Milling Machines 55143a

MILLING MACHINES INTRODUCTION

Milling machine is one of the important machining operations. In this operation the workpiece is fed against a rotating cylindrical tool. The rotating tool consists of multiple cutting edges (multipoint cutting tool). Normally axis of rotation of feed given to the workpiece. Milling operation is distinguished from other machining operations on the basis of orientation between the tool axis and the feed direction, however, in other operations like drilling, turning, etc. the tool is fed in the direction parallel to axis of rotation. The cutting tool used in milling operation is called milling cutter, which consists of multiple edges called teeth. The machine tool that performs the milling operations by producing required relative motion between workpiece and tool is called milling machine. It provides the required relative motion under very controlled conditions. Normally, the milling operation creates plane surfaces. The first milling machine came existence in about 1770 and was French origin. The milling cutter was first developed by Jacques de Vaucanson in the year 1782. The first successful plain milling machine was designed by Eli Whitney in the year 1818. Joseph R Brown a member of Brown and Sharpe Company invented the first universal milling machine in the year 1861.

Type of Milling Machines : Milling machines can be classified into different categories depending upon their construction, specification and operations. The choice of any particular machine is primarily determined by nature of the work to be done, its size, geometry and operations to be performed. The typical classification of milling machines on the basis of its construction is given below. 1) Column and Knee Type Milling Machine (a) Hand milling machine (b) Plain milling machine (c) Universal milling machine (d) Omniversal milling machine (e) Vertical milling machine 2) Manufacturing of fixed Bed Type (a) Simplex milling (b) Duplex milling (c) Triplex milling 3) Planner type milling machine 4) Special Type Milling Machine (a) Rotary table milling (b) Drum milling (c) Planetary milling (d) Pantograph, profiling and tracer controlled milling machine.

PRINCIPAL PARTS OF A MILLING MACHINE Generally columns and knee type milling machine is considered as typical milling machine. Principal parts of a typical milling machine are described as below. Base: It provides rest for all parts of milling machine including column. It is made of grey iron by casting. Column It is a type of rigid vertical long box. It houses driving mechanism of spindle, table knee is also fixed to the guide ways of column. Knee: Knee can be adjusted at a height on the column. It houses the feed mechanism of the table and other controls. Saddle: Saddle is placed at the top of the knee. Saddle provides guide ways for the movement of the table. Table: Table rests on the saddle. It consists of „T‟ shaped slots for clamping the workpiece. Movements of the table (feed motions) are given in very controlled manner be lead screw. Overhanging Arm: Overhanging arm is mounted on the column and serves a bearing for the arbor. This arm is adjustable so that the bearing may be provided near to the milling cutter. There can be more than one bearing s to the arbor. Arbor: It holds rotating milling cutters rigidly and mounted on the spindle. Sometimes arbor is ed at maximum distance from of overhanging arm like a cantilever, it is called stub arbor. Locking provisions are provided in the arbor assembly to ensure its reliability. Front Brace: Milling Front base is used to adjust the relative position of knee and overhanging arm. It is also an extra fixed between the knee and overhanging arm for rigidity. Spindle: Spindle is projected from the column face and provided with a tapered hole to accommodate the arbor. Performance of a milling machine depends on the accuracy, strength and rigidity of the spindle. Spindle also transfer the motive power to arbor through belt or gear from column. SPECIFICATIONS OF A MILLING MACHINE Along with the type of a milling machine, it has to be specified by its size. Generally size of a typical milling machine is designated as given below: (a) Size (dimensions) of the worktable and its movement range table length  table width =1100 mm× 310mm. Power traverse: Longitudinal ×cross× vertical =650mm×235mm×420mm (b) Number of feeds available. (c) Number of spindle speeds. (d) Total power available.

(e) Spindle nose taper. (f) Floor space required. (g) Net weight. CUTTING PARAMETERS There are three major cutting parameters to be controlled in any milling operation. These three parameters are cutting, speed, feed rate and depth of cut. These parameters are described below. Cutting Speed Cutting speed of a milling cutter is its peripheral linear speed resulting from operation. It is expressed in meters per minute. The cutting speed can be derived from the above formula. V= dn/1000 , where d = Diameter of milling cutter in mm, V = Cutting speed (linear) in meter per minute, and n = Cutter speed in revolution per minute. Spindle speed of a milling machine is selected to give the desired peripheral speed of cutter. Feed Rate It is the rate with which the workpiece under process advances under the revolving milling cutter. It is known that revolving cutter remains stationary and feed is given to the workpiece through worktable. Generally feed is expressed in three ways. Feed per Tooth It is the distance traveled by the workpiece (its advance) between engagement by the two successive teeth. It is expressed as mm/tooth (ft). Feed per Revolution Travel of workpiece during one revolution of milling cutter. It is expressed as mm/rev. and denoted by f(rev). Feed per Unit of Time Feed can also be expressed as feed/minute or feed/sec. It is the distance advances by the workpiece in unit time (fm). Above described three feed rates are mutually convertible. fm= n× f rev ,where n = rpm of cutter. It can be extended further as fm= n× f rev ,= Z × n×ft where z = Number of teeth in milling cutter. Depth of Cut Depth of cut in milling operation is the measure of penetration of cutter into the workpiece. It is thickness of the material removed in one pairs of cutter under process. One pairs of cutter means when cutter completes the milling operation from one end of the workpiece to another end. In other words, it is the perpendicular distance measured between the original and final surface of workpiece. It is measured in mm. MILLING CUTTERS Milling cutters are classified into different categories depending on different criteria as described below : According to the Construction of Milling Cutter (a) Solid milling cutter (b) Inserted teeth cutter (c) Tipped solid cutter

Solid cutter consists of teeth integral with the cutter body, in tipped cutter, teeth are made of cemented carbide or satellite, teeth are brazed to steel cutter body called shank. Inserted teeth cutter are larger in diameter, teeth of hard material are inserted and secured in the shank. According to Relief Characteristics of the Cutter Teeth (a) Profile relieved cutter (b) Form relieved cutter In case of profile relieved cutter, a relief to cutting edges is provided by grinding a narrow land at their back. In case of form relieved cutters a curved relief is provided at the back of the cutting edges. According to Method of Mounting the Cutters (a) Arbor type (b) Facing cutter (c) Shank cutter Arbor type cutters have a central hole and keyways for their mounting on arbor. Milling Shank type cutters are provided with straight or tapered shanks inserted into the spindle nose and clamped there. Facing type milling cutter are used to produce flat surfaces. These are balled or attached to the spindle nose or the face of a short arbor (stub arbor). According to Direction of Rotation of the Cutter (a) Right hand rotational cutter (b) Left hand rotational cutter A right hand rotational cutter rotates in an anticlockwise direction when viewed from end of the spindle while left hand rotational cutter rotates in a clockwise direction. According to the Direction of Helix of the Cutter Teeth (a) Parallel straight teeth (b) Right hand helical (c) Left hand helical (d) Alternate helical teeth Parallel or straight teeth cutter consists of teeth parallel to axis of rotation of the cutter with zero helix angle. In case of right hand and left hand helical teeth cutters, teeth cut at an angle to the axis of rotation of the cutter. Teeth have opposite inclination in both the cutters. Alternate helical teeth cutter has alternate teeth of right hand and left hand helical teeth cutters.

According to Purpose of Use of the Cutter (a) Standard milling cutter (b) Special milling cutter Special milling cutters are designed to perform special operations which may be combination of several conventional operations. Standard milling cutters are the conventional cutters which are classified as given below.

MILLING MACHINE OPERATIONS Following different operations can be performed on a milling machine : (a) Plain milling operation (b) Face milling operation (c) Side milling operation (d) Straddle milling operation (e) Angular milling operation (f) Gang milling operation (g) Form milling operation (h) Profile milling operation (i) End milling operation (j) Saw milling operation (k) Slot milling operation (l) Gear cutting operation (m) Helical milling operation (n) Cam milling operation (o) Thread milling operation

Plain Milling: Operation This is also called slab milling. This operation produces flat surfaces on the workpiece. Feed and depth of cut are selected, rotating milling cutter is moved from one end of the workpiece to other end to complete the one pairs of plain milling operation. Face Milling Operation This operation produces flat surface at the face o the workpiece. This surface is perpendicular to the surface prepared in plain milling operation. This operation is performed by face milling cutter mounted on stub arbor of milling machine. Depth of cut is set according to the need and cross feed is given to the work table. Side Milling Operation This operation produces flat and vertical surfaces at the sides of the workpiece. In this operation depth of cut is adjusted by adjusting vertical feed screw of the workpiece. Straddle Milling Operation This is similar to the side milling operation. Two side milling cutters are mounted on the same arbor. Distance between them is so adjusted that both sides of the workpiece can be milled simultaneously. Hexagonal bolt can be produced by this operation by rotating the workpiece only two times as this operation produces two parallel faces of bolt simultaneously. Angular Milling Operation Angular milling operation is used to produce angular surface on the workpiece. The produced surface makes an angle with the axis of spindle which is not right angle. Production of „V‟ shaped groove is the example of angular milling opration. Angular milling is shown in Figure 1.7. Figure 1.7 : Angular Milling Operation Angular milling cutter Work piece Work table Angular milling 17 Gang Milling Operation Milling As the name indicates, this operation produces several surfaces of a workpiece simultaneously using a gang of milling cutters. During this operation, the workpiece mounted on the table is fed against the revolving milling cutters. This operation is illustrated in Figure 1.8. Figure 1.8 : Gang Milling Operation Form Milling Operation Form milling operation is illustrated in Figure 1.9. This operation produces irregular contours on the work surface. These irregular contours may be convex, concave, or of any other shape. This operation is done comparatively at very low cutter speed than plain milling operation. Figure 1.9 : Form Milling Operation Profile Milling Operation In this operation a template of complex shape or master die is used. A tracer and milling cutter are synchronized together with respect to their movements. Tracer reads the template or master die and milling cutter generates the same shape on the workpiece. Profile milling is an operation used to generate shape of a template or die. This operation is demonstrated in Figure 1.10. Figure 1.10 : Profile Milling Operation End Milling Operation End milling operation produces flat vertical surfaces, flat horizontal surfaces and other flat surfaces making an angle from table surface using milling cutter named as end mill. This operation is preferably carried out on vertical milling machine. This operation is illustrated in Figure 1.11. Work piece Work table Gang milling Gang milling cutter Milling cutter Work piece Form milling Work table Tracer Table Gang milling 18 Manufacturing Processes-III Figure 1.11 : End Milling Operation Saw Milling Operation Saw milling operation produces narrow slots or grooves into the workpiece using saw milling cutter. This operation is also used to cut the workpiece into two equal or unequal pieces which cut is also known as “parting off”. In case of parting off operation cutter and workpiece are set in a manner so that the cutter is directly placed over one of the „T‟ slot of the worktable as illustrated in Figure 1.12. Figure 1.12 : Saw Milling Operation Slot Milling Operation The operation of

producing keyways, grooves, slots of varying shapes and sizes is called slot milling operation. Slot milling operation can use any type of milling cutter like plain milling cutter, metal slitting saw or side milling cutter. Selection of a cutter depends upon type and size of slot or groove to be produced. Right placement of milling cutter is very important in this operation as axis of cutter should be at the middle of geometry of slot or groove to be produced. The operation is illustrated in Figure 1.13. Figure 1.13 : Slot Milling Operation Gear Cutting Operation The operation of gear cutting is cutting of equally spaced, identical gear teeth on a gear blank by handling it on a universal dividing head and then indexing it. The cutter used for this operation is cylindrical type or end mill type. The cutter selection also depends upon tooth profile and their spacing. Gear cutting operation is illustrated in Figure 1.14. Indexing is explained in detail later in this unit. Milling cutter Work piece Milling cutter Saw milling operation Work piece Milling cutter Slot Work piece 19 Milling Figure 1.14 : Gear Cutting Operation Helical Milling Operation Helical milling produces helical flutes or grooves on the periphery of a cylindrical or conical workpiece. This is performed by swiveling the table to the required helix angle, then rotating and feeding the workpiece against revolving cutting edges of milling cutter. Helical gears and drills and reamers are made by this operation. Cam Milling Operation The operation cam milling is used to produce the cam on milling machine. In this operation cam blank is mounted at the end of the dividing head spindle and the end mill is held in the vertical milling attachment. Thread Milling Operation The operation thread milling produces threads using thread milling centres. This operation needs three simultaneous movements revolving movement of cutter, simultaneous longitudinal movement of cutter, feed movement to the workpiece through table. For each thread, the revolving cutter is fed longitudinal by a distance equal to pitch of the thread. Depth of cut is normally adjusted equal to the full depth of threads.