 1: 24 mm
• Peck 2: 20 mm
• Peck 3: 16 mm
• Peck 4: 12 mm
• Peck 5: 8 mm
• Remaining: 4 mm each

Why Decreasing Peck Depth Works ​

1. Entry is less constrained — at shallow depths, chips have a short evacuation path and the drill is well-supported by the guide bush
2. Deeper = more friction — at depth, chip friction against the bore wall increases, requiring shorter pecks
3. Coolant pressure drops at depth — pressure loss through the coolant channel reduces chip flushing effectiveness
4. Tool deflection increases — longer unsupported length means the drill is more vulnerable to chip packing

CNC Control Support ​

If the control does not support automatic degression, a custom macro can be written to calculate and execute decreasing peck depths.

Warning: If your CNC control does not support decreasing peck depths natively, do not use a fixed small peck depth for the entire hole. This increases cycle time unnecessarily — the entry section does not need 0.5×D pecks. Instead, program multiple G83 blocks with progressively smaller Q values, or write a parametric macro.

Material-Specific Parameters ​

Note: These are starting points. The optimal peck depth depends on coolant pressure, machine rigidity, and specific workpiece geometry. Always validate with a test hole.

Programming Methods ​

G83 (Full Retract Each Peck) — Simple but Inefficient ​

G83 Z-320.0 R2.0 Q5.0 F0.04

This retracts fully to the R-plane after every 5 mm peck. For a 320 mm hole: 64 pecks × full retract = maximum cycle time.

G73 (Chip Break Only) — Fast but Risky Alone ​

G73 Z-320.0 R2.0 Q5.0 F0.04

Retracts only 0.1–0.5 mm per peck. Fast, but may not clear chips adequately at depth.

Hybrid (Recommended Approach) ​

The most effective method for gun drilling combines both strategies.

Method 1: Multiple G83 blocks with decreasing Q

G83 Z-80.0 R2.0 Q8.0 F0.04   ; Peck 8 mm for first 80 mm
G83 Z-160.0 R2.0 Q6.0 F0.04  ; Peck 6 mm for next 80 mm
G83 Z-240.0 R2.0 Q4.0 F0.04  ; Peck 4 mm for next 80 mm
G83 Z-320.0 R2.0 Q2.0 F0.04  ; Peck 2 mm for final 80 mm

Method 2: Siemens CYCLE83 with degression

CYCLE83(2.0, 0.0, 2.0, -320.0, , 24.0, 4.0, 2.0, 0.3, 0.2, 1.0, 0)
; RTP=2, RFP=0, SDIS=2, DP=-320, FDEP=24, FDPR=4, DAM=2, DTB=0.3, DTS=0.2, FRF=1.0, VARI=0

This starts with 24 mm pecks and decreases by 2 mm each peck until reaching 4 mm minimum, with 0.3 s bottom dwell.

Method 3: FANUC custom macro

A user macro can replicate decreasing peck depth when the control lacks native support:

#1 = 320.0 (total depth)
#2 = 24.0 (initial peck)
#3 = 4.0 (minimum peck)
#4 = 2.0 (degression per peck)
#5 = 0 (current depth)
WHILE [#5 LT #1] DO1
  G83 Z-#5-#2 R2.0 Q#2 F0.04
  #5 = #5 + #2
  #2 = MAX(#2 - #4, #3)
END1

Full Retract Scheduling ​

Schedule more frequent full retracts as depth increases.

Dwell Optimisation ​

Advanced Techniques ​

Torque-Controlled Retract ​

FANUC's small-hole peck drilling cycle (Series 30i) supports automatic retract when spindle torque exceeds a set threshold:

• A torque limit is programmed for each peck
• If the limit is exceeded (indicating chip packing), the drill retracts early
• Feed rate can be reduced on subsequent entry

This is valuable for gun drilling where chip packing can happen unpredictably.

Iterative Learning Optimisation ​

Research by Han et al. (2018) proposes an iterative learning method that:

1. Models chip evacuation torque as a function of peck depth
2. Uses a modified Newton method to find the optimal peck depth for each segment
3. Updates the model based on measured torque from the previous hole
4. Converges to the optimal peck schedule within 3–5 holes

This approach can reduce cycle time by 15–30% compared to fixed-depth pecking.

Retract Speed Optimisation ​

Increasing retract speed reduces cycle time and chip recontact risk:

Ensure the control uses G0 (rapid) for retract and approach segments, not G1 (feed).

Troubleshooting ​

FAQ ​

What is the optimal peck depth for gun drilling? ​

The optimal peck depth decreases with hole depth. Start at 1–3× diameter near the entry and reduce to 0.3–0.5× diameter at depth. A decreasing peck depth strategy typically outperforms any fixed depth.

Should I use G73 or G83 for gun drilling? ​

Neither alone is optimal for deep gun drilling. Use a hybrid: G73-style chip-breaking partial retracts between most pecks, with G83-style full retracts scheduled at intervals (every 5–20 mm accumulated depth).

How much does dwell time affect cycle time? ​

Minimally. A 0.3 s dwell per peck × 80 pecks = 24 seconds total. The benefit in chip breaking reliability far outweighs the time cost.

What is the German trade school peck method? ​

A decreasing peck depth strategy: 3×D → 2.5×D → 2×D → 1.5×D → 1×D → 0.5×D (maintained). This matches chip evacuation difficulty to hole depth.

Can I use a fixed peck depth for the entire hole? ​

You can, but it is inefficient. Fixed peck depth must be set for the most difficult condition (the deepest part), wasting cycle time at shallow depths where deeper pecks would work.

What is the difference between chip breaking and chip removal? ​

Chip breaking uses a small retract (0.1–1 mm) to fracture the chip without full withdrawal. Chip removal retracts fully to clear the flute. Gun drilling benefits from both: chip break between most pecks, full removal at set intervals.

How does coolant pressure affect peck depth requirements? ​

Higher coolant pressure allows deeper pecks because chips are flushed more effectively. At 80 bar, peck depths can be 50–100% deeper than at 30 bar for the same material.

Does peck frequency affect tool life? ​

Yes — excessive pecking (very short pecks, frequent full retracts) increases tool wear because the drill re-enters the cut each peck, causing micro-impact at the cutting edge. Find the minimum peck frequency that maintains chip evacuation.

What dwell parameters are recommended for stainless steel? ​

Bottom dwell of 0.3–0.5 s per peck. Stainless steel produces stringy chips that need the extra time to fracture cleanly. Reduce dwell to 0.1 s if chips are well-broken.

Can I optimise peck cycles automatically? ​

Some modern controls (FANUC Series 30i, Siemens Sinumerik) support torque-monitored retract that automatically withdraws the tool if chip packing is detected. Research systems using iterative learning can self-optimise peck schedules.

Summary ​

Peck drilling cycle optimisation is one of the most effective ways to improve gun drilling productivity and reliability:

• Peck depth — start at 1–3× diameter, decrease progressively to 0.3–0.5× at depth
• Retract strategy — use chip-breaking partial retracts (0.3–0.5 mm) between most pecks, with full retracts only at scheduled intervals
• Dwell time — 0.2–0.3 s bottom dwell improves chip breaking with negligible cycle time impact
• Hybrid approach — combining chip break (G73 style) with scheduled full retracts is more effective than either alone
• Material matters — stainless steel and titanium need 3–5× shorter pecks than steel or aluminum
• The valve body manufacturer in the opening scenario reduced cycle time by 47% and eliminated scrap by implementing decreasing peck depths, hybrid retract strategy, and optimised dwell