GlossaryKind of term: Manufacturing termGrooving and parting off
What are grooving and parting off?
Grooving cuts a groove into a rotating workpiece with a narrow tool; parting off is the same move taken to the centre, separating the part from the bar.
The tool plunges straight in; typical grooves are those for an O-ring or a circlip and the relief behind a thread. Grooving can be radial, into the diameter, or axial, into the face. Parting off is the same move taken past the centre: the part comes free of the bar. Both are turning operations, done on a lathe, a Swiss-type machine or a mill-turn centre.
What limits grooving and parting off?
In parting off the diameter goes to zero. Under constant surface speed the spindle speed rises until it reaches the set limit; after that the cutting speed falls, to zero at the centre. Without constant surface speed it falls all the way in. The blade is narrow and has a long overhang, so stiffness, chip evacuation and coolant decide how fast it can go. An axial grooving tool must match the diameter of the groove in the face. Without a sub-spindle a small pip can remain at the centre of the parted face; an insert with an angled edge reduces it, but can leave the face less flat. With a sub-spindle holding the part, the back face can be finished in the same cycle.
How do grooving and parting off work in practice?
Parting off counts in both material and cycle time. In a common estimating convention, material per piece is the part length plus the parting width plus an allowance to clean up the face. At the end of each bar there is a remnant too short to clamp, which is spread over the pieces from that bar. On an expensive alloy or a short part the parting width weighs relatively heavily. A narrow groove, a groove with a bottom radius or a deep face groove may need a tool of its own.
What are grooving and parting off often confused with?
Grooving is not the groove: the groove is the feature on the drawing, grooving the operation that makes it. Sawing is not parting off: a saw cuts a slug from the bar before turning, while parting off separates the turned part on the machine.
How do you see grooving and parting off in Blake?
Turned work sits in Blake with material and machining as separate lines, per piece and per batch, using the hourly rate of the lathe. If the parting width or the bar remnant is not in the material line the way you count it, you adjust the line and give the reason. More on turning.
Frequently asked questions
How do you allow for parting width in the material?
Per piece, add the part length, the width of the parting tool and a small allowance for the face. Then spread the bar remnant that can no longer be clamped over the pieces per bar. On expensive alloys and short parts the difference shows.
Why is parting off so slow?
Because the cutting speed falls towards the centre once the spindle reaches its speed limit, and because the blade is narrow with a long overhang. So you feed slowly, and often slower still in tough material, to avoid chatter and breakage.