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? Lapping
Lapping is a finishing operation used on flat and cylindrical surfaces. The lap, shown in Fig.9.1a, is usually made of cast iron, copper, leather, or cloth.
The abrasive particles are embedded in the lap, or they may be carried through slurry. Depending on the hardness of the workpiece, lapping pressures range from 7kPa to 140kPa (1 to 20 psi).
Lapping has two main functions. Firstly, it produces a superior surface finish with all machining marks being removed from the surface. Secondly, it is used as a method of obtaining very close fits between mating parts such as pistons and cylinders.
The lapped workpiece surface may look smooth but it is actually filled with microscopic peaks, valleys, scratches and pits. Few surfaces are perfectly flat. Lapping minimizes the surface irregularities, thereby increasing the available contact area.
The drawing in Fig.9.1a shows two surfaces. The upper one is how a surface might look before lapping and the lower one after lapping. Lapping removes the microscopic mountain tops and produces relatively flat plateaus. Entire microscopic mountain ranges may need to be ground down in order to increase the available contact area.
Production lapping on flat or cylindrical pieces is done on machines such as those shown in Fig.9.1b and 9.1c. Lapping is also done on curved surfaces, such as spherical objects and lenses, using specially shaped laps.
? Polishing
Polishing is a process that produces a smooth, lustrous surface finish. Two basic mechanisms are involved in the polishing process: (a) fine-scale abrasive removal, and (b) softening and smearing of surface layers by frictional heating during polishing.
Electropolishing
Electropolishing is an electrochemical process similar to, but the reverse of, electroplating. The electropolishing process smoothes and streamlines the microscopic surface of a metal object. Mirror-like finishes can be obtained on metal surfaces by electropolishing.
In electropolishing, the metal is removed ion by ion from the surface of the metal object being polished. Electrochemistry and the fundamental principles of electrolysis (Faraday’s Law) replace traditional mechanical finishing techniques.
In basic terms, the object to be electropolished is immersed in an electrolyte and subjected to a direct electrical current. The object is maintained anodic, with the cathodic connection being made to a nearby metal conductor.
Smoothness of the metal surface is one of the primary and most advantageous effects of electropolishing. During the process, a film of varying thickness covers the surface of the metal. This film is thickest over micro depressions and thinnest over micro projections.
Electrical resistance is at a minimum wherever the film is thinnest, resulting in the greatest rate of metallic dissolution. Electropolishing selectively removes microscopic high points or “peaks” faster than the rate of attack on the corresponding micro-depressions or “valleys”.
Stock is removed as metallic salt. Metal removal under certain circumstances is controllable and can be held to 0.0001 to 0.0025 mm.
Chemical Mechanical Polishing
Chemical mechanical polishing is becoming an increasingly important step in the fabrication of multi-level integrated circuits. Chemical mechanical polishing refers to polishing by abundant slurry that interacts both chemically and mechanically with the surface being polished.
During the chemical mechanical polishing process, a rotating wafer is pressed face down onto a rotating, resilient polishing pad while polishing slurry containing abrasive particles and chemical reagents flows in between the wafer and the pad.
The combined action of polishing pad, abrasive particles and chemical reagents results in material removal and polishing of the wafer surface. Chemical mechanical polishing creates flat, damage-free on a variety of brittle materials and it is used extensively on silicon wafers in the manufacture of integrated circuits.
Chemical mechanical polishing is a complicated multiphase process. It mainly includes the following two dynamics. First, the active component in polishing slurry reacts with the atoms of the wafer, and the process is chemical reaction step with oxidation-reductive reaction.
The second step is the process of desorption, that is to say, the resultants gradually separate from the wafer surface and new surface is exposed to polishing slurry. If chemical reactive rate is smaller, the total removal rate of the wafer is also small; furthermore, the surface degree of finish is not good.
On the contrary, even if chemical reaction is very rapid, but desorption is very slow, the total removal rate is not good. Because resultants connot separate from the wafer surface, the active component in the polishing slurry cannot expose and react with the atoms on the new surface, which holds up chemical reaction.
The balance and compositive effects of two steps decide the total removal rate and its surface degree of finish.
The processes of surface engineering, or surface treatments, tailor the surfaces of engineering materials to: (1) control friction and wear, (2) improve corrosion resistance, (3) change physical property, e.g., conductivity, resistivity, and reflection, (4) alter dimension, (5) vary appearance, e.g., color and roughness, (6) reduce cost.
Common surface treatments can be divided into two major categories: treatments that cover the surfaces and treatments that alter the surfaces.
? Covering the Surface
The treatments that cover the surfaces include organic coatings and inorganic coatings.
The inorganic coatings perform electroplatings, conversion coatings, thermal sprayings, hot dippings, furnace fusings, or coat thin films, glass, ceramics on the surfaces of the materials.
Electroplating is an electrochemical process by which metal is deposited on a substrate by passing a current through the bath.
Usually there is an anode (positively charged electrode), which is the source of the material to be deposited; the electrochemistry which is the medium through which metal ions are exchanged and transferred to the substrate to be coated; and a cathode (negatively charged electrode) which is the substrate to be coated.
Plating is done in a plating bath which is usually a non-metallic tank (usually plastic). The tank is filled with electrolyte which has the metal, to be plated, in ionic form.
The anode is connected to the positive terminal of the power supply. The anode is usually the metal to be plated (assuming that the metal will corrode in the electrolyte). For ease of operation, the metal is in the form of nuggets and placed in an inert metal basket made out non-corroding metal (such as titanium or stainless steel).
The cathode is the workpiece, the substrate to be plated. This is connected to the negative terminal of the power supply. The power supply is well regulated to minimize ripples as well to deliver a steady predictable current, under varying loads such as those found in plating tanks.
As the current is applied, positive metal ions from the solution are attracted to the negatively charged cathode and deposit on the cathode. As a replenishment for these deposited ions, the metal from the anode is dissolved and goes into the solution and balances the ionic potential.
Thermal spraying process. Thermal spraying metal coatings are depositions of metal which has been melted immediately prior to projection onto the substrate. The metals used and the application systems used vary but most applications result in thin coatings applied to surfaces requiring improvement to their corrosion or abrasion resistance properties.
Thermal spray is a generic term for a broad class of related processes in which molten droplets of metals, ceramics, glasses, and/or polymers are sprayed onto a surface to produce a coating, to form a free-standing near-net-shape, or to create an engineered material with unique properties.
In principle, any material with a stable molten phase can be thermally sprayed, and a wide range of pure and composite materials are routinely sprayed for both research and industrial applications. Deposition rates are very high in comparison to alternative coating technologies.
Deposit thickness of 0.1 to 1mm is common, and thickness greater than 1cm can be achieved with some materials.
The process for application of thermal spray metal is relatively simple and consists of the following stages.
(1) Melting the metal at the gun.
(2) Spraying the liquid metal onto the prepared substrate by means of compressed air.
(3) Molten particles are projected onto the cleaned substrate.
There are two main types of wire application available today namely arc spray and gas spray.
ARC—A pair of wires are electrically energized so that an arc is struck across the tips when brought together through a pistol. Compressed air is blown across the arc to atomise and propel the autofed metal wire particles onto the prepared workpiece.
GAS—In combustion flame spraying the continuously moving wire is passed through a pistol, melted by a conical jet of burning gas. The molten wire tip enters the cone, atomises and is propelled onto the substrate.
Thin-Film Coatings. Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) are two most common types of thin-film coating methods.
PVD coatings involve atom-by-atom, molecule-by-molecule, or ion deposition of various materials on solid substrates in vacuum systems.
Thermal evaporation uses the atomic cloud formed by the evaporation of the coating metal in a vacuum environment to coat all the surfaces in the line of sight between the substrate and the target. It is often used in producing thin, 0.5μm, decorative shiny coatings on plastic parts.
The thin coating, however, is fragile and not good for wear applications. The thermal evaporation process can also coat a very thick, 1mm, layer of heat-resistant materials, such as MCrAIY—a metal, chromium, aluminum, and yttrium alloys, on jet engine parts.
Sputtering applies high-technology coatings such as ceramics, metal alloys, organic and inorganic compounds by connecting the workpiece and the substance to a high-voltage DC power supply in an argon vacuum system.
The plasma is established between the substrate (workpiece) and the target (donor) and transposes the sputtered off target atoms to the surface of the substrate.
When the substrate is non-conductive, e.g., polymer, a radio-frequency (RF) sputtering is used instead. Sputtering can produce thin, less than 3μm (120μin), hard thin-film coatings, e.g., titanium nitride (TIN) which is harder than the hardest metal.
Sputtering is now widely applied on cutting tools, forming tools, injection molding tools, and common tools such as punches and dies, to increase wear resistance and service life.
CVD is capable of producing thick, dense, ductile, and good adhesive coatings on metals and non-metals such as glass and plastic. Contrasting to the PVD coating in the “l(fā)ine of sight”, the CVD can coat all surfaces of the substrate.
Conventional CVD coating process requires a metal compound that will volatilize at a fairly low temperature and decompose to a metal when it contacts with the substrate at higher temperature.
The most well known example of CVD is the nickel carbonyl (NiCO4) coating as thick as 2.5mm (0.1in.) on glass windows and containers to make them explosion or shatter resistant.
Diamond CVD coating process is introduced to increase the surface hardness of cutting tools. However, the process is done at the temperatures higher than 700℃ (1300℉) which will soften most tool steel.
Thus, the application of diamond CVD is limited to materials which will not soften at this temperature such as cemented carbides.
Plasma-Assisted CVD coating process can be performed at lower temperature than diamond CVD coatings. This CVD process is used to apply diamond coatings or silicon carbide barrier coatings on plastic films and semiconductors, including the state of the art 0.25μm semiconductors.
? Altering the Surfaces
The treatments that alter the surfaces include hardening treatments, high-energy processes and special treatments.
High-energy processes are relatively new surface treatment methods. They can alter the properties of surfaces without changing the dimension of the surface. Common high-energy processes, including electron beam treatment, ion implantation, and laser beam treatment, are briefly discussed as follows:
Electron beam treatment. Electron beam treatment alters the surface properties by rapid heating—using electron beam and rapid cooling—in the order of 106℃/see in a very shallow region, 100μm, near the surface. This technique can also be used in hardfacing to produce “surface alloys”.
Ion implantation. Ion implantation uses electron beam or plasma to impinge gas atoms to ions with sufficient energy, and embed these ions into atomic lattice of the substrate, accelerated by magnetic coils in a vacuum chamber. The mismatch between ion implant and the surface of a metal creates atomic defects that harden the surface.
Laser beam treatment. Similar to electron beam treatment, laser beam treatment alters the surface properties by rapid heating and rapid cooling in a very shallow region near the surface. It can also be used in hardfacing to produce “surface alloys”.
The results of high-energy processes are not well known or very well controlled. But the preliminary results look promising. Further development is needed in high-energy processes, especially in implant dosages and treatment methods.
It has already been stated that the workpiece must be located relative to the cutting tool, and be secured in that position. After the workpiece has been marked out, it is still necessary to position it with respect to the machine movements, and to clamp it in that position before machining is started.
When several identical workpieces are to be produced the need to mark out each part is eliminated by the use of jigs and fixtures, but if a casting or forging is involved, a trial workpiece is marked out, to ensure that the workpiece can be produced from it, and to ensure that ribs, cores, etc. have not become misplaced.
Jigs and fixtures are alike in that they both incorporate devices to ensure that the workpiece is correctly located and clamped, but they differ in that jigs incorporate means of tool guiding during the actual cutting operation, and fixtures do not.
In practice, the only cutting tools that can be guided while actually cutting are drills, reamers, and similar cutters; and so jigs are associated with drilling operations, and fixtures with all other operations. Fixtures may incorporate means of setting the cutting tools relative to the location system.
The advantages of jigs and fixtures can be summarised as follows:
1)Marking out and other measuring and setting out methods are eliminated;
2)Unskilled workers may proceed confidently and quickly in knowledge that the workpiece can be positioned correctly, and the tools guided or set;
3)the assembly of parts is facilitated, since all components will be identical within small limits, and “trying” and filing of work is eliminated;
4)The parts will be interchangeable, and if the product sold over a wide area, the problem of spare parts will be simplified.
Bolt holes often have 1.5mm or even 3.0mm clearance for the bolt, and the reader may doubt the necessity of making precision jigs for such work. It must be remembered that the jigs, once made, will be used on many components, and the extra cost of an accurately made jig is spread over a large output.
Furthermore, it is surprising how small errors accumulate in a mechanism during its assembly. When a clearance is specified, it is better to ensure its observance, rather than to allow careless marking out and machining to encroach upon it．
1) The location of workpiece. Fig.13.1 represents a body that is completely free in space. In this condition it has six degrees of freedom. Consider these freedoms with respect to the three mutually perpendicular axes XX, YY, and ZZ.
The body can move along any of these axes; it therefore has three freedoms of translation. It can also rotate about any of the three axes; it therefore has three freedoms of rotation. The total number of freedoms is six. When work is located, as many of these freedoms as possible must be eliminated, to ensure that the operation is performed with the required accuracy.
Accuracy is ensured by machining suitable location features as early as possible, and using them for all location, unless other considerations mean that other location features must be used. If it is necessary, the new location features must be machined as a result of location from the former location features.
2) The clamping of the workpiece. The clamping system must be such that the workpiece is held against the cutting forces, and the clamping forces must not be so great as to cause the workpiece to become distorted or damaged.
The workpiece must be supported beneath the point of clamping, to ensure that the forces are taken by the main frame of the jig or fixture, and on to the machine table and bed. When jigs and fixtures are designed, the clamping system is designed to ensure that the correct clamping force is applied, and that the clamps can be operated quickly but with safety.
? Definition of a Drill Jig
A drill jig is a device for ensuring that a hole to be drilled, tapped, or reamed in a workpiece will be machined in the proper place.
Basically it consists of a clamping device to hold the part in position under hardened-steel bushings through which the drill passes during the drilling operation. The drill is guided by the bushings.
If the workpiece is of simple construction, the jig may be clamped on the workpiece. In most cases, however, the workpiece is held by the jig, and the jig is arranged so that the workpiece can be quickly inserted and as quickly removed after the machining operation is performed.
Jigs make it possible to drill, ream, and tap holes at much greater speeds and with greater accuracy than when the holes are produced by conventional hand methods. Another advantage is that skilled workers are not required when jigs are used. Responsibility for the accuracy of hole location is taken from the operator and given to the jig.
The term jig should be used only for devices employed while drilling, reaming, or tapping holes. It is not fastened to the machine on which it is used and may be moved around on the table of the drilling machine to bring each bushing directly under the drill. Jigs physically limit and control the path of the cutting tool.
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If the operation includes machining operations like milling, planing, shaping, turning, etc., the term fixture should be used. A fixture holds the work during machining operations but does not contain special arrangements for guiding the cutting tool ,as drill jigs do.
? Typical Jigs and Fixtures
Typical drill jig. Figure 13.2 illustrates a drilling jig for drilling four holes in the flange of a workpiece that has been completed except for the four holes.
The workpiece has an accurately machined bore, and is located from the bore and the end face, from a cylindrical post. There is no need to control the rotational position about the axis of the bore, because up to the time when the holes are drilled, it is symmetrical about that axis.
The four bushes used to control the drill are held in the drill plate, which, with the hand n