Specific Cutting Force (KC & KC1)

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Each material has a Specific Cutting Force coefficient that expresses the force in the cutting direction required to cut a chip area of one square millimeter that has a thickness of 1 millimeter with a top rake angle of 0°, hence the name KC1. In addition to KC1., each material has an MC constant that indicates how KC varies as it gets further away from its normalized point. KC1 and MC values are listed in charts like the one at the bottom of this page or in cutting tool technical guides. The specific cutting force KC is calculated from KC1, MC, and the actual chip thickness. It is a mandatory value to calculate the required machining power, cutting forces, and torque.

Definitions of terms

Specific Cutting Force (KC1 or KC1.1)

Each material has a Specific Cutting Force coefficient that expresses the force in the cutting direction required to cut a chip area of one square millimeter that has a thickness of 1 millimeter with a top rake angle of 0°, hence the name KC1.1. It ranges between 700 to 4000 N/mm^2 and is the primary factor affecting the final result of the KC calculation. (KC1 values can be found in the below charts.)

Typical values of Specific Cutting Forces for the main material groups in N/mm^2

Specific Cutting Force KC [N/mm^2] per material groups
K1C Material Groups Chart Mobile

Actual Specific Cutting Force (KC)

The Actual Specific Cutting Force (KC) expresses the force in the cutting direction, required to cut our actual chip based on KC1 and the additional parameters that are explained below.

KC Curve Factor (MC)

The MC constant indicates how KC varies as the chip thickness gets further away from the normalized point of 1 mm. MC values range between 0.2 and 0.3 depending on the raw material. When the chip thickness is one mm, the MC has no effect. As the chip thickness gets smaller, a higher MC increases the ratio between KC and KC1. You can see the behavior in the below chart. (MC values can be obtained from the below charts.)

Effect of different MC values on KC

Effect of MC coefficient on the specific cutting force

How to calculate KC

Since KC1.1 is normalized to 1 mm at 0° rake, we need to calculate the actual specific cutting force KC coefficient suitable for our conditions. To perform the calculations, we will need to obtain 4 parameters.

\( \large \bf KC = KC1.1\,\times\,HM^{-MC}\,\times\,\left (1\,- \,0.01\,\times\, GAMF\right ) \)
\( \large KC = KC1.1\,\times\,HM^{-MC}\,\times \)
\( \large \left (1 – 0.01\,\times\, GAMF\right ) \)
  1. KC1.1 – Normalized Specific cutting Force [KPSI] or [KW] – Obtained from the below chart
  2. MC – Curve slope of the KC graph. – Obtained from the below chart
  3. GAMF – Top rake angle. – Obtained from the tool/insert catalog or drawing.
  4. HM – Chip thickness [Inch] or [mm] – Needs to be calculated per application.

a & b) Obtain KC1.1 and MC from the below table.

c) Top Rake (Radial) Angle -GAMF

Radial (Top) Rake Angle - GAMF (Zoom)

Each cutting tool has a radial rake angle. The angle is measured between the cutting edge and the workpiece. Therefore when an indexable insert is mounted on a tool holder, you should use the combined angle (the angle of the top rake relative to the tool’s clamping plane when the insert is mounted in the pocket). A reputable tool supplier will provide this angle in his catalogs. If you have trouble obtaining it, use +7° as a default value since most cutting tools have a slight positive rake angle.

d) Chip thickness (HM) Is calculated differently depending on the application:

Turning Approach Angle
  • When the approach angle is 90° (or more), use the feed per revolution as the chip thickness HM=FN
  • As the approach angle gets smaller, the chip thickness is reduced according to the formula: HM = FN X SIN(KAPR)
  • Milling The chip thickness depends on two factors:
Sketch of Chamfer endmill

Approach angle (KAPR):

  • When the approach angle is 90° (Standard straight cutters), use the feed per revolution as the chip thickness HM=FN
  • As the approach angle gets smaller, the chip thickness is reduced according to the formula: HM = FN X SIN(KAPR)
  • For round shapes, the formula is more complicated and is not covered here.

K1C and MC chart for material group

  • In the below table, you can find KC1 and MC values for 41 different material groups.
  • If you want to search for KC and MC according to raw material designation (SAE/DIN/Wnr), use this Detailed Chart.

K1C and MC chart for raw materials

You can search the below chart according to SAE, DIN, or Wnr designations.

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