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PROFESSIONAL ADVICE PROFESSIONAL ADVICE

determination of chip thickness. A way to account for the involvement of a tool’s nose radius and higher cutting speed, or more productive cutting conditions, will result in longer tool life.
was developed by Swedish engineer Ragnar Woxén in the early 1960s. He provided a formula When the concept of increasing two cutting parameters and increasing metal removal rate at
for equivalent chip thickness in turning operations that calculates theoretical chip thickness the same time was introduced in the 1960s and 1970s, it was a breakthrough idea and contrary
along the tool nose. The result essentially straightens out the nose radius and enables the chip to then-current experience and intuition.
area to be described with a rectangle. Use of that description enables a model to reflect the The development of models that include multiple factors in the metal cutting process,
engagement of the tool’s rounded nose.
The Colding model
A tool life model developed by Swedish professor Bertil Colding in the late 1950s describes Equivalent chip thickness - Voxen model
the relationship between tool life, cutting speed and the equivalent chip thickness as well as
incorporates additional factors in the cutting process. These factors include tool material and
geometry, temperature and workpiece machinability. This model and the complex equation
related to it enables accurate evaluation of the effect of combined changes in multiple cutting
conditions.
Colding recognised that changing the equivalent chip thickness (feed rate) changes the
relationship between cutting speed and tool life. If equivalent chip thickness increases, cutting
speed must be lowered to maintain the same tool life. The more that chip thickness increases,
the greater the impact of changing cutting speeds. normal tool wear - Taylor model
On the other hand, if the equivalent chip thickness
decreases, tool life increases and the effect of higher
cutter speeds decreases as well. Many combinations Feed direction
of feed, depth of cut, lead angle and nose radius
can produce the same equivalent chip thickness
value. And if a constant equivalent chip thickness
is maintained at constant cutting speed, tool life constant depth of cut and feed
will remain constant as well, despite variations in
depth of cut, feed and lead angle.
The Colding model reflects the relationship of such as the Colding model, in combination with concepts of the Taylor and Archard models, has
changing equivalent chip thickness to tool life and served to bring theory and reality in line with each other.
cutting speed when machining within the steady Practical application of increasingly complex tool life models requires computer-executed
abrasive wear conditions of the Taylor model.
However, it also takes into account other wear
factors. Estimates derived from these factors are
of minimal importance when machining routine materials such as steels that produce steady Normal wear - Colding model
abrasive wear. However, the model’s projections outside the Taylor range become crucial when
working with materials such as superalloys and titanium that have a tendency to strain harden. Workpiece material machinability Tool material and construction
That is because at low equivalent chip thicknesses, the tool cuts through strain-hardened
material, raising cutting temperatures and requiring lower cutting speeds to reduce temperature Planning angle Vertex radius 5 constant
and maintain tool life.
However, through a portion of the cutting range a combination of greater chip thickness Cutting depth Advance

Metal cutting process - basic principle Equivalent chip thickness

Colding's
Cutting mode model Estimated tool life

Tool Tool Chips
Chips
Cutting speed
Required tool life

Blank Blank Blank Blank Wear criteria such as flank wear

Free rectangular Free oblique Non-free oblique Non-free oblique interrupted
cutting cutting cutting cuts
Requirements for processing Requirements for the quality Process stability
accuracy of the treated surface

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