Machine accuracy, was Re: Step resolution

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "metlmunchr"
<metlmunchr@y...> wrote:

> Depending on the type of screws, fit of components, and rigidity of
> the machine, the machine will probably be able to make parts with an
> accuracy somewhere between 5 times and 100 times the resolution.

With an unskilled operator.

> Manufacturer's claims notwithstanding, it is difficult to hold parts
> to +/-.0005" on an ongoing basis on a machine that costs $100,000
> without a lot of continual attention to the process. These machines
> today typically have resolution of .00001, or ten times greater than
> your resolution, and .0005 is 50 times their least resolution. In
> general, these machines could have a least resolution of .0001

instead

> of .00001, and the parts would come out identical, yet the accuracy
> would be 5 times the least resolution instead of 50 times.

This is because the users in modern manufacturing companies are
required to certify their processes (Cpk). It usually has very
little to do with what a skilled, or even fairly knowledgable
machinist can do with the machine for a one-off part.

To certify a process with a tolerance of less than .0005 usually
requires more technology than a manual machinist (mill or lathe) has
available, and a home CNC user certainly doesn't have it available
either. This would include precise temperature control of the
machining process parameters, the machine, the parts, and tracable,
certified guages.

However, a mediocre grinding hand can hold .0003-.0005 inch
tolerances all day on machines designed to hold that tolerance for a
given feature. He may tweak the size for every piece, and probably
won't be able to certify the process due to too many uncontrolled
variables ( like attention span).

Typically a Bridgeport mill is not used for work that is any closer
than +/- .002 inch. Any home built CNC devices will probably be less
accurate.

Fred Smith - IMService
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