Thermal Damage During the Grinding Process

Abstract:

In this lab a grinding machine was used with several work pieces with the aim of determining the limitations of increasing removal rates during the grinding process with the lab focusing on identifying the conditions in which thermal damage occurs, this can cause other problems like cracks occurring on the work-piece and softening and re-hardening of the material followed by residual stress

Figure (1) is a photo of three work-pieces taken at the lab all three pieces show different results the top piece shows no signs of burns (thermal damage) this is because there wasn’t sufficient speed on the piece or wheel. The middle piece shows signs of burns due to the speed of the wheel or the piece however the bottom piece shows no signs of burns even though the same speed was reached, this is because a coolant was used hence the blue discolouration on the piece. From this we know if grinding at high speeds coolant is necessary to reduce chances of thermal damage.

Table of contents:

Abstract……………………………………………………………………………………………………………………………………………. 2

Introduction………………………………………………………………………………………………………………………………………3

Method……………………………………………………………………………………………………………………………………………..4

Theory……………………………………………………………………………………………………………………………………………….4

Results………………………………………………………………………………………………………………………………………………5

Discussion………………………………………………………………………………………………………………………………………….6

Conclusion…………………………………………………………………………………………………………………………………………7

References…………………………………………………………………………………………………………………………………………7

INTRODUCTION:

Grinding is a process in which a work piece is engaged by an abrasive wheel rotating at a certain adjustable speed which cuts smalls chips from the work piece via sheer deformation. This process improves surface finish and abrades hard materials, this process is similar to sanding and because of the speed at which this occurs the abrasive action of grinding generates heat so it’s necessary to flood the cutting area with fluid. (Neailley K., 1988)

The grinding machine spins at a certain speed which is determined by the wheel diameter and manufacturers rations, this is usually done using a formula. The work piece is on a bed with a fixture guiding it, the grinding head is controlled to travel across or the head can be fixed and then piece is instead moved towards the head. Accuracy and fine control of the head or table is possible with the use of a Vernier calibrated wheel. The objective of this lab is to determine the specific energy and using that predict the values of infeed rate (Vƒ) and work piece speed (Vɯ) at which burns occur.

     (https://www.slideshare.net/lalukumar56027/l9-grinding2012)

Material is removed from the work-piece by abrasion which generates substantial heat; therefore a coolant is required to keep the work-piece at a safe cool temperature so it doesn’t overheat and fall outside the tolerance range. The coolant fluid is also a safety feature for the person handling the grinder as heat may cause burns in some occasions. Coolant isn’t always required for some high-precision machines as they generate so little heat that the rise in temperature is negligible, they remove about 200 nm per pass. There are different types of grinding wheels made of different materials CBN grinding wheels are the second hardest just before diamonds they were developed as a by-product in the 50s during the manufacturing of synthetic diamond however unlike diamond it isn’t a form of carbon which opens it up to use on carbon steels whereas if diamond was used, ‘graphitization’ would occur because of the affinity of the carbon in both materials this causes rapid wear of the diamond and high temperature. These problem are not found with CBN. When it comes to coolants they are effective in preventing thermal damage since fluids remove heat from the grinding zone and they lubricate to reduce friction on the wheel and work-piece. However there are restrictions when it comes to coolants ability to cool effectively in creep-feed grinding, Shafto et al. (1975) have shown us that when it comes to creep-feed grinding water-based coolants only effectively work when the surface temperature is below the critical value of 130 °C after this temperature is reached the coolant no longer works effectively allowing the work-piece temperature to increase, because of this thermal damage occurs.

Method:

Figure 3 shows a grinding machine that’s in operation. (Probst, E. 2016)

(https://www.mmsonline.com/articles/grinding-machine-increases-tool-suppliers-production)

The lab was setup using a grinding machine similar to that of figure (x) and using suitable values of infeed rate and work-piece speed given, surface grinding was carried out and 15 different results were recorded using different samples all the pieces were then evaluated for thermal damage. During the grinding cycle the peak grinding power should be taken note of and the work piece diameter before and after the cycle should be taken down.

Theory:

  _w    = thermal conductivity of the work-piece

a      =     depth of cut

θ_m^* = critical temperature at which burn occurs

R      = energy partition ratio between the wheel and the work-piece.

Approximate values of the above parameters may be obtained from the Annals of the CIRP

Using the above formulas and the results recorded in our table we calculated the required values for the following as shown in the table below.