Metal cutting Physics
Idealized Chip Formation
Built Up Edge
Calculating Speeds and Feeds
The figure below depicts an idealized, two dimensional view of the metal
cutting process. The assumptions in this model are that the tool is
perfectly sharp, that the cut depth t and the cutting speed
V are constant, and that the cut depth is small compared to the
cut width. In this idealized model, the material layer at the top is
formed into a chip by a shearing process in the primary shear zone at
AB. The chip slides up the rake face undergoing some secondary plastic
flow due to the forces of friction. This idealized model correctly
predicts that cutting force increases with cut depth, material hardness,
and friction coefficient. Cutting forces are inversely propoortional to
rake angle. Power required increases with the feed rate.
Idealized Chip Formation
One important effect that is not considered in the model above is built
up edge. Under most cutting conditions, some of the cut material will
attach to the cutting point. This tends to cause the cut to be deeper
than the tip of the cutting tool and degrades surface finish. Also,
periodically the built up edge will break off and remove some of the
cutting tool. Thus, tool life is reduced. In general, built up edge
can be reduced by:
- Increasing cutting speed
- Decreasing feed rate
- Increasing ambient workpiece temperature
- Increasing rake angle
- Reducing friction (by applying cutting fluid)
Built Up Edge
Cutting speed refers to the speed at which the tool point of the
cutter moves with respect to the work measured in feet per minute. Feed
is the rate at which the work moves into the cutter measured in feed per
tooth revolution. Feeds and speeds affect the time to finish a cut,
tool life, finish of the machined surface, and power required of the
machine.
The cutting speed is mostly determined by the material to be cut and the
material of the tool. To find the right speed for any task, refer to
the Machinery's Handbook or other reference. To calculate the proper
spindle speed, divide the desired cutting speed by the circumference of
the tool (or of the part if it is rotating) expressed in feet. The feed
rate depends on the width and depth of cut, finish desired and many
other variables. To calculate the desired feed setting from the feed
rate, multiply feed per tooth per revolution by number of teeth and rpm
of the spindle.
On to measurement.
Return to the machine shop.
Return to the Mechanical Engineering Department.
Maintained by
2.670adm@mit.edu