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Mechanical engineering |Машиностроение

Lesson 3

Read the text: Milling machines

Milling is the process of removing metal with rotating cutters. The essential features of most milling machines are a power-driven table on which the work is secured, and a spindle carrying one or more multiple-toothed cutters for the production of plane or profiled surfaces, slots or grooves.

   The work is fed slowly past the cutter in such a way that the cutting edges successively come in contact with the work and remove small portions of the surplus metal. While there are many varieties of milling machi­nes, two main types will be dealt with here, the horizontal millet and the vertical miller.

The Horizontal Milling Machi­ne consists of a main casting in which is mounted the spindle and its gear drive, and the feed gear-box. On the front of this casting is a vertical V-guide on which is mounted the knee. The knee is rais­ed or lowered by a telescopic jack screw reacting on the base. A saddle slides from front to back on V-guides on the top of the knee, being moved by a square thread screw operated from the front of the knee. The work table is mounted in V-guides on the saddle, and is moved longitudinally by a square thread screw operated from either end of the table. The table is thus provided with movement in two directions at right angles to each other in the horizontal plane, and with vertical movement relative to the cutter, whose height is permanently fixed. These movements must be true and at right angles to each other, within very close limits, while the longitudinal horizontal movement must be at right angles to the axis of the cutter spindle, and the lateral horizontal movement parallel to it. The horizontal movements are usually hand operated and automatic in both directions, while on most machines the vertical movement is hand operated only. The table is provided with longitudinal T-slots for clamping the work. The table can be swiveled horizontally on its saddle, so that it can be set at an angle to the cutter. This feature converts a plain milling machine to a universal milling machine.

The cutter is mounted on an arbor, and held in the desired position by spacing washers and a locking nut. This enables a variety of cutters to be mounted individually on the same arbor as required, the changeover being affected in a few minutes. If desired, several different cutters may be mounted in the arbor at the same time to perform an operation known as gang milling. The spindle is provided with a taper bore, into which the arbor is inserted and drawn up tightly by a draw bar passing through the spindle. The outer end of the arbor runs in a bearing carried on the overarm, which can be adjusted to suit varying lengths of arbors. For heavy cuts, additional half bearings may be placed close to the cutters to prevent upward deflection of the arbor.

    Milling requires great rigidity, hence the spindle bearings must be tight and the spindle must have no end play. All the slides should be well fitted, and those not in use during a cutting operation should be locked by the locking screws provided. For heavy cuts, particularly on repetition work, additional support may be obtained for the outer arbor bearing by slotted rods bolted to the bearing and the front of the knee pry saddle.                                                     

The Vertical Milling Machine has a vertical cutter spindle, although many types have a swiveling cutter head which swivels in the longitudi­nal plane of the table. The table is similar to that of the horizontal miller, in that it is provided with longitudinal and lateral horizontal movement. In many cases vertical movement is also provided, while in others the cutter head can be raised or lowered to suit varying heights of work.

The types of cutter mainly used are the end mill and the facing mill. These cover most classes of work for which the machine is suitable,- such as milling flat surfaces, profiling, cutting slots, keyways in shafts, and milling sunken surfaces.

Sixteen or more spindle speeds are provided, usually from a constant speed motor through a gear-box, and varying from 20 r. p. m. to 800 r. p. m. Twelve or more automatic feeds for both horizontal movements of the table are usually available, and are specified in inches per minute, vary­ing from ³/₈ in. to 18 in./min.

 

1. Match the left part with the right:

 

2. Complete the sentences with the suggested words: cutter, edges, essential, surplus, remove

The work is fed slowly past _  in such a way that the cutting __ successively come in contact with the word and __small portion of the __metal . The __features of most milling machines are a power-driven table and a spindle carrying one or more multiple-tooled ___.