In a continuous cut, a drill's cutting lips are in constant contact with the workpiece — the cutting forces are steady, the chips form and evacuate at a predictable rate, and the drill is stabilized by the friction and support of the hole wall. The system is in equilibrium.
In an interrupted cut, the drill periodically loses contact with the workpiece — one or both cutting lips exits into an air gap, the cutting force drops to zero, and then re-engages on the far side of the gap. This re-engagement is an impact load. The drill wasn't designed for impact — it's designed for sustained cutting. The impact on re-entry is a bending and torsional shock that, depending on the drill diameter, depth, and material, can snap the drill at the point, at the flute runout, or at the web.
The problem is worse than it appears because the gap is usually only visible from the outside of the workpiece. The operator sets up a seemingly straightforward drilling operation and breaks drills without a clear explanation of why.
Cross-drilling through a keyway. A shaft or bore with a keyway machined into it, when drilled perpendicular to the keyway, presents an air gap at the keyway width. The drill crosses open air, re-engages, crosses open air again on the exit side. Each transition is a shock load.
Drilling through a cross-hole. Any previous hole that intersects the drilling path creates an air gap. This is common in hydraulic and pneumatic manifolds, cross-drilled brake components, and multi-passage fittings.
Drilling across a parting line or seam. Cast or forged parts often have parting lines — slight mismatches or gaps between two halves of a mold or die. A drill crossing a parting line experiences a brief discontinuity in cutting resistance.
Drilling into a previously milled pocket. The edge of a milled pocket represents a step — the drill enters full material, reaches the pocket edge, and suddenly one cutting lip is in air while the other is still cutting. This asymmetric load causes drift.
Drilling into an angled surface or existing bore step. When the entry surface isn't perpendicular to the drill axis, one cutting lip contacts before the other. The drill has an unsupported moment at entry that deflects it off center.
The bending stress generated by an interrupted cut is concentrated at the drill tip and web. For a large diameter drill, the cross-section at the web is substantial and the material can absorb the impact load without exceeding the elastic limit. For a small diameter drill — anything under ¼" is in the risk zone, and anything under ⅛" is high risk — the web cross-section is tiny and the same impact load represents a much higher stress ratio.
This is why interrupted cuts in small diameters kill drills reliably while the same condition in ½" or ¾" drills causes increased wear but not breakage. Size matters significantly in interrupted cut planning.
Reduce speed at the interruption point. The re-engagement impact is proportional to the rotational speed at contact. Cutting speed at 50% of normal for the cross-gap passes reduces the impact energy significantly. For manual drill press work, reduce spindle speed before reaching the gap. For CNC, program a feed/speed reduction in the canned cycle or use a separate drilling move at reduced parameters through the gap zone.
Reduce feed at the interruption. A slower feed rate means the drill moves more slowly through the transition zone — less momentum going into the re-engagement. For interrupted cross-holes, some machinists use near-zero feed and let the drill "find" the far side of the gap with minimal axial force.
Fill the gap before drilling. For slots and keyways that are open at the surface, a plug of softer material (aluminum, copper, or even a hardwood plug) in the gap converts the interrupted cut to a continuous cut through dissimilar materials. The plug material drills easily and supports the cutting lip during transition. This is standard practice in toolmaking and precision work involving keyway cross-drilling.
Drill before milling the interruption. In manufacturing sequence planning, drill holes before machining the features that would create an interrupted cut. If you need a cross-hole in a shaft and the shaft will also have a keyway, drill the cross-hole first, then mill the keyway. The order of operations eliminates the problem before it exists.
Use a stub length (screw machine length) drill. Shorter drills are stiffer. A screw machine length drill in a given diameter has significantly more bending stiffness than a jobber length drill. In interrupted cuts, the shorter moment arm of a stub drill resists deflection on re-engagement better than a longer drill of the same diameter.
Use carbide rather than HSS for small diameters. Carbide is more brittle than HSS in general machining — but in small interrupted cuts, the higher modulus of elasticity of carbide (less deflection at a given force) can actually reduce breakage compared to HSS that deflects enough to set up a chatter resonance at the re-engagement. This counterintuitive result depends heavily on setup rigidity and impact severity.
CNC drilling cycles can be adapted for interrupted cuts with modest changes. The key is to treat the interrupted cut as a peck drill through the gap zone:
Program the drill to peck in small increments (0.020"–0.050") as it approaches the gap, then continue pecking through the gap transition. This reduces the drill speed relative to the spindle by adding more retract cycles per unit depth, which reduces the rotational energy at the point of re-engagement impact. It also clears any chips that might pack in the flutes at the gap.
If the CNC controller allows spindle speed override within the canned cycle at specific depth values, a programmed speed reduction of 40–60% through the gap range is more efficient than full peck drilling. For high-volume production with frequent interrupted cuts, this optimization can recover the cycle time lost to conservative feeds.
A drill that has been stressed by repeated interrupted cuts may show chipping or micro-cracks at the cutting edge tips and chisel edge without the uniform wear rounding typical of normal drilling. The damage is concentrated at the outermost corner of the cutting lip — the point of highest stress during re-engagement impact.
If you're finding unexplained drill breakage or short tool life on a particular operation, check whether an interrupted cut is in the drilling path. A quick review of the part cross-section through the hole path often reveals a gap or step that wasn't considered in the original setup.
Chipping at the outer corner of the cutting lip is the fingerprint of interrupted cut stress. Mail in your impacted drills — we'll inspect and regrind the edge geometry on the WinsloMatic.
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