Slotting, Pocketing & Profiling

Slotting, pocketing and profiling are the three operations that machine the features of a part’s face with an end mill, the everyday work of the machining centre and the mill. Together they are the 2.5-dimensional operations of milling: they cut shapes whose depth is a constant or stepped value, defined by a two-dimensional outline — the slot, the pocket, the part outline — swept down into the material to its depth. Slotting cuts a slot, a straight channel of the cutter’s width, along the work. Pocketing cuts a pocket, a closed cavity, clearing the material from inside a boundary while leaving its walls and its floor. Profiling cuts a profile, following the outline of a part to cut it out of the stock or to finish its edge. The three share the milling this wiki treats, and they share the end mill that cuts them; they differ in the shape each leaves. This entry sets out how each is done.

Slotting

Slotting is the operation that cuts a slot, and it is the simplest and one of the commonest of the three. The slot is cut by an end mill of the slot’s width, fed along the slot’s line: the cutter plunges or ramps in at the start and travels along the length, its side cutting the slot’s walls and its end cutting its floor, leaving a channel of the cutter’s diameter and the path’s length. A through slot runs from one edge of the work to another, its ends open; a closed slot starts and ends inside the work, so the cutter must enter the material first — plunging straight down if the cutter is centre-cutting, or drilling a start hole and feeding in. The slot’s width is fixed by the cutter, so the accuracy of the slot is the accuracy of the cutter’s diameter and its runout, and its depth is cut in passes when the slot is deep, because the cutter cutting a deep, narrow slot must clear its chips from a confined space — the reason deep slots are cut in steps of depth, with the chips cleared as each step is cut.

Pocketing

Pocketing is the operation that clears a closed cavity, and it is the operation that removes the most material from a face. The pocket is a region bounded by its walls — rectangular, round or any outline — and pocketing removes the material inside that boundary down to the pocket’s floor. The cutter must first enter the pocket: it cannot start on solid material in the middle of the region, so it enters by a ramp, a helical plunge or a pre-drilled hole, and then works through the pocket, cutting the material away in a series of passes that clear the interior while leaving the walls and the floor. The roughing passes remove the bulk of the material, cutting the pocket out in overlapping paths, and leave a little stock; the finishing passes then cut the walls and the floor to size, one pass round the boundary and one across the floor. The pocket’s corners carry a constraint worth knowing: an end mill cuts a concave corner of at least its own radius, so a pocket with sharp corners must have its corners finished by a smaller cutter or by a different operation.

Profiling

Profiling is the operation that cuts the outline of a part, and it is how a part is cut out of the stock it is made from. The cutter follows the profile — the two-dimensional outline of the part — round its contour, cutting with its side; when the profile is an external one, profiling cuts the part free from the surrounding material, the cutter travelling round the outline and leaving the part’s edge, and when the profile is internal it cuts a hole or a recess of the profile’s shape. Profiling is usually done in two stages: the roughing profile cuts round the outline a little outside the final line, removing the bulk of the material; the finishing profile then follows the true outline, cutting the part’s edge to its final size and finish, often in climb milling for the better surface. The finished profile’s accuracy is the accuracy of the cutter path and the cutter’s size, corrected by the cutter compensation the control applies, so the part comes off the machine to its drawn outline.

The three operations together

The three operations are the vocabulary of the flat part, and most parts are machined by a combination of them. A bracket or a plate is profiled to its outline, pocketed for the recesses that lighten it or seat a component, and slotted for the slots that adjust or guide it; a mould insert is profiled round its edge and pocketed or slotted for its features; and the same end mill, in different paths, does all three. They are the operations that the CAM systems generate from the part’s drawing, each a toolpath of a kind — the straight line of the slot, the clearing pattern of the pocket, the contour of the profile — run at the feeds and speeds the cutter and the material allow. And they are the operations that the beginner learns and the shop never stops using: the three ways an end mill machines the features of a face, cutting the slots, the cavities and the outlines that turn a block of material into a part.