Holemaking is one of the basic operations in metalworking. Think of a finished assembly — from the toaster in your kitchen to a jet airplane. It’s impossible to imagine anything made of metal without counting one to a million holes as part of its manufacturing process.

The art and science of holemaking in a machine shop is a three-step process. First, you have to create the hole, then size it, and then finish it to specifications. These steps — drilling, boring and reaming — require specific tools to achieve the desired results.

First Cut: Drilling

Holemaking begins with drilling a cylindrical hole in a solid workpiece. The machinist uses a drill bit inserted into an electric rotary drill or drill press. The bit does the actual cutting when it is pressed against the workpiece. Rotating action cuts into the material. The drilled hole is smaller in diameter than the ultimate “target” (finished) hole will be. This difference, called allowance, gives working room for the boring and reaming processes to follow.

Drilling can be done to spot a pilot hole, make a deep hole (usually 10 times the diameter of the bit), or center a hole for countersinking.

Right Sizing: Boring

A drilled hole is essentially a rough cut. The second step, boring, widens the hole to approximately its final diameter. Boring is typically done on a jog bore or lathe. Unlike drilling, where the workpiece is stationary and the drill rotates, the opposite is true for boring. A boring bar is mounted on or in a stationary port holder while the workpiece turns on the lathe making contact with the bar. Lubricant is used to keep the piece and the bar cool and to aid in chip removal. It’s important to set the bar securely and at the right distance from the workpiece to avoid chatter.

Boring can widen both blind and through holes. The most critical part of boring is achieving the proper depth to avoid stepping. Boring produces a more concentric hole with smoother sides than a drilled hole.

Smooth and Spot-on: Reaming

Reaming is the finishing process that ensures precision and smoothness in the hole. A multi-edge cutting tool called a reamer is used to precisely widen and smooth (burnish) the hole. The reamer typically provides ideal tolerances and consistency at high penetration rates and small depths of cut. As with boring, coolant is used in the reaming process.

Considerations

All three phases of holemaking require careful planning, taking into consideration, for example, the workpiece material, size and depth of the hole, speed and feed rates, geometry and grade of the cutting tool and pressure and temperature.

Drills, boring bars and reamers can be solid pieces or indexable with attached, replaceable cutting inserts. Application, tool cost and processes determine the type of tool to be used.

So, to review: drilling creates the hole, boring enlarges it and reaming finishes it. If the machinist wants to thread a finished hole, the tapping process comes into play. But that is a topic for another blog.

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