Unlike lathes, which have been around for thousands of years, milling machines are less than 200 years old. Because they require much more power than hand-driven lathes, the introduction of milling machines had to wait for the invention of industrial water and steam power. Also, their mechanical components had to first be made available, such as accurately fitted slides, large castings to resist cutting forces, calibrated lead screws and hardened steel cutting tools. Presented here are some of a milling machine’s primary components.
■ The milling machine base provides rigid support for the vertical column and knee castings. A vertical dovetail machined into the column guides the knee on the Z-axis. The vertical column also supports the turret, which in turn supports the ram and head. Most bases and columns on Bridgeport-style machines are a single gray iron casting. The combined rigidity of the structural castings, the tightness of the dovetails and lead screw backlash determine the maximum useful horsepower and largest cut that can be made without chatter.
■ The ram, also called the overarm, enables the cutting head to move in and out in the Y-axis direction. The ram gives the machine greater capacity and flexibility. Minimizing the distance between the vertical column and the cutting head increases overall machine rigidity and reduces the chances of chatter. The turret supports the ram and allows it to move in and out on its ways. It also allows the ram to swing side to side. The ram or turret position is not changed during machining.
■ The knee is a casting that supports the saddle and the milling table and prevents their movement under cutting forces on all three machine axes. After the base and vertical column casting, the knee is the second largest casting. It must support the saddle, the table and the workpiece.
■ The saddle casting has dovetails at right angles to each other that permit milling table movement along both the X and Y axes.
■ The milling table provides a base on which to clamp the workpiece and hold it rigidly against machining forces. Many milling machines provide a stream of coolant, which the milling table traps and recycles.
■ The lead screws, which move the X and Y axes, lie between the upper and lower dovetails on the saddle in the channels, or depressions, made for them. For manual machines, handwheels at the right and left ends of the milling table provide X-axis movement, and the single handwheel on the face of the knee provides Y-axis movement. Another lead screw below the knee provides vertical knee movement. In manual machines, a crank, to give the mechanical advantage required to raise the knee under heavy table loads, turns this lead screw.
■ The purpose of the quill is to support the spindle in ball bearings and guide it vertically. The spindle is bored on its lower end to accept a collet. The spindle holds a collet and the collet holds the cutting tool. A drawbar, which projects from the top front of the milling machine head housing, holds the collet in place. CTE
About the Author: Frank Marlow, P.E., has a background in electronic circuit design, industrial power supplies and electrical safety. He can be e-mailed at orders@MetalArtsPress.com. Marlow’s column is adapted from information in his book, “Machine Shop Essentials: Questions and Answers,” published by the Metal Arts Press, Huntington Beach, Calif.
Related Glossary Terms
- backlash
backlash
Reaction in dynamic motion systems where potential energy that was created while the object was in motion is released when the object stops. Release of this potential energy or inertia causes the device to quickly snap backward relative to the last direction of motion. Backlash can cause a system’s final resting position to be different from what was intended and from where the control system intended to stop the device.
- chatter
chatter
Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.
- collet
collet
Flexible-sided device that secures a tool or workpiece. Similar in function to a chuck, but can accommodate only a narrow size range. Typically provides greater gripping force and precision than a chuck. See chuck.
- coolant
coolant
Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
- milling
milling
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- milling machine ( mill)
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
ARTICLES
