Power and heat generation in machining operations (Part II)

DETERMINATION OF CUTTING TEMPERATURE

The magnitude of the cutting temperature need to be known or evaluated to facilitate

• Assessment of machinability which is judged mainly by cutting forces and temperature and tool life

• Design and selection of cutting tools

• Evaluate the role of variation of the different machining parameters on cutting temperature

• Proper selection and application of cutting fluid

• Analysis of temperature distribution in the chip, tool and job.

The temperatures which are of major interests are:

θ_s : average shear zone temperature

θ_i : average (and maximum) temperature at the chip-tool interface

θ_f : temperature at the work-tool interface (tool flanks)

θ_avg : average cutting temperature 

Cutting temperature can be determined by two ways :

• Analytically – using mathematical models (equations) if available or can be developed. This method is simple, quick and inexpensive but less accurate and precise.

• Experimentally – this method is more accurate, precise and reliable.

CONTROL OF CUTTING TEMPERATURE

It is already seen that high cutting temperature is mostly detrimental in several respects. Therefore, it is necessary to control or reduce the cutting temperature as far as possible.

Cutting temperature can be controlled in varying extent by the following general methods:

ü  proper selection of material and geometry of the cutting tool(s)

ü   optimum selection of V_C – s_o combination without sacrificing MRR

ü  proper selection and application of cutting fluid

ü  application of special technique, if required and feasible.

Role of variation of the various machining parameters on cutting temperature

The magnitude of cutting temperature is more or less governed or influenced by all the machining parameters like :

• Work material : - specific energy requirement

                              - ductility

                              - thermal properties (λ, c_v)

• process parameters : - cutting velocity (V_C)

                                         - feed (s_o)

                                         - depth of cut (t)

• cutting tool material :    - thermal properties

                                          - wear resistance

                                          - chemical stability

• tool geometry :  - rake angle (γ)

                              - cutting edge angle (φ)

                                                - clearance angle (α)

                                                 - nose radius (r)

• cutting fluid :      - thermal and lubricating properties

                              - method of application

 

HEAT GENERATED IN VARIOUS MACHINING OPERATIONS

Almost all of the heat generation models were established under orthogonal cutting condition. But in practice, there are various machining operations which cannot satisfy this condition, such as oblique turning, boring, drilling, milling, grinding, etc. Generally, the intensity of heat sources in real machining operations can be determined approximately by the external work applied, however, the distribution of the heat sources are hard to obtained by either theoretical or experimental methods.

The following listed are the simplified heat source model in real operations:

ü  Boring: A uniform moving ring heat source.

ü  End Milling: An ellipsoidal shape distribution with a distribution of uniform heat flux at milling area.(Heat source not defined by its intensity)

ü  Grinding: A circular heat source moving on the surface of workpiece.

Types of heat sources

There are several types of heat source in machining:

ü  Plastic work converted to heat.

ü  Viscous dissipation transformed into heat if the cut material is viscoplastic.

ü  Work done by friction converted to heat.

ü  Ambient heat source sometimes need be considered if thermal deformation is concerned.

ü  In non-traditional machining, other types of heat sources exist.

Heat Generated in Primary Zone

HEAT TRANSFER

Basically, heat transfer via three basic methods, all of which exist in machining processes. These three major types of heat transfer are conduction, convection and radiation. Conduction is the transmission or transfer of heat between two bodies in contact by virtue of their difference in temperature. Heat transfer inside the chip and workpiece, the tool and toolholder is by conduction. Convection in turn, is the transference of heat in a fluid (liquid or gas) by the upward movement of the heated and less dense particles. Heat transfer between coolant/air and the chip/tool/workpiece is by convection. Finally, radiation is the emission of energy as electromagnetic wave or moving particles. Radiation is rarely investigated in traditonal machining operations. But radiation techniques are widely applied in measuring the temperature distribution in various machining operations. [11, 13]

Convection by Coolant

Because convection of coolant varies with many factors, such properties of coolant, application conditions, state of coolant flow, and operation conditions, etc, it's required to investigate these corresponding issues. A Heat Transfer Performance Module, which can predict the convective heat transfer coeffients of several kinds of coolants used in some typical machining operations, can be accessible.

Another important noteworthy process is the simulation of open cutting fluid circulation system. An energy and mass flow model of cutting fluid circulation system is a very important issue in environmentally conscious machining. Sometimes, the disposal of chips and coolants needs much more energy than that in real cutting operations. Developing an effective way to utilize energy should be under consideration.

Cutting fluids' effects on heat transfer

Cutting fluids may reduce the cutting force, such as friction, therefore, heat generation is reduced to some extent. Using cutting fluids, heat generated in machining can be rapidly removed away by convection.

Generally, using cutting fluid cannot reduce the maximum temperature at the tool/chip interface, but increase the temperature gradient in both the chip and the tool because cutting fluid is not easy to access the cutting edge. [4]

POWER GENERATION

The contact between the cutting tool and the workpiece generates significant forces. These

forces create torques on the spindle and drive motors, and these torques generate power which is drawn from the motors. Excessive forces and torques cause tool failure, spindle stall (an event which is typically detected by monitoring the spindle speed), undesired structural deflections, etc.

The cutting forces, torques, and power directly affect the other process phenomena; therefore,

these quantities are often monitored as an indirect measurement of other process phenomena and are regulated such that productivity may be maximized while meeting machine tool and product quality constraints.

Cutting Force Models

There has been a tremendous amount of effort in the area of cutting force modeling over the

past several decades. However, these models tend to be quite complex and experimentation is

required to calibrate the parameters as an analytical model based on first principles is still not

available. The models used for controller design are typically simple; however, the models used for simulation purposes are more complex and incorporate effects such as tooth and spindle runout, structural vibrations and their effect on the instantaneous feed, the effect of the cutting tool leaving the workpiece due to vibrations and intermittent cutting, tool geometry, etc.

Note: this is just a highlight of the preliminaries for this project topic.............. for more info on this topic, leave a comment