machine tool: a power driven machine for shaping solid work.

inspection: a checking or testing of an individual against established standards.

Webster

A machine tool is a mechanical system built to perform some task in changing the form of

material. This includes punch presses, sheet metal brakes and engine lathes. For purposes of this

report, machining centers are focussed on. A machining center is a multi functional, computer

numerically controlled (CNC) metal cutting machine capable of milling, drilling, boring, etc.

through the use of an automatic tool changer and multiple axis operation. Many ofthe same

techniques may be applied to other types of machine tools.

Machining centers are large, complex systems of mechanical devices. They are heavy duty,

and it is hard to imagine how a machine with such controlled motion could be prone to error. It is

often assumed that the machine operates in such a manner that when it moves in a straight line, it

is very straight, without deviation. It is also assumed that when the machine moves the tool to a

prescribed position, that it does so without fail. And the same goes for any motion that the

machine is commanded to perform. However, error exists in every member and every motion of

the machine. An error free machine would be very difficult and expensive to produce. Even

inspection machines have error, contrary to the belief of many inspectors. It is important for the

manufacturing engineer to understand how machine tool error relates to economical production,

and be prepared to quantify the amount of permissible errors. Equally important is designing a

process that minimizes the effect of machine tool error on production processes. This takes many

years of experience, but begins with an understanding that error exists and how to find it.

Isolation of process error

The most common reason for machine tool inspection involves the isolation of existing

process error. It is typically prompted by production parts running out of specification, and a

machinist who is unable to correct the process using common offsets in the machine. Since the

other apparent "variable" is the CNC program, that must be the cause of the error. The

programmer or manufacturing engineer is called down to fix the program. The immediate

objective is to isolate the problem, correct it, and return the machine to production. If the

problem is the inspection method, material, set up, tools, or program, the solution should be

fairly obvious. If the machine tool is suspected, the problem will need to be isolated. It is

important to understand the Machine-Fixture-Tool-Work piece (MFTW) system. This is the

relationship of the mechanical linkage that makes up the process. Does the error correlate with

any part of the system? An example would be a hole pattern that is shifted. Inspect and isolate all

the potential causes of a shift in the direction indicated. Study the structural loop of the machine.

This is the linkage from the fixture through the machine to the cutting tool in the spindle. There

are many interconnected components, and any of them could be the cause of the error. If

potential trouble spots are found, check the error and quantify it. Usually there is a combination

of errors that are contributing, and all deviations found are worth noting. For particularly difficult

problems, the process may have to be duplicated to eliminate large groups of variables. For

example, try running the part on another machine, to isolate the machine as being the problem

and not the program or tools.