# Machining Forces and Merchant’s Circle Diagram (MCD) | Mechanics of Machining

## Benefit of knowing and purpose of determining cutting forces

The aspects of the cutting forces concerned :

• Magnitude of the cutting forces and their components

• Directions and locations of action of those forces

• Pattern of the forces : static and / or dynamic.

Knowing or determination of the cutting forces facilitate or are required for :

• Estimation of cutting power consumption, which also enables selection of the power source(s) during design of the machine tools

• Structural design of the machine – fixture – tool system

• Evaluation of role of the various machining parameters ( process – V_C, S_o, t, tool – material and geometry, environment – cutting fluid) on cutting forces

• Study of behaviour and machinability characterisation of the work materials

• Condition monitoring of the cutting tools and machine tools.

## Cutting force components and their significances

The single point cutting tools being used for turning, shaping, planing, slotting, boring etc. are characterised by having only one cutting force during machining. But that force is resolved into two or three components for ease of analysis and exploitation. Fig. ‘Cutting force R resolved into P_X, P_Y and P_Z‘ visualises how the single cutting force in turning is resolved into three components along the three orthogonal directions; X, Y and Z.

##### figure: Cutting force R resolved into P_X, P_Y and P_Z

The resolution of the force components in turning can be more conveniently understood from their display in 2-D as shown in Fig. ‘Turning force resolved into P_Z, P_X and P_Y‘.

##### figure: Turning force resolved into P_Z, P_X and P_Y

The resultant cutting force, R is resolved as,

In Fig. ‘Cutting force R resolved into P_X, P_Y and P_Z‘ and Fig. ‘Turning force resolved into P_Z, P_X and P_Y‘ the force components are shown to be acting on the tool. A similar set of forces also act on the job at the cutting point but in opposite directions as indicated by P_Z‘, P_X_Y‘, P_X‘ and P_Y‘ in Fig. ‘Turning force resolved into P_Z, P_X and P_Y‘.

### Significance of P_Z, P_X and P_Y

P_Z : called the main or major component as it is the largest in magnitude.

It is also called power component as it being acting along and being

multiplied by V_C decides cutting power (P_Z.V_C) consumption.

P_y : may not be that large in magnitude but is responsible for causing

dimensional inaccuracy and vibration.

P_X : It, even if larger than P_Y, is least harmful and hence least significant.

### Cutting forces in drilling

In a drill there are two main cutting edges and a small chisel edge at the centre as shown in Fig. ‘Cutting forces in drilling’.

The force components that develop (Fig. Cutting forces in drilling) during drilling operation are :

• a pair of tangential forces, P_T_1 and P_T_2 (equivalent to P_Z in turning) at the main cutting edges

• axial forces P_X_1 and P_X_2 acting in the same direction

• a pair of identical radial force components, P_Y_1 and P_Y_2

• one additional axial force, P_X_e at the chisel edge which also removes material at the centre and under more stringent condition.

P_T_1 and P_T_2 produce the torque, T and causes power consumption P_C as,

T = P_T x ½ (D) (8.3)

and P_C= 2πTN (8.4)

where, D = diameter of the drill

and N = speed of the drill in rpm.

The total axial force P_X_T which is normally very large in drilling, is provided by

P_X_T = P_X_1 + P_X_2 + P_X_e (8.5)

But there is no radial or transverse force as P_Y_1 and P_Y_2, being in opposite direction, nullify each other if the tool geometry is perfectly symmetrical.

##### figure: Cutting forces in drilling

### Cutting forces in milling

The cutting forces (components) developed in milling with straight fluted slab milling cutter under single tooth engagement are shown in Fig. ‘Cutting forces developed in plain milling’.

The forces provided by a single tooth at its angular position, ψI are :

• Tangential force P_T_i (equivalent to P_Z in turning)

• Radial or transverse force, P_R_i (equivalent to P_X_Y in turning)

• R is the resultant of P_T and P_R

• R is again resolved into P_Z and P_Y as indicated in Fig. ‘Cutting forces developed in plain milling’ when Z and Y are the major axes of the milling machine.

Those forces have the following significance:

o P_T governs the torque, T on the cutter or the milling arbour as

T = P_T x D/2 (8.5)

and also the power consumption, P_C as

P_C = 2πTN (8.6)

where, N = rpm of the cutter.

The other forces, P_R, P_Z, P_Yetc are useful for design of the

Machine – Fixture – Tool system.

In case of multitooth engagement;

Total torque will be D/2.ΣP_T_i and total force in Z and Y direction will be ΣP_Z and ΣP_Y respectively.

One additional force i.e. axial force will also develop while milling by helical fluted cutter

##### figure: Cutting forces developed in plain milling (with single tooth engagement)

## Merchant’s Circle Diagram and its use

In orthogonal cutting when the chip flows along the orthogonal plane, π_O, the cutting force (resultant) and its components P_Z and P_X_Y remain in the orthogonal plane. Fig. ‘Development of Merchant’s Circle Diagram’ is schematically showing the forces acting on a piece of continuous chip coming out from the shear zone at a constant speed. That chip is apparently in a state of equilibrium.

##### figure: Development of Merchant’s Circle Diagram

The circle(s) drawn taking R or R_1 as diameter is called Merchant’s Circle which contains all the force components concerned as intercepts. The two circles with their forces are combined into one circle having all the forces contained in that as shown by the diagram called Merchant’s Circle Diagram (MCD) in Fig. ‘Merchant’ s Circle Diagram with cutting forces’.

##### figure: Merchant’ s Circle Diagram with cutting forces

The significance of the forces displayed in the Merchant’s Circle Diagram are:

P_S – the shear force essentially required to produce or separate the chip from the parent body by shear

P_n – inherently exists along with P_S

F – friction force at the chip tool interface

N – force acting normal to the rake surface

P_Z – main force or power component acting in the direction of cutting velocity

The magnitude of P_s provides the yield shear strength of the work material under the cutting condition.

The values of F and the ratio of F and N indicate the nature and degree of interaction like friction at the chip-tool interface. The force components P_X, P_Y and P_Z are generally obtained by direct measurement. Again P_Z helps in determining cutting power and specific energy requirement. The force components are also required to design the cutting tool and the machine tool.

## Advantageous use of Merchant’s Circle Diagram (MCD)

Proper use of MCD enables the following:

• Easy, quick and reasonably accurate determination of several other forces from a few known forces involved in machining

• Friction at chip-tool interface and dynamic yield shear strength can be easily determined

• Equations relating the different forces are easily developed

### Some limitations of use of MCD

• Merchant’s Circle Diagram is valid only for orthogonal cutting

• by the ratio, F/N MCD gives apparent (not actual) coefficient of friction

• It is based on a single shear plane theory

The advantageous of constructing and using MCD has been illustrated as by an example as follows;

Suppose, in a simple straight turning under orthogonal cutting condition with given speed, feed, depth of cut and tool geometry, the only two force components P_Z and P_X are known by experiment i.e., direct measurement, then how can one determine the other relevant forces and machining characteristics easily and quickly without going into much equations and calculations but simply constructing a circle-diagram. This can be done by taking the following sequential steps :

• Determine P_X_Y from P_X = P_X_YsinΦ, where P_X and Φ are known

• Draw the tool and the chip in orthogonal plane with the given value of γ_o as shown in Fig. ‘Cutting forces developed in plain milling (with single tooth engagement)’ Choose a suitable scale (

• Choose a suitable scale (e.g. 100 N = 1 cm) for presenting P_X_Y in cm

• Draw P_Z and P_X_Y along normal to as indicated in Fig. ‘Merchant’ s Circle Diagram with cutting forces’

• Draw the cutting force R as the resultant of P_Z and P_X_Y

• Draw the circle (Merchant’s circle) taking R as diameter

• Get F and N as intercepts in the circle by extending the tool rake surface and joining tips of F and R

• Divide the intercepts of F and N by the scale and get the values of F and N

• For determining P_s (and P_n) the value of the shear angle β_o has to be evaluated from

• Draw the shear plane with the value of β_o and then P_s and P_n as intercepts shown in Fig. Merchant’ s Circle Diagram with cutting forces.

• Get the values of P_s and P_n by dividing their corresponding lengths by the scale

• Get the value of apparent coefficient of friction, µ_a at the chip tool

interface simply from the ratio, µ_a = N/F

• Get the friction angle, η, if desired, either from tanη = µ_a or directly from the MCD drawn as indicated in Fig. Merchant’ s Circle Diagram with cutting forces.

• Determine dynamic yield shear strength (τs) of the work material under the cutting condition using the simple expression

where, A_s = shear area as indicated in Fig. Shear area in orthogonal turning

##### figure: Shear area in orthogonal turning

### Evaluation of cutting power consumption and specific energy requirement

Cutting power consumption is a quite important issue and it should always be tried to be reduced but without sacrificing MRR.

Cutting power consumption, P_C can be determined from, PC = P_Z.V_C + P_X.V_f

where, V_f = feed velocity

= Ns_o/1000 m/min [N=rpm]

Since both P_X and V_f, specially V[/latex]_f[/latex] are very small, P_Z.V_C can be neglected and then P_C ≅ P_Z.V_C

Specific energy requirement, which means amount of energy required to remove unit volume of material, is an important machinability characteristics of the work material. Specific energy requirement, U_s, which should be tried to be reduced as far as possible, depends not only on the work material but also the process of the machining, such as turning, drilling, grinding etc. and the machining condition, i.e., V_C, S_o, tool material and geometry and cutting fluid application.

Compared to turning, drilling requires higher specific energy for the same work-tool materials and grinding requires very large amount of specific energy for adverse cutting edge geometry (large negative rake).

Specific energy, U_s is determined from

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