Milling and turning are the two primary operations performed in precision machining. Which one your parts need is dictated by their geometry, (and some will need both.) In this blog we:

• Explain the fundamental principles of these two precision machining technologies
• Discuss why almost all mills and lathes come with CNC
• Summarize their implications for part design and manufacture

Milling vs. Turning

Machining requires a cutting tool to move through the workpiece. A few highly specialized machine tools cut in only a single direction, (broaches in particular,) but rotating either the tool or the workpiece results in higher efficiency.

In milling the workpiece is fixed on a table and a rotating tool cuts away material. Milling machines have a vertical or horizontal spindle that holds the cutter, plus three linear axes. These configurations let the machine produce the straight surfaces required by prismatic parts. The cutter can be replaced with a drill for producing holes in the workpiece and taps for threading.

In contrast, turning entails bringing a cutting tool in against a rotating workpiece held at one end in a chuck or collet. This produces cylindrical parts. A basic lathe has two linear axes for moving the tool parallel and perpendicular to the workpiece axis, plus spindle rotation. (Note that the cutting tool can also move across the end face of the workpiece to produce a flat surface.)

Turning can be performed on internal as well as external diameters, in which case it’s called boring. By mounting a drill in the tailstock, (used to support long workpieces,) it’s possible to drill holes on the axis of workpiece rotation.

The Addition of CNC

Starting in the 1960s, the motorized axes of mills, lathes and other types of machine tools were placed under computer control. This computer numerical control (CNC) lets the machine follow a sequence of movements dictated by a program.

CNC brought about a dramatic improvement in productivity and quality. Productivity rose as multiple operations could be performed in a single setup. Quality improved as a result of increased precision and repeatability. Another major benefit was increased flexibility. Precision CNC machine tools are quickly changed over from one job to the next, making them cost-effective for almost any quantity of parts.

Today, speed, flexibility and precision mean almost every mill and lathe comes with CNC. However, these controllers vary widely in terms of programming methods, processing power and capabilities. When precision machine shop managers and owners shop for new CNC machines, controller evaluation is an important part of the selection process.

Implications of CNC Milling and Turning for Part Design and Manufacture

CNC milling is used to produce flat surfaces on workpieces. These are often those used for mounting, like the top and bottom surfaces of a cylinder head, the open end of a housing, or the base of a bracket. Milling may also be used to cut out a large volume of metal from a solid block, although this is usually only done for prototyping.

In contrast, CNC turning produces cylindrical shapes like valve spools, sleeves and drive shafts. Turning is performed on bar stock, typically cut to length but on some lathes it may be fed through the lathe spindle. Castings can also have cylindrical features turned into them: this needs careful mounting in the chuck.

Milling and turning operations produce very different surface finishes. Turning, which is a single point cutting process, yields a groove not unlike those seen on old vinyl records. Milling is a multipoint cutting process that leaves broad sweeps of parallel grooves across a flat surface.

In both milling and turning, groove size, and hence surface roughness, are influenced by tool tip radius, cutter speed and feed rate. A slower feed reduces effective groove depth, because the grooves overlap. However, smoother surfaces are more expensive to produce because they need more machining time.

Precision Machining Expertise at Impro

When sourcing precision machining services it pays to find a machine shop with modern milling and turning machines, and the ability to fully utilize their capabilities. Impro can offer customers extensive milling and turning capacity, performed on newer machines with advanced CNC controls. If your parts need accuracy and excellent surface finish, contact us today.