ABSTRACT Lapping is low pressure, low speed abrading operation for refining surface finish within the high dimensional and geometrical accuracies. Lapping is generally used for flat, regular surfaces, gauges, piston rings, pins. valves, roller bearings, crank shaft, cylindrical surfaces etc. centreless lapping machine is generally used for super finishing operations on cylindrical surfaces and this machine gives super finish in microns.
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Transcript
ABSTRACT
Lapping is low pressure, low speed abrading operation for refining
surface finish within the high dimensional and geometrical accuracies.
Lapping is generally used for flat, regular surfaces, gauges, piston
rings, pins. valves, roller bearings, crank shaft, cylindrical surfaces etc.
centreless lapping machine is generally used for super finishing operations
on cylindrical surfaces and this machine gives super finish in microns.
CONTENTS
1. INTRODUCTION
2. TYPES OF SURFACE FINISH
2.1 HONING
2.2 SUPERFINISHING
2.3 ROLLER BURNISHING
2.4 LAPPING
3. TYPES OF LAPPING
3.1 HAND LAPPING
3.1.1 LAPPING FLAT SURFACES
3.1.2 LAPPING OF INTERNAL CYLINDRICAL SURFACES
3.2 MACHINE LAPPING
3.2.1 VERTICAL LAPPING M/C WITH TWO CIRCULAR PLAT
3.2.2 MACHINE WITH SINGLE ROTATING CIRCULAR PLATE LAP
3.2.3 CENTRELESS ROLL LAPPING MACHINE
3.2.4 CENTRELESS LAPPING MACHINE
3.2.5 LAPPING MACHINES FOR INTRENAL CYLINDRICAL
SURFACES
4. LAPPING OF SLIP GAUGES
5. LAPPING FINISHES AND ACCURACIES
6. LAP MATERIALS
7. LAPPING MEDIUM
7.1 SILICON CARBIDE
7.2 ALUMINIUM OXIDE
7.3 BORONCARB1DE
7.4 DIAMOND
8. VEHICLES
9. LAPPING SPEEDS AND PRESSURES
10. ALLOWANCES FOR LAPPING
11. DESIGN
11.1 MOTOR SPECIFICATION
11.2 PULLIES SELECTION
11.3 CALCULATION FOR LENGTH OF BELT
11.3.1 FOR LAPPING WHEEL
11.3.2 FOR REGULATING WHEEL
11.4 CALCULATION FOR BELT TENSION
11.4.1 FOR LAPPING WHEEL
11.4.2 FOR REGULATING WHEEL
11.5 SHAFT DESIGN
11.5.1 1NTERMIDIATE SHAFT
11.5.2 SHAFT DESIGN FOR LAPPING WHEEL
11.5.3 SHAFT DESIGN FOR REGULATING WHEEL
11.6 BEARING SELECTION
12 PROCESS CHARTS
12.1 LAPPING WHEEL SHAFT
12.2 REGULATING WHEEL SHAFT
12.3 INTERMIDIATE SHAFT
12.4 BEARING HOLDRE
12.5 LAPPING WHEEL
12.6 REGULATING WHEEL
13. COST SHEET
14. CONCLUSION
15. REFERENCES
LIST OF FIGURE
TITLE
LAP FOR EXTERNAL THREADS.
LAPPING MANDREL.
VERTICAL LAPPING M/CWITII UONDLD AIJKAC'IVI-S LAPS
CENTRELESS ROLL LAPPING M/C
SPHERICAL LAPPING
DIAGRAMS FOR DESIGNS
DIAGRAMS FOR PROCESS CHARTS
LIST OF TABLES
TAB.NO. TITLE
LAPPING ACCURACY AND FINISH
RECOMMENDED LAPPING ALLOWANCES
LIST OF PROCESS CHART
CHART NO. TITLE
01 LAPPING WHEEL SHAFT
02 REGULATING WHEEL SHAFT
03 INTERMIDIATE SHAFT
04 BEARING HOLDER
05 LAPPING WHEEL
06 REGULATING WHEEL
1. INTRODUCTION
The dimensional and geometrical accuracy obtained by normal
method of machining, like turning, milling, etc. are limited. The geometrical
error includes circularity, cylindrical, flatness and parallelism of functional
surfaces. Also, the surface finish has vital influence on most important
functional properties such as wear resistant, fatigue strength, corrosion
resistance and power losses due to friction. Poor surface finish will lead to
rupture of oils films on the picks of micro irregularities, which lead to state
approaching dry friction, and results in excessive wear of rubbing surfaces.
Therefore, fine finishing processes are employed in machining the surfaces
of many critical components to obtained very high surface finish or high
dimensional and geometrical accuracies. Such processes include honing,
super finishing, burnishing and lapping.
2. TYPES OF SURFACE FINISH
There are four types of surface finishing methods, they are as follows.
1. Honing.
2. Super finishing.
3. Roller burnishing.
4. Lapping.
2.1 HONING
Honing is low velocity abrading process in which the stock is removed from
metallic or non-metallic surfaces by bonded abrasive sticks. It is a finishing
operation employed not only to produce high surface finish, but also to
correct out-of-roundness, taper and axial distortion in work pieces.
In honing, since a simultaneous rotating and reciprocating motion is given to
stick, the surface produce will have a characteristic crosshatch lay pattern.
Honing is employed very frequently for finishing of bores, but there arc
numerous external surfaces, which are honed to obtain required properties.
Some examples are gear teeth, valves setting, race of ball and roller bearing.
Moreover, bores of any size can be honed whether long, short, blind,
tandem, or with keyways. It is effective on almost any ferrous or non-ferrous
material in a hardened or soft conditioned.
2.2 SUPERFINISHING
Super finishing is an abrasive process utilizing either a bonded abrasive
stone for cylindrical surfaces or a cup wheel for flat and spherical surfaces.
It is, in a way, a fine honing operation. By this process it is possible to
achieve higher surface finish on components. In addition, Super finishing
removes chatter marks, feed spirals, grindings commas and other
imperfection left by the previous grinding operation.
Stock removal in the Super finishing process is of the order of 0.005 to
0.025 mm on diameter. The surface produce will have a mirror like finish,
and surface finish obtained is of order of 0.05 to 0.2 micrometers.
In super finishing, the stones makes contact with the work piece over a large
area. The stone is given an oscillating motion in the axial direction and
simultaneously the job is given a rotary motion about the axis. As feed
movement could also be given, if necessary. The contact pressure exerted by
the stone on the work piece is very low, and it normally depends on material
being machined. Some time a light pressure is used in the beginning and
when the operation near completion, the pressure is increased to about 3
Kg/cm". The low operating pressure does not cause any appreciable
temperature rise and has a tendency to improve (he geometrical shape of the
part.
2.3 ROLLER BURNISHING
Roller burnishing is cold working process, by which improvement in surface
finish; dimensional accuracy and work hardening can be effected, without
the removing metal. It is finishing operation, and normally done on parts
which arc turned, bored, reamed, or ground. Any ductile or malleable
material with hardness less than 40 HRC can be successfully burnished.
Surfaces that could be burnished include external and internal cylindrical
surfaces, external and internal tapered surfaces, spherical surfaces and
circular flat surfaces. Machines normally used for burnishing operations can
be drill presses, lathe, boring machine, automatic bar chucking machines,
multispindles machines, or even special purpose transfer machines.
Instead of reaming, roller burnishing is some time used for finishing, but it is
most often used to supplement reaming or boring. Since roller burnishing
improves the surface hardness, metals that work hardens rapidly must be at
lower hardness before roller burnishing is done. The depth of surface
hardness obtained by roller burnishing varies from 0.1 to 0.7 mm.
2.4 LAPPING
Lapping is a low pressure abrading process, which is employed as precision
finishing operation to:
Improve geometrical accuracy
Achieve high dimensional accuracy
Refine surface finish
Ensure close fit between mating parts
Lapping is perform manually or by machine by charging a lap made of
material softer than the works piece with abrasive particles and rubbing with
over the work piece surface with slight pressure. Special lubricating and
bonding agent, known as vehicles, are used during the process
Hand lapping is done with abrasive powder as the lapping medium, where as
machine lapping is done either with abrasive powder, or with bonded
abrasive wheels. In both manual and machine lapping operations, the quality
of surface finish and the extent of dimensional accuracy that can be achieved
mainly depend on-
1] The type of lap material. 2] Type of lapping medium. 3] Speed of lapping
motion and pressure applied. 4] Material to be lapped.
Various lapping methods are divided in two main groups.
3. Types of lapping
1] Hand lapping. 2] Machine lapping.
3.1HAND LAPPING
3.1.1 LAPPING FLAT SURFACES
Hand lapping of flat pieces is perform by rubbing the parts over the
accurately finish flat surface of a master lap, accomplishing the abrading
action by a very fine abrasive powder mixed with vehicle. The work is
moved relative to master lap, along an ever changing path to ensure uniform
abrasive of both work and lap, and to eliminate, as far as possible, parallel
grain marks. A suitable abrasive should be selected for the works and it must
used sparingly as its excessive used will increase the wear op lap.
In lapping flat surfaces, fairly thick lapping blocks made of soft close-grain
cast iron, are used as a lap. For rough works, lap works better if it surface is
serrated. The lapping compounds collects in the grooves and continuously
rolls in and out as the work is moved, getting between the plate and work,
and it is crush into the cast iron, thus charging it toughly in short time. Cast
iron Laps, with plane flat surfaces, are used for work pieces, which have
been ground on a surface grinder.
Manual lapping of flat surfaces demands high operating skill since the
proper lapping speed and pressure depend upon his feel and judgment. Ring
lapping is simplest method of lapping external cylindrical surfaces. Ring
Laps are generally made of close grain cast iron.
Ring lap should always be shorter than the work piece and if the size
permits, it should have adjustable slots. The bore of ring lap should be very
close to size of work. Precision adjustments are normally made with the
used of closing and expanding screws provide on the lap. In some design, to
cover a range of diameters, a single holder with a set of interchangeable
bushes is employed. The work to be lapped is held in the chuck of lathe and
rotated, while the split ring lap held over the cylindrical surface is
reciprocated. The abrasive and vehicle are fed through the slots in the ring
lap and, when reciprocated, the lap should overrun the work by about one
third of its length.
Ring lapping recommended for plug gauges made in small quantities and for
precision machine spindle where great roundness accuracy is required.
External threads can also be lapped by this method. The laps [fig 01] are the
once which have an interchangeable threaded bush corresponding to the
external thread to lapped, with provision for precise adjustment.
Ring lapping when performed by a skill operator, offers two advantages over
machine lapping:
1} The out-of-roundness error can be corrected to higher degree than in
machine lapping.
2] The parts can be produced to extremely close tolerances.
Since ring lapping is tedious and calls for higher skills of the operator, it
should be considered only when:
1] There is too much of out of roundness in the work piece
2] The lot size is small
3] The works piece has key way or flats or similar interruptions on the
surface
4] The work piece has two or more different diameters to be lapped.
3.1.2 LAPPING; OF INTERNAL CYLINDRICAL SURFACES
Holes or bores are lapped using solid or adjustable laps. Rolling the laps on honing
machines, lathes or polishing heads, while the works piece is reciprocated over it does
the tapping operation.
Solid lap for larger sizes are generally made of cast iron, For the smaller sizes, steel or
hard brass is used. The laps are ground straight and round. Helical grooves cut on the
surface provide clearance for feeding abrasive and vehicles.
Adjustable laps are suitable for all plasters of hole lapping. Design of adjustable laps
for hole lapping include laps with partial split sleeves with screws expansion devices,
and laps with replaceable copper sleeves on taper mandrels for expansion of sleeves
(tig.02). Ring gauges arc examples of apart that are hip to extremely accurate
dimension with adjustable lap.
To obtain good results with lapping holes, the length of the lap should longer than the
work. The lap makes contact with the full length of the hole, thus preventing the
tendency to follow any already existing curvature. In internal cylindrical lapping,
virtually no material is removed, as the ease in honing. Sometimes, lapping follows
honing has touch up operation. The widest application of internal lapping is in match
lapping. In this, the male or female parts are hipped together to obtain the matched
pairs.
3.2 LAPPING MACHINES AND MACHINE LAPPING
Lapping machine has facilitated the machining of economic batch quantity. On
lapping machine, three types of lapping media arc used. Machines using metal laps
and abrasive powder mixed with suitable vehicle are employed where extreme
accuracy is required. Machines using bonded abrasive wheels are suitable for
commercial production. More recently, machines which employ abrasive paper or
cloth as lapping medium have also been developed.
Lapping machines can be mainly classified in to three types:-
Machine using cast iron or bonded abrasive circular plates as laps for lapping
cylindrical and Hat work pieces.
Machines using cast iron or bonded abrasive wheels as laps working on centre less
lapping principle for lapping cylindrical work pieces.
Machines similar to honing machine used for lapping internal cylindrical surfaces.
3.2.1 VERTICLE LAPPING MACHINE WITH TWO CIRCULER
PLATES
This type of lapping machine consists of two opposed, heavy east iron or bonded
abrasive circular plates held in vertical spindles (12.17). When cast iron laps are used,
the lower lap drive the work pieces when the former is rotated at a speed of not more
than 100m/min. the upper lap is held stationary but is free to float in the vertical
direction . It rest upon the work during the lapping operation, thus applying the
constant pressure.
The work pieces are retained between these laps in slotted plates and made to rotate
and slide. The lower lap regulates the speed of rotation. The abrasive is used with a
paste type vehicle and is applied on the lap before the cycle is started. Oil or kerosene
is added during the cycle to prevent drying of vehicle.
Very fine finish of the order of 0.02micrometcr and tolerance of 0.5 micrometer are
feasible when cast iron laps are used. When bonded abrasive laps are used, both laps
are rotated. The laps are rigidly supported on the spindles and separately driven at
speed higher than that used for cast iron laps. Kerosene or similar lubricant is used as
coolant as well as to Hush the chips. Lapping takes place al a faster rate and
consequently, the machine does not produce extreme accuracy possible with
machines using cast iron laps. Since bonded abrasive laps must be dressed with
diamond tools, it is not possible le to make them as flat as cast iron laps on which the
machine generates flatness. Both cylindrical and flat lapping operation can be done on
this type of machine with the same arrangement of lapping plates.
Machine lapping between plates is an economical finishing method for parts like plug