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Dimensioning NO NO

Dimensioning NO NO

Dimensioning NO NO

(OP)
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.
 

RE: Dimensioning NO NO

He's almost certainly an idiot.  My understanding of ASME Y14.5M-1994 does not support this and I don't think that much changed in Y14.5M-2009.  I suggest you ask him to show you which standard (and where) promotes this scheme.

However, If you want in depth answers I'd post over in forum1103: Drafting Standards, GD&T & Tolerance Analysis.  Also state what if any drawing standards you work to.

Of course, if you don't work to any recognized standards then it's difficult to say he's explicitly wrong.

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RE: Dimensioning NO NO

Chann,

   This is a good question for the Drafting Standards, GD&T & Tolerance Analysis forum.

   All dimensions must come from a feature that can be located by fabricators and inspectors.  This can be a feature of size.   You are allowed to centre 0,0,0 on an outside width or some internal width, which would be called up as a datum.

   Locating and fixturing to features of size is not absolutely simple, especially if the FOS is not accurate.  I would say that if the FOS is at least ten times more accurate than your other tolerances, you have no problems.  After that, you need to think about MMC conditions.  You (or he) should consider designing the fabrication and inspection fixtures.  This may affect the design of the original part.

   When your designer claims that "this is what the world is going to", is he referring to his choice of zero position, or his he talking about datum dimensioning?  A lot of people do not like datum dimensioning.  Perhaps he thought you were criticising that!

               JHG

RE: Dimensioning NO NO

Maybe it's right, on _his_ planet.

Here, not so much.

All I can conjecture is that's how he thinks GD&T works, which in turn implies that he could use some training.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Dimensioning NO NO

Um, 0,0,0 on a car body is a point in space, somewhere forward of and below the car. It is after all easy to think of a plane of symmetry as your zero and there may be no features on that plane at all.

I'm not saying it is good practice, but that is the way it is.

Cheers

Greg Locock


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RE: Dimensioning NO NO

Hence I said he's 'almost certainly' an idiot.

There are applications where it can make sense, such as top level assemblies especially where there are nominal lines of symmetry, or maybe a C of G or something.

However, for a machined or similar part, it's probably not a great plan.

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What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Dimensioning NO NO

Your "drafter", who likely has the self-declared or tenure-enabled title of "designer", needs the corrective training that most hard-headed, know-it-all, "I know the CAD program so I'm smarter than you" types who make up the majority of the drafter population needs . . . a stick up side da head.

If the rest of your operation, i.e., QC, manufacturing, etc., can't use it (or have to go to extra lengths to use it), his work is, at best, a drain on the enterprise, and possibly completely worthless.  Fire his arrogant @$$!

RE: Dimensioning NO NO

GregLocock,

   I am familiar with station numbers in aircraft, and I have used them on complex opto-mechanical equipment, way back in my drafting board days.  It seems like the best way to keep track of the exact configuration of a complex assembly.

   None of this wound up on my fabrication drawings.  The assembly level station numbers had no relevance at the individual part level.  I dimensioned from existing features.   

               JHG

RE: Dimensioning NO NO

Are most of your parts symmetrical?  I have seen drawings where they use center lines to dimension from.

Tobalcane
"If you avoid failure, you also avoid success."  

RE: Dimensioning NO NO

Just because you've seen it doesn't make it right Twoballcane.

If those 'center lines' - really center planes - are explicitly derived from a datum feature then it can be correct - as drawoh alludes to with his feature of size comment.

However, arbitrarily picking a 0, 0, 0, probably based off of the 3D model by the sounds of it, that is nominally 'in the middle' is generally bad practice.

While the OP is fairly brief, and maybe I'm reading too much into it, I get the impression this is the case based on the QC inspection comment.

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RE: Dimensioning NO NO

I've seen it on aircraft parts, but not exactly as you describe.

Usually works like this:

1. 0,0,0 point is the "airplane co-ord system"
2. Drawing uses this 0,0,0 to locate ONE set of datums.
3. Every other dimension is "local" i.e. from the part datum.

J

 

RE: Dimensioning NO NO

Are these lathe turned parts? Parts with radial symmetry are often dimensioned from center because that makes it easier to program but it can be hard to inspect.  

RE: Dimensioning NO NO

I think the basic axiom is that dimensions must be inspect-able.

If these parts lend themselves to inspection by coordinate measure machines (CMM), which can record and digest hundreds of hard (x,y,z) points, input by a probe touching the surface, then the style of dimensioning you described starts to make more sense.  I've never seen it done, but yeah, maybe the world really is going that way.

Otherwise, I agree its poor practice to dimension in this manner because it makes inspection impractical.  Generally there should be tangible datum planes or points for 'open setup' in a gauge lab, or for functional Go/NoGo gauges.
 

RE: Dimensioning NO NO

Dimensioning to a theoretical point can be hard to justify at best.  ASME 14.5 does allow derivation of datum center planes from features, which could then be dimensioned from.  This is a huge pain to inspect, but I have seen circumstances where it was an appropriate expression of the design intent.  I would take issue with it being 'what the world is going to'.

I work pretty close to exclusively on automotive components nowadays, and the vehicle origin is at some defined place under the hood, with this origin used on most of the individual parts.  This doesn't mean any of the dimensions on these parts reference that origin - they are all dimensioned to appropriate features of the part.

RE: Dimensioning NO NO

Sounds to me like this designer may have come from a civil or structural engineering background.  It is quite common in buildings to use a "northings" and "eastings" approach.

RE: Dimensioning NO NO

The way inspection is done where 0 0 0 is in space is to use master control holes to orient the part (or assembly) with respect to grid. One of these is effectively the origin, the others define the axes. The inspection report is then just a list of the errors in the location of each feature with respect to the axes defined by the MCHs.

As such we don't much use dimensioned lines for inspection, everything is a list of coordinates.

Note this is from my perspective only, where I am interested in establishing the location of bolt holes and surfaces and pins that are used to locate subframes into bodies, I do not know how the general run of the body surface is inspected.

 

Cheers

Greg Locock


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RE: Dimensioning NO NO

It is done in exactly the same way Greg, the 0,0,0 is usually or possibly always the centre line of the front axle and the middle of the car. That way every single part of a car can be referenced to all other parts, it is actually hard or even impossible to think how else you could do it.

It does amuse me when people say it cannot be done like this when in reality it is exactly how every vehicle is manufactured and inspected and has been more many years.
 

RE: Dimensioning NO NO

(OP)
I have worked in Truck and Car, and it is done that way for fit location.

Every part peice is dimensiond frome the center line of the part no mater what the shape or if their is a feature or not.

I like feature to feature, and  fit to fit dimensions to check tolerence of fit to the next part.

One other thing this guy outlawed GD&T at our facility.

Sounds like he just has no clue.

RE: Dimensioning NO NO

(OP)
Thank you all for your help it was amusing!

RE: Dimensioning NO NO

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

RE: Dimensioning NO NO

"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
www.infotechpr.net

RE: Dimensioning NO NO

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.

RE: Dimensioning NO NO

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

RE: Dimensioning NO NO

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.  

RE: Dimensioning NO NO

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.

RE: Dimensioning NO NO

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..
 

RE: Dimensioning NO NO

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

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