Tool wear and wear mechanisms

TOOL WEAR AND WEAR MECHANISMS

During machining process, the tool is subjected to three important factors such as forces, temperature and sliding action due to relative motion between tool and workpiece. In aforesaid factors, the tool will be giving unsatisfactory performance after some time. It results the loss of dimensional accuracy, increased surface roughness and increased power requirements etc. The unsatisfactory performance results tool wear due to its continuous use. Therefore, the tool requires a periodical reconditioning or replacement. It will result in the loss of production and also the cost of replacing or reconditioning. Hence, the study of tool wear is important.

1. Mechanism/Forms of Tool Wear

1. Attrition:

In low cutting speeds, the flow of metal from the cutting edge is irregular and less streamlined. Sometimes, the built-up edge might be formed but the contact will not be continuous. In this situation, the tool will start to tear from the tool surface. It is called attrition. It occurs in continuous cutting but with interrupted cutting or due to lack of rigidity of the machine tool which will generate enormous vibrations and uneven work surfaces. All these reasons will lead the tool to destroy its cutting edges. Attrition could be minimized by increasing the cutting speed or using carbide tips as cutting edges where the built-up edge forms.

2. Diffusion:

Diffusion wear happens due to the diffusion of metal and carbon atoms from the tool surface into the work material and chips. It is also due to high temperature and pressure existing at the contact surfaces in metal cutting and rapid flow of chips. It mainly depends on the metallurgical properties of tool and work.

2. Classification of Tool Wear

The tool wear is generally classified as follows.

(i) Flank wear

(ii) Face wear or crater wear and (iii) Nose wear.

1. Flank wear:

This is also called "edge wear". Friction, abrasion and adhesions are the main causes for this type of wear. Flank wear is a flat worn out portion behind the cutting edge. The worn out region of the flank is known as wear land. This wear takes place when machining the brittle material such as cast iron. It also occurs when the feed is less than 0.15 mm/revolution. When the wear land increases, the frictional heat will cause the excessive temperature of the tool at the cutting edge thereby decreasing its hardness rapidly and hence, the catastrophic failure of the tool will occur. Flank wear results in a rough machined surface.

2. Crater wear:

The face of the tool is always contacted with the chip. The chip slides over the face of the tool. Due to the pressure of the sliding chip, the tool face gradually wears out. A cavity is formed on the tool face. The cavity is called crater. This type of wear is known as crater wear. The major tendency of this type of wear is abrasion between chip and face of the tool. When the cratering becomes excessive, the cutting edge may break from the tool.

Cratering is commonly occurred while machining a ductile material which produces continuous chips. Diffusion of metal may be one of the causes of this type of wear. The maximum depth of the crater is usually a measure of the amount of the crater wear. The tool life due to crater wear can be determined by fixing the ratio of the width of the crater to its depth.

3. Nose wear:

It is similar to flank wear in certain operations. The wear occurs on the nose radius of the tool. When the nose of the tool is rough, abrasion and friction between tool and workpiece will be high. Due to this type of wear, more heat will be generated. Also, more cutting force acts on the tool. This type of wear is more prominent than flank wear.