Step Drilling for Deep Holes: Multi-Diameter Strategies

On a dedicated deep hole drilling machine, the tool feeds continuously from entry to exit, stopping only when the hole is finished. On a conventional machining centre, every deep hole is a compromise between chip evacuation, tool deflection, and depth capability — managed through step drilling. The choice of step strategy determines whether a 100 mm deep hole in a 10 mm drill is a routine operation or a broken tool waiting to happen.

Why Step Drilling Is Used ​

When Step Drilling Is Necessary ​

Tip: As a general rule, any hole exceeding 4× diameter in length on a conventional machining centre should use step drilling. Even with through-coolant tooling, the chip evacuation advantage of a peck cycle provides a safety margin against the most common failure mode in deep hole drilling: packed chips.

Step Drilling Fundamentals ​

Terminology ​

Peck Depth Rules ​

The principle: peck depth should decrease as hole depth increases, because chip evacuation becomes more difficult and tool deflection risk increases with depth.

G83 Standard Peck Cycle ​

Format ​

G83 X_ Y_ Z_ R_ Q_ F_ K_

Operation Sequence ​

1. Rapid to X, Y position
2. Rapid to R-plane
3. Feed down by Q (first peck)
4. Rapid retract to R-plane (full chip clearing)
5. Rapid down to (current depth - δ) where δ is typically 0.5–1 mm
6. Feed down by next Q increment
7. Repeat until Z is reached
8. Rapid retract to R-plane

Limitations of G83 ​

G65 P9003 Custom Macro Approach ​

For Fanuc and compatible controls, G65 P9003 is a user-written custom macro for parametric step drilling:

Call Format ​

G65 P9003 T_ D_ Z_ A_ F_

Logic Flow ​

1. Calculate number of steps: #133 = FUP[#26 / #1]
2. Recalculate Z increment for even division: #134 = #26 / #133
3. Loop through G91 incremental pecks
4. Retract after each peck to clear chips

Custom Macro for Variable Peck Depth ​

O9010 (VARIABLE PECK DRILL)
#100 = #7      (PECK AMOUNT - INITIAL)
#101 = ABS[#26] (HOLE DEPTH)
#102 = #6      (RETRACT OFFSET)
#103 = #18     (RETRACT PLANE)
#104 = #100    (CURRENT PECK)
F#9
WHILE [#104 LT #101] DO1
  G1 Z-#104
  G0 Z#103
  Z-[#104 - #102]
  #104 = #104 + #100   (REDUCTION CAN BE ADDED HERE)
END1
G1 Z-#101
G0 Z#103
M99

The key advantage of a custom macro is that the peck increment (#104 calculation) can be modified each cycle to implement variable peck strategies.

Variable Peck Depth Strategies ​

Fixed Step (G83 Equivalent) ​

All pecks at the same depth:

| Peck | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | |:----😐::😐::😐::😐::😐::😐::😐::😐::😐::😐::😐 | Depth increment | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | | Cumulative depth | 5 | 10 | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 |

Progressive Reduction (Constant Value) ​

Each peck reduced by a fixed amount until a minimum is reached:

| Peck | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | |:----😐::😐::😐::😐::😐::😐::😐::😐::😐::😐::😐 | Depth increment | 8 | 7 | 6 | 5 | 4 | 3 | 3 | 3 | 3 | 3 | | Cumulative depth | 8 | 15 | 21 | 26 | 30 | 33 | 36 | 39 | 42 | 45 |

Factor Reduction ​

Each subsequent peck is a percentage of the previous peck:

| Peck | 1 | 2 | 3 | 4 | 5 | 6 | 7 | |:----😐::😐::😐::😐::😐::😐::😐::😐 | Depth increment | 10.0 | 8.0 | 6.4 | 5.1 | 4.1 | 3.3 | 2.6 | | Cumulative depth | 10 | 18 | 24.4 | 29.5 | 33.6 | 36.9 | 39.5 |

Factor: 0.8 per step. Minimum increment: 2.0 mm.

Okuma Variable Method (JP H03111116A) ​

Start with 1 peck at 2× diameter, then 2 pecks at 1× diameter, then 4 pecks at 0.5× diameter:

Initial peck = 2 × D Then halve the increment and double the count at each depth zone

This method is efficient because it automatically matches peck depth to the chip evacuation difficulty at each depth zone.

Warning: The variable peck strategies above assume that full retract to R-plane occurs every peck. If the retract is shortened to a chip-break retract (0.5–2 mm), the peck depth must be reduced proportionally because chip evacuation is less effective without full retract.

Chip Break vs Chip Clear Retract ​

Two-Stage Retract Strategy ​

The most efficient step drilling strategy combines both retract types:

Implementation in Custom Macro ​

#110 = 0      (COUNTER)
#120 = 3      (FULL RETRACT EVERY N PECKS)
WHILE [#104 LT #101] DO1
  G1 Z-#104 F#105          (FEED TO PECK DEPTH)
  #110 = #110 + 1
  IF [#110 GE #120] DO2
    G0 Z#103                (FULL RETRACT TO CLEAR CHIPS)
    #110 = 0
  ELSE
    G0 Z-#104 + #106        (SHORT RETRACT FOR CHIP BREAK)
  END2
  Z-[#104 - #102]           (RAPID BACK TO CLEARANCE)
  #104 = #104 + #100
END1

Multi-Diameter Step Drilling ​

When the final hole diameter exceeds what can be drilled in a single pass at the required depth, multi-diameter step drilling distributes the cutting load:

Pilot-First Sequence ​

When to Use Multi-Diameter Step Drilling ​

Multi-Diameter with Step Drilling (Combined Strategy) ​

For extreme deep holes on conventional machines, combine both strategies:

1. Drill pilot (50% diameter) using peck cycle to full depth
2. Enlarge to 75% diameter using peck cycle (larger pecks, since pilot is already established)
3. Final pass at 100% diameter using peck cycle (smaller pecks due to higher torque)

Feeds and Speeds for Step Drilling ​

Feed Adjustment for Peck Cycles ​

Coolant Considerations ​

Without through-coolant, the coolant must flood the hole entry so that the drill carries coolant down on each peck. Full retract every peck is mandatory in this scenario — chip break retracts are not sufficient because the coolant cannot reach the cutting zone without the tool fully withdrawing.

Programming Examples ​

Example 1: G83 with Fixed Peck ​

G90 G0 X50. Y25.         (POSITION)
G43 Z10. H1 M3 S3000     (TOOL LENGTH COMP, START SPINDLE)
G83 Z-100. R2. Q8. F200. (PECK DRILL, 8 mm PECK)
G80                       (CANCEL CYCLE)

Suitable for: L/D up to 8:1, through-coolant tool.

Example 2: Variable Peck Custom Macro ​

O1000
T1 M6
G90 G0 X50. Y25.
G43 Z10. H1 M3 S3000
G65 P9010 Z-80. D8. R2. F180. K0.5
G80 G0 Z10.
M30

O9010 (VARIABLE PECK MACRO)
#100 = ABS[#26]     (TOTAL DEPTH)
#101 = 10            (INITIAL PECK)
#102 = #101          (CURRENT PECK)
#103 = 0             (CURRENT DEPTH)
#104 = #18           (RETRACT PLANE)
#105 = #9            (FEED)
#106 = 0.8           (REDUCTION FACTOR)
#107 = 2.0           (MINIMUM PECK)
WHILE [#103 LT #100] DO1
  #103 = #103 + #102
  IF [#103 GT #100] THEN #103 = #100
  G1 Z-#103 F#105
  G0 Z#104
  Z-[#103 - 0.5]
  #102 = MAX[#107, #102 * #106]
END1
G0 Z#104
M99

Example 3: Multi-Diameter Step Drilling ​

(PILOT - 6 mm DRILL)
T1 M6
G90 G0 X0 Y0
G43 Z10. H1 M3 S5000
G83 Z-120. R2. Q4. F250.
G80

(ENLARGE - 10 mm DRILL)
T2 M6
G90 G0 X0 Y0
G43 Z10. H2 M3 S4000
G83 Z-120. R2. Q5. F200.
G80

(FINISH - 12 mm DRILL)
T3 M6
G90 G0 X0 Y0
G43 Z10. H3 M3 S3500
G83 Z-120. R2. Q3. F180.
G80

FAQ ​

What is step drilling in deep hole drilling? ​

Step drilling is any strategy where a hole is drilled in multiple passes or depth increments rather than a single continuous feed. It includes peck drilling (drilling a single diameter in multiple depth steps) and multi-diameter step drilling (drilling a pilot hole at a smaller diameter and enlarging it in subsequent passes).

When should I use step drilling instead of continuous feed? ​

Use step drilling when the L/D ratio exceeds 4:1 on conventional machines, or when chip evacuation is unreliable. Continuous feed is only safe when chips can evacuate freely — which requires through-coolant tooling, adequate coolant pressure, and a chip-breaking feed rate.

What is the difference between chip break and chip clear retracts? ​

A chip break retract moves the tool 0.5–2 mm off the hole bottom to fracture the chip without fully withdrawing. A chip clear retract moves the tool completely out of the hole (typically to the R-plane) to allow full chip evacuation. The most efficient strategy alternates between them: chip break every peck, chip clear every 3–5 pecks.

What is G65 P9003 used for? ​

G65 P9003 is a user-written custom macro (on Fanuc controls) that implements parametric step drilling. It accepts tool number, hole diameter, depth, step increment, and feed rate as parameters, then automatically divides the total depth into calculated pecks. It offers more flexibility than the standard G83 canned cycle, particularly for variable peck depth and custom retract strategies.

How should peck depth change as the hole gets deeper? ​

Peck depth should decrease as the hole gets deeper: 1–2× diameter for the first 5× diameter, 0.5–1× diameter for 5–10× diameter, and 0.25–0.5× diameter beyond 10× diameter. Variable peck strategies (progressive reduction, factor reduction, or the Okuma method) automate this.

What is multi-diameter step drilling? ​

Multi-diameter step drilling drills a pilot hole at 50% of the final diameter, then enlarges it in one or more steps (typically to 75%, then 100%). This distributes the cutting load across multiple passes, reduces torque on each pass, and enables deeper holes than single-pass drilling at the final diameter.

Does step drilling require through-coolant tooling? ​

Through-coolant is strongly recommended for step drilling beyond 5:1 L/D. Without through-coolant, the tool must fully retract every peck to allow coolant to reach the cutting zone, which approximately doubles cycle time. The coolant must flood the hole entry so the drill carries coolant down on each re-entry.

How do I program variable peck depth on a Fanuc control? ​

Use a G65 custom macro with a WHILE loop. The macro tracks cumulative depth and reduces the peck increment each cycle. The example in this article shows a macro that starts at a 10 mm peck and reduces by a factor of 0.8 per step until a 2 mm minimum is reached.

What are the risks of step drilling? ​

The primary risks are: chip washing back into the hole on tool re-entry (mitigated by adjusting R-plane clearance), work hardening of the material at the hole bottom from repeated pecking (mitigated by adequate feed rate and sharp tooling), and tool edge fracture from interrupted cutting on re-entry (mitigated by reduced feed rates for step drilling).

When should I switch from step drilling to gun drilling or BTA? ​

When the L/D ratio exceeds 10:1 on a conventional machine, or when production volume justifies dedicated equipment. Gun drilling becomes cost-effective above 300 holes per year at L/D > 10:1. BTA becomes cost-effective above 20 mm diameter and 100 holes per year.

Conclusion ​

Step drilling is the bridge between conventional drilling and purpose-built deep hole drilling equipment. On standard machining centres, three strategies cover the range: G83 peck cycles for simple holes up to 10:1 L/D, custom macro variable peck drilling for holes requiring graduated peck depth and mixed retract strategies up to 20:1 L/D, and multi-diameter step drilling for holes where single-pass torque or chip load exceeds the machine or tool capacity. The most efficient approach combines all three: a pilot hole drilled with variable peck depth, enlarged in progressive diameter steps, using a two-stage retract strategy (chip break every peck, chip clear every third peck) and through-coolant tooling wherever possible. For production volumes that justify dedicated equipment, gun drilling and BTA systems eliminate the need for step drilling entirely — but for the majority of machining operations, step drilling remains the practical solution for every deep hole.