Machinability
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The machinability of a material is a measure of how economically it can be machined into a finished part. Materials with good machinability require little power to cut, the resulting surface has a good finish, and the cutting tool lasts a long time; such materials are said to be free machining. The factors that typically improve a material's performance often degrade its machinability. Therefore, to manufacture components economically, engineers are challenged to find ways to improve machinability without harming performance.
Machinability can be difficult to predict because machining is such a complex process. In most cases, the strength and toughness of a material are the primary factors. Strong, tough materials are usually more difficult to machine simply because greater force is required to cut them. Other important factors include the chemical composition, thermal conductivity and microstructure of the material, the cutting tool geometery, and the machining process parameters. Sometimes, especially for non-metals, the ancillary factors are most important. For example, soft materials like plastics can be difficult to machine because of their poor thermal conductivity.
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[edit] Quantifying Machinability
There are many factors affecting machinability, but no widely accepted way to quantify it. Instead, machinability is often assessed on a case-by-case basis, and tests are tailored to the needs of a specific manufacturing facility. Common metrics for comparison include: tool life, surface finish, cutting temperature, and power consumption. [1]
Tables and charts that provide a reference for comparing the machinability of different materials do exist, but they are necessarily vague because process variables and other external factors can have a large impact. These tables usually measure the machinability in terms of the cutting velocity for a given tool life. For example, the relative machinability might be given as (v_60_1)/(v_60_2), where v_60 is the cutting speed for a 60 minute tool life. [2] The most famous machinability tests were conducted by FW Taylor and became the basis for the Taylor tool life equation. [3]
[edit] Steels
The carbon content of steel greatly affects its machinability. High-carbon steels are difficult to machine because they are tough and because they may contain carbides that abrade the cutting tool. On the other end of the spectrum, low-carbon steels are troublesome because they are too soft. Low-carbon steels are "gummy" and stick to the cutting tool, resulting in a built up edge that shortens tool life. Therefore, steel has the best machinability with medium amounts of carbon, about 0.20%. [1]
Chromium, molybdenum and other alloying metals are often added to steel to improve its strength. However, most of these metals also decrease machinability.
Inclusions in steel, especially oxides, may abrade the cutting tool. Machinable steel should be free of these oxides.
[edit] Additives
There are a variety of chemicals, both metal and non-metal, that can be added to steel to make it easier to cut. These additives may work by lubricating the tool-chip interface, decreasing the shear strength of the material, or increasing the brittleness of the chip. Historically, sulphur and lead have been the most common additives, but bismuth and tin are increasingly popular for environmental reasons.
Lead can improve the machinability of steel because it acts as an internal lubricant in the cutting zone. [4] Since lead has poor shear strength, it allows the chip to slide more freely past the cutting edge. When it is added in small quanities to steel, it can greatly improve its machinability while not significantly affecting the steel's strength.
Sulphur improves the machinability of steel by forming low shear strength inclusions in the cutting zone. These inclusions are stress risers that weaken the steel, allowing it to deform more easily.
[edit] Stainless Steel
Stainless steels have poor machinability compared to regular carbon steel because they are tougher, gummier and tend to work harden very rapidly. [1] Slightly hardening the steel may decrease its gumminess and make it easier to cut. AISI grades 303 and 416 are easier to machine because of the addition of sulphur and phosphorus. [5]
[edit] Aluminum
Aluminum is much softer than steel, and techniques to improve its machinability usually rely on making it more brittle. Alloys 2007, 2011 and 6020 have especially good machinability. [5]
[edit] Other materials
Thermoplastics are difficult to machine because they have poor thermal conductivity. [4] This creates heat that builds up in the cutting zone, which degrades the tool life and locally melts the plastic. Once the plastic melts, it just flows around the cutting edge instead of being removed by it. Machinability can be improved by using high lubricity coolant and keeping the cutting area free of chip build up.
Composites often have the worst machinability because they combine the poor thermal conductivity of a plastic resin with a tough or abrasive ceramic matrix.
The machinability of rubber and other soft materials are sometimes machined with improves by using a very low temperature coolant, such as liquid carbon dioxide. The low temperatures chill the material prior to cutting so that it is more brittle and requires less energy to machine.
[edit] See also
[edit] Notes
- ^ a b c Bakerjian, Ramon; Cubberly, W. H. (1989). Tool and manufacturing engineers handbook. Dearborn, Mich: Society of Manufacturing Engineers, 15-3, 15-10, 19-13 to 19-18. ISBN 0-87263-351-9.
- ^ Bacon, David; Dieter, George Ellwood (1988). Mechanical metallurgy. New York: McGraw-Hill, 698. ISBN 0-07-100406-8.
- ^ Taylor, F.W. (1907). "On the Art of Cutting Metals". Transactions of the ASME 28: 31-350.
- ^ a b Kalpakjian, Serope; Steven R. Schmid (2003). Manufacturing Processes for Engineering Materials. Pearson Education, 437-440. ISBN 8178089904.
- ^ a b McMaster-Carr Catalog. Retrieved on 2008-04-01.
