suface finishing

8/7/2019 Suface Finishing

1/11

Surface finishing is a broad range ofindustrial processes that alter thesurface of a manufactured item to achieve a certain property. Finishingprocesses may be employed to: improve appearance, adhesion, solderability,corrosion resistance, chemical resistance, wear resistance, hardness, modifyelectrical conductivity, remove burrs and other surface flaws, and control the

surface friction In limited cases some of these techniques can be used torestore original dimensions to salvage or repair an item.

Surface finishing processes can be categorized by how they affect theworkpiece:

y Removing or reshaping finishingy Adding or altering finishing

Mechanical processes may also be categorized together because of similaritiesthe final surface finish.

Adding and altering

y Blanchingy Case hardeningy Ceramic glazey Claddingy Corona treatmenty Diffusion processes:

o Carburizingo Nitriding

y Electroplatingy Galvanizing

Removing and reshaping

y Abrasive blastingo Sandblasting

y Burnishingy Chemical-mechanical planarization (CMP)y E

lectropolishingy Flame polishingy Gas cluster ion beamy Grindingy Industrial etchingy Linishingy Mass finishing processes

o Tumble finishing


2/11

o Vibratory finishingy Picklingy Polishing

o Buffingy Peening

o Shot peeningy Superfinishing

Mechanical finishing

Mechanical finishing processes include:

y Abrasive blastingo Sandblasting

y Burnishingy Grindingy Mass finishing processes

o Tumble finishingo Vibratory finishing

y Polishingo Buffing

Lapping is a machining operation, in which two surfaces are rubbed together with an abrasivebetween them, by hand movement or by way of a machine.

This can take two forms. The first type of lapping (traditionally called grinding), typically

involves rubbing a brittle material such as glass against a surface such as iron or glass itself (alsoknown as the "lap" or grinding tool) with an abrasive such as aluminum oxide, jeweller's rouge,

optician's rouge, emery, silicon carbide, diamond, etc., in between them. This producesmicroscopic conchoidal fractures as the abrasive rolls about between the two surfaces and

removes material from both.

The other form of lapping involves a softer material such as pitch or a ceramic for the lap, which

is "charged" with the abrasive. The lap is then used to cut a harder materialthe workpiece. Theabrasive embeds within the softer material which holds it and permits it to score across and cut

the harder material. Taken to the finer limit, this will produce a polished surface such as with apolishing cloth on an automobile, or a polishing cloth or polishing pitch upon glass or steel.

Taken to the ultimate limit, with the aid of accurate interferometry and specialized polishing

machines or skilled hand polishing, lensmakers can produce surfaces that are flat to better than30 nanometers. This is one twentieth of the wavelength of light from the commonly used

632.8 nm helium neon laser light source. Surfaces this flat can be molecularly bonded (opticallycontacted) by bringing them together under the right conditions. (This is not the same as the

wringing effect ofJohansson blocks, although it is similar).


3/11

Contents

[hide]

y 1 Operationy 2 Two-piece lappingy 3 Accuracy and surface roughnessy 4 Measurement

o 4.1 Flatnesso 4.2 Roughness

y 5 See alsoy 6 References

Operation

Small lapping plate made of cast iron

By way of example, a piece oflead may be used as the lap, charged with emery, and used to cuta piece of hardened steel. The small plate shown in the first picture is that of a hand lappingplate. That particular plate is made ofcast iron. In use, a slurry of emery powder would be spread

on the plate and the workpiece simply rubbed against the plate, usually in a "figure-eight"pattern.

Small lapping machine


4/11

The second picture is that of a commercially available lapping machine which is needed for thisprocess. The lap or lapping plate in this machine is 30 centimetres (12 in) in diameter. For a

commercial machine, that is about the smallest size available. At the other end of the sizespectrum, machines with 8-to-10-foot-diameter (2.4 to 3.0 m) plates are not uncommon, and

systems with tables 30 feet (9.1 m) in diameter have been constructed. Referring to the second

picture again, the lap is the large circular disk on the top of the machine. On top of the lap aretwo rings. The workpiece would be placed inside one of these rings. A weight would then beplaced on top of the workpiece. The weights can also be seen in the picture along with two fiber

spacer disks that are just used to even the load.

In operation, the rings stay in one location as the lapping plate rotates beneath them. In thismachine, a small slurry pump can be seen at the side, this pump feeds abrasive slurry onto the

rotating lapping plate.

Lapping machine and retention jig

When there is a requirement to lap very small specimens (from 3" down to a few millimetres), a

lapping jig can be used to hold the material while it is lapped (see Image 3, lapping machine andjig). A jig allows precise control of the orientation of the specimen to the lapping plate and fine

adjustment of the load applied to the specimen during the material removal process. Due to thedimensions of such small samples, traditional loads and weights are too heavy as they would

destroy delicate materials. The jig sits in a cradle on top of the lapping plate and the dial on thefront of the jig indicates the amount of material removed from the specimen.

Two-piece lapping

Where the mating of the two surfaces is more important than the flatness, the two pieces can be

lapped together. The principle is that the protrusions on one surface will both abrade and beabraded by the protrusions on the other, resulting in two surfaces evolving towards somecommon shape (not necessarily perfectly flat), separated by a distance determined by the average

size of the abrasive particles, with a surface roughness determined by the variation in theabrasive size. This yields closeness-of-fit results comparable to that of two accurately-flat pieces,

without quite the same degree oftesting required for the latter.


5/11

Schematic of two-piece lapping

One complication in two-piece lapping is the need to ensure that neither piece flexes or isdeformed during the process. As the pieces are moved past each other, part of each (some areanear the edge) will be unsupported for some fraction of the rubbing movement. If one piece

flexes due to this lack of support, the edges of the opposite piece will tend to dig depressions intoit a short distance in from the edge, and the edges of the opposite piece are heavily abraded by

the same action - the lapping procedure assumes roughly equal pressure distribution across thewhole surface at all times, and fails in this manner if the workpiece itself deforms under that

pressure.

Accuracy and surface roughness

Lapping can be used to obtain a specific surface roughness; it is also used to obtain very accurate

surfaces, usually very flat surfaces. Surface roughness and surface flatness are two quite differentconcepts. Unfortunately, they are concepts that are often confused by the novice.

A typical range of surface roughness that can be obtained without resort to special equipment

would fall in the range of 1 to 30 Ra (average roughness in micrometers or microinches).

Surface accuracy or flatness is usually measured in Helium Light Bands, one HLB measuring

about 0.000011 inches (280 nm). Again, without resort to special equipment accuracies of 1 to 3HLB are typical. Though flatness is the most common goal of lapping, the process is also used to

obtain other configurations such as a concave or convex surface.

Measurement

Flatness

The easiest method for measuring flatness is with a height gage positioned on a surface plate.

Note that you must setup the part on three stands and find the minimum variation while adjusting

them, just placing the part on the surface plate and using a dial indicator to find TIR on theopposite side of the part measures parallelism. Flatness is more easily measured with a co-

ordinate measuring machine. But neither of these methods can measure flatness more accuratelythan about 0.0001" (2.5m).


6/11

optical flats in wooden case

Monochromatic light unit

Another method that is commonly used with lapped parts is the reflection and interference ofmonochromatic light.

[1]A monochromatic light source and an optical flat are all that are needed.

The optical flat which is a piece of transparent glass that has itself been lapped and polished onone or both sides is placed on the lapped surface. The monochromatic light is then shone down

through the glass. The light will pass through the glass and reflect off the workpiece. As the lightreflects in the gap between the workpiece and the polished surface of the glass, the light will

interfere with itself creating light and dark fringes. Each fringe or band represents a changeof one half wavelength in the width of the gap between the glass and the workpiece. The light

bands display a contour map of the surface of the workpiece and can be readily interpreted for

flatness. In the past the light source would have been provided by a Helium lamp or tube,

[citation

needed] but nowadays a more common source of monochromatic light is the low pressure sodiumlamp.

[citation needed]The picture to the right shows a typical monochromatic light unit used in

workshops and laboratories.

For a more thorough description of the physics behind this measurement technique, see

interference.


7/11

Roughness

Surface roughness is defined by the minute variations in height of the surface of a given material

or workpiece. The individual variances of the peaks and valleys are averaged (Ra reading), orquantified by the largest difference from peak-to-valley (Rz). Roughness is usually expressed in

microinches. A surface that exhibits an Ra of 8 consists of peaks and valleys that average nomore than 8 microinches over a given distance. Roughness may be also measured by comparing

the surface of the workpiece to a known sample. Calibration samples are available usually sold ina set and usually covering the typical range of machining operations from about 125 Ra to 1 Ra.

Surface roughness is measured with a profilometer, an instrument that measures the minutevariations in height of the surface of a workpiece.

Honing is an abrasive machining process that produces a precision surface on a metal workpiece

by scrubbing an abrasive stone against it along a controlled path. Honing is primarily used toimprove the geometric form of a surface, but may also improve the surface texture.

Typical applications are the finishing ofcylinders forinternal combustion engines, air bearing

spindles and gears. Types of hone are many and various but all consist of one or more abrasivestones that are held underpressure against the surface they are working on.

In everyday use, a honing steel is used to hone knives, especially kitchen knives, and is a fineprocess, there contrasted with more abrasive sharpening.

Other similar processes are lapping and superfinishing.

Contents

[hide]

y 1 Honing stonesy 2 Process mechanicsy 3 Honing configurationsy 4 Economicsy 5 Performance advantages of honed surfacesy 6 Cross-hatch finishy 7 Plateau finishy 8 See alsoy 9 Notes

[edit] Honing stones


8/11

Superabrasives and hone head for cylinders.

Honing tools

Honing uses a special tool, called a honing stone or a hone, to achieve a precision surface. Thehone is a composed of abrasive grains that are bound together with an adhesive. Generally,honing grains are irregularly shaped and about 10 to 50 micrometers in diameter (300 to 1,500

mesh grit). Smaller grain sizes produce a smoother surface on the workpiece.

A honing stone is similar to a grinding wheel in many ways, but honing stones are usually more

friable so that they conform to the shape of the workpiece as they wear in. To counteract theirfriability, honing stones may be treated with wax or sulfur to improve life; wax is usually

preferred for environmental reasons.[1]

Any abrasive material may be used to create a honing stone, but the most commonly used are

corundum, silicon carbide, cubic boron nitride, ordiamond. The choice of abrasive material isusually driven by the characteristics of the workpiece material. In most cases, corundum orsilicon carbide are acceptable, but extremely hard workpiece materials must be honed using

superabrasives.[1]

The hone is usually turned in the bore while being moved in and out. Special cutting fluids areused to give a smooth cutting action and to remove the material that has been abraded. Machines


9/11

can be portable, simple manual machines, or fully automatic with gauging depending on theapplication.

Modern advances in abrasives have made it possible to remove much larger amount of material

than was previously possible. This has displaced grinding in many applications where "through

machining" is possible. External hones perform the same function on shafts.

[edit] Process mechanics

Since honing stones look similar to grinding wheels, it is tempting to think of honing as a formof low-stock removal grinding. Instead, it is better to think of it as a self-truing grinding

process.[2]

In grinding, the wheel follows a simple path. For example, in plunge grinding a shaft, the wheelmoves in towards the axis of the part, grinds it, and then moves back out. Since each slice of the

wheel repeatedly contacts the same slice of the workpiece, any inaccuracies in the geometric

shape of the grinding wheel will be transferred onto the part. Therefore, the accuracy of thefinished workpiece geometry is limited to the accuracy of the truing dresser. The accuracybecomes even worse as the grind wheel wears, so truing must occur periodically to reshape it.

The limitation on geometric accuracy is overcome in honing because the honing stone follows acomplex path. In bore honing for example, the stone moves along two paths simultaneously. The

stones are pressed radially outward to enlarge the hole while they simultaneously oscillateaxially. Due to the oscillation, each slice of the honing stones touch a large area of the

workpiece. Therefore, imperfections in the honing stone's profile cannot transfer to the bore.Instead both the bore and the honing stones conform to the average shape of the honing stones'

motion, which in the case of bore honing is a cylinder. This averaging effect occurs in all honing

processes; both the workpiece and stones erode until they conform to the average shape of thestones' cutting surface. Since the honing stones tend to erode towards a desired geometric shape,there is no need to true them. As a result of the averaging effect, the accuracy of a honed

component often exceeds the accuracy of the machine tool that created it.

The path of the stone is not the only difference between grinding and honing machines, they alsodiffer in the stiffness of their construction. Honing machines are much more compliant than

grinders. The purpose of grinding is to achieve a tight size tolerance. To do this, the grindingwheel must be moved to an exact position relative to the workpiece. Therefore a grinding

machine must be very stiff and its axes must move with very high precision.

A honing machine, ironically, is relatively inaccurate and compliant. Instead of relying on theaccuracy of the machine tool, it relies on the averaging effect between the stone and theworkpiece. In fact, compliance is a requirement of a honing machine that is necessary for the

averaging effect to occur. This leads to an obvious difference between the two machines: in agrinder the stone is rigidly attached to a slide, while in honing the stone is actuated with

pneumatic or hydraulic pressure.


10/11

High-precision workpieces are usually ground and then honed. Grinding determines the size, andhoning improves the shape.

The difference between honing and grinding is not always distinct. Some grinders have complex

movements and are self-truing, and some honing machines are equipped with in-process gaging

for size control. Many through-feed grinding operations rely on the same averaging effect ashoning.

1. Surface finishing process-LappingLapping is a surface finishing process used to give a surface finish to articles. It is

essentially an abrading process. Very less material can be removed from this process only

around 0.005 to 0.01 mm is removed. This is a low efficiency process and is used only when

a required amount of surface finish is required.

In this process, the laps i.e.; lapping shoes are charged with abrading powders like emery,

iron oxide, chromium oxide. Laps are made from any material like soft cast Iron soft steel,brass etc. These laps are made to rub against the surface so that the irregular surface

erodes. For better surface finish, the laps can be given reciprocating motion etc. In short

two types of motions can be combined

The lapping machine is available in three types.

1.Vertical axis lapping machine

2. Center less lapping machine

3.Abrasive belt lapping machine.


11/11

suface finishing

Documents