Turning is a machining process in which the workpiece rotates while a cutting tool removes material. It is principally used to produce cylindrical, conical and faced surfaces, although a lathe can perform a much wider range of machining operations.
Common lathe operations include turning, facing, boring, drilling, reaming, counterboring, countersinking, threading, grooving, knurling and parting. The basic principles of the centre lathe remain relevant to both conventional manual machines and modern CNC turning centres.
Lathes have developed into many forms to suit different component sizes, production quantities and levels of automation. Important types include:
The conventional centre lathe provides a useful basis for understanding the principal elements and motions involved in turning.
The headstock contains the main spindle and provides rotation of the workpiece. Depending on the machine design, the spindle may accept chucks, faceplates, centres, collets and other workholding equipment.
The carriage supports and moves the cutting tool along the bed. The cross-slide provides movement transverse to the spindle axis, while the compound or top slide can provide additional angular adjustment and controlled tool movement.
The tailstock can support long workpieces using a centre and can also hold tools such as drills, reamers and countersinks for axial machining operations.
Lathe capacity is commonly described using dimensions related to the maximum workpiece diameter and length which the machine can accommodate. When assessing whether a component can actually be machined, allowance must also be made for the chuck or fixture, tool access, tailstock and required carriage movement.
The method used to hold the workpiece affects accuracy, rigidity, concentricity and the operations which can safely be performed. Typical workholding methods include:
Long or slender workpieces may require additional support from the tailstock, fixed steady or travelling steady to control deflection and vibration.
| Operation | Purpose |
|---|---|
| Turning | Produces an external cylindrical or tapered surface. |
| Facing | Produces a surface approximately perpendicular to the axis of rotation. |
| Boring | Enlarges or finishes an existing hole using a single-point tool. |
| Drilling | Produces a hole using an axially fed drill. |
| Reaming | Sizes and finishes an existing hole. |
| Threading | Produces an internal or external screw thread. |
| Grooving | Produces a narrow recess in the workpiece. |
| Parting | Separates the finished component or section from the remaining stock. |
| Knurling | Produces a patterned surface by forming rather than conventional chip-removal cutting. |
Traditional single-point turning-tool terminology provides a useful basis for understanding how cutting geometry affects chip formation, cutting forces, tool strength and clearance.
| Term | Description |
|---|---|
| Face | The surface of the tool over which the chip flows. |
| Cutting edge | The edge which performs the cutting action. |
| Flank | The tool surface adjacent to and below the cutting edge. |
| Nose | The region where the principal and secondary cutting edges meet. |
| Nose radius | The radius joining the cutting edges. It influences tool strength, surface finish and the permissible feed. |
| Rake angle | Influences chip flow, cutting forces and the strength of the cutting edge. |
| Clearance or relief angle | Provides clearance between the tool flank and the newly machined workpiece surface. |
| Cutting-edge angle | Describes the orientation of the cutting edge relative to the workpiece and feed direction. |
Modern indexable inserts use additional standardised geometries and designations, but the fundamental functions of rake, clearance, cutting edge and nose radius remain important.
Cutting speed is the relative surface speed between the workpiece and the cutting edge. In turning it is normally expressed in metres per minute (m/min).
For a workpiece diameter D in millimetres rotating at N revolutions per minute, the cutting speed is:
Vc = πDN / 1000
where:
The corresponding spindle speed is:
N = 1000Vc / (πD)
For general information on cutting-tool materials see Cutting Tool Materials. For information on cutting fluids see Cutting Fluids.
Cutting speed is only one of the parameters controlling a turning operation. Feed and depth of cut must be selected with it.
Feed in turning is commonly expressed as the axial movement of the tool per revolution of the workpiece. Increasing feed generally increases material removal rate but also affects cutting force, surface finish and tool loading.
Depth of cut is the radial thickness of material removed in a turning pass. Roughing operations normally aim to remove material efficiently, while finishing operations use conditions selected to achieve the required dimensional accuracy and surface condition.
The surface produced by turning depends on feed, tool geometry, nose radius, cutting-edge condition, machine rigidity, workpiece material and vibration.
A larger tool nose radius can improve the theoretical geometric finish for a given feed, but it can also increase cutting forces and the tendency for vibration on a flexible workpiece or machining system. Consequently, increasing nose radius is not automatically an improvement.
Turning forces can deflect the workpiece, cutting tool and machine structure. This is particularly important when machining long slender shafts, boring deep holes or using tools with substantial overhang.
Deflection can produce dimensional error, taper, poor surface finish and chatter. Where possible, work should be supported close to the cutting region and tool overhang kept to the minimum necessary for access.
The basic mechanics of cutting are common to manual and CNC lathes. Modern CNC turning centres, however, can combine several operations in one controlled machining cycle and may include automatic tool changing, powered tooling, additional axes, bar feeding, probing and automated component handling.
The engineering principles illustrated by the centre lathe therefore remain useful even when the actual component is manufactured on a modern CNC turning centre.
The following external resources were included on the original RoyMech page and provide additional information relating to lathes and turning.