Regular subscribers to the Portable Performance Blog will know we like to provide insight into how our machines work and describe their applications on-site. Everything we make is designed with machinability in mind, so this time we get back to basics and cover the definition of machinability, what effects it, and how its measured.
Definition of machinability
The machinability of a material is the ease with which it can be cut by a tool to achieve the required finish. A material that is easy to machine requires less power, takes less time to cut, it doesn’t wear the tooling quickly and it produces a good surface finish. Manufacturers of metal products face the challenge of balancing machinability with performance, as typically, the easier a component is to machine the less ‘performance’ it delivers. By performance, in this instance we mean the strength and durability.
Materials and their effect on machinability
- The properties of a material that impact its machinability include; its modulus of elasticity, thermal conductivity, thermal expansion, tensile strength and work hardening.
- The crystal structure and arrangement of atoms within the crystal grains and their boundaries has a significant impact on the ease of material removal.
- Heat treatment, work hardening and fabrication change the crystal structure, making the metal more difficult to machine.
- Chemical composition also changes machinability, for example adding carbon increases the strength but makes the metal more difficult to machine.
Tool Geometry: The design of the tool has a major influence on machining. The angle of the rake face, clearance faces and ‘chip breaker’ each play their part in creating a trouble-free and clean cut.
Tool Material: Selecting a cutting tool with correct right hardness, toughness and wear resistance of the are key considerations. High speed steel is available in M and T types (Molybdenum and Tungsten) and provide better performance than carbon steel. Tungsten Carbide tool bits are an alternative which last longer but are more brittle.
Speeds: The cutting speed is the most important factor for extending tool life. The cutting speed must be set to suit the machinability of the material. A high cutting speed initially may produce a good finish, but at the expense of excessive tool wear making it difficult to maintain the correct dimensions.
Feed rate: This is feed the speed at which the material is fed into the tool. In the case of drilling this means how fast the operator or auto feed is pushing a drill bit into the job.
Depth of cut: The depth of cut refers to the amount of material in contact with the tool. For example, the depth of cut set on a milling machine using the Z axis on the spindle.
Rigidity of the machine: Even if everything else correct, a machine that is not robust and is unstable will produce poor results. Machine design factors affecting accuracy include; ball screw drives, a large spindle power, the quality of the bearings, the clamping design and the overall weight and robustness of the machine.
How is machinability quantified?
The American Iron and Steel Institute (AISI) carried out turning tests on many metals at 180 surface feet per minute (sfpm). The weighted averages of normal cutting speed, surface finish and tool life were then determined for each material. The metal 160 Brinell B1112 steel was set as the 100% reference point and used to compare the machinability rating of other metals. Metals with a value of more than 100% are softer and therefore easier to machine. Those less 100% are harder materials and more difficult to machine.This method provides a useful reference point as an approximate guide, but it should be noted that if the exercise was repeated using machining types other than turning, the results may throw up some inconsistencies.
Metal Machinability Ratings Chart
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