Blind-Hole Drilling: The Chip Volume Math That's Snapping Your Drills
Why a drill that runs clean through-hole packs, walks, or snaps the moment the hole goes blind, and the peck-cycle math that fixes it...
Category: Drilling • Reading Time: ~9 minutes • Audience: Intermediate to Advanced Machinists
Most of the shops that snap drills in blind holes are not doing anything egregiously wrong. The SFM comes off a real chart. The chip load matches the tool manufacturer’s data sheet. The peck cycle is copied from a program that ran clean on a similar job. The drill still packs, walks off center, or snaps two-thirds of the way to depth, and the easy conclusion is a bad choice of drills. Not necessarily the case… The variable that predicts blind-hole failure is chip transport against depth, and a lot of folks overlook it before hitting cycle start.
This is the math tool catalogs do not print, because it depends on the hole, not just the drill. Get it right and a standard jobber drill will clear blind holes several diameters deep without drama. Get it wrong and the exact same drill, running the exact same parameters that worked through-hole, will choke before it gets there.
Why Blind Holes Break Drills That Cut Fine Through-Hole
The Evacuation Path Only Gets Longer
In a through-hole, chip evacuation gets easier as the drill approaches breakout. Resistance drops, and chips have a shrinking column of material ahead of them and an open exit close behind. In a blind hole, the opposite happens. Every chip cut at the bottom of the hole has to travel the full depth already drilled to reach the flute opening at the top, and that distance is longest at the exact moment the tool is deepest, most loaded, and least able to absorb a problem.
The Flute Is a Conveyor That Gets Harder to Run With Depth
A drill’s flute is an open channel with a fixed cross-sectional area, set by diameter, web thickness, and helix angle, and it works like a conveyor. As depth increases, the chip column riding in the flute gets longer. Friction against the hole wall builds, coolant reaches the point less effectively, and the force needed to push chips out rises until they stall and pack. Past a certain depth-to-diameter ratio (L/D), transport can no longer keep up, and the flute starts doing something it was never designed to do: hold material instead of moving it.
Torque Climbs Before You Hear It
Once chips start packing against the flute wall instead of exiting, they get re-cut, welded to themselves under heat, or wedged between the margin and the hole wall. Torque and thrust climb sharply, and by the time it shows up as noise, chatter, or a spindle load spike, the tool is usually already past the point where backing off the feed saves it.
The Chip Volume Math
The solid metal removed from any hole is fixed by the hole itself:
Solid volume removed = π/4 × D² × depth
That solid volume does not change no matter how the hole is cut, but the space the chips occupy does. Tightly broken chips may take a few times the solid volume, while long, loose curls can take 10 times or more, and feed and point geometry control which you get. What also changes is how much of it the flute has to carry at once, and that is where peck strategy decides the outcome.
A full-retract peck (G83) withdraws the drill completely after every increment, clearing the chip column out of the hole and letting coolant back in on every cycle. The flute never has to carry more than one peck increment’s worth of chips at a time, regardless of how deep the hole eventually goes.
A chip-breaking peck (G73) only backs off a short “kick,” commonly a few thousandths to a couple hundredths of an inch, enough to snap the chip but not enough to clear the flute. Chips are still being conveyed up the flute during G73, but the chip column gets longer as the hole gets deeper. This is exactly why a G73 program that cuts clean, curled chips to two or three diameters deep can start packing and stalling by five or six diameters on the identical drill, speed, and feed. Nothing about the cutting action changed. The chip column simply got long enough that friction against the hole wall and poor coolant access stalled the chips.
Why G73 Works Until It Doesn’t
A chip-breaking peck interrupts the chip. It does not clear the flute. As the chip column gets longer, friction builds and coolant reaches the point less effectively, and once the chips stall, the next peck packs instead of cutting. Switch to a full-retract cycle before you reach that depth, not after the drill tells you.
Tool Geometry That Changes the Math
Point geometry (a 135-degree split point versus a standard 118-degree point) affects thrust at initial engagement and self-centering, but it does not change flute capacity, so it will not fix a packing problem on its own.
Flute design does. Parabolic, deep-hole flutes are cut with a wider, more open channel specifically to push the L/D where packing starts further out than a standard jobber-length drill allows. A standard jobber drill is a reasonable choice to roughly four to five diameters deep in most steels. Past that, the flute geometry itself becomes the limiting factor, not the peck cycle.
Web thickness works against you here. A thicker web, often added for rigidity on shorter production drills, shrinks the open flute channel and lowers the L/D where packing starts, even though the drill looks identical in diameter and length class.
Coolant-through drills change the equation entirely. Instead of relying on flute geometry to carry chips out by contact, a coolant jet at the point physically ejects them. A coolant-through drill with adequate pressure can often run a continuous or lightly pecked cycle to depths that would pack a standard drill within the first few pecks.
Peck Strategy by Hole Depth
| Hole Depth (L/D) | Recommended Strategy | Peck Increment | Notes |
| Up to 2 x D | Continuous, no peck required | N/A | Most materials clear on their own at this depth |
| 2 x D to 4 x D | Chip-break peck (G73) | 0.5 to 1.0 x D | Short-chipping materials (cast iron, free-machining steel, brass) can push toward the upper end before switching |
| 4 x D to 8 x D | Full-retract peck (G83) | 0.5 to 0.75 x D | Standard for mild and alloy steel, stainless, and most blind-hole production work |
| Beyond 8 x D | Full-retract, reduced increment, or coolant-through drill | 0.25 to 0.5 x D | Standard jobber-length drills are marginal here; parabolic or coolant-through geometry is worth the tooling cost |
Ductile, stringy-chip materials push the switch to full-retract earlier than these ranges suggest. Chapman’s SFM, RPM, and chip load guide covers material-specific speed and chip load targets; use those chip load values to calculate the actual chip volume per revolution for your material and diameter before assuming a peck strategy from a different job will carry over.
Coolant and Evacuation Strategy
Through-coolant pressure has to be adequate to physically eject chips, not just cool the tip. Many machining centers deliver 300 psi or better through the spindle, and dedicated deep-hole drilling systems run well beyond that. If the drill is coolant-through but the program never turns the coolant on before the plunge, none of this matters.
A brief pause at the retract plane lets coolant flush the flute before the next plunge. Too long a pause just burns cycle time without added benefit; a fraction of a second is usually enough once coolant pressure and flow are correct.
On tooling without coolant-through capability, an air blast at full retract is the fallback. It is weaker than flood or through-coolant evacuation, and it pushes the safe L/D back down from the ranges above. Coolant chemistry and delivery matter as much here as tool geometry; see Chapman’s coolant guide, Is Your Coolant Killing Your Tools?, for the broader strategy.
Reading the Failure Before It Happens
Chip color and shape are the earliest warning. Chips that come out discolored, crushed, or powdery (rather than cleanly broken or curled) mean recutting is underway.
A change in sound between the plunge and retract stroke, a higher pitch or more resistance on the way in, usually means chips are already dragging against the flute wall instead of clearing it.
A thrust or torque spike on the spindle load meter, even without an alarm threshold tripped, is typically the last warning before the tool snaps. Treat it as a stop signal, not a data point to note and continue past.
A Practical Checklist Before You Run
- Calculate actual L/D for the hole being drilled, not the drill’s rated length
- Confirm peck strategy: chip-break (G73) only below roughly 4 x D in short-chipping materials, full-retract (G83) beyond that or in ductile materials
- Set peck increment from the table above as a starting point, then adjust to actual chip form
- Verify coolant-through pressure and flow are actually commanded in the program, not just plumbed to the spindle
- Match drill geometry (parabolic flute or coolant-through) to any hole beyond 6 to 8 diameters deep
- Watch first-article chip form before running the job unattended
Chapman Can Help
Solving blind-hole drilling is half tool geometry and half peck strategy, and getting one right without the other still breaks drills. Chapman stocks coolant-through and parabolic-flute drills from Dormer Pramet, Mitsubishi Materials, and Garr Tool, all built with the flute capacity to carry chips out of deep blind holes instead of packing them. Our team can help match drill geometry, peck strategy, and coolant delivery to the actual L/D you are running, not just the diameter on the print.
Browse our CNC Cutting Tools catalog at shop.wcchapman.com, or call us at 410.686.6860 to talk through your application with a member of our team.