July 2009 / Volume 61 / Issue 7|
Parts of the milling machine
By Frank Marlow, P.E.
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. CTEAbout 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.
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