[Tutor] class question

[Elwin Estle]

--- On Wed, 1/26/11, Alan Gauld <alan.gauld at btinternet.com> wrote:

From: Alan Gauld <alan.gauld at btinternet.com>
Subject: Re: [Tutor] class question
To: tutor at python.org
Date: Wednesday, January 26, 2011, 1:10 PM
>>Is this really a series of different types of casting or a single Workpiece going through a sequence of Actions
>>each Action having a set of quality data associated?

>>If so it may be that you have a single Workpiece class and a relatively small heirarchy of Actions.

No, there are multiple castings, anywhere from smallish parts you can hold in the palm of your hand, up to engine blocks weighing nearly 2 tons.

>>Is it the casting or the machining that has the quality related data? ie Is it casting specific or operation specific?

The casting can have quality data associated with it, tho rarely.  If there is a machining issue that can be traced back to defective castings, then yes (but we don't really have any process in place for this, normally that sort of thing is the casting supplier's problem).  Normally, tho, it is the machining that has the associated quality data., i.e., measure a 2.500 machined bore with a +/- .001 tolerance, using such and such gage, giving some result (like 2.503, for example), measured on such and such date by such and such machine operator.  Various bits like this will be associated with a given machining operation.  Sometimes machining operations are all done on a single machine, sometimes they span multiple machines.

>>Do you know in advance what these operations will be? Can you assign a WorkPlan to the Workpiece? Each step of the plan having an associated Action?

I would say yes.  However, and this ties in with something you ask further down, sometimes that plan may sort of "overlap"  For example, say we have a raw casting, 3402963R, that when machined, becomes part # 3402963.  But that same raw casting may also be machined into another part number # 3332965, simply by adding or omitting hole or some other feature.  So, I suppose one could have separate plans for each finished part number, in which case there might be some duplication of things for each plan.

> Each of those operations on separate machines will have a different set of quality checks associated with it.

So here we clearly say it is the Action that has the data associated...

> ...or it might be a part that goes from a raw casting to a sort of "mini-assembly" such as a rocker lever ...
> but it is still the same thing, and each one of those steps may have some sort of quality check involved.

So a workpice can consist of other WorkPieces.
And again tyhe Actions associated have quality data

>>> Lets complicate things even further. One raw casting may be machined into multiple part numbers.

>>Is that true? Or is it one type of casting?
>>Each instance of casting will surely onmly go through one specific set of actions? If we treat them all as generic >>workpieces then the Actions are different but the workpiece concept remains constant?

>>Or, are we saying that a single original workpiece can be cut into several distinct workpieces? Again thats not >>necessarily a problem, objects can clone themselves and have the clones diverge thereafter.

Yes, that is the case, as described above.

>>> This is done through something called a "tabbed" blueprint, wherein there is a master number, but there are "tabs" indicating that if you changes such and such feature, then the part number isn't the master number, but the tabbed number.

>>I'm not sure I follow that bit.

Say you have a master part, # 3245671, this is sort of a "primary machined number".  On the blueprint for this part, there will be certain features that don't have any dimensional information, but instead have a callout of some kind, i.e., "Hole D", or somesuch.   At the top of the blueprint is a "tab block" that is a table of "alternate part numbers", vs these "named features".  Each of these part numbers will have differing information as to the nature of "Hole D".  Perhaps the size of "Hole D" differs from one part number to the next, or perhaps it is omitted all together.  It goes along wth the "one casting makes multiple part numbers" thing described above.  In and of itself, this perhaps may not seem that complicated...but the blueprints aren't filed under the the actual part number for a given tabbed part, but under the master number.  So...you may have part 3458760, but the blueprint for it is filed under 3245671, so you have to go hunt up that
 blueprint, then hunt up the information on it that describes the actual part in question.  All this ties in to your quality control documentation.  You have to make sure that when you check a hole diameter, you are checking the correct one as per the tabbed print.

Why not have separate prints for each part number?  This was originally a convenience/cost saving thing, I suspect.  Back in the days when blueprints were hand drawn, having someone do a separate print for all the part numbers on a tabbed print would have been expensive, and also introduced the potential for human error.  The blueprints in question are all scanned to pdf, but there are thousands of them, and paying someone to go in and somehow modify them for multiple parts would be a lot of work.

>>> So, in essence, there's a sort of "network" of "is-a" and "has-a" information.

>I see a lot of "has-a" but very little "is-a" at this stage.
In fact I don't see any "is-a" yet, althopugh I can see where some might emerge in the future.

I see what you mean.  They are all "parts" and a part might have "parts", but they are still all parts.

>>You may also find it useful to think through the state model for your castings/workpieces. That may help to constrain the number of Actions/operations required.

ummm...what's a "state model"?  I have kind of an idea, but ...I will do a google and see what pops up.

Thanks!