17-05-2012, 04:28 PM
Milling Machines
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A milling machine is a power driven machine that cuts by means of a multitooth
rotating cutter. The mill is constructed in such a manner that the fixed workpiece is fed
into the rotating cutter. Varieties of cutters and holding devices allow a wide rage of
cutting possibilities.
The mills in the Student Shop are vertical milling machines, commonly called
“Bridgeport” style mills. These versatile mills are capable of performing many
operations, including some that are similar to those performed on the drill press like
drilling, reaming, countersinking, and counterboring. Other operations performed on the
mill include but are not limited to: side and face milling, flycutting, and precision boring.
Mills are classified on the basis of the position of their spindle. The spindle
operates in either a vertical or horizontal position. The amount of horsepower the mill is
able to supply to the cutter is also often important.
Mill Construction;
The vertical milling machine is made up of five major groups: base and column,
knee, saddle, table, and head, (see figure). The base and column are one piece that forms
the major structural component of the milling machine. They are cast integrally, ad
provide the mill with its stability and rigidity. The front of the column has a machined
face which provides the ways for the vertical movement of the knee. The knee supports
the saddle and table. It contains the controls for raising and lowering the saddle. Sitting
atop the knee is the saddle which supports the table. The saddle slides in dovetailed
grooves into and away from the machine, providing the mill with its Y-axis movement.
On top of the saddle sits the table. Being moved side-to-side, left-right, over the saddle
furnishes the mill with its X-axis movement. The workpiece is secured to the table
through the use of various types of holding devices.
The head is the most complex assembly in the major parts groups. This contains
the following components:
1. The drive motor and on/off switch.
2. Drive belt, gear train, and range lever selector.
3. Quill, spindle, and draw bar.
4. Quill feed, lock, and digital depth read out (Z-axis).
* The on/off switch is an electrical switch, with positions marked high and low.
This is nothing more than an electrical reversing drum switch. Its position selection is
done to match that of the range lever selector. Changing the range from high to low or
vice-versa is done through the use of the range lever selector and not the on/off drum
switch. This is a static adjustment, and done with the motor turned OFF! Check to be
sure the gears are fully engaged before turning on the motor. Do this by manually rotating
the spindle. Spindle speeds are adjusted with the hand wheel on the right front part of the
head. This is a dynamic adjustment, and done with the motor ON!
Procedures;
Proficiency in milling operations involves more than simply cutting metal. There
are two main categories of procedures when machining on a milling machine: the
preliminary operations, and the machining operations.
Preliminary Operations-
Cleaning- The first, (and last), procedure in any machining operation. Without
clean equipment and tools, the accuracy of the finished product diminishes quickly. The
accuracy, durability, and longevity of the equipment and tools depend on being kept
clean. In today’s high tolerances in engineering, cleanliness is critical.
Set-up- For most jobs performed on a milling machine, setting up the workpiece is
the most difficult, critical, and time consuming part of the job. The workpiece must not
only be securely clamped, but held in such a way so that very surface to be machined will
accurately align with other surfaces when finished. Several types of holding devices are
used in mounting the workpiece o the milling machine.