The Definitions of Lapping
Lapping Machines & Specification questionnaire
Lapping Rules of Thumb
Laps are usually made of soft cast
iron, copper, brass, lead or ceramic. In general, soft close-grained
iron is the best material to use for a higher degree of accuracy
in the finished product. Whatever material is used, the lap should
be softer than the work or the work will become charged with the
abrasive and cut the lap. External laps are commonly made in the
form of an outer ring or holder and an inner shell, which forms
the lap proper. This inner shell is made of cast iron, copper,
brass or lead. Ordinarily, the lap is split and screws are provided
for adjustment. The length of an external lap should be equal
to the diameter of the work or longer. Large ring laps usually
have a handle for moving them across the work. Laps for producing
plane surfaces are made of cast iron. In order to secure accurate
results the lapping surface must be a true plane.
A flat lap that is used for roughing
or blocking down will cut better if the surface is scored by narrow
grooves. They are usually located about 1/2 inch apart and extend
both lengthwise and crosswise thus forming a series of squares
similar to those of a checkerboard. For fine work a lap having
an unscored surface is used and the lap is charged with fine abrasive.
After the lap is charged all the loose abrasive should be washed
off with gasoline or solvent.
When lapping, the surface should
be kept moist with a light oil. Loose abrasives should not be
applied while lapping. If the lap is well charged with an abrasive
in the beginning and is kept well moistened during moderate use,
it will cut for a longer time. The pressure upon the work should
be just enough to insure constant contact. The lap can be made
to cut only so fast, and if excessive pressure is applied, it
will become stripped in places. The cause of scratches can result
from loose abrasive on the lap, poorly graded abrasive, or too
much pressure on the work.
To produce a perfectly smooth surface,
free from scratches, the lap should be charged with a very fine
abrasive. When the entire surface of the lap is charged, clean
and examine the lap for bright spots. If there are any visible
bright spots, continue the charging until the entire surface has
a gray appearance. If a lap is once charged is should be used
without applying more abrasives until it ceases to cut. If a lap
is overcharged and an excessive amount of abrasive is used, there
is a rolling action between the work and the lap, which results
The surface of a flat lap is usually
finished true prior to charging by scraping and testing with a
standard surface plate or by a method of scraping where three
plates are lapped together to secure a plane surface. In any case,
the varying marks or spots must be uniform and close together.
These spots can be blended, by covering the plates evenly with
a fine abrasive and rubbing them together.
With the wet method of using a
surface lap there is a surplus of oil and abrasive on the surface
of the lap. As the specimen is moved there is shifting of the
abrasive particles. With the dry method, the lap is first charged
by rolling or rubbing the abrasive into the surface. All surplus
oil and abrasive are then washed off, leaving a clean surface
and one that has a uniform embedding of small particles of abrasive.
It is like the surface of a very fine oilstone and will cut away
the hardened steel that is rubbed over it. While this had been
called the dry method, in practice the lap is kept moistened with
gasoline, naptha or any other thin-filmed oil.
In lapping it is desirable to avoid
unnecessary scratching. For a fine background even a minute scratch
is evident and unsightly. In some cases it is detrimental to the
general functioning of the part. To prevent scratching, several
conditions must be observed and maintained. The room where lapping
is being performed must be reasonably free from dust. The proper
abrasive must be selected and a suitable vehicle or carrier used.
The proper amount of abrasive must be applied and used at the
Generally speaking, the most suitable
abrasive is one that is readily embedded in the surface of the
lap. Soft cast iron is used for making laps on mechanical machines
with a fine abrasive. To insure straight work the lap surface
must be perfectly flat. To prevent undue wear the lap width should
be made to conform with the length of the work being lapped. The
laps may be made flat with true plane surfaces by dressing together
in sets of three as in making master surface plates. Dress plate
No. 1 with plate No. 2, No. 1 with No. 3 and then No. 2 with No.
3. The surface condition of the laps will determine the quality
of the finish. By the use of multi-motion and ever changing work
movement relative to the work, a true lapping action is obtained,
and uniform lap wear is assured.
The advice given to a machine operator
should be similar to that given to the chef who makes the salad.
"Be a miser with the vinegar and a spendthrift with the oil,"
but in this case it should be, "Be a miser with the abrasive
and a spendthrift with the oil." This is the secret of good
lapping. When there is a super abundance of abrasive rolling loosely
on the laps and the work, it is almost impossible to obtain a
good finish. The lap wears unduly and the accuracy of the product
An old method of lapping consisted
of lapping with a soft metal lap, into which was rolled an abrasive
such as diamond dust, Silicon carbide or one of the natural abrasives.
The metal laps serve much the same purpose as the bond in a grinding
wheel. In the modern method, the abrasive is embodied in a suitable
carrying base or vehicle and allowed to roll between the lap and
Using this method, it is essential
to select the lap material with care, considering the material
of which the work is made. Strange as it may seem, in this process
the greatest amount of stock is removed from the harder of the
two parts. Therefore the lap must be softer. The reason for this
is that in either very soft or very porous laps the abrasive grains
are held temporarily stationary by being embedded in the lap.
Since their position is fixed with respect to the lap but moving
with respect to the work, they cut the work. Thus it is very easy
to see that the lap material has a great influence on the speed,
accuracy and finish of the work.
Open grained cast iron will work
faster than dense cast iron or steel. For certain jobs laps are
made of the softer metals or even wood. The speed depends upon
several variables such as the material to be lapped, the material
of the lap, the type of vehicle, the type and size of abrasive,
the pressure on the lap and the personal skill and even the psychology
of the operator.
THE DEFINITIONS OF LAPPING:
LAPPING is the balancing of abrasive grit size and proper hardness
or kind of abrasive against lapping time and the pressure of the
part on the plate or lap.
The proper balance is achieved when all abrasive particles break
down completely into inert sizes, while removing the desired amount
of metal and wearing out the abrasive power of the particles.
There are no hard, fast rules to follow and each operator must
experiment to find the proper compound and proper lap. He then
combines them with his own skill and experience to solve his lapping
Lapping has four main purposes:
• to obtain greater accuracy as to dimensions
• to improve imperfections of shape
• to obtain a smoother polished surface
• to improve the fit between surfaces.
LAPPING PLATES: Good lapping plates
are made of soft, close-grained cast iron or meehanite. A scleroscope
hardness of 27 to 32 has proven best for cast iron surfaces. Harder
laps will often cause glazing and scratching while softer laps
cause a loss of flatness and parallelism and will produce grayer
The plate should be heavy
enough and properly designed so it will not distort in use. Its
surface may be plain or grooved. Plain plates are best suited
for lapping cylindrical work and for extreme accuracy. Grooved
plates, which have a working surface serrated with vee-grooves
V.040 deep and 1/2" apart, are preferred for lapping flat
work only. A rotating plate may have a spiral groove leading from
the center to the outside edge.
LAPPING MACHINES & SPECIFICATION
questionnaire (hand lapping with a rotating plate):
Vertical-spindle lapping machines, with single horizontal
rotating plates, are made in sizes from 10" to 56" in
diameter. These machines can be equipped with any desired lap
material, bonded abrasive disc or lapping wheel. As the lap rotates
the work, held by hand, is moved at random across the lapping
wheel. If the size or weight of the part does not provide proper
pressure, the operator must apply whatever is necessary. Light
pressure gives greatest accuracy, heavy pressure removes stock
MECHANICAL LAPPING: This method includes
the addition of a mechanical means of applying pressure and motion.
The work or parts are carried in a holder of the planetary type
that revolves or rotates about the center of the lap. Mechanical
lapping can also be performed with non-rotating laps for certain
types of flat lapping: for gage blocks, etc. Machines of this
type have a stationary lower and a removable non-rotating upper
lap. The weight of the upper lap can be adjusted to meet lapping
requirements. Such lapping machines may also use bonded abrasive
Another method of lapping
is a refinement of cente-less grinding to produce straight cylindrical
objects. Such a machine has two rotating rollers. One roller is
twice the size of the other. Both revolve slowly at the same speed
and in the same direction. The work is held in the throat or V
between them and is caused to rotate at the speed of the smaller
roller. Because of its increased surface speed over that of the
work, the large roller causes a rapid lapping action. The lapping
pressure is applied manually through a notched fiber stick held
against the work and moved evenly over the entire surface. It
is desired to correct taper or other error, the stick is held
longer on the portion requiring more stock removal.
Cylindrical parts to be lapped
by this method must be ground within 0.005" of the finished
size and well within the tolerance for the roundness desired.
Usually tow machines are available, or two sets of laps-one set
for roughing and the other for finishing. Diamond compounds must
be used for tungsten carbide parts. Several of the alumina compounds
can be used to impart color or luster to the part and in most
cases this finishing is done with a slight addition of naptha
or gasoline to the rotation rollers.
EXTERNAL LAPS: These laps are usually
in the form of a ring which is split and adjustable by means of
a screw arrangement. Some external laps have an inner-ring of
copper, brass, lead, tin, etc. External lap length should be at
least equal to, or longer than, the diameter of the part being
WET LAPPING: In wet lapping, the
abrasive is mixed into a carrier or vehicle. This even film of
abrasive covers the total lap area. The carrier acts as a lubricant
and absorbs the heat generated by the abrasive against the metal.
It allows the abrasive to cut the material by resisting the rolling
of the abrasive granules.
DRY LAPPING: This
method is used for the final finishing of a part. Laps are charged
with the proper abrasive and the surface is barely moistened with
a very thin oil such as spindle oil, gasoline, naptha, mineral
seal oil, etc.
Caution: Kerosene may cause
Just enough pressure to insure contact is used. If too much pressure
is used, or if the lapping operation is too fast in motion, the
lap may strip. Foreign matter, such as dirt and lint from the
operators clothing and various sources, can also cause scratches
in the work. A wash stand for frequent use by the operator is
recommended for all lapping departments.
NOTE: Lapping often precedes
polishing, and the quality of a polished surface is largely dependent
on the lapping process. The selection of lapping abrasives, vehicles,
additives, pressures, speed, plates and many other factors play
an important role in this complex, controlled stock removal process.
When the quality of a polished surface is critical, we strongly
recommend selecting the finest uniform-graded abrasive compound
RULES OF THUMB:
(knowledge gained through practical experience)
• A lap should be softer than the part to be lapped.
• The abrasive in the compound should be as hard as the metal
being lapped. Hard abrasives will charge or embed in a softer
metal. A non-embedding or non-charging compound should be used
for soft metals, for instance, Garnet abrasive for brass or bronze.
The softer the metal, the softer the abrasive. The harder the
metal, the harder the abrasive.
• A high lap speed will increase stock removal. For a rotary lap,
speed of approximately 275 rpm is recommended. If the lap pressure
is too high it will score the part.
• An increase of pressure, of the part again the lap, will increase
the speed of the cut.
• Serrated or grooved laps are best for flat surfaces with large
areas or flat areas with holes in the surface.
• Laps with no serration or grooves are preferred for cylindrical
• Abrasives when mixed into an oily paste or greasy vehicle will
give better results than just a mixture of fluid oils and abrasives.
• Tungsten carbide is best lapped with a Diamond compound. A Norbide
or Boron-Carbide abrasive will slowly abrade the metal, but the
finish is not equal.
• A gray or frost-like surface may be as smooth and accurate as
a bright finish. A bright or polished surface does not indicate
that it is smooth, but a smooth surface may be bright or polished.
• A polished surface is harder to produce than a gray matte finish
and will show scratches more readily.
• It is more difficult to lap soft metals than hard metals.
• A soft lap will cut faster, wear longer and give a brighter
surface than a hard lap. A hard lap cuts slowly, wears faster
and gives a duller finish, but its accuracy of lapping is greater.
• Use our lapping compounds sparingly. Too much is worse than
too little. Excess compound removes metal but does not correct
the error in parts.
• Final finishing should be done only with a charged lap lubricated
with thin oil, naptha, gasoline, spindle, etc.
• Thin work pieces can be lapped parallel but not necessarily
• Final finishes are best obtained with no loose abrasive on the
plate. Use the embedded abrasive granules in the surface of the
plate and a very thin lubricant.
• When soft metal parts such as brass or bronze running seals
are lapped, a non-embedding abrasive should be used. We recommend
compounds containing a Garnet abrasive such as our "GK"
series of compounds.
• Red Rouge (Ferric Oxide) is recommended as a polishing medium
for soft metals.
• Green Rouge (Chromium Oxide) is well suited for the polishing
of stainless, chrome and other hard metals. It is also used for
• Different kinds of abrasives or grit sizes should not be used
at the same time on the same plate. It is recommended to use one
plate for any roughing operation and another plate for finishing
• Diamond compounds should be used for lapping tungsten carbide
or other metals when the Rockwell C hardness exceeds 64.
• An abrasive particle will produce a scratch one-half of its
size. Thus a 10 micron sized particle will produce a scratch of
• Brightness does not indicate flatness. A gray or matte finish
can be just as smooth and accurate as a bright finish.
• Fine, abrasive grit sizes do not mean fine finishes. Abrasives
can be too fine. The abrasive should be just coarse enough to
abrade or remove the desired amount of stock or metal which causes
the abrasive granules to break down into inert sized particles.
These fine particles produce the desire finish.
• When measuring flat lapped surfaces, a polish is necessary and
an optical flat is used. An optical flat is a transparent disc,
preferably quartz. With proper reflective surface conditions,
the phenomenon of interference bands is created with an optical
flat. When viewed through the optical flat, these bands appear
as a pattern of dark strips on the illuminated work surface and
can be used with great accuracy to determine the flatness of a
surface, that is, the location and amount of concavity or convexity.
Straight band indicates a flat surface. Curved or irregular bands
show a lack of flatness. Bands that curve toward the line of contact
indicate a convex surface and bands that curve away from the line
of contact indicate a concave surface.
• To obtain a reading with an optical flat, on a flat work piece,
the surface must have a reflective finish and be flat within 0.00005
• Every abrasive has a different finishing quality as to its brightness
or reflective ability. Silicon Carbide abrasive, no matter how
used, will only produce a gray, frosted or matte finish.
• A mixture of just oil and abrasive is not a good lapping medium.
The abrasive granules should be held in a film on the plate that
resists the movement or rolling of abrasive granules. This resistance
causes the cutting edges of the abrasive to abrade or remove the
• Our compounds have a pasty base or vehicle which creates an
important film. The crushed abrasive granules in this film create
a protective medium between the lap and part.
• For lapping various grades of aluminum metal the Garnet and
Silicon Carbide compounds are recommended. Line powders are used
to create a high polish or mirror finish.
• For lapping hardened steels, stainless, chrome plate, etc. (Rockwell
C 55-63), the No. 38, white Aluminum Oxide abrasive compounds
are recommended. The No. 38, white Aluminum Oxide abrasive is
recommended also for beryllium and ceramics. It is available in
all standard sizes.
• The Corundum abrasive is well suited for softer steels.
• Carbon seals should not be lapped with loose abrasive on a plate.
The lapping plate should be a bonded abrasive disk using a thin
• Surface finish is a function of abrasive grit size, part material
and part hardness. Coarse abrasives, produce higher r.m.s. finishes
than finer abrasives. Harder materials exhibit lower r.m.s. finishes
from a given abrasive than softer materials.