Grinding - a Finishing Process

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Name: Yap Chan Fai

ID: 2014-05-0069

Pensyarah: Mr Bala

Content

                                                            Page

Introduction                                                             3

Equipment                                                         4-6

Method and theory                                               7-10

Advantages of grinding                                     10

Discussion                                                     11-13

Reference                                                           14

Conclusion                                                           14

Introduction

Grinding is a finishing process used to improve surface finish, abrade hard materials, and tighten the tolerance on flat and cylindrical surfaces by removing a small amount of material In grinding, an abrasive material rubs against the metal part and removes tiny pieces of material The abrasive material is typically on the surface of a wheel or belt and abrades material in a way similar to sanding. On a microscopic scale, the chip formation in grinding is the same as that found in other machining processes. The abrasive action of grinding generates excessive heat so that flooding of the cutting area with fluid is necessary.

The grinding machine consists of a power driven grinding wheel spinning at the required speed (which is determined by the wheel’s diameter and manufacturer’s rating, usually by a formula) and a bed with a fixture to guide and hold the work-piece. The grinding head can be controlled to travel across a fixed work piece or the work piece can be moved whilst the grind head stays in a fixed position. Very fine control of the grinding head or tables’ position is possible using a Vernier calibrated hand wheel, or using the features of numerical controls.

Equipment

Grinding Machine

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Grinding machines remove material from the workpiece by abrasion, which can generate substantial amounts of heat; they therefore incorporate a coolant to cool the workpiece so that it does not overheat and go outside its tolerance. The coolant also benefits the machinist as the heat generated may cause burns in some cases. In very high-precision grinding machines (most cylindrical and surface grinders) the final grinding stages are usually set up so that they remove about 200 nm (less than 1/100000 in) per pass – this generates so little heat that even with no coolant, the temperature rise is negligible.

CBN grinding wheels

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1. CBN is second only to diamond in hardness.
2. It was developed as a by-product of research in the 1950s into the manufacture of synthetic diamonds.
3. Unlike diamond, it is not a form of carbon.
4. This allows it to be used on carbon steels.
5. If diamond were used on carbon steels a process known, as ‘graphitization’ would occur due to the affinity of the carbon in the two materials.
6. This would lead to rapid wear of the diamond and increased temperatures.

Coolant or lubricant

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The use of grinding fluids is effective in preventing thermal damage since fluids not only remove heat from the grinding zone but also lubricate to reduce the friction between the wheel and the workpiece. There are, however, limitations on the ability of the fluid to cool. Shafto etc. (1975) have shown that for creep-feed grinding, water-based fluids cool effectively only when the workpiece surface temperature is below a critical value of about 130°C. Once this temperature is exceeded, the fluid loses its ability to cool, and the workpiece temperature rises steadily to a temperature, which could be expected under dry grinding conditions. Depending on this temperature, various degrees of thermal damage may result.

Method and theory

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Surface grinding was carried out using grinding machine, the material was used in this experiment was passed through the grinding machine for fifteen different runs. This was done using different samples. After passing the work piece through the grinding machine, all work-pieces were then evaluated after machining and signs of burn marks identified.

For each of the material sample, values of specific energy were obtained in both experimental and theoretical cases using equation (i) and (ii) respectively. For example, for work piece number seven, the calculations were done as follows:

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The above equation was used to determine the theoretical value of specific grinding power. For the experimental as the equation used is shown below:

Experimental Specific Grinding energy,

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• For each complete cycle, the workpiece was visually inspected for signs of burn and the result was recorded. Chatter occur on workpiece number 14, this was caused by low speed.
• The data obtained is converted to the all parameters involved in the various calculations to units of millimeters (mm), seconds (s) and watts (W)
• The desired values of specific energy in J/mm3 are calculated in the same manner to yield the result.

The experimental values of specific energy at which burn occurs is compare with the theoretical values that is obtained using the equation shown below

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Where: [pic 12]

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Problems that occur during this lab (grinding process) include:

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