Drill Bit Geometry for Left-Hand Drilling: Reversing the Rules

Why Left-Hand Drills Exist

Most machinists never reach for a left-hand drill until they have a broken bolt staring back at them. That is the scenario that sells probably 90% of left-hand drill sets — a snapped fastener, flush or slightly below the surface, and a desperate need to get it out without destroying the surrounding material.

The logic is elegant: left-hand drills rotate counterclockwise. If you are drilling into a broken right-hand bolt, the cutting action of the drill works against the thread direction instead of with it. In many cases, especially when the bolt broke under torque and still has rotational tension stored in it, the drill will catch and spin the broken piece out before you even reach full depth. It is not magic — it works maybe 40 to 60 percent of the time — but when it works, it saves hours of misery with an extractor set.

The second major use case is spindle-down work. Certain machine configurations, notably some horizontal mills and specialized fixtures, run with the spindle oriented so that gravity works against you and a right-hand drill would tend to pull back. A left-hand drill running in reverse creates the right cutting action for the setup. This is less common in small job shops but worth understanding.

How Left-Hand Geometry Differs from Standard RH

Here is where it gets interesting. A left-hand drill is not simply a right-hand drill running backwards. The geometry is mirror-imaged, which sounds simple but has real consequences for how you sharpen and evaluate the bit.

The helix angle flips. On a standard right-hand drill, the flutes spiral upward to the left as you look at the cutting end. On a left-hand drill, they spiral upward to the right. This is the most visually obvious difference and the first thing to check when you pull an unknown bit out of a drawer.

The cutting lips are also reversed. The leading edge on a left-hand drill is on the opposite side compared to an RH bit. This matters when you are checking lip height with a drill point gauge or measuring relief angles. Everything that is standard on an RH bit gets mirrored on an LH bit, including point angle, lip relief, and chisel edge geometry.

Point angles are typically the same — 118 degrees for general purpose, 135 degrees for split-point versions — but the relief must be ground in the opposite rotational direction. This is not something you can fudge. If the relief is wrong, the drill either rubs instead of cuts, or it grabs and chatters. Both outcomes are bad, and one of them tends to snap the drill.

Resharpening Geometry for Left-Hand Bits

Most machinists who try to sharpen left-hand drills freehand end up with scrap. The muscle memory built up from sharpening hundreds of right-hand drills works against you completely. Every instinct about which way to rotate your wrist is backwards.

A drill grinding machine with proper fixtures handles this without drama. The fixture holds the drill at the correct angle and the rotation is controlled by the machine, so the geometry comes out right regardless of which direction the helix runs. This is one of the reasons that machine-sharpened left-hand drills are worth the cost — the geometry is actually correct, which is not something you can guarantee from freehand work unless you have spent significant time training yourself to sharpen both directions.

Key geometry targets for a properly sharpened LH drill:

Point angle: 118 degrees standard, equal on both lips
Lip relief: 8 to 12 degrees, consistent across both lips
Chisel edge angle: 130 to 135 degrees
Web thickness: should not be thinned unless the original was thinned

Left-hand drills are typically run at the same speeds and feeds as equivalent right-hand drills in the same material. The geometry differences are about cutting direction, not cutting aggressiveness. If you find yourself running an LH drill much slower or faster than you would an RH drill of the same diameter, the geometry is probably off.

HSS Left-Hand Drills vs Carbide

Standard HSS left-hand drills are the right tool for the broken bolt extraction scenario. You are usually drilling into mild steel or a stainless fastener, at relatively low speeds, often in a handheld drill or a drill press without great rigidity. HSS handles this fine and costs a fraction of carbide.

Cobalt HSS (M35 or M42) makes sense when you are dealing with stainless steel fasteners or any hardened material. The extra heat resistance matters when you are drilling slowly into tough material, which is exactly the broken-bolt scenario in stainless.

Carbide left-hand drills exist but they are specialty items. They make sense in CNC environments where you are doing production work with a left-hand spindle orientation, not in the typical broken-bolt scenario. Carbide is brittle enough that using it in a handheld drill chasing a broken fastener is a good way to snap the drill and make the situation worse.

For resharpening economics: left-hand drills are worth resharpening in the same size ranges as right-hand drills. Anything 1/4 inch and above is worth the cost of a proper resharpen rather than replacement, especially in cobalt HSS where the raw material cost is significant. The geometry just needs to be handled by a machine that can accommodate the reversed helix — not all grinders are set up for LH work, but a proper drill grinder handles it cleanly.

Practical Notes for Shop Use

Keep left-hand drills clearly separated from right-hand drills. This sounds obvious but mixed storage is how you end up trying to figure out why your drill press is making a terrible sound. Marking the shank with a paint pen or storing LH drills in a separate case solves this immediately.

When using an LH drill for broken bolt extraction, center punch the broken fastener as precisely as you can before you start. The drill needs to start on center or it will walk off the fastener and into the surrounding material. A sharp center punch and a light hammer strike is not optional — it is what makes the difference between the drill catching the fastener and the drill destroying the threads.

Start slow. The goal in broken bolt extraction is not material removal rate, it is catching the broken piece and letting the drill's counterclockwise rotation do the extraction work. High speed generates heat and reduces the chance of the piece spinning out. Low speed, firm pressure, steady feed.

If you have left-hand drills that have been sitting unused and are dull from age or past use, get them resharpened before you need them in an emergency. A dull left-hand drill in a broken bolt situation is worse than no drill at all — it generates heat, work-hardens the material, and tends to wander. Sharp geometry is what makes the extraction technique work.