Definition Precision Machining - Definition

Big Jim

Super Moderator
#11
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.
Perhaps "chemical milling" should be added to the list too, particularly in the making of micro-miniture parts through photo etching. No parts to wear, but resulting parts are very precise (and tiny).
 
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bobdoering

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#12
I agree, this too is a precision process, but should it be "machining" if there is no tool and no tool wear involved - or is it another category altogether (along with laser, et. al.)?
 

Jim Wynne

Staff member
Admin
#13
It's an interesting question. I think the language hasn't caught up with technology, or there's no need for more specificity. In general, we think of "machining," in its simplest form, as strictly an erosive or abrasive material removal process, as opposed to something like stamping which (in addition to removing material in some cases) forms the metal into a specified shape. For your purposes of accentuating the proper SPC methods, perhaps it would be best to say, "machining (the adjective "precision" is redundant, I think) with perishable tooling." That's quite a mouthful, but at least the distinction is preserved.
 

bobdoering

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#14
For your purposes of accentuating the proper SPC methods, perhaps it would be best to say, "machining (the adjective "precision" is redundant, I think) with perishable tooling."
I do not think that precision is redundant, in that you can have sloppy machining processes, where there is sufficient variation to mask the tool wear (bad bearings, sloppy ways, etc.). It may not matter, if your specifications are wide open. So, the precision portion of the definition infers the portion of the definition " all variation is statistically insignificant except for tool wear."


That's quite a mouthful, but at least the distinction is preserved.
Yes, it sure doesn't roll of the tongue, but it has good specificity. :agree1:
 

Jim Wynne

Staff member
Admin
#15
I do not think that precision is redundant, in that you can have sloppy machining processes, where there is sufficient variation to mask the tool wear (bad bearings, sloppy ways, etc.). It may not matter, if your specifications are wide open. So, the precision portion of the definition infers the portion of the definition " all variation is statistically insignificant except for tool wear."
It sounds like tautology to me:
Q: How do you know it's precision machining?
A: You have to use the uniform distribution, because the normal distribution is misleading.
Q: How do you know when to use the uniform distribution?
A: You use it when you're doing precision machining.

If you don't want "precision" to be redundant in the phrase "precision machining," you need a definition that doesn't go in circles.
 

bobdoering

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#16
I think you missed some steps - it is more like this:

Q: What is precision machining?
A: A machining process the only statistically significant variation is tool wear.
Q: How do you control a process where the only statistically significant variation is tool wear?
A: You start at one control limit, and allow the dimension to vary to the opposite control limit, then reset. For outer dimensions, start at the lower control limit, for inner dimensions, start at the upper control limit.
Q: What is the resulting curve for the process?
A: The sawtooth curve.
Q:What is the distribution for the sawtooth curve?
A: The continuous uniform distribution.

Does not seem that circular to me.

Conversely,

Q: What is not precision machining?
A: A machining process where the most significant variation is not tool wear, such as vibration, worn bearing, manual operation, etc.
Q: How do you 'control' a process where the statistically significant variation is not tool wear?
A: You can only adjust to the target value when the value exceeds a control limit as the process randomly varies about it.
Q: What is the resulting curve for the process?
A: Random variation about the target.
Q:What is the distribution?
A: The normal distribution.
 
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