To study about the Milling machine.
Milling is a machining process which is performed with a rotary cutter with several cutting edges arranged on the periphery of the cutter. It is a multiple point cutting tool which is used in conjunction with a milling machine. This process is used to generate flat surfaces or curved profile and many other intricate shapes with great accuracy and having very good surface finish. Milling machines are one of the essential machines in any modern machine shop.
Basic Milling ProcessGenerally, there are two types of milling processes. These are called (a) Up milling or conventional milling process, and (b) Down milling or climb milling process.
Generally, there are two types of milling processes. These are called (a) Up milling or conventional milling process, and (b) Down milling or climb milling process.
In up milling, the direction of rotation of milling cutter and the direction of work piece feed are opposite to each other; whereas in down milling, they move in the same direction at the point of contact of the cutter and the work piece. In up milling, the thickness of chip at the start is nil and is maximum when the cutting teeth leave the surface of the work piece. In down milling, it is vice-versa. In up milling, the cutting teeth try to up root and lift the work piece from the machine table, in down milling, reverse happens. Technically, down milling is a superior process, but up milling is commonly used. Down milling is not used unless the milling machine is fitted with a backlash eliminator.
From Fig. 4.1, basic milling operation can also be understood. The milling cutter is circular and a large number of cutting edges (or teeth) are arranged along its circumference. The cutter is rotated at a speed of N r.p.m. If the cutter diameter is D, then cutting speed at the tip of teeth can be calculated as
π DN meters/minute and it should conform to the recommended values. The depth of cut is clearly shown in the figure and the thickness of the work piece will reduce by this amount in one pass. Usually, the width of the milling cutter is more than the width of the work piece; hence one pass is all that is required.
Feed of the work piece is measured in terms of mm/minute. Actually, the correct measure of feed is movement of work piece per revolution of cutter per teeth. If a milling cutter has z number of teeth and if the table feed is ‘f’ mm/minute, feed per rev per teeth will be f / NZ mm. It should therefore be clear that metal removal rate in milling operation is much higher than in shaping or planing operations.
However, as in shaping or planing operation, the stroke length is always a little more than the length of the job, in milling operation also, the minimum table traverse required is L+ D, where L is the length of job and D is the milling cutter diameter. D /2 are the minimum overlap required on either side of job, so that the cutter becomes clear of the job.
Unlike turning, the milling process involves intermittent cutting and the chip cross-section is not uniform. The high impact loads at entry as well as fluctuating cutting force make milling process subject to vibration and chatter. This aspect has great influence on design of milling cutters.
Rotary movement to the cutters, and feed to the work piece and arrangement for clamping, automatic feed etc.
Milling machines come in three basic models:
Other configurations of the milling machine have been developed for special applications, but above three are most common.
Horizontal Milling Machine
The most common type of milling machine is the horizontal knee type; so called, because of the over-hanging ‘‘knee’’ which can slide up and down the front of the machine and which carries the cross slide and the table. A diagram of the horizontal milling machine is given in Fig. 4.9.
Horizontal milling machines may be either plain or universal type. The main difference between the two is that the table of the universal type is mounted on a turn table and may be swiveled in a horizontal plane. This feature permits the cutting of helix. In addition, the standard accessories provided on the universal machine include a ‘dividing head’ for indexing. There are some other minor refinements, which make the universal horizontal machine very useful for tool room work.
The plain version of the horizontal machine is much more robust and more suitable for production work. In the diagram, the arbor on which peripheral cutters are mounted is not shown. It is fitted in the spindle nose ‘C’ and extends a little beyond arbor supporting bracket ‘B’.
The table of the horizontal milling machine can be given either hand feed or auto feed. It is also capable of being traversed at high speed.
With these features, the machine proves really useful.
Vertical milling machine:The application and technique of this machine differ from that of a horizontal milling machine. It does not have an arbor and instead has a vertical spindle into which the taper shank of end mills and facing milling cutters and drills etc. can be fitted. The arrangement and movement of table are similar to the table of a horizontal milling machine. This machine used for making, flat surfaces, grooves, slots, pockets and guide ways etc. in work pieces using end mills and facing mills etc.
There are many accessories and fittings, which, if provided greatly improve the performance and range of work which can be carried out on a milling machine.
Get all latest content delivered to your email a few times a month.