Lapping is the process of rubbing two surfaces together with an abrasive
and a lubricant to improve the quality of at least one of the surfaces.
Although lapping can be used to create flat surfaces, in the context of
woodworking, lapping better serves to minimize the roughness of a surface
known as surface conditioning.
By minimizing the roughness in the sole of a plane, there is reduced
friction between the plane and the workpiece, which in turn reduces abrasion.
For blades or chisels, the cutting edge can be made sharper if both intersecting
surfaces are free of scratches, even if the back of the blade isn't perfectly
Figure 1: A ground blade versus a lapped blade.
Lapping can remove only small amounts of material. If the sole of your
plane or the back of your blade is twisted, wavy or bowed, it will be
necessary to sand or grind off the high points prior to lapping.
Lapping is always performed with an abrasive oil slurry, which not only
allows the object to slide about the lapping plate (called a lap), but
also provides a means to remove abraded particles and worn abrasive.
Figure 2: Lapping mechanics.
The lapping plate is made of soft iron and will wear over time. These
instructions provide information on how to ensure the lap remains flat
for a lifetime. Furthermore, the lap should not be used
- for flattening water stones, as the oil lubricant will ruin the water
- with water as a lubricant, as water will rust the lap.
- for flattening oil stones, as any residual grit on the lap can be
transferred to the stone, resulting in scratches during honing.
- as a replacement for a honing stone, as the grooves are unsuitable
for a honing guide and the blade bevel can be seriously damaged.
Choosing an Abrasive
While silicon carbide and aluminum oxide are ideal lapping abrasives
(since they become finer with use), diamond should not be used because
it will impart new scratches onto the surface as the object is lapped.
||When a flat and ground blade needs only 0.001"
or less of material removal to condition the surface.
||When the slightly rougher texture of a plane sole requires more
||For twisted, wavy or linished objects, or when considerable material
removal may be required.
While all the abrasives can achieve the same result, the coarser the
abrasive, the longer it will take to achieve a smooth surface. Then again,
a coarser abrasive is far more efficient at eliminating high points.
Choosing a Lubricant
Although any oil is suitable for lapping, the ideal lapping oil is of
medium viscosity, similar to that of olive oil. If the oil is too thin,
it will flow away easily, making it difficult to maintain a film on the
lap. If it is too thick, it can be difficult or impossible to clean the
lap and the object that was lapped. The oil must also be chemically compatible
with the abrasive and environmentally benign for subsequent clean-up.
Mineral oil is a good overall choice.
Conditioning the Lap
As received, the lapping plate will need to be conditioned prior to first
use. Conditioning coats the lap with a uniform film of oil and abrasive
and beds some of the abrasive into the surface of the lap. This optimizes
the time spent lapping and ensures uniform results. Conditioning is needed
only when the lap surface is free of abrasive. The residual abrasive left
on the lap from a previous session eliminates the need to recondition
the lap (unless you are switching to a finer grit).
||Figure 3: Conditioning the lap.
To condition the lap:
- Apply a thin coat of oil onto the entire lap surface. This can be
done with a spray or squirt bottle. The object being lapped can be used
to spread the oil. This coating provides adhesion for the abrasive as
well as preventing most of the abrasive from simply filling up the grooves.
Oil should be added until the entire lap has been coated.
- Place a teaspoon of abrasive in the center of the lap. Use the object
to distribute the abrasive. Most of the lap surface should become coated
with a thin layer of moist abrasive. It may be necessary to add more
- Wet any of the dry abrasive areas with additional oil, if necessary,
and move the object about to mix the oil and abrasive together. The
lap has now been conditioned.
If lapping is a frequent activity, time can be saved by pre-mixing slurry
one part abrasive to roughly ten parts of oil. This slurry can
be used to bring a lap back up to fresh working grit more quickly than
a complete reconditioning. Pre-mixed oil slurry can be used only on a
lap that has already been conditioned and has a light coating of oil and
abrasive. Slurry applied to a lap which has been cleaned or never used
will not coat the lap surface.
Gripping the Object
Prior to lapping, determine how you will be holding the object. Since
most lapping sessions can last upwards of half an hour, this can become
very important. Also, once lapping commences and the object becomes coated
in oily slurry, it can be difficult to change to a completely different
means of holding the object.
A plane sole should be lapped while the plane is assembled, but with
the blade retracted. Not only does this provide a convenient means to
hold the plane, but also takes into account any distortion resulting from
the plane assembly. An adjustable toe or frog that extends to the sole
should be lapped together with the rest of the plane.
To fully lap the back of a small blade (e.g., block plane or spokeshave),
an auxiliary blade holding jig, such as one made from a block of wood
with double-sided tape, is highly recommended.
Figure 4: Simple blade holding jig.
Since there is little reason to lap the entire back of a large plane
blade, as the benefit is limited to the leading edge, the rest of the
blade can be used as a handle.
Figure 5: Lapping larger blades.
This approach can work well for short lapping sessions, but if considerable
lapping is expected, the same blade holding jig used for the small blades
can be used to guide the working end of a larger blade.
The cross section of a chisel blade makes it difficult to grip directly,
but a chisel does come with a handle. Use one hand to guide the chisel
and the other to apply pressure, keeping the blade in contact with the
Figure 6: Lapping a chisel.
Lapping entails moving the object around on the lap surface until you
have achieved the desired condition. The exact technique used is up to
whoever is lapping and what feels comfortable. However, the following
guidelines should help to achieve good results.
- Use a newsprint underlay. Lapping is a rather messy process. Oil laden
with metal, dust and abrasive tends to permeate the immediate environment.
Using a newsprint underlay (four or so sheets thick) will keep the mess
to a minimum by absorbing oil and make clean-up that much simpler.
- Clamp the lap in place. Despite its weight, the lap can shift and
slide about in use. Clamping it in place alleviates the frustration
of unexpected shifting and eliminates the possibility that the lap will
accidentally slide off the working surface.
- Raise the lap to a proper height. Lapping requires repeating the same
motion for a long period of time. Raising the lap so that the working
surface is a few inches below the users elbow can reduce the risk
of injury associated with this activity.
Figure 7: Lapping set-up.
Be patient. When using grit that is properly matched to the object being
lapped, a typical minimum lapping time is 20 minutes for fine abrasives
and 30 minutes for coarse abrasives. After the initial lapping session,
clean the object and check the progress. Additional 10 to 15 minute
sessions may be needed to achieve the desired results.
||Figure 8: Figure-eight pattern.
- Use a figure-eight pattern. Regardless of the object being lapped,
optimal results are achieved when the object is moved about the lap
in a long, slow figure-eight pattern, as shown in Figure 8. This
technique continuously redistributes the slurry around the lap, keeping
it uniformly oiled. It also prevents a uniform scratch pattern from
being imparted to the object or the lap, as well as extends the life
of the lap by encouraging uniform wear.
- Use the entire surface. Ideally, the entire lapping surface should
be used, but for small objects, this may not be feasible. When lapping
a plane sole or the entire back of a blade, the stroke pattern should
cover the entire lap and extend beyond the edge of the lap a few times.
Not only does this help in extending the life of the lap, but also it
ensures that, when the object being lapped is slid off the edge of the
lap (hydraulic lock makes it difficult to lift straight off), there
are no burrs or dry abrasive along the edge that could scratch the object.
- Avoid adding grit halfway through a lapping session. Since the abrasive
grit becomes finer over time, adding grit will have the effect of restarting
that session, increasing the time needed to lap, but not improving the
results. However, should you not hear the distinctive grinding sound,
it indicates that there is not enough grit on the lap. In this instance,
more abrasive will be needed. Also, as you examine the results of the
first lapping session and you discover that the surface is more wavy
or curved than initially thought, it may be necessary to switch to a
Figure 9: Partially lapped blade.
- Occasionally add oil. A well-oiled surface is essential to quality
lapping results. Over time, the lap will naturally lose oil (typically
by spilling off the sides). Allowing the lap to dry out can spoil both
the lap and the object being lapped.
- Apply sufficient force. When lapping the sole of a plane or the entire
back of a blade (using a suitable jig), the operators hand and
the weight of the object impart enough force to result in useful strokes.
For larger objects for which only a portion of the surface is being
lapped, it will be necessary to impart some downward force to keep the
object in contact with the lap. See Figure 6.
Cleaning the Lapped Object
To clean slurry off a plane sole, it is best to wipe it with a paper
towel or facial tissue saturated with water. It will take some time to
fully remove the slurry. With each wipe, use an unused portion of the
paper towel (or a fresh tissue). Rubbing risks imparting new scratches,
so wipe gently. Clean blades with soap under running water. Dry all lapped
objects with paper towel or cotton rags.
Note: Do not use Kraft paper towel, as it contains abrasive
contaminants that can easily scratch a lapped surface.
Cleaning the Lap
After each session, tip the lap up on its side and allow the oil to drain
for one to two hours. This provides enough time for the slurry to flow
through the grooves and drain out the side. Ideally, this should be done
over several sheets of newsprint to capture the excess oil.
If desired, wipe the lap surface once with paper towel or newsprint to
remove any excess grit. This should be done while the lap is still oily
to minimize the scratching that will occur.
Note: Do not scrape or rub the lap, as this can damage the
Storing the Lap
Store the lap in a dust-free location, in a sealed bag or a suitable
container. Ideally, the lap should be lightly oiled before storage to
prevent rusting. However, if the lap is merely drained after use but not
wiped dry, the oil that remains will provide sufficient protection and
the abrasive that remains will minimize the conditioning that will be
necessary during the next lapping session.