Dimensioning NO NO 1

[Chann]

We have a senior designer that dimensions parts with a theoretical starting point and dimensions from a 0,0,0 from the center of a part and dimensions everything off the theoretical center line. Is this wrong? No features to dimension from. I think this is the poorest dimensioning that I have ever seen. (Lazy) He says this is what the world is going to. QC cannot measure parts with no features.

 

[Chann]

Thank you all for your help it was amusing!

 

[KENAT]

I'd just like to point out to all the automotive folks, that the OP was talking about "from the center of a part", not from some 'global' 0,0,0 or similar.

I've worked aerospace stuff where there was some use of global datums.

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[drawoh]

KENAT,

On a optical sensor I worked on, I used the front lens of a big telescope as my coordinate zero. Later, the optical designer moved the front lens. At the system level, locating zero ahead of and below the car as Greg Locock noted, probably is based on much painful experience, on drafting boards.

This ought to have no effect on fabrication drawings, and on people running 3D parametric CAD.

JHG

 

[PeterStock]

"One other thing this guy outlawed GD&T at our facility." - Update your resume and get out ASAP.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[DELurker]

To be honest, as a designer, I would have to ask if this method allows the machinists to make the same functional part over and over. If it does, then it's OK in my book. However, if the machinists cannot make a functional part repeatedly from the print (or evaluate if a part is correct in the first place), then it's not the right way to do things.

 

[drawoh]

DELurker,

If you hand your machinists crappy drawings, they will often get the job done anyway. Of course, you will be the butt of jokes, and you will have no control over the final product, and the guy who joins you on the job will post to EngTips about the idiot he has to work with...

JHG

 

[DELurker]

drawoh,

I agree. Crap in gives crap out. However, that wasn't quite what I meant, but my post wasn't clear on that.

The measure that I have used to evaluate whether or not a drawing is well-drawn is to look at from a machinist's perspective. If the drawing gives me every dimension that I need to fabricate the part with the tolerances either indicated or defaulted without requiring additional calculations or translations, then it's probably usable. If it indicates design intent in the process, even better.

Basically, I should be able to take the print to any machinist in the shop, tell them how many parts that I need and when I need them by, and never hear from them until they drop that many parts on my desk. I should be able to determine, using calipers, micrometers, radius gauges, etc, whether or not each part is correct.

However, from the OP's original description, the senior designer's technique would force me to calculate various dimensions so that I could actually both fabricate and check the part. This would not be acceptable to me (YMMV). If the machinists have to "make it work", then the drawing is not right, because each machinist will "make it work" in a different way, thereby generating a different and possibly incompatible part.

As for the "banning" of GD&T, I cannot express the depth of my feelings on that. While I still to this day have to look up the GD&T information when I need it or come across it, it's too useful a tool to get rid of. The only thing that I can think of is that he so completely cannot understand GD&T that he doesn't want anyone to use it and show him up, so to speak. But I'm just guessing there.

 

[gadkinsj]

We have 2 departments that make similar equipment in my company. We make a smaller version and they make a larger version. To a man (35 year old veteran to a 1 year out of school) they take the time to scribe a centerline on everything they make and dimension everything off of this. Nobody in our team does this because why add an extra cut to the part? There are arguments for why it makes sense but none of them are good.

 

[321GO]

Bit late to the party and not read all the postings,

but IMHO dimensioning from a virtual reference can be done perfectly in digital world, but cannot be transferred to a non-digital world, at least not that easily/intuitive. Most of the time it simply does not exist, as on the drawing.

The quality control person will sratch hit head first and then step in your office with a question..

 

[MechE2]

Back to basics (even for Geo Tol)

Dimension the part as it would be made and QC'd (re: functional gaging). If one feels the need to dimension to an abstract point, make them reference dimensions in addition to measurable features.

Example - I dimension a slot to be measured length and width with calipers. I reference dimension the center of the slot for our nc machinist.

MechE2

 

[KENAT]

"Dimension the part as it would be made " not per asme Y14.5, particularly 1.4e

Dimensioning for function is primary per American drawing standards way of thinking.

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