Twist Drills and Drill Sizes

The lips cut; the flutes give chips a way out

A twist drill makes a round hole with cutting edges at its point. The lips run from the center region toward the outside diameter. The helical grooves are flutes: they provide space for chips and a route for cutting fluid. The web is the central core between the flutes.

The chisel edge crosses the center of a conventional point. It pushes material aside with substantial thrust. Narrow strips called margins run along the outside of the drill body and help guide it in the hole. The relieved body behind them reduces rubbing. The shank is the portion held by the machine.

Point angle and split point describe different features

118° and 135° are included point angles, measured across the point in an axial view. A 135° point is flatter and shorter than a 118° point of the same diameter. A 118° point is common on general-purpose drills; 135° points are common on drills intended for tougher materials. Select the complete tool for the material and setup, not by angle alone.

A split point has extra grinding near the center to reduce the chisel edge and improve starting behavior. “135°” does not automatically mean “split point.” A split point is also not a guarantee that the hole will be located accurately without a suitable setup.

Read the diameter system before selecting the drill

Read the marking and confirm the cutting diameter when identification is uncertain. A drill index is storage, not proof that someone returned the drill to the correct slot. Use the specified diameter and a checked chart when selecting a tap drill.

Match the shank to the holder and the hole to the job

A straight shank fits an appropriate chuck or collet. A reduced shank is smaller than the cutting diameter; it does not increase the machine's drilling capacity. A taper shank fits its matching taper socket or approved adapter, rather than being gripped as though it were a straight shank.

Choose enough flute length and reach for the hole, with as little unnecessary projection as the setup permits. A drill's nominal diameter does not guarantee a finished hole's size, location or surface finish. Plan the drilling sequence, then choose a finishing operation if the drawing calls for it.