×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Centre line symmetry

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.

RE: Centre line symmetry

If I am assuming right basing on the dimensioning style, you are not working in accordance with ASME Y14.5 standard. There is a large group of ISO standards (ISO 128) which define drawing practices. I do not have any copy here but you should check for instance ISO 128-20:1996 "Technical drawings -- General principles of presentation -- Part 20: Basic conventions for lines.". Maybe there will be something interesting in it.

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

One more word as a summary: the way you made it is absolutetly acceptable IMO. You should try to explain to your interlocutor that the centerlines are not extending outer contour of the part so they can not be considered as symmetry lines for whole component.

RE: Centre line symmetry

(OP)
Thank you for your comments pmarc.

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

My first suggestion was to remove the dashed lines between holes, but later, as I thought about it more, I concluded it would be good to keep them (to show that the holes are in line) and that's way I said your dimensioning scheme is OK.

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

While the text doesn't appear to go into great detail justifying it, figure 1-48 in ASME Y14.5 shows similar to what you've done, as do a few following figures.  Due to what 1.1.4 says about figures you have to be careful in using them to justify things but in this case I really think you are fine.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Centre line symmetry

I agree with pmarc... your drawing does not imply symmetry as shown, and using one common centerline to locate the holes is not just acceptable, but preferred.  Even if the centerline extended below the part outline (for additional dimensions perhaps), this should not be interpreted as defining a symmetrical part without the presence of a symbol or other indicator.
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

I agree with the other responses that the drawing as done is acceptable.  A lot will depend on what standard(s) are referenced in the full drawing (I'm presuming some of the drawing was removed for public posting).  I would interpret the line as meaning the features connected to it are in line, and nothing more - but a referenced standard or other document that does define more meaning to this type of line would obviously take precedence and clarify intent.

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

Everyone else here is correct. The centerline just notes that all features along that centerline are all to the same dimension.  Though it is not in ASME, Genium Publishing stated in their manual that if centerlines are used, quantity is not needed in the associated dimension.  If the centerline is not there, then put in the quantity.  That's pretty much all the centerline means.  No implied symmetry.  Symmetry is only applied if GD&T FCF symmetric symbol is used.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

&

RE: Centre line symmetry

(OP)
Steve as you suggested, drawing border was removed for public posting.

Thank you all for your comments, much appreciated. :)  

RE: Centre line symmetry

Quote:

if centerlines are used, quantity is not needed in the associated dimension
This serves to confuse the matter (which is one of my dislikes of said manual), so I find it best to think of the dimension as locating the centerline, not the features.  If you have more than one set of aligned features (holes in this case) then a quantity may indeed be appropriate.  Don't consider each aligned feature in the quantity, just the centerlines.
 

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Centre line symmetry

ewh,

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

Issues such as that caused the director of engineering to task me with compiling a company standard referencing the best of the Genium manual and the bulk of the ASME specs refined to best address our niche in industry.  I see a lot of cut and paste in my future. winky smile  

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Centre line symmetry

Quote:

Actually the way you did it does not show symmetry lines for whole part,...
I am wondering will designer intend be correct interpreted if I modify previous sample
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

RE: Centre line symmetry

That is a thorough investigation. I'm not an ISO guy, but based on what you present, I do not believe their is an implied dimension from the centerline. Line of symmetric is a specific term with a specific definition that doesn't appear not apply to your drawings based on the BSI 2008 4.3 in your scan.

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

Actually, the use of centerlines does show a visual symmetry, but NOT a GD&T symmetry.  Sorry, don't have time to dig out sections, but if you look at Implied Basic Zero and a number of other subtle details, plus the recurring use of visual symmetry in the graphics (e.g. Fig. 1-57; the 4X 10 dimension locating the sides of the slot visually [basic dimensionally?] symmetrical about the centerline), then you get the value and validation of visual 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

Jim,

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

You're right, Matt, it doesn't say it specifically in the standard.  Nor does it say that visually coaxial diameteral features are implied to be centered on a centerline if they aren't dimensioned explicitly (basic zero).  Fig. 1-57 does not reference keyways (i.e. 1.8.17), so it is not clear that it is a keyseat; couldn't it just be a slot?  In fact, since 1.8.17 references Fig. 1-46, it is clear that the intent of Fig 1-57 is to show something other than a keyseat.

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

MechNorth,

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

Implied symmetry/center-lines or whatever you call it works fine when used with appropriate GD&T controls such as position because that's where the tolerance comes in.

If using +- tolerancing then I'd say you need to add a centering dimension.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Centre line symmetry

My assumption that drawing is symbolic media. Every symbol have a means. If this type of line was introduced in standard as line of symmetry why do not use as interpretation of 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

Agree with Kenat. CL symbol does not specify how symmetrical features are in relation to each other. There has to be geometric tolerance specified. There has to be datum feature also assigned to precisely inform from which feature's centerline/centerplane measurements should be taken. If these two elements are not on a drawing, the drawing is ambiguous.   

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources