Centre line symmetry
Centre line symmetry
(OP)
I am currently working on a pretty basic label drawing which contains differing hole sizes for push buttons and mounting screws. The difficulty has arisen in the dimensioning of the drawing (although this is a formality as the label makers will transfer straight from CAD)
I have been pulled up on the fact I have used a centreline as a projection line through 4 differing holes which run vertically up the centre of the label on the same horizontal dimension. I have been told that I cannot do this as either side of the vertical centre line I have differing hole sizes and engravings therefore the label is not symmetrical. My understanding is that the centreline can be shown in this case to define the centre of the holes and not the part in general.
I know this is a minor point in engineering terms but can anyone clarify this? Any reference material would be appreciated.
I have added a copy of the drawing for reference.
I have been pulled up on the fact I have used a centreline as a projection line through 4 differing holes which run vertically up the centre of the label on the same horizontal dimension. I have been told that I cannot do this as either side of the vertical centre line I have differing hole sizes and engravings therefore the label is not symmetrical. My understanding is that the centreline can be shown in this case to define the centre of the holes and not the part in general.
I know this is a minor point in engineering terms but can anyone clarify this? Any reference material would be appreciated.
I have added a copy of the drawing for reference.





RE: Centre line symmetry
Actually the way you did it does not show symmetry lines for whole part, because the lines are not extending the outer contour of the part. You just connected in line holes together with a long-dashed dotted line and this indeed can be confusing for somebody who really wants to find something incorrect on a drawing.
My approach would be to use 6 separate axes crosses for every hole and attach dimension lines only to the ones which are on the very outside.
RE: Centre line symmetry
RE: Centre line symmetry
The difficulty is that wherever I have worked people have different personal preferences on how things are shown on drawings whether they work to a standard or not. In an ideal world I would have enough time to read/reference every standard and do everything to the letter of the law but sadly I dont have enough time in my day to do that so most of my knowledge has been through experience.
I am glad to hear that you believe what I have shown is acceptable. I will try and find out if we have a copy of the standard here as it would be an interesting read.
From your initial response I gather that you would suggest removing the dashed centre line altogether but leave the centre marks on the holes themselves with the same dimensions shown? My reasoning was that the centre line confirmed the holes were on the same dimension, perhaps this is implied and not necessary.
On one hand I feel it is just nit picking from a senior engineer, on the other I would like to produce drawings to the correct standard and appreciate the feedback as it keeps things in check.
His suggestion was to show a separate view so that more dimensions could be added for each hole without crossing extension lines etc. but I just feel that would lead to more confusion and it would still leave me with having to dimension the central holes on top of the label itself.
RE: Centre line symmetry
Adding extra views to put dimensions to every hole can indeed cause more confusion than clarity, not mentioning the time needed for preparing such drawing will increase significantly.
There is a fundamental rule in dimensioning that says when elements are shown in line on a drawing 0 basic dimension between them applies. But this require using basic dimensions and tolerance of position for holes location.
RE: Centre line symmetry
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Centre line symmetry
This is based on my experience with ASME, however... ISO may address this in other ways.
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Centre line symmetry
To the best of my (admittedly far from perfect) knowledge the ASME standards don't define any sort of implied symmetry about a centerline. I'll actually send back drawings I get that rely on an implied symmetry about a line.
RE: Centre line symmetry
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
&
RE: Centre line symmetry
Thank you all for your comments, much appreciated. :)
RE: Centre line symmetry
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Centre line symmetry
I know. I'm not a fan of their Drafting Manual either. I used to think they were a good source until I notified them of obvious (as least to me) unvetted material in their publication. They were not receptive.
ASME Y14.5-2009 language on how/when to apply quantity is soft. The word "may" is used a couple of times. They may (or may not) be in response to the "common practice" that the Genium manual mentions.
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
&
RE: Centre line symmetry
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Centre line symmetry
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Centre line symmetry
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
&
RE: Centre line symmetry
I drag centerline outside of part contour and remove some dimensions because with reference at ISO center line can be interpreted as symmetry line
add comments please to my own sample
from my opinion it is quite enough specs for define thread hole in center of part
RE: Centre line symmetry
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RE: Centre line symmetry
4.3 covers aligned features, perpendicular features, parallel features, equispaced about pitch circle features and thru-ness of holes. There is no implied "line of symmetry" for linear dimensions. In fact, using the centerline in the way you are attempting appears to be a violation of "Tolerances shall never be implied, and shall always be indicated". If you use only a centerline, how is one to know how to apply a tolerance? Does one half the tolerance of the symmetric dimension, or does one apply haft that dim and full tolerance to that unstated number? If that is the case, which side does one tolerance or start the 1/2 dim from? There are likely many discussions on this forum about this topic alone.
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
&
RE: Centre line symmetry
As for the OP's graphic, to me it's overdone by adding the 20mm & 33mm dimensions. Even the 93 and 80mm dimensions aren't necessary though they do make it easier to confirm the intent.
As for whether the part is entirely symmetrical, that's irrelevant to visual symmetry. Now, whether or not everyone using the print can spatially perceive visual symmetry is a valid and relevant consideration. I have dealt with a number of people, including engineers, who can't pick out patterns, symmetry, perpendicularity, etc. So, while the extra dimensions may not be necessary, they may be useful to some.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Centre line symmetry
I see that you are both P.Eng, GDTP-S. So, perhaps you can go into more details about implied or visual symmetry when you get a chance. I've not found direct mention of implied or visual symmetry in the standard. I see that 1-57 seems to have implied symmetry, but that is a situation that is specifically covered under the keysets in 1.8.16 section. 1.9.1 shows a similar scenario with all dims are stated, as do many other examples throughout ASME Y14.5-2009.
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
&
RE: Centre line symmetry
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
&
RE: Centre line symmetry
Fig. 4-9 is mostly symmetrical, but for the notch on the side. Fig. 7-37, though missing any dimensions, clearly invokes visual symmetry to effectively understand the design. Fig. 7-64 doesn't need any dimension from the centerline to the edge of the groove to imply it is centered on the width (i.e. the two sides are visually symmetrical about the centerline as shown, or even about the visible centerplane established by the width datum feature of size.
I think the standard still needs a better definition of implied conditions, but at least '09 started to improve on what was posted in '94.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Centre line symmetry
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Centre line symmetry
The figures are known to be incomplete, and this is stated so in the standard. However, 4.9 does have a dim to the centerline, meaning symmetry is stated, not implied...though this is a different scenario because this are POS tol'd holes. 7-37 is also POS tols, and 7-64 is a method to find the centerplane for GD&T symmetry based on a FOS (again, a different scenario).
That aside, the remaining problem with implied symmetry is that the tolerances are also implied. Is there something in the standard that defines how linear dimensions which are split by symmetrically-placed centerlines are interpreted? Are the tolerances halved from the centerline; or is the dim halved with full tolerance ranges applied to both halves; or is there no tolerance at all thus leaving the feature or set of feature to float around?
I'm not challenging you on this to be a pain. I am looking for core justification for this methodology so I can considering using (or allowing) it on our drawings. As far as I can tell, relying on centerlines for visual symmetry creates ambiguities; and ambiguties are not allowed by the standard.
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
&
RE: Centre line symmetry
If using +- tolerancing then I'd say you need to add a centering dimension.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Centre line symmetry
I have noticed for primitive brick parts even someone depict dimension from corner to center line for specify symmetry instead of spec overall dimension.
and contrary for round part nobody specify radius dimension from silhouette edge to center line for define axis.
I believe we come back to terms of ambiguous and unambiguous and where is a border between.
What is your opinion if specify center line with CL annotation can it clarify design intend for above mentioned samples?
RE: Centre line symmetry
It's a question of what is the tolerance on 'how symmetrical' it is and how is that specified.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Centre line symmetry