What is a slot?

The walls are a set width apart, and the length exceeds that width. A slot closed all round and running right through is, for example, an elongated hole for an adjusting bolt. The drawing gives width, length or centre distance and depth, and on a guide a tolerance on the width, such as 10 H9. On a prismatic part it is a slot; on a turned part people tend to say groove, though shops use the words loosely.

What does a slot depend on?

Milling a slot at full width puts half the cutter's circumference in the cut: climb milling on one wall, conventional on the other. The cutter therefore deflects differently on each side, and together with runout the slot often ends up wider than the cutter with uneven walls. Chip evacuation is harder too, especially in a deep slot. For a tight width you therefore rough with a smaller cutter and finish each wall separately, or use a trochoidal strategy with low engagement. A closed slot needs plunging with a centre-cutting end mill, ramping, a pre-drilled hole or, in a slot wider than the cutter, helical entry; an open slot can be entered from the side.

How does a slot work in practice?

Width, depth and tolerance together decide the route. A loosely toleranced slot can be cut to width in one pass; a guide at H9 or tighter often needs a separate finishing pass and a check with a gauge or gauge blocks. A T-slot or dovetail needs its own cutter form and a pre-milled slot to start from. A long, narrow slot in a thin part can let the part distort as residual stresses are released. The time a slot takes depends on material, the ratio of depth to width, tooling and machine.

What is a slot often confused with?

A slot is not a pocket: a pocket is a recess with a closed contour of any shape, while a slot has two parallel walls at a set width. Nor is a slot a keyway: a keyway is a slot sized and toleranced to a key standard, for a key between shaft and hub. And on a turned part, a circumferential cut is called a groove.