Definition Precision Machining - Definition

Precision Machining - a process where material is removed by a cutting surface, such as grinding, honing, turning, milling, etc. The process must be controlled in a manner that all variation (vibration, bearings, measurement error, etc.) is statistically insignificant except for tool wear.

bobdoering said:

Precision Machining - a process where material is removed by a cutting surface, such as grinding, honing, turning, milling, etc. The process must be controlled in a manner that all variation (vibration, bearings, measurement error, etc.) is statistically insignificant except for tool wear.

Click to expand...

Well, OK, as far as it goes. I would have included lasers and electric arc discharge machines, too. Maybe even water jet cutting as well.

Frankly, I think the definition of precision machining is not so much about the devices used to remove material from stock (essentially, either the material moves against the cutting device or the cutting device moves against the material) as it is about the precision of the tolerances which are held in the process. Sometimes, of course, the Control Plan for precision machining factors in the tool wear and the tool undergoes regular adjustment throughout the production cycle to maintain the tight tolerances (which may seem blasphemous to folks who live by SPC, Cpk, and the other alphabet soups of reporting whether a process is under control.)

What do you say, Bob?

bobdoering said:

Precision Machining - a process where material is removed by a cutting surface, such as grinding, honing, turning, milling, etc. The process must be controlled in a manner that all variation (vibration, bearings, measurement error, etc.) is statistically insignificant except for tool wear.

Click to expand...

There are those that would argue that "precision" machining only applies when the tolerances are less than 10 thousands of an inch. If the tolerance is measured in thousands of an inch it is just machining.

I'm not claiming that this is my definition though.

Well, looks like we can have a lively conversation about this!

First - precision: If we define precision by the tightness of the tolerance, in 1930 precision may have been a tolerance of .1", while today it could be .0005" - or whatever target it may be. Therefore, it would be a moving target that would be hard to define, depending on the processes of the day. Which is my point, I am focusing on that the process that is precision, not the result.


Second - machining: I think wire EDM, laser and waterjet are dumped into the "machining" bucket because the result of the process is similar to the mechanical cutting devices - but they do not exhibit tool wear. They may be better suited for another category. Electrodischarge grinding may behave like precision machining. I did not have a chance to work with it long enough to determine if the wheel does decay like plunge EDM electrodes, in which case its behavior would be consistent witht he definition.

I think that some of the essence of this discussion is in the fact that the short term capability (less tool wear) of a precision machining process is extremely high relative to what ever the tolerance may be. The short term variation of the process (less tool wear may in fact be normally distributed. However, for perfectly valid, economical reasons, a decision is made to allow variation from tool wear to enter the picture over a longer term.

This decision should be based on whether a part anywhere within the tolerance is equally acceptable to another part elsewhere in the tolerance. Assuming these are all economically and practically valid reasons, the end result is a short term, normal (or skewed, or etc.) distribution that is deliberately allowed to drift with tool wear to predetermined limits whereupon it is adjusted to the other decision limit.

The long term effect of this is a pseudo-uniform distribution. I use the term pseudo-uniform because unlike a true uniform distribution that has vertical tails, this distribution will have tails that reflect the underlying short term distribution. These tails may not be large from a practical view, but they do exist.

In short, precision machining has extremely high short term capability that is allowed to drift according to tool wear then adjusted resulting in a sawtooth pattern.

Miner said:

In short, precision machining has extremely high short term capability that is allowed to drift according to tool wear then adjusted resulting in a sawtooth pattern.

Click to expand...

I am not sure how one can eliminate tool wear from the machining process consideration. It is the process. As far as precision machining capability, it really has no difference between the short term and long term capability, the capability is always (USL-LSL)/(UCL-LCL) based on the uniform distribution. Theoretically, there are no 'tails', but in practice, thanks to total variation, sum of the lesser variations, such as the inability to hit the control limits exactly , measurement error, etc. - some of the lesser variations are normal and will contribute 'tails', the overall composite distribution will have some tails. That is why the recommended control limits are 75% of the tolerance to allow for that.

Yep. In 1999, my shop had machines that could routinely hold 0.000050 (50 millionths of an inch.) When I told this to my then 90-year old great uncle, who had been a machinist during WWII, his first comment was, "How do you even measure 50 millionths of an inch?"

He hadn't been inside a machine shop for 30 years - when I brought him a little titanium doodad (cup-shaped, looking somewhat like a bullet casing he made in the 40s) our shop made for a satellite which we shaped on a Swiss turning center, heat-treated, had a small bridge cut out of the bottom with an electric arc discharge machine, then electropolished to a mirror finish, he was absolutely flabbergasted. He simply couldn't grasp the change in machinery from the Browne & Sharpe lathes, Model B Davenports, etc. of his day in terms of accuracy, stability, precision.

Miner said:

<SNIP>
Assuming these are all economically and practically valid reasons, the end result is a short term, normal (or skewed, or etc.) distribution that is deliberately allowed to drift with tool wear to predetermined limits whereupon it is adjusted to the other decision limit.

The long term effect of this is a pseudo-uniform distribution. I use the term pseudo-uniform because unlike a true uniform distribution that has vertical tails, this distribution will have tails that reflect the underlying short term distribution. These tails may not be large from a practical view, but they do exist.

In short, precision machining has extremely high short term capability that is allowed to drift according to tool wear then adjusted resulting in a sawtooth pattern.

Click to expand...

As the professor in "My Fair Lady" says,
"By Jove, I think he's got it!"

Good job, Miner!

Wes Bucey said:

As the professor in "My Fair Lady" says,
"By Jove, I think he's got it!"

Good job, Miner!

Click to expand...

Yep, pretty darn close! I hope we can get others on board!

Wes Bucey said:

He hadn't been inside a machine shop for 30 years - when I brought him a little titanium doodad (cup-shaped, looking somewhat like a bullet casing he made in the 40s) our shop made for a satellite which we shaped on a Swiss turning center, heat-treated, had a small bridge cut out of the bottom with an electric arc discharge machine, then electropolished to a mirror finish, he was absolutely flabbergasted. He simply couldn't grasp the change in machinery from the Browne & Sharpe lathes, Model B Davenports, etc. of his day in terms of accuracy, stability, precision.

Click to expand...

How do you hold .00005 when you're electropolishing?