Factors Affecting Tool Life

Factors Affecting Tool Life

Factors Affecting Tool Life -The following factors affect tool life:

(i) Cutting speed The most significant factor affecting tool life when the work material, tool material and tool shape are chosen for a particular machining operation is the cutting speed.

In 1906, Taylor showed that a relationship existed between tool life and cutting speed as follows:

VTⁿ = C where v is cutting speed, mpm.
T is tool life, min
n is exponent depending upon cutting conditions,
C is constant = the cutting speed for a tool life of I min.

It can be seen that as the cutting speed is reduced, the tool life increases.

(ii) Effect of feed and depth of cut An increase in feed and depth of cut will shorten tool life but not nearly as much as an increase in cutting speed. A empirical formula that includes the effect of speed, feed and depth of cut on tool life is

V = 257 / T^0.19 * s⁰³⁶ * t^0.08 mpm

V is cutting speed, mpm
T is tool life, min
s is feed, mm per min t is depth of cut, mm

(iii) Cutting temperature High temperature in cutting is a result of other factors, but is also a limiting factor in tool life. Anything that affects the temperature will affect tool life. In general, higher temperatures cause shorter tool life.

In general, the following are the maximum safe operating temperatures:

The cutting temperature may be measured by the tool-work thermocouple method. With a given combination, e.g., medium-carbon steel and high speed steel tools, the cutting temperature depends upon

Cutting speed
Depth of Cut
Cutting fluids
Feed
Tool Geometry

The relationship of cutting temperature to cutting speed (in the realm of a continuous chip) is of the form

θ_i = CVⁿ

Where θ_i is tool chip interface temperature
v is cutting speed, feet per minute
C is a constant (depends on tool work pair and cutting variables other than speed)
n is an exponent, whose value for H58 tools, cutting steel ≈ 1/2 ; for sintered carbides on steel it is about 1/5.

An empirical equation of similar form relates cutting temperature to feed. In this case, however, the exponent is somewhat lower, being about 3/8 for high speed steels on steel, and 1/7 to 1/8 for sintered carbides on steel. From these values it is seen that the increase in feed is less hazardous tempera­ture wise than a corresponding increase in speed.

The depth of cut will influence the cutting temperature in a limited manner. If the depth of cut is more than twice the nose radius, a further increase in depth will have little or no effect on the cutting temperature.

Tool geometry, will affect cutting temperature somewhat, since images in side cutting edge angle, rake angle, etc., alter chip formation and flow. If the change in tool geometry results in less plastic deformation in the chip, the heat generated there is reduced, and one could expect a lower cutting temperature.

However, if this is accomplished by an increase in the rake angle ex, one must keep in mind that large angles reduce the mechanical strength and heat conducting capacity of the cutting edge. In work materials of low shear strength this is usually not serious, but with such materials as medium carbon steels the reduction in strength and in heat conducting capacity may cause early tool failure.

 The effect of cutting fluid on cutting temperature may be due to either direct cooling or to reduction of the energy required in cutting. At low speeds the cutting fluid has time to penetrate and react to provide friction reducing effects and decrease the cutting temperature.

At high speeds, little effect on the cutting temperature is noted. However, the cutting fluid will remove heat from the tool and the workpiece, and avoid heat accumulation and temperature buildup in the vicinity of the active cutting edge of the tool.

(iv)Abrasive action of the work on the Tool The source of the abrasive action of the work on the tool is at least twofold. It is made up of abrasion due to

(1) the inherently hard constituents present in the microstructure of the metal being cut and

(2) the hardening induced in the chip and work surface by the cutting process. The first of these needs little further explanation. Hard inclusions in the microstructure ordinarily will produce rapid wear of the tool.

The amount of abrasion produced by the second of these factors the hardness induced by the cutting process depends on

(1) the initial hardness of the metal being cut (as measured by the Brinell hardness value H) and

(2) the amount of shearing strain that the metal undergoes during machining. This last is a function of the friction between the chip and tool. Thus, other things being equal, tool wear is reduced by a reduction in the initial hardness of the metal or a reduction in the amount of deformation taking place during cutting.

To summarize, the basic mechanical variables whose effect on tool life is known are the shear strength of the metal being cut the friction between chip and tool, its initial Brinell hardness, and the amount and nature of the hard constituents in its microstructure.