CHAPTER 12

Milling, shaping and grinding

12.1 Introduction Milling is a process in which the work piece is advanced with a suitable feed rate to a multiple point cutter rotating at comparatively high speed, with the cutter axis of rotation in a fixed position.

The rotational axis of the milling cutter may be horizontal (i.e. parallel to the machined surface), fig.12.1. This is called plain milling. Plain milling is usually carried out on a horizontal milling machine, fig 12.3a.When the rotational axis of the milling cutter is perpendicular to the machined surface, the process is called face milling or end milling, fig 12.2 Vertical milling machines are the machines mostly used for this type of milling, fig 12.3b

Plain Milling face Milling or end Milling fig.12.1 fig12.2

12.2 Milling Machines

Fig.12.2 shows the basic features of a horizontal milling machine, while fig.12.4 shows the basic features of a vertical milling machine.

Fig 12.3 (a) Schematic illustration of a horizontal-spindle column-and-knee-type milling machine. (b) Schematic illustration of a vertical-spindle column-and-knee-type milling machine. Source: G. Boothroyd, Fundamentals of Machining and Machine Tools.

12.3 Milling Cutting MechanismIn milling each tooth on a tool removes part of the stock in the form of a chip.• There are two types of cutting actions,Peripheral - The teeth at the periphery do the cuttingFace - The teeth on the face of the cutter remove metal.

• The basic interface between tool and work is pictured below. This shows a peripheral milling tooth.

Fig. 12.4 Up cut (conventional ) milling

Up-Cut MillingThe milling method shown above in fig. 12.4 is called up-cut (or conventional) milling. In this case the tableis moving towards the cutter, opposing the cutter direction. The basic steps of chip cutting hereare,1. As the tooth makes contact with the surface, the tooth begins to push down. As the tooth continues to turn, it reaches a point at which the pressure has built up to a high level, and the tooth begin to dig in.2. As the tooth starts to dig, it cuts down, and the metal chip begins to shear off.3. The tooth continues to cut the chip off, until it reaches the surface of the material. At this point the chip breaks free, and the cutting forces drop to zero. Because the cutter does not start to cut when it makes contact, and because the advance moves

Down-Cut MillingWhen the cutter rotation is in the same direction as the motion of the work being fed, it is referred to a Down-cut, or climb milling. When this cutter makes contact with the work, it must begin cutting at the maximum torque. As a result, a back-lash eliminator must be used to take play out of the system.• This method has advantages,

Fig. 12.5 Up cut and

down cut milling

- The cutter forces are directed into the table, which reduces

fixture forces, and allows

thinner workpieces- There is less radial pressure on the arbor- Better surface finishes obtained because there is no “dig-in”

12.4 Types of Milling Cutters 1) plain and face Milling Cutters

Fig12.6 shows types of milling cutters.

a) Plain milling cutters have helical or straight teeth; they are used for roughing and finishing of plain surfaces on horizontal machines.

Fig. 12.6 Different types of milling cutters

b) Interlocking plain milling cutters with helical teeth opposite each other to compensate for axial forces. Suitable for heavy cuts.

c) Face milling cutters have teeth on both the periphery and on one end. For milling plane surfaces and shoulders on horizontal and vertical machines.

2) Side Milling Cutters

a) Saws for parting off and for narrow slots. Straight teeth slotters for shallow slots.

b) Side milling cutters with teeth on three sides for deep slots.

Fig12.7 Side milling cutter

3) End Milling Cutters

a) End mills with left or right hand spiral for finish machining.b) T-slot cutters for milling. T slots.c) Two teeth end mills for rough milling of slots.

Fig12.8 End milling cutter

4) Profile Milling CuttersFig. 12.9 illustrates different types of profile milling cutter

a) Convex form milling cutter.b) Gear milling cutters.c) Gang milling cutters.d) Angle milling cutters for V grooves.e) Dove-tail milling cutters.f) Single edged cutter for small profiles .

Fig. 12.9 Profile milling cutter

12.5 Milling Machine Operations END MILLING OPERATION: as pictured in fig. 12.10

Milling cutter used is called end mill End mill resembles a drill bit in shape but having no pointed edge Used for removing material from sides of work piece and for cutting

complicated profiles

Fig. 12.10 End milling operation

Plain Milling Operation: as shown in fig.12.11 It is used for removing material from upper surface of the work piece(flat

Surface) It is also called slab milling For Plain milling Up milling or Down milling process is used

Fig. 12.11 Plain milling operation

SIDE AND FACE MILLING: as depicted in fig. 12.12 In this operation both horizontal surface and vertical surfaces are machined. It removes material from side and face of the work piece

Cutter used is side and face cutter

Fig. 12.12 Side and face milling Specialized Milling operations: as shown in fig. 12.13

Straddle Milling (two or more cutter) Form Milling (cutter with specially shaped teeth) Slotting and slitting (Slot cutter and slitting saw) T-slot Milling (Shell mill)

Fig. 12.13 Profile milling

Fig. 12.14 illustrates some special operations performed on the milling machines, and shows that the tracer 6 follows the contoured profile 5 of a master die. The path of the tracer is transmitted through the power connection 4 to an end mill 3 which cuts the work piece 2 mounted on the table 1.

Fig. 12.15 shows other milling operations such as T slot milling, Key way milling and gear milling which need indexing machine shown in fig. 12.16

Fig. 12.14 Special milling operation

Angular milling Gang milling Form milling

Profile milling End milling Saw milling

2

1

Fig. 12.15 T slot, Key way and gear milling

Fig. 12.16 Indexing device

12.6 Shaping, planning and SlottingShaping, planning and slotting are methods of usually flat surfaces shown in fig.12.17 by rectilinear main motion where the work piece and the tool move relatively opposite to each other by rectilinear interrupted and reversible motion as illustrated

The main motion shown in fig. 12.18 is performed either by the tool (in shaping and slotting) or by the work piece (in planning). Is consists of two strokes, the working (cutting) stroke and the return (idle) stroke. The speed of the return stroke ‘vz’ is greater than that of the cutting one ‘vp’ for saving the time. The feed ‘s’ is performed either by the work piece (in shaping and slotting) or by the tool (in planning). The feed is given in millimeters per one double stroke as shown in fig. 12.19.

Fig. 12.17 Shaped, planned and slotted components

Fig. 12.18 Main motion during shaping, planning and slotting

Fig. 12.19 Forward and return stroke

12. 6.1 Shaping processes

A shaper operates by moving a hardened cutting tool backwards and forwards across the workpiece. On the return stroke of the ram the tool is lifted clear of the workpiece, reducing the cutting action to one direction only.The workpiece mounts on a rigid, box shaped table in front of the machine. The height of the table can be adjusted to suit this workpiece, and the table can traverse sideways underneath the reciprocating tool which is mounted on the ram, the table motion is usually under the control of an automatic feed mechanism which acts on the feedscrew. The ram slides back and forth above the work, at the front end of the ram is a vertical tool-slide that may be adjusted to either side of the vertical plane. This tool-slide holds the clapper box and toolpost from where the tool can be positioned to cut the straight, flat surface on the top of the workpiece. The tool-slide permits feeding the tool downwards to put on a cut it or may be set away from the vertical plane, as required. The ram is adjustable for stroke and, due to the geometry of the linkage, it moves faster on the return (non-cutting) stroke than on the forward, cutting stroke. This action is via a slotted link or whitworth link as shown in fig.12.20

Cutting fluid may be employed to improve the finish and prolong the tool's life.

Fig. 12.20 Shaping mechanism

12.6.2 Shapers

In operations carried out on shapers, the tool is reciprocated at the required cutting velocity while the work piece is fed into the cutting tool. The feed in shaping is intermittent and represents the width of cut, its unit is mm/stroke.

Fig. 12.21 Conventional shaper Fig12.21shows the conventional shaper.The return stroke is made faster than the

cutting stroke to increase productivity.

12.6.3 Planers

planer is a type of metalworking machine tool that is analogous to a shaper, but larger, and with the entire workpiece moving beneath the cutter, instead of the cutter moving above a stationary workpiece. The work table is moved back and forth on the bed beneath the cutting head either by mechanical means, such as a rack and pinion gear, or by a hydraulic cylinder.

Planers and shapers were used generally for two types of work: generating accurate flat surfaces and cutting slots (such as keyways). Planers and shapers are now obsolescent, because milling machines have eclipsed them as the machine tools of choice for doing such work. However, they have not yet entirely disappeared from the metalworking world

The design of a planer allows the machining of several surface of long heavy work piece at the same time. Unlike the shaper and the slotter, the work piece on a planer is reciprocated and the cutting tool is fed into it.

The mechanical drive of a planer consists of a powerful electric motor, gearbox and control equipment for the stroke length and the speeds of the cutting and return stroke. This enables machining several meters long work piece. Fig.12.22 shows a double housing planer which allows planning on the top and side surfaces.

Fig. 12.22 Double housing planner

Open-side planers, permit machining of very wide work piece. The work can extend far beyond the left side of the table and may have an additional support on an auxiliary rolling table placed on the left of the planer and extending parallel to the bed.

12.6.4 Slotters ( Vertical Shapers ) A slotting machine is simply a vertical shaper. Fig12.23shows a slotting machine and shows the machining of slots using the vertical shaper.

Fig. 12.23 Slotting machine

6.6.5 Cutting Tools

The cutting tools used in shaping, planning and slotting, Fig 12.24 are similar to those used when turning, but these processes are accompanied by shocks when starting each cutting stroke. Therefore, their tools have to be sufficiently solid and large in cross-sections.

Fig 12.24 Shaping, planning and slotting tools

The planning machines are constructed to up heavy cuts and coarse feeds during a long cutting stroke than in case of shaping. Therefore, the planning tools are heavier and more rigid than the shaping tools.

12.7 Grinding processes12.7.1 Basic concepts Grinding is a metal cutting operation performed by means of a rotating abrasive wheel that acts as a cutting tool. It is used to finish work pieces which must show a high surface quality, accuracy of shape and dimensions. It is considered as a finishing operation because it removes comparatively little metal, usually 0.25 to 0.50 mm and the accuracy in dimensions is in the order of 0.000025 mm, it is also done to machine materials which are too hard for other machining methods that use cutting tools. Fig. 12.25 1illustrates basic concepts when grinding.

Fig. 12.25 Grinding operation basic concepts

12.7.2 Machine typesTypical categories of grinders include,

Surface Surface Finish Formed Grooves Internal (rounds) Cylindrical Internal Center Centerless

Surface grinding - Surface grinders have a few basic types,- Horizontal Spindle with Reciprocating Table shown in fig. 12.26- Horizontal Spindle with Rotary Table- Vertical Spindle with Rotary Table- Vertical Spindle with Reciprocating Table

Fig. 12.26 Horizontal grinding machine

Center grinding • With centers parts are mounted so that they may rotate about fixed centers and then ground externally.

Centerless grinding • Centerless grinding shown in Fig. 12.27 is popular as a high speed, low cost operation. In this operation there is a grinding wheel and a governing wheel. The part sits between the wheels and is ground by the grinding wheel. The governing wheel acts to slow the rotation of the part so that it does not spin at the same speed as the grinding wheel and reduce the surface speed of the grinding operation.If the part has a uniform cross section through feed grinding can be used. Otherwise in feed grinding will have to be used. For infeed grinding the parts are placed between the wheels, ground, and then pulled out. Through feed grinding has the parts move in a steady flow between the wheel.

Fig. 12.27 Centerless grinding

Internal grinding • Internal grinding is similar to other forms of

rotational grinding, except that as the part rotates as illustrated in fig. 12.28. The internal features are ground by a smaller wheel. Using a smaller wheel requires higher grinding speed which increases the challenge of this process

Fig. 12.28 Internal grinding

The grinding wheel performs always the rotary main motion at high cutting speed, the work piece performs usually the needed secondary motion. The cutting part of a grinding wheel consists of a large amount of cutting edges of individual grinding grains which are of irregular shape and are displaced no uniformly. The rake angles of grains are usually negative with respect to the surface to be ground. When grinding, a great amount of heat originates and chips are heated to such a temperature (800-1200C0) at which they are melted and apart of them burn. When grinding, parts of cutting edges are broken away step by step, the cutting edges are rounded, their cutting properties become worse step by step and grains are worn-out.Regarding the required accuracy of dimensions, surface finish and layer to be taken-off, we distinguish, rough, fine and finest grinding.

12.7.3 The Grinding WheelThe Grinding wheel is a multi-teeth cutter made up of many hard particles known as abrasives, crushed to leave Sharpe edges which do the cutting The abrasive grains are mixed with a suitable bond which acts as a matrix or holder of grains. The wheel may consist of one piece or of segments of abrasive blocks built up into a solid wheel. Abrasives, bond, grit, hardness, and structure are the main features specifying grinding wheels beside their shapes and dimensions.

Abrasives Abrasives are either natural or artificial. The natural abrasives as sandstone, emery, corundum or diamond are used to produce only a very small percentage of grinding wheels due to the impurities they contain and the lack of uniformity of these natural abrasives.

The most commonly used artificial abrasives are :

A) Aluminum oxide( a1203):

According to the percentage of impurities it can be black, gray, pink or whit, is the purest one.

b) Silicon carbides(Sic):

It can be green or blue-black. The former is the purest one.

Applications

A1203 abrasives Sic abrasives

They are tough, thus they are they are very hard, thus Suitable to grind materials to they are suitable to grind

High tensile strength such as materials of low tensile steels, high speed steels, strength such as sintered wrought iron and tough bronze, carbides, stone, ceramic etc. materials, gray cast iron, brass, bronze, copper, aluminum, etc.

Bonds

A bond is an adhesive substance that is employed to hold abrasive grains together in the form of grinding wheels.

The used bonds are :

- vitrified bond : It gives the grinding wheel good strength as well as porosity which allows high depth of cut. Vitrified bonded wheels are preferred for grinding because they are not affected by rapid changes in temperature and contact with water and oils. A vitrified bonded wheel is denoted by the petter(v)

- silicate bond : silicate bonded wheels are applied when grinding edged tools and other operations where heat must be held to a minimum. They are denoted by the letter “S” .

- Shellac bond : The elasticity of this bond is greater than in any other type and it has considerable strength. The shellac bonded wheels, also known as elastic bonded wheels, are not as thin wheels, are not intended for heavy duty. They are frequently manufactured as thin wheels. A shellac bonded wheel is denoted by the letter “E”.

- Resinoid bond : It is synthetic resins, such as bakelite. It is used to manufacture strong grinding wheels used at high cutting speeds. A resinoid bonded wheels is denoted by the left “B”. - Rubber bond : The rubber bonded wheels are strong and tough enough, thus they can be produced extremely thin . They are used where good finish is a primary requisite. A rubber bonded wheel is denoted by the letter “R”. Characteristics of Grinding Wheels:Grinding wheels are characterized by

Grit size Grade StructureGrit or Grain size

Grain size shows the size of the sieve through which grains pass. For example: ‘10’ is coarse, ‘100’ fine, ‘600’ is very GradeThe hardness with which bond holds the abrasives grains in placeDesignated by letters A to Z - softest to hardestStructureSpacing between abrasive particles Number of cutting edges per unit area and size of void spaces (pores or porosity)

Fine. Fig. 12. 29 shows standard marking of grinding wheels

Fig. 12.29 Standard marking of grinding wheels12.7.4 Wheel shapes Grinding wheels are made in many different shapes and sizes to adapt them for use in different type of grinding machines and on different classes of work. The size of a wheel is given in terms of its diameter, diameter of spindle hole or opening at the center and width of face. Fig.12.30 illustrates the standard grinding wheel shapes.

Fig. 12.30 Different shapes of Grinding wheels

12.7.5 Safety is extremely important at all times, especially when grinding wheels are involved. They are a mainstay of the engineering and construction industry and the risks involved with them may tend to be taken for granted. Safety awareness and an understanding of the materials and processes involved is essential. Appropriate safety gear must be worn and all safety procedures must be followed. Always err on the side of caution.Before mounting and balancing a grinding wheel, the wheel must be sounded. Sounding is loosely suspending the wheel by a bit of twine or other material so that it hangs free, and giving the wheel a very light tap with a hard object. Care must be taken not to damage the wheel when sounding. A wheel that is safe to use will ring clearly and solidly, like a bell or tuning fork. A damaged wheel will not make any resonating sound. Damaged wheels must not be used under any circumstances and are best discarded or returned to the manufacturer. This process is critically important for surface and diameter grinding, where frequent handling, storage, and changing of wheels increases the risk of fatal wheel damage.Contact with a spinning grinding wheel will produce a cut. Because of the heat generated in the grinding process, a burn might also be produced. Wood should never be ground on a wheel, as it can clog the wheel's pores and cause the wheel to burst, with fatal results.