How To Page
EXPANDING LAPPING TOOLS use a taper to expand a cylinder shaped lap. The inside
diameter of the Lap and the outside diameter of the Arbor have a mating locking
taper. The outside diameter of the Lap is ground straight and is split via a
thin slot. The slot allows the Lap to expand by moving the Lap up the taper of
the Arbor. Tools that are fully supported by the taper, and lock on it, produce
the best geometry. There are mechanical tools that use fingers to hold the lap
available. These tools allow the lap to be expanded while the lapping process is
in motion so the spindle need not be stopped. Hydraulic expansion is available
for high production use in a lapping machine. Some of these tools have cycled
over 100,000 times
SPEEDS Lapping rotation speeds fall in the range of 750 RPM for 1/4 diameter
bores and shafts and 300 RPM for a 3/4" diameter work part. Some materials can
be lapped faster and slower than the above. Experimenting with the fastest speed
possible is up to each individual job.
TOLERANCES Most tool work can be lapped straight and round to 25 millionths of
an inch. (.000025"). Gage work type parts can be held to 2 - 3 millionths of an
inch if the material is stable enough.
FINISHES The best finish possible is determined by the abrasive compounds used.
Finishes to less than 1 micro inch are easily obtained if the material lapped is
dense enough. Lapping is usually specified when 2 - 8 micro inch surface is
FLAT LAPPING A flat surface lapping plate or rotation table is used to lap flat
parts. Most production parts are lapped on rotary table machines. Hand lapping
can be done on a flat lapping plate. These plates usually have slots or grooves
machined in the surface to assist in compound and lubricant distribution. Plates
without grooves lap much slower than plates with grooves.
Mount the Lap Arbor onto or into a spindle that rotates. Prime spindle examples
are; lathe, hone, mill, drill press or even a hand held drill.
The next step is to slide the Lap over the Arbor so the tapers are mated. The
large inside diameter end of the Lap goes on the Arbor first. Once the Lap is on
the Arbor a Lap Expander is then used with a hammer to drive the Lap up the
taper of the Arbor. When the end of the Lap and the end of the Arbor are flush
the diameter of the lap will be at the size that it was originally ground to.
The technique for sizing the lap to the work part is as follows. With the Lap
mounted on the Arbor apply a thin layer of lapping compound with a brush or
other means. Move work-part onto Lap and start to expand the Lap with the Lap
Expander until an interference fit is achieved.
Once the fit is achieved turn the spindle on and move the work-part back and
forth over the Lap. The desired stroke length is to go one third the length of
the work-part off of each end of the Lap. Once the abrasive is spent new
abrasive needs to be applied and the Lap needs to be expanded, usually every 5 -
15 seconds of lapping.
While the spindle is under rotation move the work-part off of the Lap. Using a
shop towel wipe the spent compound off of the Lap and apply new lapping
compound. Then using the Lap Expander tap the Lap a few times to expand the Lap
to compensate for wear and to increase the work-part's size. The desired amount
of lapping compound to apply is as follows. Use the minimum amount of lapping
compound as possible. Enough to keep the Lap moist and so the compound does not
collect at the ends of the lap. Using to much compound usually results in a
bellmouth condition in the work-part.
Repeat the above steps until the desired diameter and finish are achieved. To
start another work part pull the Lap back down the taper using a Lap Puller.
This is done as follows. Insert the tip of the Lap Puller screw into the center
hole of the Arbor. Adjust screw so the Lap Puller Disc is past the back end of
the Lap. While holding the Lap Puller so the disk has as muck bearing surface
against the Lap as possible turn the T-Handle screw clock wise to move the Lap
down the taper of the Arbor.
After a few work-parts are lapped the operator will achieve a feel for how far
to expand the Lap to achieve the desired diameter.
For external lapping the work part is usually put under rotation. Using some
kind of chuck or work-part holding fixture mount a work-part into the spindle.
Now take an External Lap and slide it inside an External Lap Holder. Turn the
adjusting screw of the Holder so the Lap will no longer move in the Holder. Next
the Lap needs to locked into the holder to prevent it from rotating. Insert a
work-part into the Lap and tighten the Holder Adjusting Screw until the
work-part will no longer move inside the Lap. With work-part inside the Lap and
the Lap inside the Holder tighten the locking set screw into the Lap. The set
screw has a cup point and will leave an indentation in the Lap. Loosen and
tighten the screw a few times to leave an indent for the set screw to hold the
lap from rotation. After the final tightening of the set screw back the set
screw off so it is still making contact with Lap but is not distorting the Lap,
about 1/16 of a turn.
To begin the lapping process spread a small amount of Lapping Compound on the
work-part while it is under rotation. With the spindle stopped slide the Lap and
Holder over the work-part and turn the adjusting screw clock wise until an
interference fit is achieved. Now turn spindle on and stroke the Lap and Holder
back and forth. The ideal stroke length is to go one third the length of the Lap
off of each end of the work-part.
Once the interference fit has been diminished it is time to apply new compound
and adjust the Lap. The regular cycle time for External Lapping is between 10
and 20 seconds. With the spindle still under rotation slide the Lap and Holder
off of the work-part. Using a shop towel wipe off swarf and re-apply compound.
Adjust the Lap by turning the adjusting screw in the holder and slide Lap and
Holder over work-part and complete another cycle.
The ideal amount of Lapping compound being applied is so that the Lap is moist
and there is no excess compound on the ends of the work-part. Continue the
process until the desired size and surface finish are achieved.
IRON - The most popular material for I.D., O.D., and flat laps is cast iron. The
metallurgical structure provides for soft areas for the lapping compound to
embed. The compound then is backed up by harder areas to hold it in place so it
will abrade harder materials. This is the basic reason a softer lap will cut and
wear hardened parts faster than the lap wears. The iron lap is also hard enough
to maintain shape which is essential to producing good geometry while lapping.
It can produce very good finishes.
BRASS - Brass is also a popular lap material. It holds shape well but does not
have a structure to hold the abrasive in place when embedded. It allows an
abrasive grain to be torn out as fast as it is embedded. This provides slow
cutting, but in some materials it can produce a very good finish.
COPPER - Copper is softer than brass and holds it's shape well. The softness
allows it to be used for lapping soft materials without embedding abrasive into
the work-part. It can produce very good finishes.
LEAD - Lead was a popular choice many years ago. Lead laps are easy to produce,
but will not hold abrasive well. Abrasive grains tear out as fast as they are
embedded. Lead is used very little today.
DIAMOND PLATED - Diamond plated lap tools eliminate the need for loose abrasive
to be applied continually. They will produce astounding stock removal speed and
excellent geometry. These type tools are the star of the future , as one tool
will produce over 10,000 parts.
NYLON - The softness and density allow nylon laps to produce lustrous finishes;
but at the cost of good geometry. It has some advantages when used on soft
BAKELITE RESIN FIBER - Bakelite resin fiber has a good material structure that
allows laps to be embedded with abrasive easily. The fiber helps hold the
abrasive from tearing out. It can produce very good finishes, but geometry can
FILLED PLASTICS - Filled plastics can have specialty uses on ceramics and
carbides. Usually the filler material is one of the above metals. Filled
plastics require engineering to each individual job.
WOOD FIBER Wood fiber is usually used to polish and improve brilliance and
finish. Wood will not produce good geometry.
SILICON CARBIDE - Silicon carbide is a sharp, fast cutting abrasive with long
sliver like pieces. The sharp grains produce deep scratches and cut marks.
Finishes obtained are average and can be dull. Silicon carbide is best used when
stock removal is paramount.
ALUMINUM OXIDE - Aluminum oxide is cubic in grain size and is mostly used for
hardened steel work-pants. It can produce excellent finishes and geometry. It is
preferred for valves and gage type work.
DIAMOND - Diamond is the fastest cutting abrasive. It will produce excellent
finishes and geometry. It is one of the few abrasives that will cut ceramics and
BORON CARBIDE - Boron carbide is very hard and is a substitute for diamond, but
does not cut like diamond. Its uses are limited.
KEROSENE JELL - Kerosene jell and kerosene are very good lubricants and give the
best results on steel and iron.
MACHINE OIL - Machine oil works almost as well as kerosene and is a good
substitute when kerosene jell is not available.
MINERAL OIL - Mineral oil works well and has some advantages in special uses. It
keeps the abrasive free cutting longer.
LARD OIL - Lard oil is as good as the above for lapping uses. It does have shelf
life and sanitary considerations. Vegetable shortening is used when a thick
water soluble lubricant is needed.
SOLVENT BRUSH - Solvent brush cleaning is the easiest. Kerosene and nylon
brushes work well to loosen the swarf and wash it away. Sometimes a clean rinse
follows the brushing.
SOLVENT VIBRATION - Solvent vibration is also available especially if diamond
abrasive is used. The vibration will loosen embedded abrasive grains.
SOLVENT SWAB - Solvent swab is used with Q-tips in small bores and cotton balls
in medium size bores. Parts are usually blown dry with compressed air.
DIPPING - Dipping and soaking in solvent works for large volumes of parts.
Sometimes vibration is added to the dipping
Diamond plated tools cuts fast and last a long time. Diamond Plated Tools do not
require abrasive compound to cut. The plating can last for 10,000 or more bores
and cuts quickly because the abrasive grains are held in place so they cannot
move to the ends of the tool and be lost with lubricant wash. Roundness and
straightness are very good and bell mouth ends, caused by compound collection on
the lap ends, is nonexistent. Some borazon plating is used in special