Left-Hand Drill Bits: What They Are, When to Use Them, and How They Work

Left-hand drill bits look like normal drills at first glance. Same twist, same point angle, same shank. The difference is that the flutes spiral in the opposite direction — counterclockwise when viewed from the point — and the drill cuts when the spindle runs in reverse. That single design inversion makes left-hand drills uniquely useful in specific situations where a standard right-hand drill makes the problem worse instead of better.

The Geometry

A standard right-hand twist drill has flutes that spiral clockwise (left to right as you look at the shank). The drill turns clockwise when viewed from the shank end — the same direction as a right-hand screw thread. A left-hand drill mirrors this: counterclockwise flute spiral, cutting in counterclockwise rotation.

The cutting geometry is otherwise identical: same point angles, same relief angles, same lip configuration. A left-hand drill drills a hole just as effectively as a right-hand drill, provided you're running the spindle in the correct (reverse) direction. The hole quality, diameter accuracy, and cutting efficiency are the same.

The Primary Application: Extracting Broken Fasteners

The reason most shops keep left-hand drills in stock is for broken bolt and stud extraction. When a bolt snaps flush or below surface, the standard approach is to drill down the center and use an extractor. The problem: if you use a right-hand drill to pilot the extractor hole, the drill's tendency to "walk" and the slight biting action of the drill's rotation can actually tighten the remnant in the bore — driving it deeper and making it harder to extract.

In practice, a significant percentage of broken fastener jobs finish at the drilling step when a left-hand drill is used. The drill catches the remnant, the counter-rotation unscrews it, and the job is done. This saves the time and risk of a separate extractor operation — extractors are easy to break and very difficult to remove if they break in a bore.

Procedure for Broken Fastener Removal

1. Center punch the broken fastener as close to center as possible.
2. Start with a small left-hand drill — 1/8" or slightly under the extractor's pilot diameter — running in reverse at low speed.
3. Apply moderate pressure and watch for any rotation of the fastener remnant.
4. If the remnant begins to turn with the drill, maintain pressure and let it unscrew out.
5. If it doesn't unscrew, continue drilling the pilot hole, then use your extractor.

The left-hand drill doesn't guarantee success on every broken fastener, but it provides a meaningful chance at a faster resolution before committing to the extractor step.

Other Applications

Drilling Near Left-Hand Threads

In assemblies with left-hand threaded components, drilling operations nearby can be complicated if you're concerned about inadvertently backing out a nearby fastener. Running the drill in reverse (left-hand drill) keeps the cutting torque consistent with the thread engagement direction of left-hand fasteners, reducing risk of unintended loosening.

CNC Operations With Specific Climb/Conventional Requirements

In some CNC setups involving simultaneous turning and drilling, left-hand drills allow the spindle to run in M04 (reverse) direction during a drilling cycle, which is required in certain multi-turret configurations where spindle direction affects synchronization.

Drilling Through Left-Hand Threaded Materials

Some exotic situations — drilling through a hex nut with left-hand threads, for instance — benefit from left-hand drills simply to avoid the drill pulling itself into the thread in an uncontrolled way.

What Left-Hand Drills Are Not For

Left-hand drills are specialty tools. They're not a general-purpose substitute for right-hand drills, and they shouldn't be used in standard right-hand spindle drilling operations. Attempting to use a left-hand drill in a forward-spinning right-hand drill press produces a cutting edge that's running backwards — the drill will rub, not cut, and will damage the edge rapidly.

Keep left-hand drill sets separate and clearly labeled. A common shop problem is left-hand drills accidentally mixed into the general drill supply and then used incorrectly.

Sizes and Availability

Left-hand drills are available in most standard size series — fractional, wire gauge, and metric — though availability decreases significantly below 1/8" diameter. Standard jobber length is the most common. Long-series left-hand drills exist but are specialty items. Most shops maintain a set from 1/8" to 1/2" in 1/32" increments, which covers the vast majority of broken fastener work.

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