What Happens at an Interruption
Interrupted cuts in deep hole drilling happen when the drill passes through a cavity, cross hole, keyway, or any gap in the material. The cutting edge hits the far wall of the cavity, loses contact, and then slams back into the material on the other side. That shock loads the edge and chips it.
I have seen a sharp C2 carbide edge chip on the first interruption of a 10mm hole through a valve body with a 6mm cross hole. The chip happened in the first 0.1 seconds of re-entry. After that, the drill was cutting with a damaged edge and the bore finish was shot.
The severity depends on the gap width and the speed. A narrow gap of 2-3mm at moderate RPM causes minor edge chipping. A gap wider than half the drill diameter at high RPM breaks the tool.
Feed Reduction Strategy
I drop the feed by 30% starting 5mm before the interruption and hold it reduced for 5mm after re-entry. This softens the impact when the cutting edge meets the far side of the cavity.
| Gap Width Relative to Drill Dia | Feed Reduction | Distance Before | Distance After |
|---|---|---|---|
| Less than 25% | 20% | 3mm | 3mm |
| 25% to 50% | 30% | 5mm | 5mm |
| More than 50% | 50% | 8mm | 8mm |
For predictable interruptions like cross holes in valve bodies, I program the feed reduction directly into the CNC cycle using the M-codes that trigger at specific Z depths. The control executes the reduced feed automatically without operator intervention.
For unpredictable interruptions like casting voids, I use the spindle load monitor as a backup. If the load spikes more than 30% above baseline, the control retracts the drill and alarms out. This has saved me from breaking drills on porous castings more times than I can count.
Tool Geometry and Grade Selection
Standard C2 carbide has a sharp edge that handles continuous cutting well but chips on interruptions. I switch to a tougher grade with a larger edge radius.
| Grade | Hardness (HRA) | Edge Prep | Best For |
|---|---|---|---|
| C2 (fine) | 92.5 | Sharp as-ground | Continuous cuts, tight tolerances |
| C5 (medium) | 90.5 | 0.03mm radius | Interrupted cuts, general purpose |
| C6 (coarse) | 89.0 | 0.05mm radius | Heavy interruptions, roughing |
I typically run C5 grade for valve body work with cross holes. The edge wears slightly faster in continuous cutting but survives the interruptions that would destroy a C2 edge. The tradeoff is about 15% shorter tool life on the straight sections, but I avoid catastrophic breakage.
I have also used a micro-edge hone on the cutting corner. A 0.02mm radius at the corner distributes the impact force over a larger area. Every interrupted cut job I run gets a honed edge.
Drilling Speed Adjustments
I reduce spindle speed by 15-20% through the interruption zone as well. Lower speed reduces the kinetic energy of each impact. I keep the speed reduction going until the drill is 3mm past the far edge of the gap.
For example, on a 12mm gun drill running at 2000 RPM in 4340 steel, I drop to 1600 RPM through a 8mm cross hole. The feed drops from 0.04 mm/rev to 0.028 mm/rev. Together, the two reductions cut the impact energy by about 35%.
Drilling Through Multiple Interruptions
Some parts have multiple interruptions along the bore path. A valve body might have three or four cross holes at different depths. Each interruption puts the same stress on the cutting edge.
I handle multiple interruptions by treating each one independently. I reduce feed 5mm before each interruption and hold it for 5mm after. The total cycle time adds about 10-15% over an uninterrupted hole, but the tool survives.
The risk with multiple interruptions is cumulative edge damage. Even if the feed reduction protects the edge at each individual interruption, the edge can fatigue from repeated impacts. I inspect the edge after every 10 parts on multi-interruption jobs.
| Number of Interruptions | Edge Inspection Interval | Expected Tool Life vs Continuous |
|---|---|---|
| 1 | Every 20 parts | 70% of continuous |
| 2-3 | Every 15 parts | 55% of continuous |
| 4-6 | Every 10 parts | 40% of continuous |
| 7+ | Every 5 parts | 25-30% of continuous |
Material-Specific Interruption Behavior
Different materials respond differently to interrupted cuts. The material’s ductility determines how the edge loads when it re-enters the cut.
Steels below 30 HRC are ductile and allow the cutting edge to dig in on re-entry. The edge sees a higher shock load. Steels above 35 HRC are more brittle, and the re-entry shock causes micro-chipping rather than digging in.
| Material | Re-entry Behavior | Feed Reduction Needed | Best Grade |
|---|---|---|---|
| 1018 mild steel | Digs in, shock loads edge | 30% | C5 |
| 4140 (28-32 HRC) | Moderate dig-in | 25% | C5 or C2 with hone |
| 4340 (35-40 HRC) | Micro-chipping risk | 30% | C5 |
| 304 stainless | Work-hardens on impact | 40% | C5 with 0.04mm hone |
| Cast iron | Brittle, chips easily | 20% | C2 with light hone |
| Aluminum | Soft, edge loads moderate | 25% | C2 or uncoated carbide |
I use the largest feed reduction on stainless steels. The material work-hardens instantly at the re-entry point. The harder surface then chips the cutting edge. The reduced feed lowers the impact force and reduces the work-hardening effect.
Coolant Flow Through Interruptions
The interruption also affects coolant flow. When the drill passes through a cavity, the coolant pressure drops because the seal between the drill and the bore wall is lost at the gap.
The pressure drop at the gap can be 30-50% of system pressure. The reduced pressure means less coolant reaches the cutting tip and chip evacuation suffers.
| Gap Width | Coolant Pressure Drop | Effect at Tip |
|---|---|---|
| 2mm | 10-15% | Minor chip evacuation issue |
| 5mm | 20-30% | Chips may pack temporarily |
| 10mm | 30-45% | Significant chip flow reduction |
| 15mm+ | 50%+ | Risk of chip packing and tool damage |
I compensate for the pressure drop by extending the dwell at the bottom of each peck after the interruption. The extra dwell time lets the coolant pressure rebuild and flush any chips that accumulated during the gap crossing.
Key Takeaways
Interrupted cuts are the most stressful condition for a gun drill. I use a 30% feed reduction and a tougher C5 carbide grade with a honed edge. For gaps wider than half the drill diameter, I change the process sequence or bridge the gap with a sacrificial sleeve. The feed reduction and grade selection together make the difference between a re-sharpenable tool and a broken drill.