Datums and centerlines 2

[urkson]

My current argument with some of our older engineers is showing centerlnes as datums on their prints.I know it has been an accepted practice by some over the years even though frowned upon by some others. I have explained that under GD&T it is basically forbidden. As head of QC Mechanical I attend GD&T seminars and it has always been explained that a datum has to be something you can touch and you cannot touch a centerline.I am having this argument because I see the younger engineers are taking the lead from from their 'Mentors' and getting into this 'bad' habit. Do you think I right to press the point?

 

[BiPolarMoment]

Just keep asking them, "Centerline of what?". The shoulder? The slot? The big part? The little part?

I like to use a similar tact when a designer/drafter I work with resists adding positional FCFs on "symmetric" parts.

Another good example our Drafting Supervisor likes to use when there is a bunch of cylindrical features nominally concentric with no GD&T is to take out the change in his pocket, stack the different coins concentrically and the conversation typically goes like:
"Image this is your part, would you accept this part?"
"Sure."
--Takes the stack of coins and taps them on the nearest flat surface so that all the ODs are flush to one side[therefore not even remotely concentric]--
"What about this one?"
"No, I'd reject it."
"Why? It meets your drawing specification."

And then there would be some argument about "implied" concentricity. It's entertaining to watch.

 

[powerhound]

It's perfectly fine to imply concentricity/coaxiality of features that are shown coaxial. That's even one of the fundamental rules. The kicker is that there is no implied tolerance and some designers just can't wrap their heads around that.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[johnchrc]

If he is using concentricity instead of positional tolerance, he isn't getting a bad part, he just added a lot of expensive to check CLs. He now has perfect axis and cylindricity with regard to the two CL axis and represented surfaces for DRF. I have not used GD&T in some time after changing companies but we need to adopt it. That type of tol is always unclear.

Will SW 14 incorporate it into CAD like Pro/E?

- CJ

 

[SeasonLee]

@urkson

Drawing how bad is not what I am trying to talk about, perhaps nobody knows your post figure is only the shank portion of a lever, it’s very hard for all to understand the design intend and its functionality, please see this thread (5 years ago) linked below, it will help you on how to well define the DRF and the positioning parts feature in a DRF.

Well, any comments on the lever DRF callout are welcome.

Season

 

[SeasonLee]

Oh, my bad, forgot the link

http://www.eng-tips.com/viewthread.cfm?qid=240138

Season

 

[powerhound]

Seasonlee,
I'm not really sure about the point you are trying to make in your post but it sounds like you're trying to say that the drawing could be okay depending on the function of the part. Is that what you're saying?

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[JNieman]

For turned parts, I find it pretty simple/quick to mark 'datum A' on every part as the centerline of the most prominent feature, or easiest feature to set up on in an inspection room. There is a default note "All features concentric to Datum A within .xxx unless otherwise noted". Thus the less critical features on such parts will be constrained in SOME fashion, whereas the the critical ones are hopefully noted individually.

General notes and default tolerances can be super great. There is a lot of "implied" relationships but notes can give them tolerances and really keep a drawing clean.

_________________________________________
Engineer, Precision Manufacturing Job Shop
Tool & Die, Aerospace, Defense, Medical, Agricultural, Firearms

NX8.0, Solidworks 2014, AutoCAD LT, Autocad Plant 3D 2013, Enovia DMUv5

 

[SeasonLee]

@John

The drawing could not be okay, as I mentioned "any comments on the lever DRF callout are welcome",expect to hear your comment regarding to the DRF callout.

Season