Carbide vs. Cobalt HSS for Interrupted Cuts: Which Survives

Interrupted cuts are the drill killer. Keyways, slots, crossing holes, welds, stack plates with gaps — any time the drill exits and re-enters material mid-cut, the cutting edge takes an impact load. Carbide handles impact differently than cobalt HSS, and choosing the wrong tool for interrupted conditions is how you turn a 10-second drill cycle into a tool breakage investigation.

What "Interrupted Cut" Means for Drilling

An interrupted cut is any situation where the drill periodically loses contact with material during a cut:

• Drilling across a keyway or longitudinal slot
• Drilling through a stack of plates with a gap between them
• Drilling into a weld — hard spots interspersed with softer base metal
• Drilling near the edge of a feature where the drill momentarily breaks out
• Drilling through castings with voids or inclusions
• Crossing a previously drilled hole at a perpendicular (hole crossing hole)

Each time the drill exits material and re-enters, the cutting edge goes from loaded to unloaded instantly — then back to loaded. This produces a shock load at the cutting edge. How the tool material responds to that shock load determines whether you finish the hole or fish a broken drill out of your part.

Carbide: Hardness Without Toughness

Tungsten carbide is significantly harder than HSS — 1,400–1,600 HV vs. 800–900 HV for HSS. This hardness is why carbide drills last longer in continuous cuts through hard materials and can run at significantly higher speeds. However, hardness and toughness are inversely related in tool materials. Carbide is brittle — it doesn't bend, it shatters.

In an interrupted cut, the impact load at re-entry can exceed the transverse rupture strength of carbide — especially at the cutting edge where the geometry is thin and the stress concentration is highest. The result is edge chipping or catastrophic fracture. A carbide drill that works perfectly through 1,000 continuous holes in alloy steel may break on the 5th hole if that hole crosses a keyway.

Cobalt HSS: Toughness That Absorbs the Hit

M42 cobalt (8% Co) and M35 cobalt (5% Co) are HSS grades with enhanced heat resistance over standard M2 HSS, achieved through the cobalt addition. More importantly for interrupted cuts, they retain the fundamental toughness of HSS — they deform plastically under overload rather than fracturing.

When a cobalt HSS drill hits the shock load of an interrupted cut re-entry, the cutting edge absorbs the load through a combination of elastic and plastic deformation. The edge may wear faster than in a continuous cut, but it doesn't shatter. You get wear instead of breakage.

Comparison Table

When Carbide IS the Right Choice for Interrupted Drilling

The rules aren't absolute. Carbide succeeds in interrupted conditions when:

• The interruption is minor: A small keyway crossing a large-diameter hole — the interrupted portion is brief and the carbide's rigidity controls deflection better than HSS
• The setup is extremely rigid: A boxway CNC machining center with through-spindle coolant and a solid collet hold is a very different environment from a drill press. Rigid setups reduce the vibration that causes chipping
• Solid carbide with chip-breaking geometry: Some solid carbide drills are designed with geometry that manages interrupted entry better — the point angle distributes the impact load
• The material is very hard: In materials above 45 HRC where HSS won't cut effectively, carbide may be the only viable option even with interruptions — run it slow and feed light

Point Geometry for Interrupted Cuts

Regardless of material, point geometry matters in interrupted cutting:

• 135° point angle over 118° — the shallower included angle distributes the re-entry load across a wider area of cutting edge, reducing stress concentration
• Conventional chisel edge over split point — the chisel edge provides a small amount of protection at the center where split points are most vulnerable to chipping on re-entry
• Heavier web thickness when possible — don't thin the web for interrupted cuts; the extra material strength at the center pays off

Speed and Feed Adjustments

In interrupted cuts, regardless of tool material:

• Reduce speed 20–30% vs. continuous cut recommendation — lower peripheral velocity = lower impact energy on re-entry
• Reduce feed 15–20% — lighter chip load at re-entry means less deflection
• Use cutting fluid aggressively — interrupted cuts generate uneven heat cycling that ages tool material faster
• Listen for chatter — any resonance in an interrupted cut will eat the cutting edge; adjust speed to move out of the resonant frequency

The bottom line: save the carbide drills for the long continuous runs through hard material where their speed advantage pays off. For keyway crossings, stack drilling, and casting work, cobalt HSS gives you the toughness to actually finish the hole.