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.
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Dimensioning NO NO 1
- Thread starter Chann
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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. 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.
However, If you want in depth answers I'd post over in forum1103. 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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1
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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
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!
MikeHalloran
Mechanical
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
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
GregLocock
Automotive
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
New here? Try reading these, they might help FAQ731-376
I'm not saying it is good practice, but that is the way it is.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
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.
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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tr1ntx
Mechanical
- Jul 20, 2010
- 285
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 @$$!
If the rest of your operation, i.e., QC, manufacturing, etc., can't use it (or have to go to extra lengths to use it), his work is, at best, a drain on the enterprise, and possibly completely worthless. Fire his arrogant @$$!
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
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.
Twoballcane
Mechanical
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."
Tobalcane
"If you avoid failure, you also avoid success."
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.
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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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
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
Wrenchbender
Mechanical
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.
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.
SteveMartin
Mechanical
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.
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.
Wouldn't be the same guy you mention in this one would it?
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GregLocock
Automotive
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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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
New here? Try reading these, they might help FAQ731-376
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.
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.
- Thread starter
- #20
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.
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.
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