In this article we will discuss about:- 1. Manufacture Process for Cemented Carbide Tools 2. Types of Cemented Carbide Tools 3. Applications 4. Precautions.
Manufacture Process for Cemented Carbide Tools:
Cemented carbide tools (which are very suitable for machining cast iron, non-ferrous metals and such non-metallic substances as plastics and marble) are formed by pressing mixture of tungsten carbide and cobalt together in hydraulic press and then heating the compact in a hydrogen atmosphere. Tungsten carbide is an extremely hard substance. Cobalt acts as a binder for the hard carbide grains.
Since tungsten carbide has tendency to cratering, it alone is not suitable for machining steel. The addition of titanium carbide helps to reduce this problem. Titanium carbide and molybdenum carbide bonded in nickel are also very suitable for machining steel. Most of the cemented carbides used today are made predominantly from the carbides of tungsten, titanium, and tantalum usually with cobalt as the binder metal.
The various stages in the process of manufacturing hard carbide tool tips are shown below in the form of flow diagram.
Tips may also be pressed direct to shape from hard metal powder in a tableting machine. This method of shaping is particularly useful in preparing a large quantity of tips of the same size. Cemented carbide tips are very strong in compression, but relatively weak in impact or shear.
The cemented carbides can be applied to most chip- forming machining such as turning, boring, reaming, shaping, planning, milling, drilling, etc., as well as to many other applications such as wire-drawing and deep-drawing dies, cold heading and stamping.
Partly for reasons of economy and partly because of the comparatively low impact strength of carbides, these are almost invariably used in the form of tips or inserts secured to steel shanks or other members of suitable form.
These tips are available in the standard forms having upper and lower surfaces parallel and relief angles provided on side and front. The correct top rake on the tool can be obtained by milling the base of the seating in the shank at the required angle.
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On account of poor impact strength of these tips the tool must be designed with a view to stressing the tip in compression only, and to give it the greatest possible support by means of suitable shaped seating in the shank.
It is important that in deciding the type of seating, depth and shape of shank, thickness of tip, and other points in design, due consideration be given to the magnitude and direction of the resulting force (due to feed, radial pressure against the nose of the tool and tangential force).
The type of seating most generally used on turning tools, especially for machining steel, is produced by end- milling the recess in the side of the shank as shown in Fig. 27.1. Good support by the shank is given on three sides of tip.
There is another form of seating (shown in Fig. 27.2), which gives the maximum possible protection to the tip. This is best suited on planning tools.
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The usual shank material is 0.5% carbon steel in the normalised or annealed condition. The depth of the shank is generally kept about four times the tip thickness. The base of the shank should be milled or ground flat so that it will sit firmly in the tool box without rocking.
Types of Cemented Carbide Tools:
1. Diamond Tools:
Shaped diamond tools are used in production turning of light metals, plastics and other specialised structure materials for obtaining highest possible degree of dimensional accuracy and surface finish. Nearly all types of bearings, light-alloy pistons and those having high-silicon alloys are machined by diamond tools. Diamond turning is also used where very low cutting pressures are desired to avoid distortion.
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Diamond tools are available in single-point form with multiple facets of varying curvature of cutting edge. These are also available in shapes suitable for boring, grooving and forming operations. Very high cutting speed with feeds about 0.02 to 0.1 mm per revolution and depth from 0.05 mm to 0.5 mm are used.
The suitable values of cutting speeds for single-point diamond tools with some materials are given below:
Carbides—12 to 24 smpm, ceramics—24 to 90 smpm, bronze—150 to 300 smpm, aluminium alloys—200 to 300 smpm, white metal—240 to 350 smpm, magnesium alloys— 300 to 375 smpm, pure aluminium—450 to 550 smpm, lead bronze and silver—500 to 600 smpm.
The various recommended angles are:
Throw Away or Disposable Inserts:
Cemented carbide and oxides tools are usually made in the form of inserts. These inserts are usually index able type and have a number of cutting edges. Once an edge is worn out, the insert is indexed to present new cutting edge. When all cutting edges are worn out, the insert is discarded.
These are available with both + ve and —ve rake angles. Negative rake angle insert has double the cutting edges because cutting edges on both top and bottom surfaces can be used whereas in positive rake inserts only the top edges on face are usable. Some inserts have built in chip breakers ground on the rake face or separate plate type chip breakers may be attached over it.
ANSI (American National Standard Institute) specifies index able inserts by using an identification system comprising of 10 alphanumeric characters, the details of which are given below:
1st Character:
(Shape of insert) like 85° parallelogram (A), diamond (D), pentagon (P), round (R), square (S), triangular (T), etc.
2nd Character:
(Relief angle) 0° (N), 3° (A), 5° (B), 7° (C) II° (P).
3rd Character:
(Tolerance on size and thickness) ± 0.025 mm on both size and thickness (A), + 0.025 mm on size and 0.125 mm on thickness (B), etc.
4th Character:
(Type of insert). Hole (A), Hole and countersink (B), 0° top rake land, and hole and grooves on both surfaces (G), etc.
5th Character:
(Size of inserts). It specifies diameter of inscribed circle, or width and length.
6th Character:
(Thickness of insert).
7th Character:
(Cutting point configuration). 1 for 1”/32 nose radius, 2 for 1”/16 nose radius and so on, ‘A’ for square insert with 45° chamfer, etc.
8th Character:
(Dimension parallel to edge). Same as for nose radius for 7th character.
9th Character:
(Surface or edge treatment). Honing from 0.025 to less than 0.075 mm (A), etc.
10th Character:
{Whether tool is for right-handed (R) or left handed operation (L)}.
2. Kolesov Tools:
Experiments have been carried out from time to time by various researchers and tool shapes with other than conventional geometry tried out.
In Kolesov tool geometry, three cutting edges are incorporated in place of two.
The three cutting edges are:
(i) Side cutting edge,
(ii) End cutting edge, and
(iii) Nose (land) parallel to the axis of work piece.
Last edge is wider than the feed rake employed and is provided to improve surface finish by decreasing roughness due to feed marks.
3. Gustin Tool:
In this tool, a sinusoidal profile is provided on the rake face in order to reduce cutting forces and tool wear. This becomes possible due to shifting of normal pressure and frictional stress away from the cutting edge.
4. Antichatter Tool:
In this tool shown in Fig. 27.14, a narrow land with 5- 10° negative relief is provided at the side flank beside the cutting edge. Similarly the side rake can also be made negative for a small portion of the rake face adjoining the cutting edge. These features make tool to reduce vibration/ chatter.
5. Cylindrical Cutters:
Carbide tipped milling cutters have been successfully used for the face-milling of cast iron, light alloys and steel. A tipped milling cutter can be regarded as a collection of single point tools, because the principles that apply to the design of single point tools for interrupted cutting can be used equally will for milling cutters. Fig. 27.6 shows the similarity between single point turning tool angles and the corresponding angles on a milling cutter blade.
It may be noted that the side top rake angle in single- point tool becomes the axial (or helix) angle on the milling cutter; and similarly the front top rake angle becomes the radial (or shear) angle, and the plan approach angle becomes the bevel angle. The various angles on the tip of a cylindrical cutter are shown in greater detail in Fig. 27.7.
The commonly used values of axial or helical rake on milling cutters are given below:
The commonly used values of axial or helical rake on milling cutter are given below:
The bevel angle takes the initial impact of the cut before the point or toe of the blade. A simple chamber or secondary angle is given on the heel of a milling cutter blade and it serves the same purpose as the nose radius on turning tools.
The length of the bevel on the cutting edge must be longer than the greatest depth of cut. For reamers, core drills and counter bores, a bevel angle of 45° is satisfactory for most metals. For steel this is reduced to 15 to 20°.
On milling cutters the clearance angles on the periphery should be from 4 to 6° with the steel body backed off by an additional 3 to 5°; the softer the metal the greater the clearance.
6. Throw-Away Tips:
Much research and development has led to the successful application of throw-away tips which has the obvious advantages of lower initial costs, elimination of regrinding and rapid tool changing. Tool changing time in case of throw-away tips is about one-third that for changing a brazed tool. There are other factors also which influence their economic justification. Some of them are discussed below.
Fig. 27.8 shows the effect of cutting speed on tool cost. It may be seen that the lower cost of throw-away tips enables the speed to be increased and the tools to wear faster without increasing total machining cost.
From Fig. 27.8 it will be noted that the minimum machining cost in case of throw away tips is achieved at a higher cutting speed, i.e. the best economy is obtained if the tools are allowed to wear more rapidly. The savings resulting from the faster machining time outweigh the increased cost of tool.
Throw-away tips have renewed interest in negative rake cutting since on a negative rake holder both sides of the tips can be used. The cost per cutting edge is therefore, half that of a throw away tip with positive rake. Negative rake tools do, however, bring about an increase in cutting load which in turn may lead to deflection and vibration, increased power requirements, heat development and wear.
Potential differences in cutting force and wear are illustrated in Fig. 27.9. The tangential cutting force is shown as a function of cutting speed when steel is turned with – ve and + ve rake tools at two different feeds.
The difference in tangential cutting force between the two is on average 10 to 15% for steel. Similar tests on cast iron show the difference to be 20-30%. But increased cost as a result of the increased power consumption is relatively insignificant, provided it is within the machine’s capacity.
Fig. 27.10 shows the difference in crater depth and flank wear when turning steel with – ve and + ve rake tools. The difference in flank wear is comparatively insignificant as against that in crater depth.
On the other hand, the true edge-angle is approximately the same and since this aspect affects edge life, greater crater wear can take place before the destruction of negative rake tips. This greater carter depth is of no importance with throw-away tips as there is no question of regrinding, neither does it affect carbide economy.
Finally due to absence of brazing and grinding stresses on throw-away tips, harder and more wear resisting carbide grades can be used.
Savings can also be made in the cost of the tip itself by leaving clearance faces in the sintered condition. Tips with utility finish, that is, with only the top and bottom faces ground, generally cost about 50-60% of a fully ground tip.
7. Indexable Tooling:
Cemented carbide is extensively used as cutting tool material because of the advantages it offers. Initially brazed cemented carbide tips were used which could be renewed by grinding when cutting edge turned blunt. Brazing introduces stresses in the carbide matrix. Grinding of cutting edge also introduces grinding stresses in carbide.
Indexable tooling using indexable inserts, also known as throw-away cutting edge concept does away with brazing and grinding of cutting edges. Cemented carbide tip in form of triangle of square is held mechanically in a holder.
The worn out cutting edge is replaced by indexing and after using all the available edges, the insert is discarded. This concept has proved to be highly economical and is, therefore, adopted world over in all types of turning, boring, milling drilling tools, etc.
Indexable inserts are held mechanically in a special tool holder. Fig. 27.11 (a) and (b) show the design of two types of tool holders for holding indexable inserts manufactured by Sandvik Asia Ltd. In it, the cutting edge geometry remains the same with every indexing because of on grinding, thereby ensuring consistency in performance. Chip-breaker can also be incorporated to break the chips.
The same tool-holder can be used for machining different materials simply by changing the appropriate grade insert, which results in reduction in inventory cost. Higher cutting speeds can be used to increase productivity.
Tool changing time is considerably reduced as tool need not be taken out for grinding, but the insert can be indexed or replaced on the machine itself without resetting the tool-holder. Insert is clamped with the lever and through the hole, which does not allow the insert to shift during operation, thereby ensuring positive clamping.
However, with indexable inserts, it is essential that machine tool is sufficiently right and have sufficient power. Machine slides should be in good condition. Tool holder should be cleaned and set to the correct height, ensuring minimum over-hang.
The clamping mechanism of tools shown in the Fig. 27.11 consists of a L-shaped lever. One end of this lever fits into the recess firmly against the two shoulders of the pocket seat thus providing an accurate location of the cutting edge every time it is indexed. The chip-breaker is form sintered. For clamping of insert, the screw is fixed first and lever fitted loosely in the screw recess.
The countersunk side of the shim is placed upwards. The shim pin is put in place using proper plastic punch. The open side of shim pin is turned in the direction of the seating. It should be ensured that the insert and seating are well cleaned when indexing or replacing inserts.
Application of Cemented-Carbide Tools:
Here we will study the more importance points to be observed during machining operations. Good carbide-tool design aims at exploiting to the full the high cutting capacity of the carbide, and at the same time takes into account the brittleness and low tensile strength of carbide.
A machine in bad condition or under-powered can very easily damage the best of carbide tools, hence great care must be taken in their use. The drive of the machine must be capable of transmitting the load without slip, since stalling under the cut will, in almost every case, damage the tool.
It is, therefore, also necessary to disengage the feed before the machine is stopped. Proper selection of cutting angle and setting of tool for getting best cutting efficiency cannot be over-emphasised. The tool post must be capable of holding the tool firmly, and as close to the work as is practicable.
A flat bottomed tool post fitted with blunt ended clamping screws is best. The tool nose should be made to be on the centre of the work since high cutting speeds are involved, work should be firmly secured, the dead centre should be of rotating type and travelling steady rest used to reduce danger of deflection.
Suitable peripheral speed is a very critical factor in determining the tool life between regrinds. Feeds and speeds recommended by the carbide manufacturer should be strictly adhered to. Machining of steel at too low speed results in development of built-up edge on the carbide tool.
On the other hand excessive speed results in the burning of cutting edge and they wear away rapidly. For steel, usually speed of 60 to 65 m/sec is used. In case of cast iron and non-ferrous material the speed in not so critical.
The feed chosen should always be sufficiently coarse to ensure that the tool is cutting and not rubbing. A safe general rule is 0.25 to 0.50 mm per revolution. If lower feed to be employed for better surface finish, then special attention should be given to lapping and polishing the cutting edge by diamond wheels. In the case of steels, chip disposal is an important factor in fixing the rate of feed.
The depth of cut is generally determined more by other factors in the machining operation than any physical limitation in the tool itself. For roughing, the depth is mainly limited by the power available on the machine tool. Rigidity of the cutting tool, the machine, the chuck and all holding fixtures also affect the depth of cut which may be safely executed.
In roughing cut it is important that depth of cut be sufficient to keep the nose of the tool underneath the surface scale. For finishing cut a minimum depth of 0.1 mm be maintained and if a shallow cut of 0.05 mm depth is taken then particular attention must be paid to maintaining a good finish to the cutting edge.
The best all-round coolant for carbide tools is soluble oil. It increases the tool life and maintains a constant surface finish. If coolant is used it should be supplied in plenty at a high velocity right at the nose of tool. At all costs a trickle should be avoided as it may lead to cracking. Better than this would be to machine dry and have shorter tool life.
Nose radius of the carbide tool should be adequate. If nose radius is too large, a thin chip is produced which will cause abnormal wear, especially on the end of the tool. Too small a nose radius will also cause rapid wear due to burning or collapse of the end of the tip. Insufficient top rake with a fine feed may be the cause of wear.
Setting the tool above centre, or inadequate clearance angles, will cause the tool to drag and show sign of bad wear in particularly difficult causes. On castings wear can often be caused by sand in the skin of the casting abrading the cutting edge.
Application of Cemented Carbide Tips in Milling Cutters:
Cemented carbide finds extensive application on face milling cutters though it can be applied to side-and face cutters, straddle milling and end milling also. Spiral-fluted cutters are not practicable, partly because of the difficulty of mounting and grinding the tips and partly because of the difficulty of getting sufficient feed per tooth.
Cutters below 150 mm in diameter are usually made by brazing carbide tips into a forged steel or high tensile cast iron body. The main disadvantage in brazing the tips for milling cutters is that it is very difficult to recondition the cutter if one of the tips gets smashed or badly chipped.
This difficulty is easily overcome in the inserted blade type of cutter in which a blade contains carbide tip at its end, but it becomes feasible only on cutters greater than 150 and 200 mm diameter. The blade has transverse serration which engages similar serrations in the cutter body. The purpose of serrations is to afford an adjustment to the blades which are secured by a clamp and socket-head screws.
Methods of Mounting Tool Tips:
Usually the shank is made of 0.5% steel which is considered to be the most suitable material for this purpose and this quality of steel is generally known as “Shanking steel”. Many times high-tensile cast iron (Mechanite) is also used for shanks because of its greater damping capacity.
The shank is cut to length, and for cranked tools the neck is hot-forged or bent cold. Front and side clearances are then milled or shaped (according to the convenience) on to the end of the shank. All shank angles are made 2 to 4° greater than the corresponding top, front and side rakes. This ensures that only the carbide is ground after brazing, thereby speeding the grinding operation and preventing undue wear of the grinding wheels.
The base of the seating is milled in the shank at the same angle as the top rake, so that the tip which has paralleled top and bottom surfaces, is tilted at the required angle. When the seating are being milled, car must be taken to ensure that the base is flat and that the tip is a good fit without rocking, otherwise the tip may crack in service.
The tip surfaces which are to be brazed are ground with a silicon-carbide wheel in order to remove the thin surface layer which is produced during sintering and which is not readily wetted by the brazing alloy. There should be an adequate seating chamber on the edge of the tip and care must be taken that the seating radius is a snug fit in the shank.
Brazing can be carried out by copper, Sif bronze or silver solder. Copper gives the strongest joint, but it is necessary to use a controlled-atmosphere furnace because of oxidation at the high temperature which is necessary to melt the copper.
Sif bronze requires a slightly lower temperature and can be used successfully for torch brazing. Silver solder has a much lower melting point and is probably the most satisfactory brazing metal to use is small tool rooms as it is simpler to use.
The operation of brazing may be carried out by using either gas or electric furnace designed especially for this purpose or by oxy-acetylene blow pipe. The seating, tip and brazing alloy must first be thoroughly degreased by carbon tetrachloride or trichloroethylene. A flux (borax for copper) should be used to remove oxides formed on the tool shank during heating.
When a cemented tip is brazed on to a shank, stresses are set up due to the difference between the expansion rates of the two materials. The straight tungsten carbide grades used for cast iron and non-ferrous metals are able to withstand these brazing stresses better than the carbides used for machining steel.
Brazing stresses are usually shown up during grinding as cracks running parallel with the joint. Due to low temperature used for silver solder, sometimes it is considered to be the best material on long thin tips such as reamer blades, and on tips having a sudden change of section as in some form tools.
Brazing stresses on tip may be reduced by placing a thin sheet of suitable material (tinned, 40 mesh sheet iron gauge) placed between two sheets of brazing metal between the tip and shank. This is essential on all tools having tip longer than 25 mm or its largest dimension.
Grinding and Lapping of Carbide Tipped Tools:
Cemented carbide being hard and brittle, and having a lower thermal conductivity than the steel alloys is much more liable to cracking when being grinded. For this purpose, operator has to master up the special technique as carbide grinding is a slow process and attempts to speed it up by crowding the cut will usually result in cracked tips. For grinding cemented carbides, silicon-carbide wheels (the so-called green-grit wheels) are used, followed by diamond-impregnated wheels for finish grinding and lapping.
The spindly of the grinding machine should be in good condition and free from end play. The tool fixture should be fitted with a graduated scale and capable of adjustment to any required angle. The grinding wheel should have soft bond so that the wheel structure at the surface breaks down as soon as the particles become worn and dull, thus keeping the wheel in free-cutting condition.
The proper selection of the grit and grade of the wheel is of vital importance for efficient grinding. It is best to use wet grinding wherever possible and sufficient quantity of coolant should be available.
Chatter can be caused by excessive overhang. It the set-up does not permit this to be reduced, the cross-section of the tool shank should be increased. Large nose radius also causes chatter.
Precautions in the Use of Carbide Tools:
(i) Base of tool should be flat and rigid. The rocker support should not be used under tool. The tool should be set approximately on horizontal plane.
(ii) Tool should not be set above or below the centre line.
(iii) Inclined tool holders should not be used; instead tool holders designed to hold tool on horizontal plane should be used.
(iv) If necessary tool must be set short of desired length and adjusted from rear, i.e. overhang should be minimum.
(v) Tool should not be held against work when tightening clamping screws.
(vi) Hammer should never be used on cutting end of tool.
(vii) Dog point or flat clamping screws or clamping plate should be used. Pointed clamping screws should be avoided.
(viii) Tool should be allowed to cool naturally. It should not be dipped in any liquid when hot.
(ix) Generous coolant flow pointed towards under chip and against cutting edge should be used. Weak stream of coolant is dangerous.
(x) Spindle should never be stopped before disengaging feed.
(xi) Only silicon-carbide or diamond wheels should be used for grinding tips.
(xii) These should be sharpened at regular intervals to get long life.
(xiii) The tool should be kept moving across the wheel when grinding to avoid localised over-heating.