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FACTORS THAT CAUSES TOOL WEAR IN INDUSTRIAL MACHINES OPERATIONS

 

Industrial machines are single unit of complex machining tools. The machines are made up of various functional units coupled together to carry out specific purposes. A good example of industrial machine is the lathe machine. The tailstock and headstock lye along the bed of the machine but perform different functions one part of the industrial machine may not work or perform its function without the help of another.

The major part of the industrial machines is the cutting unit. This is the unit that does the main cutting operation known as removal or reduction unit of any machine. The cutting tool may consist of the drill bit, grinding stone, cutting disk, or cutting bit, etc. any metal removing component of the industrial machines could be termed as its cutting tool unit.

The units mentioned above do have problem of wear and tear from time to time based on some factors affecting its smooth operations. Here are some of the factors affecting the cutting tools of the industrial machines;

Temperature

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Tool point rupture

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Wear and Fatigue

Without any influence of the above factors, machine tools are expected to perform satisfactorily. When failure occurs in machines tools the following problem become observable;

The tool may cease to produce workpiece according to the required dimension. i.e, misalignment and inaccuracy could become a regular problem.

Constant overheating could be associated with the tool while in operation.

Increased power consumption with lower work output could be an evidence of a machine tool that has gone weak.

A failing machine tool may have a burnishing band appearing on its surface.

The judgement decision as to how long a tool should be permitted to operate after preliminary indications that the tool is beginning to fall. The tool specialist must weigh the factors of the economics of stopping production and regrinding the tool versus allowing it to continue to operate.

IMPACT OF TEMPERATURE

Heat produced during metal cutting is mainly responsible for tool failure. When the temperature during cutting becomes very high, the tool becomes too soft to function properly and will lead to failure. This type of tool failure occurs easily and frequently in cutting tools. It could be accompanied with sparking and red tips signal such that it could be recognized easily by mere observation.

Every cutting tool has its maximum operating temperature that it can withstand during operation based on the type of material they are made from. During the temperature it could still retain its hardness and strength but beyond its maximum temperature the tool will fail to perform satisfactorily. Example of cutting tools and their maximum working temperature are:

Carbon tool steel   ----    200 to 250 degree Celsius

High speed steel   -----    560 to 600 degree Celsius

Cemented carbide   ------800 to 1000 degree Celsius

 

IMPACT OF RUPTURE

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Based on the high hardness required from cutting tools, the tip is mechanically weak and brittle. The failure associated with this is commonly observed in carbide and diamond tipped cutting tools. Failure can occur when the cutting forces exceed the critical value for a given tool when small portions of the cutting edge begins to chip off or the entire tip may break away in one piece.

For a given tool material, the tendency for rupture can be diminished either by reducing the causal forces, redirecting them or redesigning the tool to withstand them. The tendency for rapture can be reduced by considering the difference in the two diagrams below.

IMPACT OF WEAR AND FATIGUE

Fatigue is an unseen weakness in cutting tool which could be caused by the materials grain or crystal structure. It could be an unseen porous, cracks, or slag inclusion in the internal part of the tool originating from the manufacturing process of the tool or based of its handling and longtime of service life. Fatigue is the cause of most of the untraceable mechanical failures coming from tools and most machine components.

Wear is a direct reduction of parts of the cutting tool. It is a visible aspect of failure while its micro size could be detected using micro-reading instruments such as the micrometer. When a tool has been used for some time, wear becomes evident. In metal cutting operations, the major cause of wear are; adhesion, abrasion, and diffusion.

For adhesion, the junction between the chip and tool material are formed as part of friction mechanism. When these junctions are fractured small fragments of tool material can be torn out and carried away on the underside of the chip or on the new workpiece surface.

For abrasion wear, it occurs when hard particles on the underside of the chip pass over the tool face and remove tool material by mechanical action.

For diffusion, it happens when there is a diffusion process where atoms in a metallic crystal lattice move from a region of high atomic concentration to one of low concentration. During cutting when temperature is quite high at interface of tool and work piece, the atoms move from tool material to work piece material and thus weaken the surface of the tool.

Major types of wear found in cutting tools are crater wear and flank wear.

In crater wear, the main cause of could be abrasion between the chip and the face of the tool. A short distance from the cutting edge will result in a crater on the surface.  The crater formation is caused by the action of the chip particles flowing over it because of very high temperature. When cratering become excessive, the cutting edge may break from the tool. It a common wear seen when machining ductile materials that produces continuous chips. The maximum depth of the crater is usually a measure of the amount of the wear and can determined with a surface measuring instrument.

In flank wear, it takes place on the flank below the cutting edge resulting from the abrasive contact with the machined surface. Brittle materials tend to cause excessive flank wear because tool cutting edge tends to scrape over the machined surface and due to low abrasive action of loose fractured chips on the tool face while the flank is in constant contact with work.

The worn region at the flank is called wear land. The increased wear land means that frictional heat will cause excessive temperature of the tool at the cutting point and therefore, the tool will rapidly loose its hardness and fail.

 

 

Originally posted 2018-11-21 08:17:10.

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