Wednesday, December 2, 2009

Tool wear

Tool wear describes the gradual failure of cutting tools due to regular operation. It is a term often associated with tipped tools, tool bits, or drill bits that are used with machine tools.

Tool wear phenomena



Types of wear include:


* flank wear in which the portion of the tool in contact with the finished part erodes. Can be described using the Tool Life Expectancy equation.
* crater wear in which contact with chips erodes the rake face. This is somewhat normal for tool wear, and does not seriously degrade the use of a tool until it becomes serious enough to cause a cutting edge failure.

Can be caused by spindle speed that is too low or a feed rate that is too high. In orthogonal cutting this typically occurs where the tool temperature is highest. Crater wear occurs approximately at a height equaling the cutting depth of the material. Crater wear depth ~ t0 t0= cutting depth

* built-up edge in which material being machined builds up on the cutting edge. Some materials (notably aluminum and copper) have a tendency to anneal themselves to the cutting edge of a tool. It occurs most frequently on softer metals, with a lower melting point. It can be prevented by increasing cutting speeds and using lubricant. When drilling it can be noticed as alternating dark and shiny rings.
* glazing occurs on grinding wheels, and occurs when the exposed abrasive becomes dulled. It is noticeable as a sheen while the wheel is in motion.
* edge wear, in drills, refers to wear to the outer edge of a drill bit around the cutting face caused by excessive cutting speed. It extends down the drill flutes, and requires a large volume of material to be removed from the drill bit before it can be corrected.

Effects of Tool Wear

Some General effects of tool wear include:

* increased cutting forces
* increased cutting temperatures
* poor surface finish
* decreased accuracy of finished part

Reduction in tool wear can be accomplished by using lubricants and coolants while machining. These reduce friction and temperature, thus reducing the tool wear.

Tool Life Expectancy

The Taylor Equation for Tool Life Expectancy provides a good approximation.

VcTn = C

A more general form of the equation is

V_c T^n \times D^x f^y=C

where

* Vc=cutting speed
* T=tool life
* D=depth of cut
* F=feed rate
* x and y are determined experimentally
* n and C are constants found by experimentation or published data; they are properties of tool material, workpiece and feed rate.

Temperature Considerations

At high temperature zones crater wear occurs. The highest temperature of the tool can exceed 700 °C and occurs at the rake face whereas the lowest temperature can be 500 °C or lower depending on the tool.ram

Energy Considerations

Energy comes in the form of heat from tool friction. It is a reasonable assumption that 80% of energy from cutting is carried away in the chip. If not for this the workpiece and the tool would be much hotter than what is experienced. The tool and the workpiece each carry approximately 10% of the energy. The percent of energy carried away in the chip increases as the speed of the cutting operation increases. This somewhat offsets the tool wear from increased cutting speeds. In fact, if not for the energy taken away in the chip increasing as cutting speed is increased; the tool would wear more quickly than is found.

Effects of cutting speed V and cutting time T on crater wear depth KT


Notch wear

This is a special type of combined flank and rake face wear which occurs adjacent to the point where the major cutting edge intersects the work surface.

The gashing (or grooving, gouging) at the outer edge of the wear land is an indication of a hard or abrasive skin on the work material. Such a skin may develop during the first machine pass over a forging, casting or hot-rolled workpiece. It is also common in machining of materials with high work-hardening characteristics, including many stainless steels and heat-resistant nickel or chromium alloys. In this case , the previous machining operation leaves a thin work-hardened skin.

Chipping
Chipping of the tool, as the name implies, involves removal of relatively large discrete particles of tool material. Tools subjected to discontinuous cutting conditions are particularly prone to chipping. Chipping of the cutting edge is more like micro-breakages rather than conventional wear.

Built-up edge formation also has a tendency to promote tool chipping. A built-up edge is never completely stable, but it periodically breaks off. Each time some of the built-up material is removed it may take with it a lump (piece) of tool edge


Ultimate failure

The final result of tool wear is the complete removal of the cutting point - ultimate failure of the tool.
This may come about by temperature rise, which virtually causes the tool tip to soften until it flows plastically at very low shear stress. This melting process seems to start right at the cutting edge and because material flow blunts the edge, the melting process continues back into the tool; within a few seconds a piece of tool almost as large as the engaged depth of cut is removed.

An alternative mechanism of ultimate failure is the mechanical failure (usually a brittle fracture) of a relatively large portion of the cutting tip. This often results from a weakening of the tool by crater formation.

Ultimate failure by melting and plastic flow is most common in carbon and high-speed-steel tools, while fracture failures are most common in sintered carbide or ceramic tools.

Typical stages of tool wear in normal cutting situation

1- Initial (or Preliminary) wear region:

Caused by micro-cracking, surface oxidation and carbon loss layer, as well as micro-roughness at the cutting tool tip in tool grinding (manufacturing). For the new cutting edge, the small contact area and high contact pressure will result in high wear rate. The initial wear size is VB=0.05-0.1mm normally.

2- Steady wear region

After the initial (or preliminary) wear (cutting edge rounding), the micro-roughness isimproved, in this region the wear size is proportional to the cutting time. The wear rate is relatively constant.

3- Severe (or Ultimate or catastrophic) wear:

When the wear size increases to a critical value, the surface roughness of the machined surface decreases, cutting force and temperature increase rapidly, and the wear rate increases. Then the tool loses its cutting ability. In practice, this region of wear should be avoided.

Flank wear and chipping will increase the friction, so that the total cutting force will increase. The component surface roughness will be increased, especially when chipping occurs.
Flank wear will also affect the component dimensional accuracy. When form tools are used, flank wear will also change the shape of the component produced.

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