Machine tool basics--Part 1

This article series is not a comprehensive explanation of manufacturing equipment, but a once-over-lightly discussion of how the major chipmaking machine tools, and supporting elements function. We hope they will help two groups of people: The first consists of recent manufacturing engineering graduates who might need more information about the tools of their trade. The second group are those who have worked in nonmanufacturing fields, but are transferred to a position of responsibility in manufacturing, and find themselves challenged when it comes to knowledge of industry basics.

Turning, drilling, and milling are the three basic manufacturing techniques that use a tool to remove metal. Recently there has been a move to multitasking machine tools in which a single machine performs all three functions, plus grinding in some cases. These multifunction machines can work on a variety of parts and carry out more operations in a single setup. Despite this blurring of distinctions among machine tools the basic operations are still unchangedThis process uses the relative motion between a rotating, multiedge cutter and a workpiece to generate both flat and curved surfaces. During rotation, each tooth of the cutter alternately enters and leaves the cut, removing small amounts of material (chips). Called interrupted cutting, it results in more mechanical shock to the part and tool than occurs with continuous cutting, which is characteristic of turning.

Milling is done by a variety of machines. Simple stand-alone machines perform a minimal number of functions and are chiefly used in job shops. Machining centers have a wider variety of tools and sometimes have additional live spindles for drilling, turning, and even grinding. Milling functions can also be a major part of transfer lines. In these high-volume applications milling is often simple and repetitive.

The capabilities of a milling machine or machining center are measured by motor horsepower, maximum spindle speed, spindle taper size (which determines the size of the toolholder and tool), worktable size, and the amount of cutting tool travel.

Much toolmaking, prototype machining, and low-volume milling machining is done on small, lightweight, vertical spindle ram-type machines called knee mills. They are rarely used for production work.

This design includes a knee-and-column support for the machine table, hence the name, "knee mill." Base and column are one piece and the knee travels vertically on the column. The knee supports the saddle and table. The saddle moves in-and-out from the column, and the table moves side-to-side.

The ram atop the column supports the head and provides horizontal motion in-and-out from the column, parallel to the saddle movement.

At the front of the ram is the milling head, with motor, toolhead, speed-and-feed controls, quill, and spindle. The nonrotating quill holds the rotating spindle and allows the spindle to be fed on its own axis. Tilting the spindle axis allows milling or drilling at an angle to the table. Cutting tools are held in drill chucks or in collets, that are, in turn, held in the spindle, or mounted directly in the spindle.

On manual mills, the operator sets the machine parameters for each cut, positions the tool for the start of the cut, directs all of the machine's motions manually, and changes the settings and tools after each operation.

The two main types of machining centers are the vertical spindle machining center (VMC) and the horizontal spindle machining center (HMC). A third, less-- common type is the universal machine, which is capable of both vertical and horizontal spindle orientations.

Verticals may be preferred over HMCs when working primarily on a single work face. When a rotary table or indexer is added to the VMC machine table, more than one side of a workpiece or a multiple-part setup can be machined without operator intervention. Rotary devices either index the part to present a new work surface to the spindle, or they rotate it slowly, under full CNC control, while it's machined.

There are the three linear axes:

X defines side-to-side table motion,

Y defines in-and-out table motion,

Z defines head movement up and down the column,

B and C may be added as rotary axes, usually as a rotating spindle or mounting table.

T-slots in the machine table are still the primary means of holding work and workholding devices to a machining center table.

Because an HMC's spindle is orientated horizontally, it may be preferred for heavy, boxy parts. Chips fall out of the way better on a horizontal machine, and more workholding and automation options may be possible than on a vertical machine.

The HMC table typically rotates to expose four sides of the workpiece, or fixture, to the tools.

Tombstones, commonly used on HMCs, come in a wide variety of configurations to hold multiple parts. The part program is written to machine all parts on the tombstone before shuttling it out of the machine.