Peck Drilling Strategy for Long-Chip Materials

Peck drilling is one of the most misused cycles in CNC machining. Many shops apply it everywhere "just in case," with parameters that were set once years ago and never revisited. Others skip it in situations where it is genuinely necessary. The result in both cases is either wasted cycle time or broken drills and scrapped parts.

The core purpose of a peck cycle (G83 in most controls) is chip control. In materials that produce long, continuous chips — primarily aluminum alloys and austenitic stainless steels — those chips pack the flutes and jam inside the hole. A packed flute cannot clear more material. The result is increasing torque, heat, and eventually breakage. Pecking interrupts the chip and gives it a chance to clear.

Understanding Long-Chip Materials

Chip morphology is primarily a function of material ductility and cutting geometry. Aluminum alloys (especially 6061 and 7075) are highly ductile and produce extremely long, continuous chips. These chips curl along the flute but do not break — they grow into long spirals that eventually pack solid in the flute and begin rotating with the drill rather than clearing.

Austenitic stainless steel (304, 316) is also highly ductile and work-hardens at the cutting zone. It produces long, tough chips that are difficult to break even with geometric chip-breakers. The combination of long chips and work-hardening behavior makes stainless one of the most demanding drilling materials from a chip control standpoint.

By contrast, cast iron, brass, and hardened steel produce short, segmented or powdery chips that clear easily. These materials rarely require peck drilling except at extreme depth-to-diameter ratios.

Chip-Breaking Depth (the Q Value)

In G83, the Q value specifies the peck increment — how deep the drill advances before retracting. This is the most important peck parameter. Too large a Q value and the chip grows long before the retract interrupts it — defeating the purpose of pecking. Too small a Q value and you are retracting constantly, wasting cycle time without meaningful benefit.

A practical starting point for aluminum: Q = 1.0 to 1.5 times the drill diameter. For a 0.375" drill, start with Q0.375 to Q0.500. Aluminum chips are long but clear relatively easily due to the low cutting forces, so moderate peck depths are effective.

For stainless steel: Q = 0.5 to 1.0 times diameter. The work-hardening behavior means you want to break the chip more frequently. A 0.375" drill in 304 stainless typically benefits from Q0.200 or less at depths beyond 2×D.

Observe the chips. If chips coming out of the hole are short and segmented, your Q value is working. If chips are long spirals, reduce Q. If the drill is dwelling in the hole longer than necessary, increase Q for efficiency.

Retract Distance

G83 fully retracts the drill to the R-plane (the initial reference position above the hole) on every peck. This full retract clears chips thoroughly but costs time. The alternative cycle G73 (chip-breaking cycle or high-speed peck) retracts only a small increment — typically 0.020" to 0.100" — intended to break the chip without full clearance.

G73 is appropriate for shorter holes in moderate materials where partial retraction is sufficient to break the chip. In deep holes or highly ductile materials, G73 does not adequately clear chips and should not be used. Use G83 (full retract) for stainless, for depths beyond 4×D, and any time chip packing is observed with G73.

The R-plane in G83 affects total cycle time significantly. Setting R too high wastes time on each peck retract and re-approach. Setting it at a reasonable clearance above the part surface (typically 0.100") without excessive standoff minimizes air-cutting time on each retract.

Feed Rate During Peck

Some shops reduce feed rate for peck drilling cycles, reasoning that the interrupted cutting needs more conservatism. This is usually not necessary and always costs time. Peck cycles are already conservative by definition. If the peck depth and retract strategy are correct, running full feed during each peck increment is appropriate and keeps cycle times competitive.

One valid exception: the initial entry pass on the first peck in hardened stainless or work-hardened material. A slightly reduced feed on entry — the first 0.050" of each peck — can reduce the shock load on the drill at re-entry, which is slightly higher than steady-state cutting force. Some controls support this automatically; others require a manual feed-in move before the G83 begins.

Through-Spindle Coolant and Peck Elimination

As noted in our TSC article, through-spindle coolant at adequate pressure continuously flushes chips from the cutting zone, potentially eliminating the need for peck cycles even in long-chip materials. For production environments drilling many identical holes, replacing G83 with a straight G81 drill cycle and TSC is worth testing. Verify chip clearance on the first article — if the hole walls are clean and drill life is maintained, the productivity gain is significant.