In
machining
Machining is a manufacturing process where a desired shape or part is created using the controlled removal of material, most often metal, from a larger piece of raw material by cutting. Machining is a form of subtractive manufacturing, which util ...
, specifically
cutting
Cutting is the separation or opening of a physical object, into two or more portions, through the application of an acutely directed force.
Implements commonly used for wikt:cut, cutting are the knife and saw, or in medicine and science the sca ...
operations, a built-up edge (BUE) is an accumulation of material against the
rake face that seizes the
tool tip, separating it from the
chip.
[The Open University (UK), 2001. T881 Manufacture Materials Design: Block 2: Cutting, page 14. Milton Keynes: The Open University.]
Formation
Because
shear is strongest at the initial contact surface with the cutting tool, the first layer of metal impacting and seizing on it work-hardens more than the rest of the volume of metal. As a consequence of this
work hardening, this first layer of
metal
A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
is stronger than the adjacent metal moving away from the workpiece. Effectively, said first layer becomes part of the tool. The process repeats itself and, after some time, a built-up edge (which could be several hundred
micrometres thick) forms.
The conditions necessary for a noticeable edge to build up are that:
* The cutting speed is low.
[The Open University (UK), 2001. T881 Manufacture Materials Design: Block 2: Cutting, page 14 and 15. Milton Keynes: The Open University.] This is because, at high cutting speeds, the metal moving away from the workpiece becomes hot enough to
recover before seizing onto the tool, preventing the formation of a BUE.
[The Open University (UK), 2001. T881 Manufacture Materials Design: Block 2: Cutting, page 15. Milton Keynes: The Open University.]
* The metal being cut is one that work-hardens and is reluctant to recover.
A BUE will not form with pure metals since they do not work-harden much. Conversely,
alloy
An alloy is a mixture of chemical elements of which in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...
s, such as
steel
Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...
, do work-harden and recover less, so they are prone to forming a BUE.
Effects on the cutting process
The built-up edge effectively changes tool geometry and rake steepness. It also reduces the contact area between the chip and the cutting tool,
leading to:
* A reduction in the power demand of the cutting operation.
* A slight increase in tool life, since the cutting is partly being done by the built up edge rather than the tool itself.
However, the formation of BUEs have negative effects on the quality of the workpiece,
specifically:
* Excessive work hardening at the surface of the workpiece.
* Poor
surface finish
Surface finish, also known as surface texture or surface topography, is the nature of a interface (matter), surface as defined by the three characteristics of lay, surface roughness, and waviness.. It comprises the small, local deviations of a ...
, since bits of the BUE eventually break off and stick to the workpiece. These bits tend to be problematic since, due to the work-hardening they underwent, they are very
hard and so become
abrasive.
* A reduction in the dimensional control of the process, due to the dynamically changing geometry of the cutting tool.
See also
*
*
References
{{DEFAULTSORT:Built Up Edge
Metalworking terminology