Machinability
Machinability is the name the shop gives to how easily a material is cut, and this entry, the last of the materials group, sets out what the name means and why some materials are hard to machine. When a machinist calls a metal “hard to machine”, they are not speaking of its hardness alone: they mean that it resists being cut well — that it wears the tool, refuses a good surface, fights the chip, or will not hold size — and that the shop must cut it slowly and carefully to get a part at all. Machinability is a rating of that ease, and it is not a single property of the metal but the meeting of the metal’s properties with the machine, the tool and the cut. The same alloy that is “hard to machine” in one shop with one setup becomes ordinary work in another, which is why this wiki treats machinability as a matter of materials and of method together.
What machinability measures
Machinability is measured in the practical terms of the shop, and its rating expresses how a material compares with a benchmark set at 100 percent — commonly the free-cutting brass of the copper family, or a free-machining steel, given that rating because they cut with such ease. A material rated at 50 percent machines at roughly half the speed of the benchmark with comparable tool life; a rating is a shorthand for the whole difficulty of the cut. But behind the number stand the things the shop actually sees: the cutting speed the material will stand before the tool fails, the tool life it allows, the power the cut demands, the surface finish it will give, and the way it forms a chip. A material is easy to machine when it cuts at speed with long tool life, low power, a good finish and a chip that clears itself; it is hard when any of those fails, and the hardest materials fail at several at once.
The properties that resist the cut
The material properties that make a metal hard to machine are a short list, and each appears in the materials of this group. Strength and hot hardness resist the cutter’s entry — the nickel superalloys that keep their strength where the cut is hot shear like nothing else. Work-hardening hardens the surface under any rubbing, so a tool that dwells or takes too light a cut finds harder metal beneath it, as with the stainless steels and titanium. Poor thermal conductivity keeps the heat of cutting at the tool edge instead of carrying it away in the chip, softening the tool and driving wear — again the titanium and nickel habit. Chemical reactivity makes a hot metal weld and gall to the tool, building an edge that tears instead of cutting. Abrasiveness — the hard carbides of an iron, the fibres of a composite, the inclusions of a free-machining grade’s opposite — grinds the edge away. And ductility at the soft end makes a metal like pure copper flow and smear instead of shearing cleanly. Each of these is a different reason a material is hard to machine, and a hard material usually combines two or three.
The different difficulties
Because the causes differ, “hard to machine” is not one difficulty but several, and the shop meets them differently. There is the material that is hard but clean — cut it slowly, it obeys, and it is only speed that is sacrificed. There is the material that is soft but gummy — copper, the low-alloy aluminiums — where the battle is with the chip and the built-up edge, not with the tool’s hardness. There is the material that work-hardens — stainless, nickel, titanium — where the battle is with rubbing, dwell and the skin each pass leaves. There is the abrasive material — cast iron’s skin, carbon fibre — where the battle is with tool wear. And there is the heat-sensitive material — the plastics, the hardened alloys — where the battle is with the temperature of the cut. The shop that dismisses a material as simply “hard to machine” has missed which of these difficulties it presents, and the entry for each material in this group names its own.
The shop’s share of machinability
Machinability is set by the metal, but it is decided by the shop, and the same material can be moved from one end of the scale to the other by the method. A rigid machine and workholding, a tool held short and sharp with the right rake and edge, a grade of carbide chosen for the metal, cutting speeds and feeds set from the material’s nature rather than a steel habit, and coolant applied where the heat gathers: each turns a part of the material’s resistance into ordinary work. The free-machining grades exist for this reason — the steel, brass and aluminium alloyed with a little lead, sulphur or bismuth to break the chip and lubricate the cut, trading a little strength for ease — and the choice of supply state that this group has described is a choice of machinability before the tool ever arrives. What the machinist calls a difficult metal is usually a metal whose difficulty has not yet been met with the method it needs; and the study of machinability is the study of matching the two. This entry closes the materials group on that note: the material sets the problem, the shop sets the answer, and the metals that are hard to machine are the ones that most reward a shop that understands both.