Grooving & Parting

Grooving and parting are the turning operations that cut across the rotating work with a narrow, plunging tool, and they are close kin. Grooving cuts a channel — a groove — into the surface of the work: a recess round the outside diameter, inside a bore, or across a face, cut by feeding a tool of the groove’s width straight into the spinning work. Parting (the part-off) is the same motion taken to its end: a narrow tool fed straight in until it has cut through the work, severing the finished part from the bar. The two share the same tool and the same mechanics — a blade-like cutting edge fed radially into the work, cutting a slot of its own width — and they differ only in depth and purpose: the groove stops at its designed depth and leaves the part whole, while the parting cut goes through and makes two parts of one. This entry sets out how each is done and how the two are made to work reliably.

The narrow tool and the plunge cut

The grooving and parting tool is built for one kind of cut: it is a narrow blade whose cutting edge spans its full width, fed straight into the work along the radius, cutting a slot the width of the blade. Unlike the turning tool, which cuts along the work with its side, the grooving tool cuts only with its end, plunging into the rotating surface; its narrow body and its full-width edge let it enter the work and cut a channel of its own width in a single feed. The tool’s geometry is made for the confined cut: its edges have side clearance so that the blade does not rub on the walls of the groove it is cutting, and its top is ground with a chip-forming shape, because the chip of a plunge cut is narrow and confined and must curl and break to clear the deepening slot. The tool is held rigidly at centre height, and it carries a carbide insert or a high-speed-steel edge ground to the width of the groove to be cut.

The grooving cut

Grooving is a plunge to a set depth, and its working follows the tool’s narrow shape. The tool is fed straight into the rotating work to the depth the groove calls for — the width of the groove is set by the tool, its depth by the feed — and then withdrawn; a groove round the outside of a shaft, a recess at the root of a shoulder, a channel inside a bore, is cut in one careful feed. The limiting facts of the groove are those of any plunge: the tool cuts with its full width in contact, so the cutting forces are high for the tool’s size, the chip is confined in a narrow slot and must be coaxed out, and the tool, cutting at the bottom of a deepening channel, has little room for error. Grooving therefore runs at a moderate speed and feed with the tool fed steadily, and with coolant or air to clear the chip; the finish of the groove’s walls and floor is set by the tool’s edge and the feed of the plunge.

Where grooves are cut

The grooves that a shop cuts are the features that a turned part needs for assembly, and each has its purpose. The O-ring groove is the channel that seats a sealing ring, its width and its bottom diameter sized to the ring so that the seal compresses as it should; the retaining-ring groove carries the snap ring that locates a bearing or a component on the shaft; the oil groove is the shallow channel that carries lubricant along a journal or a bore; and the relief groove is the recess at the root of a thread or a shoulder that lets a mating part or a following tool seat fully. The groove may be cut on the outside diameter — the common external groove — inside a bore with a grooving tool that reaches in, or across a face, cutting a circular recess in the end of the work. In every case the groove is a feature of size and place: its width, depth and position set by the tool and the program.

Parting the part off

Parting takes the plunge to its end: the tool feeds in until it has cut through the bar, and the finished part falls free. The parting cut begins at the outside of the work, at the point where the part is to be separated, and feeds straight in towards the axis, cutting a deep, narrow slot all round the rotating bar; as the tool nears the centre, the remaining metal weakens and finally tears, leaving the part separated with a small pip at its centre, which the subsequent facing operation removes. Parting is the operation that makes a part from a bar in one continuous job — the lathe or the turning centre turns the part, grooves its features and parts it off, and the bar advances for the next — which is why parting is the workhorse of the bar-fed machine, cutting hundreds of identical parts, each parted from the stock that fed it.

Making parting work

Parting is the turning operation that most tests the setup, and its practice is a set of precautions. The parting tool is slender for its depth — it must reach to the centre of the bar, so it is long and thin, and a long, thin blade fed into the work is prone to flex, to chatter and to break, which is why the tool is held with the shortest possible overhang and fed steadily at a speed and feed that keep it cutting rather than rubbing. The tool must be set exactly at centre height — a parting tool set low will ride up and grab the work, a tool set high will push down and dig in — and the work must be held firmly, with the parting point close to the chuck so that the bar does not deflect as the tool cuts through. And the cut needs coolant or a sharp, steady feed to clear the chip from the deepening slot, because a chip packed in the parting slot is the common cause of a broken blade. Set up with care, the parting tool severs part after part cleanly; set up carelessly, it is the tool that breaks, which is why the machinist treats the parting cut with respect.

Grooving and parting in the job

Grooving and parting are the operations that finish a turned part and release it. The part is turned to its diameters, its grooves cut where the drawing calls and its end faced — and then it is parted from the bar, the last operation of its making. On the bar-fed turning machine the cycle repeats part after part: turn, groove, part, and the bar advances. The narrow plunge tool that cuts the groove is the same tool that severs the part from the bar.