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Various factors go into quantifying machinability. For example, a material  may be easy to cut, but might be rough on the machines used to cut it. This  would make for poor machinability because it's not worth it to use a material  over the long term if it continually bends or chips the power tools used in the  process.

Even materials that can be broken down easily in machining centres may not  make for good machinability. An example is thermoplastics, since they tend to  melt and then flow around the blades of saws instead of being cut and then  easily removed.

Quantifying machinability is a classically difficult process and it's usually  focused on one particular process in machining centres. The first and most  obvious way to quantify machinability is the "tool life method." This method  focuses directly on how long machine tools last when they work on the material.  This method is best when it's necessary to compare the machinability of one  substance against another substance of very similar quality.

It's useful to know which material will damage the equipment of machining  centres. The downside is that this approach could give false readings if the  testers don't carefully account for other factors that could increase damage,  like the geometry of the cutting tool, the speed of the machines, and so on.

Tool forces and power consumption is another way to evaluate a substance's  machinability. The amount of energy it costs to cut through a material with  machining tools will give a good idea for how suitable the substance is for  machining in general. This gives a measurable number that will let machinists  compare substances against each other.

The surface finish method for machinability measures how much of a  troublesome edge materials build up during the machining process. This edge  makes it more difficult to machine the material. Stainless steel is an example  of a material that tends to produce built-up edges in machining centres, and so  is unsuitable for the machining process. This makes materials like aluminum  alloys and cold worked steels, that don't tend to get that edge, more suitable  for use with machine tools.

Although it's difficult to be exact, there is a 'machinability rating' that  is used to give a number value to machinability by the American Iron and Steel  Institute. The number is determined by doing a turning test at 180 SFPM (surface  feet per minute). The arbitrarily selected material 160 Brinell B1112 steel was  given a rating of 100 per cent, and all other materials are given a number  relative to that. A material less machinable than Brinell might have 90 per  cent, one that is more might have 110 per cent.