CMfgEProfessor Emeritus Engineering Technology Lawrence Technological University Former Chairman Detroit Chapter ONE Society of Manufacturing Engineers Former President International Exc
Trang 2George Schneider, Jr CMfgE
Professor Emeritus
Engineering Technology
Lawrence Technological University
Former Chairman
Detroit Chapter ONE
Society of Manufacturing Engineers
Former President
International Excutive Board
Society of Carbide & Tool Engineers
Lawrence Tech.- www.ltu.edu
Prentice Hall- www.prenhall.com
CHAPTER 18 Lapping and Honing
Metal Removal Cutting-Tool Materials
Metal Removal Methods
Machinability of Metals
Single Point Machining Turning Tools and Operations
Turning Methods and Machines
Grooving and Threading
Shaping and Planing Hole Making Processes Drills and Drilling Operations
Drilling Methods and Machines
Boring Operations and Machines
Reaming and Tapping Multi Point Machining Milling Cutters and Operations
Milling Methods and Machines
Broaches and Broaching
Saws and Sawing Abrasive Processes Grinding Wheels and Operations
Grinding Methods and Machines
Lapping and Honing
FIGURE 28.1: Typical lapping machine.
(Courtesy Engis Corp.)
18.1 Introduction
Lapping is a final abrasive finishing operation that produces extreme dimensional accuracy, corrects minor imperfections of shape, refines surface finish, and produces close fit between mating surfaces Most lapping is done with a tooling plate or wheel (the lap), and fine-grained loose abrasive particles suspended in a viscous or liquid vehicle such as soluble oil, mineral oil, or grease A typical lapping operation is shown in Figure 18.1
Honing is a low velocity abrading process Material removal is accomplished at lower cutting speeds than in grinding Therefore, heat and pressure are minimized, resulting in excellent size and geometry control The most common application of honing is on internal cylin-drical surfaces The cutting action is obtained using abrasive sticks mounted on a metal mandrel Since the work is fixed in such a way
as to allow floating, and no clamping or chucking, there
is no distortion
18.2 Lapping Processes
The principal use of the lapping process is to obtain surfaces that are truly flat and smooth Lapping is also used to finish round work, such as precision plug gages, to tolerances of 0.0005 to 0.00002 inches
Work that is to be lapped should be previously finished close to the final size While rough lapping can remove considerable metal, it is customary to leave only 0.0005 to 0.005 inches of stock to be removed
Lapping, though it is an abrasive process, differs from grinding or hon-ing because it uses a ‘loose’ abrasive instead of bonded abrasives like grind-ing wheels (Fig 18.2)
These abrasives are often purchased
‘ready mixed’ in a ‘vehicle’ often made with an oil-soap or grease base These vehicles hold the abrasive in suspen-sion before and during use The paste abrasives are generally used in hand-lapping operations For machine lap-ping, light oil is mixed with dry abra-sive so that it can be pumped onto the lapping surface during the lapping op-eration
18.2.1 Lapping Machines
These machines are fairly simple pieces of equipment consisting of a rotating table, called a lapping plate, and three or four conditioning rings Standard machines have lapping plates
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from 12 to 48 inches in diameter Large
machines up to 144 inches are made 1
to 20 HP motors run these tables A
typical lapping machine is shown in
Figure 18.3
The lapping plate is most frequently
made of high-quality soft cast iron,
though some are made of copper or
other soft metals This plate must be
kept perfectly flat The work is held in
the conditioning rings These rings
ro-tate as shown in Fig 18.4 This rotation
performs two jobs First it ‘conditions’
the plate, that is, it distributes the wear
so that the lapping plate stays flat for a
longer time Secondly, it holds the
workpiece in place The speed at which
the plate turns is determined by the job
being done In doing very critical
parts, 10 to 15 RPM is used, and when
polishing, up to 150 RPM is used
one step Also, less time is required for cleaning parts and processing waste; throughput, along with overall produc-tivity, is increased
Lapping plates are manufactured from various materials as described below, and are available in standard sizes from 6 to 48 inches in diameter Plates are supplied with square, spiral, and concentric and radial grooves as shown in Figure 18.5
Iron - Aggressive Stock Removal:
• Excellent primary/roughing lap plate, with long service life
• Often used as an alternative to cast iron plates
• Produces a good surface finish on most materials, especially metals and ceramics
Copper - Moderate to Aggressive Stock Removal:
• Most widely used, universal com-posite lap plate
• Excellent when primary and fin-ishing lap are combined in a one step operation
• Suitable for virtually any solid material: metal, ceramic, glass, car-bon, plastic, etc
Ceramic - Moderate Stock Removal:
• Generally used to lap/polish ce-ramic parts and other stain- sensitive materials
• Used in applications where metal-lic-type contamination cannot be tol-erated
• Affordable, more machinable al-ternative to ‘natural’ ceramic plates
Figure 18.3 Typical dual plate lapping
ma-chine (Courtesy Engis Corporation)
Figure 18.2 Abrasive grit must be uniformly graded to be effective
in lapping.
Workpiece
Lap
A pressure of about 3 pounds per square inch (PSI) must be ap-plied to the
w o r k p i e c e s Sometimes their own weight is suf-ficient If not, a round, heavy pres-sure plate is placed in the con-ditioning ring The larger machines use pneumatic or hydraulic lifts to place and remove the pressure plates Figure 18.5 shows various lapping plates
The workpiece must be at least as hard as the lapping plate, or the abra-sive will be charged into the work It will take from 1 to 20 minutes to complete the machining cycle Time depends on the amount of stock re-moved, the abrasive used, and the qual-ity required Figure 18.6 shows a pro-duction-lapping machine
18.2.2 Grit and Plate Selection
Flatness, surface finish, and a pol-ished surface are not necessarily achieved at the same time or in equal quality For example, silicon carbide compound will cut fast and give good surface finish, but will always leave a
‘frosty’ or matte surface
The grits used for lapping may occa-sionally be as coarse as 100 to 280 mesh More often the ‘flour’ sizes of
320 to 800 mesh are used The grits, mixed in slurry, are flowed onto the plate to replace worn-out grits as the machining process continues
The case for using diamond super abrasives rather
than conventional abrasives such as aluminum oxide
or silicon carbide can be summed
up in three words
Diamonds are
faster, cleaner,
and more
cost-ef-fective.
With diamond slurries, the lap-ping and polish-ing phases of a finishing opera-tion can often be combined into Figure 18.4 Conditioning rings used in lapping operations.
Trang 4Tin/Lead - Fine Stock Removal:
• Most widely used finishing lap/
polishing plate
• Often used in place of polishing
pads
• Suitable for metal, ceramic and
other materials
Tin - Fine Stock Removal
• Often used where lead-type
con-tamination cannot be tolerated
• Suitable for charging of extra-fine
particulates
18.3 Advantages and Limitations
Any material, hard or soft, can be lapped, as well as any shape, as long as the surface is flat
Advantages: There is no warping,
since the parts are not clamped and very little heat is generated No burrs are created In fact, the process re-moves light burrs Any size, diameter, and thickness from a few thousandths thick up to any height the machine will handle can be lapped Various sizes and shapes of lapped parts are shown
in Figure 18.7
Limitations: Lapping is still
some-what of an art There are so many variables that starting a new job re-quires experience and skill Even though there are general recommenda-tions and assistance from the manufac-turers, and past experience is useful, trial and error may still be needed to get the optimum results
18.4 Honing Processes
As stated earlier, honing is a low velocity abrading process Material re-moval is accomplished at lower cutting speeds than in grinding Therefore, heat and pressures are minimized, re-sulting in excellent size and geometry control The most common application
of honing is on internal cylindrical surfaces A typical honing operation is shown in Figure 18.8
Machining a hole to within less than 0.001 inch in diameter and maintain-ing true roundness and straightness
with finishes less than 20 u inches is
one of the more difficult jobs in manu-facturing
Finish boring or internal grinding may do the job, but spindle deflection, variation in hardness of the material, and difficulties in precise work hold-ing, make the work slow and the re-sults uncertain Honing, because it uses rectangular grinding stones in-stead of circular grinding wheels, as shown in Figures 18.9a and 18.9b, can correct these irregularities
Honing can consistently produce
finishes as fine as 4 u inches and even
finer finishes are possible It can re-move as little as 0.0001 inch of stock
or as much as 0.125 inch of stock
However, usually only 0.002 to 0.020 inch stock is left on the diameter for honing As shown in Figure 18.10, honing can correct a number of
condi-tions or irregularities, left by previous operations
18.5 Honing Machines
For most work, honing machines are quite simple The most used honing machines are made for machining in-ternal diameters from 0.060 to 6 inches However, large honing
ma-chines are made for diameters up to 48 inches Larger machines are some-times made for special jobs
The length of the hole that can be honed may be anything from 1/2 inch
to 6 or 8 inches on smaller machines, and up to 24 inches on larger ma-chines Special honing machines are made which will handle hole lengths
up to 144
18.5.1 Horizontal Spindle Machines
Horizontal-spindle honing ma-chines, for hand-held work with bores
up to 6 inches, are among the most widely used The machine rotates the hone at from 100 to 250 FPM
Figure 18.5 Typical lapping plates
(Cour-tesy Engis Corporation)
Figure 18.7 Various sizes and shapes of lapped
parts (Courtesy Engis Corporation)
Figure 18.8 Typical vertical honing operation.
(Courtesy Sunnen Products Co.)
Figure 18.6 Single plate lapping
produc-tion machine equipped for diamond abrasive
slurry use (Courtesy Engis Corporation)
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The machine operator moves the
work back and forth (strokes it) over
the rotating hone The operator must
‘float’ the work, that is, not press it
against the hone or the hole will be
slightly oval Sometimes the
work-piece must be rotated
Horizontal-spindle honing machines
are also made with ‘power stroking’ In these, the work is held in a self-align-ing fixture and the speed and length of the stroke are regulated by controls on the machine
As a hone is being used, it is ex-panded by hydraulic or mechanical means until the desired hole diameter
is achieved Various mechanical and electrical devices can be attached to the honing machine to control the rate
of expansion, and stop it when final size is reached
On the simplest hand-held ma-chines, the operator may check the bore size with an air gage, continue honing, recheck, etc until the size is correct A horizontal-spindle honing machine is shown in Figure 18.11
18.5.2 Vertical Spindle Machines
Vertical-spindle honing machines are used especially for larger, heavier work These all have power stroking at speeds from 20 to 120 FPM The length of the stroke is also machine controlled by stops set up by the operator
Vertical honing machines are also made with multiple spindles so that sev-eral holes may be machined at once, as
in automobile cylinders (Figure 18.8)
Hone Body: The hone body is made
in several styles using a single stone for small holes, and two to eight stones
as sizes get larger (Fig 18.9b) The stones come in a wide variety of sizes and shapes Frequently there are hard-ened metal guides between the stones
to help start the hone cutting in a straight line
Cutting Fluid: A fluid must be used
with honing This has several pur-poses: to clean the small chips from the stones and the workpiece, to cool the work and the hone, and to lubricate the cutting action
A fine mesh filtering system must
be used, since recirculated metal can spoil the finish
A vertical honing operation was shown in Figure 18.8 A few of the parts honed on such a machine are shown in Figure 18.12
18.6 Abrasive Tool Selection
The abrasive honing stone must be selected for the proper abrasive type, bond hardness and grit size to deliver the fastest stock removal and desired
surface finish This selection is simple
if done in the following three steps:
Step One: Select the abrasive type
with respect to the material composi-tion of the bore There are four differ-ent types of abrasives: aluminum ox-ide, silicon carbox-ide, diamond, and CBN All four of these were discussed
in the previous chapter Each type has its own individual characteristics that make it best for honing certain materi-als Some simplified guidelines for their use are:
• Mild steel hones best with alumi-num oxide
• Cast iron, brass, and aluminum hone best with silicon carbide
• Glass, ceramic, and carbide hone best with diamond
• High speed tool steels, and super alloys hone best with CBN
Mandrel Honing shoe
Honing stone
Workpiece
(a)
Figure 18.9a Schematic illustration of the
com-ponents of an internal hone.
Figure 18.9b Typical honing tool is shown
be-ing Checked (Courtesy: Gehrbe-ing L.P.)
Alignment of tandem holes Correcting bellmouth Correcting taper Correcting rainbow-shaped holes
Figure 18.10 Undesirable conditions that can
be corrected by honing.
Trang 6Diamond and CBN are considered
super abrasives because they are much
harder than conventional abrasives
They cut easily and dull slowly,
there-fore allowing them to hone certain
materials much faster and more
effi-ciently than conventional abrasives
However, as shown above, super
abra-sives are not suited to honing all
mate-rials For instance, diamond does not
hone steel very well, and CBN may not
be as economical as using aluminum
oxide to hone soft steel
Step Two: Use the stone hardness
suggested in the manufacturer’s
cata-log If the stone does not cut, select the
next softer stone; if the stone wears too
fast, select the next harder stone Stone
hardness does not refer to the hardness
of the abrasive grain, but to the
strength of the bonding material
hold-ing the abrasive grains together, as
discussed in the previous chapter A
bond must be strong enough to hold
sharp abrasive grains in position to cut,
but weak enough to allow dulled grains
to be sloughed off to expose underly- Figure 18.13 Plateau honed finish surface at 100x (Cour- tesy: Gehring L.P.)
Figure 18.12 Parts honed on a vertical honing
ma-chine (Courtesy Sunnen Products Co.)
Figure 18.11 Horizontal-spindle honing
machine (Courtesy Sunnen Products Co.)
ing sharp grains If the bond is too hard, the dulled abrasive grains will not be allowed to fall off, and the stock removal rate will be reduced If the bond is too soft, the stone will wear excessively because sharp abrasive grains fall off before they are fully used
Diamond and CBN abrasive grains dull so slowly that standard ceramic or resin bonds may not be strong enough when honing rough out-of-round bores
in hard materials, or when CBN is used
to hone soft steel Metal bonds are best suited for these applications because the grains are held in a sintered metal matrix that is much stronger than stan-dard bonds As with choosing abrasive type, stone bond hardness must be matched to the application to maxi-mize life and stock removal rates
Step Three: Select the largest
abra-sive grit size that will still produce the desired surface finish Surface finish is
a function of the height of microscopic peaks and valleys on the bore surface
and honing can produce al-most any degree of rough-ness or smoothrough-ness through the use of different abrasive grit sizes
Honing oil can improve stock removal rates by help-ing the cutthelp-ing action of the abrasive grains It prevents pickup (spot welding of tool
to bore) and loading (chips coating the stone) Honing oil does this, not by acting as
a coolant, but through chemical activity The ingredients in the oil produce this chemical activity
Whenever the temperature rises at one
of the microscopic cutting points, the sulfur in the oil combines with the iron
in the steel to form iron sulfide, an unweldable compound, and weld-ing is prevented The antiwelding property of honing oil also pre-vents chips from stick-ing together and coat-ing the stone Water based coolants cannot produce this type of chemical activity Use
of water-based coolants will result in welding
of metallic guide shoes
to the part and loading of vitrified abrasive honing stones
18.7 Cylinder Block Honing
Bores sometime require a prelimi-nary rough honing operation to remove stock, followed by finish honing to get the desired surface finish A character-istic feature of a honed surface finish
is crosshatch, which makes an excel-lent oil retention and bearing surface The crosshatch pattern is generated in the bore surface as the workpiece is stroked back and forth over the rotat-ing honrotat-ing tool
Plateau Honing: A few years ago a
special surface finish generated inter-est in the engine rebuilding market With this finish, the valleys are deep and the peaks have been removed to form plateaus, giving the name plateau honing or plateau finish as shown in Figure 18.13 A recent test by a ring manufacturer has shown that an engine with a true plateau finish consumed one-tenth the oil and had 80 percent less cylinder bore wear than the en-gines with conventional finishes
Laser-Honing: With this process,
considerably better results are achieved compared to traditional hon-ing Precisely defined surface struc-tures can be obtained with Laser tech-nology Laser-honing is a combination
of honing and Laser processing This process generates Laser-produced lu-bricant reservoirs into a specifically defined area in order to achieve an ideal plateau surface finish Such a hydrodynamic system can be produced exactly where it is required as shown in Figure 18.14
Application of the Laser-honing process requires three steps In the first step – rough honing – the macro-form
Trang 7Chap 18: Lapping and Honing
Figure 18.15 Single-stroke honing tools use
expandable diamond-plated sleeves on a ta-pered arbor (Courtesy Sunnen Products Co.)
Figure 18.14 Laser generated honed finished surface
(Cour-tesy: Gehring L.P.)
of the bore is produced In the second
step, precisely defined lubricant
reser-voirs are produced with the Laser In
step three –finish honing – an
ex-tremely fine surface finish is obtained,
resulting in increased engine life by
reduction of wear in the cylinder
sur-face and on the piston rings
18.8 Production Honing
Honing will not only remove stock
rapidly, but it can also bring the bore to
finish diameter within tight tolerances
This is especially true if the honing
machine is equipped with automatic
size control With every stroke, the
workpiece is pushed against a sensing
tip that has been adjusted to the finish
diameter of the bore When the bore is
to size, the sensing tip enters the bore
and the machine stops honing Size
repetition from bore to bore is 0001
inch to 0002 inch The operator
sim-ply loads and unloads the fixture and
presses a button; everything else is
automatic
Single-Stroke Honing: A still faster
and more accurate method of honing a
bore to final size is Single-Stroke
hon-ing The Single-Stroke tool (Fig
18.15) is an expandable diamond
plated sleeve on a tapered arbor The
sleeve is expanded only during set up,
and no adjustments are necessary
dur-ing hondur-ing Unlike conventional
hon-ing, where the work-piece is stroked back and forth over the tool,
in Single-Stroke hon-ing the rotathon-ing tool is pushed through the bore one time, bring-ing the bore to size
The return stroke does nothing to the bore ex-cept get the workpiece off the tool Single-Stroke honing is so ac-curate and consistent, that honed bores do not require gaging
Although Single-Stroke honing has many advantages, it is limited in the types and volumes of material that can be removed The size and overall volume
of chip produced in one pass must be
no more than the space between the diamond grits, or the tool will seize in the bore
Workpieces are best suited for Single-Stroke honing when they are made of materials that produce small chips, such as cast iron, and that have interruptions that allow chips to be washed from the tool as the bore is being honed Conventional honing should be used whenever the material
to be honed produces long stringy chips, or the amount of stock to be removed is large
18.9 Advantages and Limitations
Honing has developed into a produc-tive manufacturing
Process, some advantages and limi-tations will be discussed below:
Advantages: The workpiece need
not be rotated by power, there are no chucks, faceplates, or rotating tables needed, so there are no chucking or locating errors The hone is driven from a central shaft, so bending of the shaft cannot cause tapered holes as it does when boring The result is a truly round hole, with no taper or high or low spots, provided that the previous operations left enough stock so that the
hone can clean up all the irregularities Honing uses a large contact area at slow speed compared with grinding or fine boring, which use a small contact area at high speed Because of the combined rotating and reciprocating motion used, a cross hatched pattern is created which is excellent for holding lubrication Diameters with 0.001 to 0.0001 inch and closer accuracies can
be repeatedly obtained in production work
Honing can be done on most materi-als from aluminum or brass to hard-ened steel Carbides, ceramics and glass can be honed by using diamond stones similar to diamond wheels
Limitations: Honing is thought of
as a slow process However, new ma-chines and stones have shortened hon-ing times considerably Horizontal honing may create oval holes unless the work is rotated or supported If the workpiece is thin, even hand pressure may cause a slightly oval hole