CAST IRON LAP
HOW TO MAKE A CAST IRON LAP
by Robert Giesbrecht
You must first find the right kind of cast iron, either in ingot form or
cast in a disk shape. Depending on your shop equipment put the iron in a
chuck or on a face plate. I would suggest that a chuck only be used to take
off all of the saw marks and scale so that the piece is close to the final
shape and clean.
Finishing the lap on a face plate is essential for
accuracy. Adjust the iron in the chuck until it is running as true as
possible. Most ingots are slightly oval shaped and the goal is to get the
largest lap from the piece.
Using a facing tool true up the front face.
Place a 1/4" centering drill in the tailstock chuck and start the hole then
use a slightly larger drill say 3/8" and drill all the way through. It is
important that you finish the hole later on for accuracy, the hole is only
used for a reference at the moment.
Now that one face is flat and smooth you
can finish the rest of the lap on the face plate. The face plate must be
very accurate and I have found the best way to do this is to attach a
sacrificial piece of metal to the face plate and then face this piece of
metal every time a lap is made.
The face plate must be smaller than the lap
being turned so that the turning tool does not come in contact with the face
plate. Apply some superglue to the face plate in small dabs all around the
face plate. Put the lap near the face plate and use the live center in the
tailstock to support the lap in the reference hole.
Bring the lap to the
face plate by advancing the tailstock until the lap is in contact with the
face plate. The superglue will bond instantly and this is why it is
important to have the hole centered properly as you will not get a second
chance. It is a good idea to let the glue set for a while to ensure that it
has set properly.
Support the iron with the tailstock during the roughing
operations as the glue is only for light machining.
Turn the outside diameter until the lap is round and smooth.
Use a facing tool and face off
this side and get as close as possible to the live center.
Now remove the
live center and use the facing tool to cut a circular depression in the face
of the lap about an inch from the center and 1/8" to 1/4" deep. Using the
tailstock chuck, drill the hole to 30/64ths to 31/64ths diameter.
Now set the
lathe to the lowest speed and ream the hole to 1/2" using lots of cutting
Now use a chamfering tool to bevel off the sharp corners near the
center and the outside of the lap, a 1/16" bevel is sufficient.
The lap is finished.
Starting from an ingot has the advantage that you get the right kind of
iron. The iron that I used is designated 65-45-12 iron. It seems to have the
right balance of properties to make a good lap. I am sure there are other
types of cast iron that would make a good lap but whichever type is used it
should be a fine grained iron. If you can find a foundry that will take on
the job of casting laps then the iron will have to be consistent from pour
to pour. The larger the lap you make the thicker it should be. My lap is 6"
diameter and 5/8" thick. Diamond cutters use laps (scaives) that are 12"
diameter and 2" thick.
The type of superglue I use is Lepages super 8. The
surfaces need to be very clean to make the glue stick and it is a good idea
to have clean dry hands when placing the lap on the face plate. To speed
things up I have a small face plate that can be held in the jaws of the
lathe chuck, it is an old bronze bushing 4" diameter and 2" long. I face
this piece of bronze every time I use it and that way I know the face will
When removing the lap from the face plate do not just smack the
lap with a hammer to break the glue bond as this could damage the lap. Place
a small piece of wood or rubber on the back of the lap and using a pry bar
gently break the glue while supporting the lap with your other hand. If the
lap falls off of the face plate and hits the cross slide there will be dents
on the lap and it will have to be resurfaced.
USING THE LAP
This is how Robert Giesbrecht and others prepare a Cast Iron lap:
Clean the lap with soap and water to remove oil and contamination and
then dry with a towel.
Wipe lap with a paper towel damp with acetone to remove the last traces
Spray or sprinkle diamond on surface of lap and let dry, do not use water
based diamond sprays.
Apply a few small drops of olive oil to the lap and rub on lap with a
clean finger tip. Do not use too much oil as it will just make a mess when
the lap is running.
The lap is ready to use, if polishing slows down spray on more diamond
by Robert Giesbrecht
- Get a small piece of SiC or
Norbide dressing stick say 3/8" sq and 1" long
and attach to a dop stick.
Set the protractor to about 45 degrees and cut a
flat on the dressing stick using a rough lap (260 mesh) and lots of water.
Dry off the dressing stick.
Gently place the iron lap on the machine. Apply
olive oil (other types are OK) to the lap and spin at the highest speed.
Carefully drag the dressing stick across the surface of the lap working back
and forth from slighly over the rim to slighly over the center depression.
This operation smoothes the surface of the lap. If you are fortunate enough
to have access to a surface grinder or Blanchard grinder then this step is
not important although it is far less costly.
Take the lap to the sink and
clean with soap and water, I use Dutch cleanser because it cuts through oil
Dry the lap then dap on some olive oil and rub all over the lap.
Wipe the lap with a clean towel to remove excess oil. It is a good idea to
have a very thin film of oil on all surfaces of the lap to prevent rust.
Now it is time to put diamond and oil on the lap. I have a small spray bottle
with 1 part olive oil and 8 to 10 parts acetone and this is used for
lubricant for the diamond. The purpose of the acetone is that it dilutes the
oil so that it is easy to spray and leaves a very thin film on the lap.
Spray a very small amount of oil on the lap and then spray some diamond and
work it around the lap with a clean finger.
- Note: I have tried diamond from 8000
mesh to 100000 mesh and I have settled on 8000 to 14000 diamond for cutting
sapphires. Try cutting a sapphire with a 600 mesh lap and then prepolish the
facets with 8000 diamond on the cast iron. You will find that the 8000 mesh
diamond easily removes the scratches and pits from the 600 mesh stage and
leaves such a good finish that the facets only need to be touched up with a
tin lap to get a mirror finish.
If you have another way or a better way, please let us know.
Robert Giesbrecht has not had any heat problems while running a 6 inch lap at up to 1000
rpm. He does not use any coolant on the lap other than the thin film of olive
oil. He does not use any special techniques other than making small sweeping
motions on the lap to prevent grooving the lap and facets.
Some people may use a slight amount of Teflon oil or spray from auto stores.
Others may use small amounts of WD40 or mineral oil.
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