Millturning | Turning

Henry Stier

Millturning

When turning softer materials, such as low-carbon steels, it is sometimes impossible to break up chips even with special inserts. The resulting "bird's nests" of long stringy chips can cause damage to both the cutting tool and the workpiece. Production interruptions and short tool life are often the result. In fully automatic turning operations, positive chip breaking is essential.

A relatively new manufacturing process called "millturning" has emerged in Europe. It is the use of a milling cutter in place of the single-point tool normally used on a lathe and it assures the formation of short chips.

The powered milling cutter, in the station of a tool turret, rotates against the turning direction of the part as established by the lathe spindle. Thus several milling cutter teeth are cutting in succession and removing short chips.

With different diameters and types of face milling cutters, or by changing the cutting angles, a wide range of part configurations and surfaces may be machined and finished. Surface finishes of Ra 3 to 8 and part roundness of 0.00005-inch can be achieved by this approach with a rigid setup.

Cutting depths of 1/8 to 5/8-inch with small feed rates per revolution are the norm. Feed rates of 3/8 to 2.0 inches per minute are possible with a combined cutting speed of the part and cutter adding up to 800 to 1500 surface feet per minute.

With this combined turning milling approach, rigidity is essential in the entire setup. Both the lathe and milling spindle must be stepless and adjustable so that the turning and milling rotations oppose each other. This means that when the lathe spindle turns in a normal right hand mode, the milling spindle must rotate against the part surface direction.

With this type milling, the chips are thin at the cutting tooth entry, thick at the middle, and thin at the exit. The chip width also varies throughout. As the speed of part rotation increases in proportion to the cutting tool rotating speed, the length of the chip increases proportionally.

Some users in Europe have been able to match all the variables to finish to size in one cut, thus avoiding the roughing/finishing sequence.

The process is especially economical when large amounts of material must be removed and rough forgings are either too expensive or lead times are too long for short runs.

Millturning is a relatively new process, it is not widely known or used in the United States, but the process of millturning shows great promise for the shops that run fully automatic operations.

Henry Stier 4126 Sheppard Drive, Spartanburg, SC 29301