Anti-kickback was developed to ensure safety
and is considered a standard feature on larger diameter CMT bits
where risk of kickback is the highest. Kickback occurs when the
bit makes the initial bite into the wood and jumps back. Due
to the large amount of space between the flutes on the cutter
head. CMT bits are designed with generous rounded shoulders between
the flutes to stay in almost constant contact with the wood when
cutting and also to make a smoother initial connection with the
CMT designed rounded shoulders
between the flutes to radically reduce dangerous kickback.
Hold a bit by the sank and look at the cutting
edge, you will notice that it goes either straight up and down,
left to right (negative shear) or right to left (positive shear).
The angle cutting edge or its "shear" will determine
how the cut is made. Specific shear angles are designed to work
best in certain type of materials. A shear angle gives a less
choppy cut and give a cleaner finish, thus making them preferred
over straight cutting edges for edge work. Negative shear cutting
angles give the smoothest finish and are recommended for laminate
Our CMT Mortising bit
(left) has a negative shear angle of 6 degrees and our Cavetto
Edge Mold bit (right) a positive shear angle of 15 degrees.
The hook angle is sometimes called the rake
ad you can see it by looking down at the top of your bit - notice
the angle the cutting edge makes with the overall bit diameter
(see drawing). This is the angle at which the cutting edge meets
the stock and its purpose is to help leave a smoother, splinter-free
Radial relief is the curved grind on the
outer cutting edge of the carbide. The presence of extra mass
behind the cutting edge gives the carbide many added advantages:
-strength behind the edge
-stays sharper longer
-gives more resharpening
CMT includes radial relief on all straight
edge bits to guarantee durability and the best highest quality
The hook angle is determined
by the slant of the cutting edge toward the center of the bit.
Radical relief grind
compared to flat grind.
A major feature that makes a major difference
in tool performance is clearance space. As the bit rotates, the
tip of the cutting edge makes an outer circle as it removes stock,
and the rotation of the body makes a smaller inner circle. The
little space created between the rotation of the tip and the
rotation of the body (see photos right) is known as the clearance
ad it serves as a free space for the bit to avoid coming into
contact with the work piece and burning, and as an exit for sawdust.
Notice the space created
when measuring the diameter from the cutting edge (left) and
from the body of the bit (right). This little space makes a big
difference in the efficiency of chip and sawdust ejection.