design and selection of single and multipoint cutting tool
here you will find the design procedure of the single and multipoint cutting tools........
PRACTICAL NO: 11 AIM: To study the design and selection of the single point cutting tool. INTRODUCTION: The cutting tool can be classified in different categories. Depending upon the number of cutting points on the tool, the tools are divided as: 1) single point cutting tool 2) Multi point cutting tool. The tool having only one cutting point or edge is called single point cutting tool such as tool used for turning, boring shaping or planning, while the tool having more than one cutting point or edge is called multi point cutting tool such as tool used for drilling, milling broaching. Single point cutting tools
Multipoint cutting tools
FIGURE 11.1 - TYPES OF CUTTING TOOLS Depending upon the construction of the cutting tool, it is classified as: 1) Solid tools 2) Tipped cutting tools. All the cutting tools are designed to meet the purpose of removing material from work piece. All the cutting tools contain: 1) Cutting element 2) Mounting element. The design of cutting tools means the determination of all the dimensions and the shapes of all the elements of a tool, either analytically or graphically. The following functions should be performed to design cutting tools:
• To determine the forces acting on the cutting surfaces of the tool. • To find the most producible shapes of the cutting and the mounting elements of the tool. • To determine the strength and rigidity of the cutting and mounting elements of the tool. • To make a working drawing of the tool and compile the manufacturing specification.
DESIGN OF CUTTING ELEMENT:
To design the cutting element of the tool, it is necessary to know the kinetics of cutting. The motions in various metal cutting machines tools are made up of linear and rotary motions. The following design factors should be kept in mind when designing the cutting element: 1. The load distribution may be different in different tools. So the load distribution should be kept in mind. 2. The roughing tool should be capable of removing the largest amount of stock at minimum forces and power. Finishing tools should be able to attain the required surface finish. 3. Sharpening of tools. When a tool wears out, it is sharpened by grinding off a layer of metal that has become worn during the cutting process. The method of sharpening determines the dimensions of the tooth or blade and shape. 4. Chip disposal. The free and unrestricted disposal of chips from the cutting edges of the tool and ample chip space are one of the important design features. For the continuous chips the chip breaker should be provided. 5. Heat disposal. Lot of heat is produced during the cutting process. This heat can lead to intensive wear of the cutting tool. In single point cutting tool, it is achieved by grinding the tool point to certain angles and additionally by an ample supply of cutting fluid. In the case of complex tools, proper heat removal is achieved by maintaining the body of the cutting tooth to the sufficient size and by making passages to deliver the cutting fluid. 6. Strength and rigidity. The variables affecting the cutting process are so complex that it is impossible to do the exact force analysis. It is even more difficult to check the rigidity and vibrational response of a cutting tool. 7. Heat treatment of tools. Heat treatment has a great effect on the strength of a cutting tool. High internal stress developed in hardening a tool may lead to the information of cracks and to failure. Residual stresses get concentrated at sharp corners and at points of abrupt section changes. Hence, all the sharp corners should be rounded off in properly designed cutting tools.
Prac 11 3 8. Economical utilization of tool material. The tool designer should keep the point of economical utilization of tool material in mind. The various materials used for tools are very expensive. Due to this, the tool industry follows the economical route of constructing tools in which the cutting element is made of H.S.S. or cemented carbide and the body of the tool is made of structural steel. 9. Built up tools. Built up or assembled tools have a great advantage over solid tools, namely, the possibilities of size adjustment. Size adjustment of tools increases their life. •
DESIGN OF MOUNTING ELEMENT
The mounting element of a tool of a shank-or-arbor-type should be capable of transmitting the power developed by the machine tool spindle to the cutting edge of the tool. If mounting element is not sufficiently strong, it will limit the capacity of the cutting tool. The form of the mounting element should be such that it enables the tool to be clamped with a minimum loss of time in the machine tool. The most widely applied constructions of mounting elements are given below: 1. A square on a straight cylinder shank, a taper shank with or without a tang. 2. Quick change clamping devices of various constructions for shank type tool for rotary motion. 3. A cylindrical mounting hole with a longitudinal key or with drive keys on the end face of a tool, locking devices of various constructions, and taper holes for arbor type tools with rotary motion. 4. A pull end of shank with a tapered key, rapid change clamping devices of various constructions for tools with a lengthwise motion, for, push or pull broaches.
• DESIGN OF SINGLE POINT CUTTING TOOL Tipped single point cutting tools. The carbides, ceramics, cast alloys, diamond, CBN, UCON are used as tips or inserts which are either brazed into a prepared seat machined on a tough steel tool shank or are clamped to the shank. 1. Brazed Tipped Tools: Here suitable shapes of inserts are brazed to a steel shank. When the insert gets worn out, it is resharpened with the help of special grinding wheel.
Prac 11 4 The main draw back of a brazed tip is that because of different coefficient of expansion of tip material and tool material the brazing has to be done very carefully.
FIGURE 11.2 – BRAZED TIPPED TOOLS AND INSERTS 2. Mechanically clamped tip tools. In these tools, the tips or inserts are clamped mechanically on to the tool shank. These tips are known as indexable because these have more than one cutting edges which are used one by one by indexing the tip and these tips are known as throwaway type because once all the edges of the tip have been used, the tip or insert is removed from the tool shank and thrown away.
FIGURE 11.3 – MECHANICALLY CLAMPED TOOL The most common shapes in which these tips are available are : square, triangular, and diamond. In the first case, the tips will provide a negative rake angle because these will have to be clamped on to shank with the seating sloping downwards to provide a clearance angle.
FIGURE 11.4 – TYPES OF RAKE ANGLES When a cutting edge on the tip gets worn, the clamp is released and the tip is rotated to bring a new cutting edge into the cutting position. When all the edges have been used the tip is thrown away.
• CHIP BREAKER During machining ductile materials, continuous chips are produced which are difficult to handle and a sharp, hot chip in motion is a hazard. To handle and dispose off the chips conveniently, the continuous chips are broken up into short segments. This is achieved with the help of chip breakage. The various types of chip breakers are: 1. A groove may be ground into the top face of the tool after leaving a small land from the tip. 2. A step may be ground into the tool. 3. By providing a secondary rake angle and chip breaker projection. 4. In the case of carbide tipped tool, a chip breaker groove is made all around the boundary or a separate plate or step may be clamped on top of the tool.
FIGURE 11.5 – CHIP BREAKERS TABLE – 11.1 Dimension of the chip breaker. Depth of cut 1 4 9
0.2 b, mm 1.5 2.5 3.0
Feed, mm/rev 0.35 t, mm 0.3 0.5 0.5
b 2.0 3.0 4.0
t 0.4 0.5 0.6
0.55 b 3.0 4.0 4.5
t 0.5 0.6 0.6
• DIMENSIONS OF TOOL SHANK The tool shank can be square, rectangle, or circular in section. Rectangular section is commonly used since reduction in its strength is less as compared to other sections, when a seat is cut on it for an insert. The dimension of the tool shank will depend upon the cutting force, overhang of the tool shank from the tool post and the material of the shank. The tool acts as a cantilever. If Fc is the cutting force and l is the overhang of the tip of tool, then, bending moment on the tool shank is, M = Fc × l Moment of resistance = Bh 2 / 6 × б b , for rectangular section. = π / 32 × б b , for circular section . Where B is the width and h is the height of rectangular tool shank and б b is the safe stress in bending for the material of the tool shank. The values of h / B are taken as follows: H / B = 1.25 for roughing operations = 1.6 for semi finishing and finishing operations. The overhang is usually kept as (1 to 1.5) × h. The deflection of the tool point should also be checked. For cantilever, it is given as, δ = 4× Fc × l 3 / E × B × h 3 The standard cross sections of the rectangular tool shanks are, B × h, 10×16, 12×16, 12×20, 16×20, 16×25, 20×25, 20×32, 25×32, 25×40, 32×40, 32×50 and 40×50.
• DESIGN OF MULTI POINT CUTTING TOOL 1. MILLING CUTTERS Milling cutters are multi point cylindrical cutting tools with cutting teeth spaced around the periphery. The most appropriate way of classifying milling cutters is on the method of providing relief on the tools. According to this, the milling cutters are classified into two main categories: 1. Profile relieved cutters 2. Form relieved cutters. The profile milling cutters are obtained by sharpening a narrow land behind the cutting edge. Form relieved cutters have a curved relief behind the cutting edge. The milling cutters can also be classified according to the method of their mounting, for example, arbor type, shank type or spindle type.
The design features of a milling cutter will be illustrated by considering a plain milling cutter. The main elements to be considered for the design are: size of cutter, tool angles, number of teeth, flutes, and material.
FIGURE 11.6 – MILLING CUTTER
(a) SIZE OF CUTTER. The outside diameter of the cutter D depends upon the arbor diameter, d, thickness of the cutter ring, t, and the height of the cutting tooth, h, or the depth of the flute. It is given as: D = d + 2t + 2h Generally the cutter diameter, D, is taken about 2.5 to 3 times the arbor diameter. The face width of the cutter should be adequate so as to give sufficient support to the cutting edges. The arbor is usually selected from the commercially available standard arbor sizes: 16, 22, 27, 32, 40, 50 and 60 mm. Cutter hub diameter = d + 2 × thickness of body. = (1.5 to 2.5) × d Diameter of plain milling cutters and the depth of cut are also inter related as follows: D= 60 to 90 mm for depth of cut up to 5 mm. = 90 to 110 mm for depth of cut up to 8 mm. = 110 to 150 mm for depth of cut up to12 mm. Face width of milling cutter = width of w.p. + (2 to 5) mm. Diameter of face milling cutter is selected depending on the width B of the surface to be milled: D = (1.06 to 1.10) × B for H.S.S. cutters = (1.20 to 107.) × B for carbide tipped cutters.
(b) TOOL ANGLES. A plain milling cutter may have either straight or cylindrical teeth. The helix angle is taken as: Helix angle = 20º to 30º for plain helical cutters = 10º to 15º for side and end mill cutters. The radial rake angle varies from 10º to 20º, larger values for cutting softer materials and smaller values for cutting harder materials. Carbide tipped cutters has negative rake angles, -10º to -15º. The relief angle is provided to eliminate heel drag. Relief angles should be small to ensure greater strength of the cutting edge and for better heat dissipation. Larger relief angles reduce the strength of cutting edge leading to its failure and also increase the tendency of chatter. TABLE 11.2 Average relief angles Type of Tool material Work material cutter Steel Cast iron Non ferrous and non metallic 1.peripheral H.S.S. 5-10º 5-10º 7-12º Carbide 4-6º 4-6º 5-10º Cast alloys 4-6º 4-6º 5-10º Side or end All 1-4º 1-4º 2-7º cutting edges
(c) WIDTH OF LAND. To give strength to the cutting point, a narrow land is provided immediately behind the cutting edge. This land is ground to the relief angle. Its values are: Width of land = 0.127 to 0.254 mm for small end mills = up to 3.2 mm on large diameter cutters = 0.80 to 1.6 mm (average)
(d) NUMBER OF TEETH. The number of teeth on a milling cutter will depend upon the work material and the surface finish required. For rough cuts fewer numbers of teeth are required while for finer cut greater numbers of teeth are required. The number of teeth in a milling cutter is given as: n = f / (f t × N) Where f= feed rate, mm/min, f t = feed rate per tooth, mm N = cutter speed, rpm For H.S.S. plain milling cutters, ft = 0.05 to 0.6 mm/ tooth for milling steel = 0.1 to 0.8 mm/ tooth for milling C.I. The metal removal rate is given as, MRR= w × h × f, mm 3 /min Where w = width of cut, mm h = depth of cut, mm = 3 to 8 mm for roughing
= 0.5 to 1.5 mm for finishing To prevent overloading the machine motor, the variables are connected as given below: n = (K × hp c ) / (ft × N × h × w) Where hp c = horse power available at cutter K =machinability factor, mm 3 / min/ hp c According to cutter diameter, the number of cutter teeth may be taken as given below: n = C× (D) ½ For solid cutters: C = 2 to 2.8 for fine tooth cutter = 0.6 to 1.05 for coarse tooth cutter Solid end cutters: C = 1.2 for large teeth = 2.0 for fine teeth = 2.5 to 2.8 for angle milling cutters = 1.5 to 2 for form milling cutters = 2 for disk milling cutters. For inserted blade cutters, n = 0.04D for d 200mm = 0.10D for cutting C.I.
(e) POWER REQUIREMENTS FOR MILLING. The total horsepower required at the cutter can be found as hp c = (metal removal rate, cm 3 /min)/ K
(f) FLUTES. The flutes of a milling cutter may be straight, helical or angular. Helical flutes are most common, since the entire cutting edge does not come into contact with work material at one time. The helix can either be right handed or left handed.
2. BROACH DESIGN A broach is a multipoint cutting tool consisting of a bar having a surface containing a series of cutting teeth or edges which gradually increase in size from the starting or entering end to the rear end.
FIGURE 11.7 – BROACHING TOOL
(a) DETAILS OF INTERNAL BROACH. A typical broach is shown in figure above. To machine an internal hole, the broach is gripped by a puller at the shank end. The front pilot centers the broach in the hole before the teeth begin to cut. The front taper facilitates the insertion of the front pilot in the hole. The first set of teeth behind the front pilot, removes most of the material and are called “roughing teeth”. These are followed by a few teeth called “semi finishing teeth” where the depth of cut of individual tooth is quite small. Finally, there are finishing or sizing teeth which are all of the same size and have the shape of the finished hole. These have no cutting edges but are bottom shapes and from 0.025 to 0.075 mm larger than the size of the hole.
(b) TOOTH ELEMENT. The front rake angle refers to rake angle of a single point cutting tool and the back off angle is provided to prevent rubbing of tool with work piece.
(c) MATERIAL. H.S.S. is far by the most widely used material for the broaches. Brazes carbides or disposable inserts are sometimes used for the cutting edges when machining cast iron parts which require close tolerances and production rates. Carbide tools are also used to an advantage on steel casting to offset the damaging effect of local hard spots.
(d) CONSTRUCTION. A broach may be either solid or assembled or built up from shells, replaceable sections or inserted teeth. Replaceable sections, teeth or shells make a broach easier to repair.
(e) BROACHING ALLOWANCE. Or the stock left for broaching is defined as the total thickness of the metal to be removed by broaching. For example, for round broaches, it is the difference between the maximum permissible diameter of the broached hole and the diameter of the hole previous to broaching. The nominal allowance for round holes, machined by drilling or core drilling previous to broaching is, A b = 0.005D + (0.1 to 0.2) (L) ½ Where D = basic diameter of hole, mm
11 L = length of hole to be broached, mm
(f) PITCH OF TEETH. The pitch of broach is decided in such a manner that any time at least two teeth are cutting in order to keep the cutting operation smooth. The other factors affecting are: depth of cut or chip thickness, length of cut, cutting force required and power of the broaching machine. A good approximation for pitch is: p = (1.25 to 1.50) (L) ½ , for plain broaches. = (1.45 to 1.90) (L) ½ Where L is the length of hole or length being cut. The minimum permissible value for ‘p’ should be less than 5 mm unless a smaller pitch required for very short cuts to provide at least two teeth in contact simultaneously with the part machined. For surface broaching, the pitch is given as, p = (length of cut × ratio of gullet area to chip cross section)
The ratio of gullet area to chip cross section is taken as = 3 to 5 for rough = 8 for finishing
(g) RAKE AND RELIEF ANGLES. The front rake angle falls in the same range as used for another tools, but, the back off angle is quite low, (0.5 to3.5). A large relief angle weakens the tooth.
(h) DEPTH OF CUT PER TOOTH. Depth of cut per tooth or chip thickness or rise per tooth depends upon the shape of hole, size of hole, type oh material being cut and the force available at the machine. It is generally very small, of the order of 0.025 mm for finishing to 0.15 mm for roughing. In progressive broaching, the cut per tooth can be made more and may be between 0.1 to 0.35 mm or even more.
(i) WIDTH OF LAND. An average land is 0.12 mm on the roughing teeth and 0.25 to 0.75 mm gradually increasing through the finishing teeth. It is selected as a compromise between tooth strength and chip space. In general, it is taken as, w = (0.25 to 0.333) × p Land at zero clearance, that is, parallel to the broach axis is taken as 0.13 mm to 0.50 mm.
(j) DEPTH OF CUTTING TEETH. The size of the depth of cutting teeth is directly related to tooth size of pitch. The usual value of the tooth depth is given below: h = (0.35 to 0.40) × p
Prac 11 12 The depth of tooth gullet can also be found in the following manner: When the job is getting broached, the metal chips get collected in the gullet space. It is highly desirable that the chips don’t get compressed and thereby jams up the entire gullet space. Now, the effective cross sectional area of the gullet space can be taken as, A g = π/4 × h 2 Now to allow for clearance between the coils of continuous chips and for free formation of the discontinuous chips, the effective gullet section is taken to be 3 to 6 times the actual chip section. Thus, if the uncut chip thickness is‘t’, and job length is L, the chip section area A c is, Ac = L × t Ag = K A c Where K is 3 to 6, is the volume factor. Its value depends upon chip thickness and material. Depth of gullet will be given as, h = (4×k×L×t/π) ½
(k) TOOTH FILLET RADIUS. It is provided between the teeth to strengthen the broach tooth. It also helps in the curling of the chips. The various empirical relations to calculate teeth fillet radius are: r = (0.20 to 0.35) × p = (0.40 to 0.60) × h = 1/3 × (land width) +1/2 × (tooth depth) + ¼(pitch)
(l) CHIP BREAKER. Rounded chip breaking grooves are provided at interval along the cutting edges to break up the wide curling chips and preventing them from clogging the chip space, thus reducing the cutting pressure and strain on the broach. These grooves are provided only on roughing teeth. The more ductile the material the wide the chip breaker grooves should be, the smaller the distance between them. The number of the chip breaker for round broaches can be selected from the following table: TABLE 11.3 Broach diameter, mm 10 to 13 13 to 16 16 to 20 20 to 25 Number of chip breakers 6 8 10 12
(m) TOTAL LENGTH OF BROACH. The total length of broaches given as, Lb = length of toothed portion + length of shank + length of rear pilot. Now, the length of toothed portion will depend upon the number of cutting teeth, the number of finishing teeth and pitch of the teeth. Now, the number of cutting teeth can be calculated as, n c = T/s + (2 to 4) Where T = metal thickness to be removed = A b /2, for round broaches.
Thus the total number of teeth is, n t = n c + (3 to8) Length of the toothed portion, l = nt × P = (P × T/s) + (5 to 12) × P
(n) CUTTING SPEED. Cutting speed in broaching process is low and seldom exceeds 15 mpm. For steel castings and forgings, the range is 6 to 10 mpm and for C.I. brass and aluminium it can be up to 12 mpm.
(o) LOAD ON BROACH. The load on the broach or the force needed for broaching may be found by the simple relation. F = Area of metal removed by the teeth in contact with the work × shear strength of the material being cut. = n × A × τs Where n = number of teeth at a time A = cross sectional area of cut. The above relation does not consider the fact that the resistance to broaching varies with rise per tooth, being higher for lower values of rise per tooth. Considering this fact, the relation becomes, F = n A C Where C = specific cutting force, that is, force remove 1 mm 2 of metal at a given size per tooth. Considering the blunt broach factor, the expression finally becomes, F = n A C B Where B = Blunt broach factor (1.25 to 1.40) For round broaches, F = n × πDs × C × B For surface broaching, F = n × s × b × C × B For spline broaching, F = n × z × s × w × C × B Where D = finish diameter of hole b = width of contact of each tooth z = number of splines w = width of splines. Power needed for broaching = F×V / 1000, kw Where F is in Newton and V is in m/s. MRR per pass = n A V, mm 3 /s.
3. DRILL DESIGN. Drilling is the process of cutting or originating a round hole from the solid material. The tool is revolved and is fed in to the material. There are many ways of classifying drills, according to: material, number of types of flutes, drill size, type of shank, and cutting point geometry. However the most common type of drill is the fluted twist drill.
FIGURE 11.8 - DRILL
DESIGN FEATURES. (a) MATERIALS. Carbon tool steel drills have a low first cost, but these should be used occasionally and at slow speeds. High speed steel drills are economical for high production but are expensive and must be handled carefully to avoid breakage. These are used mainly for drilling magnesium, zinc and also plastics, hard rubber etc. these are not used for steel components as there is likelihood of their breakage due to high tip pressure.
(b) SPEEDS AND FEEDS. TABLE 11.4 Work material Stainless steel C-steel(0.4-0.5c) C-steel(0.2-0.3c) Soft grey C.I. Brass and bronze Magnesium alloy
V 9-12 21-24 24-33 30-45 60-90 75-120
Drill size 0-3.2 3.2-6.35 6.35-12.7 12.7-25.4 >25.4 -
F 0.025-0.05 0.05-0.10 0.10-0.175 0.175-0.375 0.375-0.625 -
(c) DRILL SIZE. Standard drills are available in four size series, the size indicating the diameter of the drill body: 1. Fractional size. Size range is 1/64” to ¼” with increments of 1/64”. 2. Millimeter size. Size range is 0.50 to 10 mm with increments of 0.1 mm. 3. Numbered size. Size range is 0.0135” to 0.228” with very slight increment. 4. Lettered size. Size range is 0.234” to 0.413” with very slight increment. The diameter should always be slightly smaller than that of the hole it is to be drilled, since drills always cut oversize.
(d) LIP RELIEF ANGLE. The heel of the drill point is backed off when ground to give relief behind the cutting lips. This will allow the cutting edges to cut without interference. This is equivalent to end relief angle of a single point cutting tool. It is kept 12º to 15º.
(e) POINT ANGLE. The point angle is selected to suit the hardness and brittleness of the material being drilled. It is 116º to 118º for medium
Prac 11 15 hard steel and cast iron, 125º for hardened steel and 130º to 140º for brass and bronze. It is only 60º for wood and fiber. This angle refers to side cutting edge angle of the single point cutting tool.
(f) HELIX ANGLE. This angle is equivalent to back rake angle of a single point cutting tool. It is 24º to 30º for most drills.
(g) WEB THICKNESS. It is an important element of twist drills. If the web is too thin, the drill will not be rigid enough to withstand a high drilling torque. But, with a thinner web, the axial thrust is reduced and the drilling is easier since the chisel edge becomes shorter. Following are the recommended values for web thickness: 1. For carbon steel and H.S.S. drills. Web thickness = (0.2 to 0.25) × D for D = 6 to 10 mm. = (0.13 to 0.16) × D for D>10 mm 2. Carbide-tipped drills. Web thickness = (0.27 to 0.30) × D for D up to 10 mm. = (0.2 to 0.26) × D for D > 10 mm. 3. Twist drills with milled flutes. The web thickness increased by 1.4 to 1.8 mm per 100 mm towards the shank. It increases the strength and rigidity of the drill.
(h) CHIESEL ANGLE. This is the angle which the raised line at the dead center makes with cutting edge. It is 120º to 135º.
(i) LAND WIDTH. Land width affects the strength and rigidity of the drill body. It is usually taken to be equal to the flute width if drill diameter is more than 20 mm. for smaller diameter drills; the land width is made larger than the flute width. As a guide, the following values for land width should be taken: Land width = 0.62 × drill diameter, for drill diameter 3 to 8 mm = 0.59 × drill diameter, for drill diameter 8 to 20 mm = 0.58 × drill diameter, for drill diameter > 20 mm
(j) MARGIN. Margin is usually kept as 0.06 to0.07 times the drill diameter. Its height is from 0.03 to 0.02 of the diameter.
(k) BACK TAPER. As mentioned above, the back taper is usually kept as 0.0075 mm per cm of drill body. The usual values are given below: Back taper (mm per 100 mm length of drill) = 0.03 to 0.07 for drill diameter 1 to 6 mm = 0.04 to 0.08 for drill diameter 6 to 18 mm = 0.05 to 0.10 for drill diameter > 18 mm
(l) TORQUE AND THRUST. The geometrical analysis of a drill is very complex, because, the inclination angle, the normal rake angle and the effective rake, vary radically as we go from the inner most part to the
Prac 11 16 outer part. The chisel edge does not cut in the usual sense, but rather displaces the metal sideways. The total torque and thrust can be divided into two components: (1) due to cutting edge (2) due to chisel edge. The contribution of chisel edge to total torque is only small but to thrust is considerable. The depth of cut d in drilling from the solid metal is one half the drill diameter. The feed f is the movement of the drill along its axis in mm per revolution. The chip thickness, t = (f/2) × sin α p and width of cut, b = D / (2 × sin α p ), where 2 α p is the point angle. Thus F p = σ c × chip cross section = σ c × (f/2) × (D/2) = σ c × b ×t Where σ c is the contact stress, equal to Brinell hardness H B . The moment due to two forces, F p separated by half the drill diameter is M = (σ c × D 2 × f) ÷ g = (H B × D 2 × f) ÷ 8 The thrust force T 1 due to cutting edges can be estimated if the mean rake angle is known. However, the effective rake angle goes from a positive value near the outer radius to a negative value near the chisel edge. Taking F t / F p = 0.05 to 1.0 T 1 = (0.5 to 1.0) × 2 × sin α p × (f/2) × (D/2) × σ c For 2α p = 118º T 1 = (0.21 to 0.42) × σ c D × f = (1.7 to 3.5) M/D It is difficult to estimate T 2 as it is difficult to ascertain the area of contact between the chisel edge and metal. The web thickness w, is w = 0.2 D for D < 3.2 mm = 0.1 D for D > 25.4 mm As mentioned above, the cutting action of chisel point is very similar to a hardness test, so. T2 = (0.1 to 0.2) × π/4 × w 2 × H B T = (0.7 to 3.5) × M/D + (0.1 to 0.2) × π/4 × w 2 × H B Power required for a drilling by a two flute drill can be calculated from the following empirical formula: Drilling power, kW = 1.25 ×D2 ×K×N× (0.056 + 1.5f) / 105 Drilling thrust = 1.16 × K × D × (100f) 0 . 8 5 MRR = π/4 × D 2 × f × N, mm 3 /min
4. REAMERS DESIGN. A reamer is a rotary cutting tool generally of cylindrical shape, which is used to enlarge and finish holes to accurate dimensions to a previously formed hole. It is a multiple formed hole. It is a multipoint cutting tool. A reamer consists of three main parts: fluted section, neck and shank.
FIGURE 11.9 – REAMING TOOL
DESIGN FEATURES. 1. Reaming Allowance. Reaming is a finishing operation and hence the reaming allowance or the cutting allowance, that is, the material to be removed is very small. Following are the average values: (a) Machine Reaming: Reaming allowance = 0.125 to 0.25 mm for up to 6.25 mm hole = 0.25 to 0.375 mm for up to 12.5 mm hole = 0.375 to 0.75 mm for up to 37.5 mm hole. (b) Hand Reaming: Reaming allowance = 0.025 to 0.125 mm 2. Diameter of Reamer. In general, a reamer cuts a hole slightly larger than its diameter. Hence to find the diameter of the reamer, the following factors should be considered: tolerance of the hole to be produced, the amount the reamer cuts oversize, wear allowance for reamer, manufacturing allowance for reamer. Limits of tolerance on the reamer diameter in relation to those on the hole are determined as given below: Let δ m i n = the minimum amount the reamer cuts oversize. δ m a x = the maximum amount the reamer cuts oversize. Hole tolerance zone has been shown. Line AB represents the H.L. of the reamer diameter size and CD represents the L.L. of the reamer diameter size. Thus the tolerance zone for reamer diameter = CD – AB. This tolerance zone can be further subdivided in to: Manufacturing allowance for reamer and wear allowance for reamer.
FIGURE 11.10 – TOLERANCE ON REAMER DIAMETER
FIGURE 11.11 Recommended tolerance values for the reamer diameters are shown. Maximum diameter of the reamer = Maximum diameter of hole – 0.15 IT Minimum diameter of the reamer = minimum diameter of hole – 0.35 IT 3. Length of body. Length of the fluted portion = 1.5 × reamer diameter. Length of guiding or sizing section of reamer = (1/4 to 1/3) × reamer diameter. Length of the cutting section = (1.3 to 1.4) × reaming allowance × cot φ + (1 to 3) mm, where φ is the entering angle. 4. Back taper or relief. It is a good practice to relieve the reamer with a back taper. It is provided to reduce friction of the sizing section of the reamer on the surface of the hole being reamed. It is: (a) 0.008 to 0.005 mm for hand reamers. (b) 0.04 to 0.06 mm for rigidly mounted machine reamers. (c) 0.06 to 0.10 mm for floating machine reamers. 5. Front end. The front end of the reamer should be smaller than the hole diameter by 0.3 to 0.4 times the reaming allowance to ensure the free entry of the reamer into the drilled or bored hole. 6. Shape of the teeth. The reamers may be straight helical fluted, with straight fluted reamers being more common. Helical fluted reamers are
used for holes with straight slots or with sheet metal. The helix angle varies from 30º to 45º. Hand of flute is generally opposite to the hand of cut in order to avoid the screwing action. 7. Number of teeth. The number of teeth on reamer can be found out as, Z = 1.5 × (D) ½ + 4, for cutting brittle material (C.I. and Bronze) = 1.5 × (D) ½ + 2, for cutting other materials. (D is the diameter) TABLE 11.5 Reamer diameter, mm 6 to 12 7 to 30 32 to 45 45 to 56 56 to 60 70 to 75
Number of teeth HSS reamer Cemented carbide tipped reamer 6 4 8 6 10 6 12 8 14 8 16 8
8. Various angles. (a) Rake angle. Reamers are generally provided with a zero rake. But proper rake angle can be chosen depending on the work piece material. Following are the average values of the rake angles: . TABLE 11.6 0º to 5º For steel 5º to 8º For grey C.I. 5º For brass 8º to 12º For aluminium (b) Clearance angle. (i) For the cutting portion of the reamer. Primary clearance angle = 5º to 7º Secondary clearance angle = 10º to 12º (ii) For guiding section. A circular land is provided for guiding the reamer in the hole. Circular land or width of margin = 0.08 mm to 0.20mm for reamers 3 to 10mm For very accurate holes, land = 0.02 to 0.03 mm. Secondary clearance behind the circular land = 10º to 12º. (c) Taper lead angle or Plan approach angle. The cutting edges of the starting taper on a reamer make an angle with the tool axis (point or included angle 2φ). The angle is called the taper lead angle or plan approach angle. It is taken as: 2φ = 1º to 3º for hand reamers = 30º for machine reamers in reaming through holes in ductile material
Prac 11 = 10º = 60º = 90º accuracy
20 for reaming through holes in C.I. to 90º for carbide tipped reamers to 120º in reaming blind holes as well as through holes to the 3 r d grade.
(d) Chamfer or bevel angle. The chamfer forms a truncated cone on the starting end of the reamer. It is provided to further facilitate the entry of the reamer into the hole and make the cutting action more convenient. The chamfer angle generally used is 45º. 9. Material. The common materials for reamers are H.S.S. and cemented carbide tipped. Reamers are frequently tipped with cemented carbides to increase their production capacity. 10. Cutting speeds and feeds. Cutting speed = 4 to 6 m/min for H.S.S. reamers = 10 to 15 m/min or cemented carbide tipped reamers. Feed = 0.0375 mm to 0.125 mm per flute per rev.
5. FORM TOOLS. Form tools are cutting tools used to machine complex shaped surfaces with the cross section outlined by curves or broken lines. The cutting edge of the form tool is of a shape that the desired contour or the work piece in the turning operation. Form tools are commonly used in large lot and mass production. Their use ensures: -> A high output -> Uniform contour of all work pieces -> Accurate dimensions.
FIGURE 11.12 – TYPES OF FORM TOOLS
Most form tools are made of H.S.S. however cemented carbides are increasingly being used for this purpose. The use of contoured cemented carbide tips for form tools enables the productivity to be raised by 30 to 40 percent, as compared to H.S.S. form tools. A form tool should have the proper rake and relief angles, so that, the metal is cut under sufficiently advantageous conditions.
The relief angle depends upon the type of the form tool. Relief angle is: = 10º to 12º on circular form tools. = 10º to 15º on flat form tools. = 25º to 30º on form tools used for relieving form milling cutters. TABLE 11.7 Materials Aluminium Bronze, leaded brass Mild steel Medium hard steel Hard steel Very hard steel Soft cast iron Hard cast iron Very hard cast iron
Rake angles 20-25 0-5 25 20-25 12-20 8-12 15 12 8
TO DETERMINE OUTSIDE DIAMETER OF A CIRCULAR TOOL. It is determined graphically. It is explained below for a circular tool of positive rake angle. It will depend on the height of the profile to be turned.
FIGURE 11.13 – CIRCULAR FORM TOOL. 1. With O as centre of the work piece, draw two concentric circles with radii equal to the maximum and minimum radii of the contour to be machined. 2. Through point A, draw a line at the angle, α, representing the trace of the plane ground to produce the tool face. Draw another line through A at an angle equal to the relief angle, θ. 3. Draw a line normal to OO, from a point at a distance of t, from the contact point B. The distance t, is the minimum amount that will permit chip disposal from the too face. It is taken from 3 mm to 12 m depending upon the chip thickness and the amount of chips to be cut. 4. From the point of intersection C of the vertical line and the line of action of the tool face, draw a line bisecting angle β. 5. The point of intersection of this bisector and the line drawn at an angle, θ, O2, is the centre of the circular tool.
6. With O2 as centre, draw a circle of radius R. then determine all other dimensions graphically, 7. To determine the diameter of the mounting hole, the wall thickness, t1, is taken as 6 to 10 mm. The profile of a form tool, as a rule, does not coincide with the required profile of the work piece, it must be determined graphically or analytically. However, this is out of the scope of the book.