Effects of hardness
When machining hard workpiece materials, cutting tool wear can develops easily. Additionally, the cutting edge is exposed to severe impacts, this in turn will result in high cutting resistance.
While for softer workpiece materials, welding can occur. Welding being a phenomenon where the elements of the material melt and adhere to the cutting edge. This can then peel off together with a part of the cutting edge, causing chipping. When chipping occurs, the substrate is exposed leading to an increase in welding and eventually abnormal cutting edge damage that could leave a poor surface finish. Chip control should also be taken into consideration because the chips formed when machining softer materials are difficult to break. This can also lead to a poor surface finish.
Therefore if the hardness is too high or too soft problems with machining can arise. Generally speaking a hardness of between 180HB~280HB is deemed a hardness region that is termed most suitable for machining.
Effects of adhesiveness
Workpiece material adhesiveness relates to cutting resistance. Adhesive materials are difficult to cut, because of high cutting resistance. Chips of adhesive materials are also difficult to break, therefore requiring careful chip control. Care should be taken to prevent chipping of the cutting edge due to the large vibrations that can occur when the chips are broken.
(Difficult to manipulate or deform)
Material strength refers to the resistance of a workpiece to being manipulated or deformed. As machining is a process in which the workpiece is forcibly manipulated or deformed then as the workpiece strength increase so to does the cutting force required to machine it. As the cutting forces increase the cutting edge is subjected to high load and temperatures. This can lead to the development of plastic deformation, oxidation and crater wear of the cutting edge.
Effects of malleability
Malleable workpiece materials are soft materials such as aluminium and copper. When machining such materials it is necessary to bear in mind that chip control is very important. As the material is soft the chips generated are difficult to break. This results in continuous chips that will if not correctly controlled, reduce the quality of the surface finish.
Effects of thermal conductivity
When machining materials with low thermal conductivity, the heat generated when machining is not effectively diffused, therefore the cutting edge temperatures tend to become very high.
When the cutting edge temperature becomes extremely high, the hardness of the tool material reduces leading to wear. In addition, as the cutting edge softens plastic deformation can occur. When this happens the cutting resistance will increase leading to increased wear.
On the contrary, when machining a material with high thermal conductivity, the heat of the cutting edge is removed by chips or diffused into the workpiece material. And as such the cutting edge is not affected by heat.
Effects of work hardening properties
Work hardening is a material hardening phenomenon that can be caused by several factors. Almost all materials have a degree of a work hardening property.
When machining easy-to-work-harden materials the cutting tool can easily create or encounter a work hardened section. If this occurs then the cutting edge will be subjected to abrasive damage. This damage can often occur when machining at small depths of cut or at low feed rates. Additionally when machining at large depths of cut, the work hardened section creates wear at the depth of cut line causing notching.
Effects of affinity
Affinity is a term used to describe the ease of which materials tend to bond, or their compatibility with each other. If a workpiece material has high affinity, then when the cutting edge is subject to high cutting edge temperatures, the cutting edge and the workpiece react. This will result in adhesion and large rake wear.
Effects of hard particles
Materials that include hard particles such as carbide, cause the cutting edge to suffer from abrasive wear.<br>Additionally materials with many hard particles have a high hardness value and therefore accelerate tool flank wear.
The term “machinability” refers to either how hard or how easy it is to machine a workpiece material. A material with good machinability is known as easy-to-cut material, while material with poor machinability is known as difficult-to-cut material.
The criteria that determine the “machinability” of a workpiece material include tool life, difficulty of chip control, attainability of accurate machining, and attainability of acquiring a good surface finish.
Various factors determine whether or not these criteria can be achieved, such as the machining method, the machine being used, the cutting tool and the cutting conditions to be used.
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