Vibration of Machine Tools: Effects and Sources | Industrial Engineering

In this article we will discuss about the effects and sources of vibration of machine tools.

Effect of Vibration:

In machine tools attempts should be made either to eliminate the sources of vibration or to reduce them. To achieve this, it is essential that first the effects of vibration on machine tools are studied. The effects of vibrations can be considered on the machine tool, work-piece, tool life and the cutting conditions.

(i) Effect of vibrations on machine tool:

The machine tool is made of various parts and when vibrations are pro­duced, they also start vibrating at same frequency. If this frequency approaches the natural frequency of vibration of that part then amplitude of vibrations will be very excessive and the part may break even.

(ii) Effects of vibrations on work-piece:

Due to pres­ence of vibrations the surface finish obtained will be very poor and thus this aspect is very important for fine finishing operations of grinding and boring etc. In such machines the vibrations have to be completely eliminated. Due to vibra­tions, the dimensional accuracy of the job is also affected.

(iii) Effect of vibrations on tool-life:

As the tool-life is a function of the cutting variables only, the tool-life is greatly affected by presence of vibrations in machine tools. It is found out that the tool life is decreased by about 70—80% of the normal value if vibrations are present.

(iv) Effect of vibrations on cutting conditions:

By pres­ence of vibrations in machine tools, the chip thickness as removed by the cutting tool does not remain constant and due to it the cutting forces also vary. Also due to vibrations, vibratory displacement of tool takes place in the direction of motion of the job which results in the chatter of tool. The penetration rate also varies and therefore penetration force does not remain constant. Further due to vibration of the tool, cutting velocity does not remain constant and it varies about the correct value.

By studying the effects of vibrations, the tolerable limit of vibration can be fixed up for particular machines depending upon their purpose.

Sources of Vibrations:

In order that vibrations may be controlled or eliminated, it is desirable that first their sources be found out.

Vibrations in machine tools are mainly caused due to five reasons: 

(i) Generation of vibrations from the cutting process (tool chatter). These types of vibrations are self-induced and are supplied from the cutting process under some particular conditions.

(ii) In homogeneities in the material being machined and build up edges on the cutting tools. Due to sudden incre­ment in hardness of tool or job, impulsive force is generated which causes vibrations.

(iii) Intermittent cutting as in milling. Due to it, forced vibrations may be generated due to elastic nature of system.

(iv) Unbalance and disturbances in the drives caused due to rotating unbalanced masses, faulty arrangement of drive and fault in the supporting bearings.

(v) Transmission from the ground which is vibrating due to some other reasons. This can be minimised by isolat­ing the machine tools from the ground.

Vibrations and Dynamic Stability:

In dynamically stable system, the amplitude of vibration keeps on decaying with time whereas in dynamically unstable system, it exponentially increases with time and becomes susceptible to self-excited vibrations. In dynamically unstable systems, the amplitude of vibrations does not grow indefinitely but when it becomes larger and larger, non-linearity of the system becomes more and more prominent and growth of amplitude is arrested by that.

There are various stability criteria, the study of which reveals as to whether the system is stable or unstable. The structure of any machine tool has considerable influence on its dynamic stability and so the consideration of dynamic stability becomes important in the design procedure. With some damping in the machine tool structure chances of dynamic instability of the system becomes less and less.

There are mainly two theories about machine tool chatter. In first it is assumed that the modes of vibration of the system are sufficiently separated and no coupled oscillations take place. The machine tools is excited in only one mode. This is known as single degree of freedom theory. In second, the vibrations take place in two modes and theory is known as two degree of freedom chatter theory and it gives complex solutions.

Elimination of Vibrations:

An effort is made at the time of design of machine tool that the unwanted vibrations are absent and the various parts are dimensioned accordingly. Secondly knowing the sources of vibrations, the machine and the machining operations are so adjusted that no effective vibration takes place.

If the vibration is as a result of transmission from the ground then machine tool can be isolated from the ground so that no appreciable vibrations are transmitted. The machine tool is placed on springs of total stiffness (K) and total damping factor (γ).

Then if the vibration of the ground be of the form x = X cos ωt, the ratio of amplitude of the vibration of the machine and of ground will be given by:

ω is frequency of vibration of ground and

ω_n = natural frequency of vibration of machine.

m = mass of machine.

From above expression it is obvious that in order that machine body vibrates least due to ground vibrations, then value of γ, i.e., damping should be high. Also the value of ‘r’ , i.e., ω/ω_n should be high; in other words, the springs should not be having high stiffness.

In the design of the machine tools, the stiffness and damping capacity are two predominant features for determining the dynamic characteristics of the machine tools and components.

In machine tools there are mainly three sources of damping: 

(i) Energy dissipation due to rubbing between sur­faces of two elements at the junction during vibration.

(ii) Internal friction of the vibrating structure.

(iii) Resistance due to presence of surrounding medium, air.

Probably best way of increasing the damping is by intensifying the rubbing effect between the mating surfaces and this can be easily done by attaching strips of sheet of metal or plastic material at the component junctions.

The other important factor (stiffness or rigidity) should also be properly chosen for machine elements to minimise vibrations.

Isolation of Machine Tool Vibrations:

Well-designed supports are essential to ensure that neither the machine tool transmits vibrations through floor to other machines, nor it receives vibrations/noise from other machine tools. Thus to ensure smooth operation of a machine tool on shop floor, resilient and well-designed supports need to be provided to control transmission of vibration.

Computer aided designs have been developed to check suitability of all types of mounting systems. Safe passing over the resonance zone and ease of maintainability and replacement are the unique features of all mountings.

Attempts need to be made to keep the natural frequency of the isolator low enough as to control transmission of vibrations over a wide range of operating frequencies. The transmissibility of resonance can be kept to minimum by using suitable type of damping material.

Further the vibrations can be isolated in a number of stages, e.g., upper trough of isolator can be filled with concrete over which a chequered plate can be embedded for proper sealing so that concrete pack in absorbs vibration at the first stage. Spring-damping material combination can reduce vibration in the second stage, and low cast iron inverted trough can absorb vibrations in the third stage.

The guiding parameters are:

i. Low natural frequency of isolator and system

ii. Low transmissibility of resonance

iii. Flexibility of isolator in vertical direction with suit­able constraints

iv. Presetting of overall height of isolator.

v. Use of PVC helmet compound for smooth passing over resonance and easy maintenance.