Centre line symmetry

[Richard82]

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.

 

[pmarc]

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.

 

[pmarc]

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.

 

[Richard82]

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.

 

[pmarc]

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.

 

[KENAT]

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.

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[ewh]

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.

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[SteveMartin]

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.

 

[fcsuper]

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
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[Richard82]

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

Thank you all for your comments, much appreciated.

 

[ewh]

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." - [small]Robert Hunter[/small]

 

[fcsuper]

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
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[ewh]

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. ;-)

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[KENAT]

Ah, preparation of company standards, I miss doing that so much, honestly.

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[fcsuper]

It should be noted that the ASME Y14.5-2009 images consistantly show the use of common centerlines with no quantity in their examples.

Matt Lorono
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[ak762]

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

 

[ak762]

don't know how to edit post but here links for all pictures what I tried to upload

http://files.engineering.com/getfil...731-be0f-cab26e080961&file=my_centre_line.pdf

http://files.engineering.com/getfil...2-42e3-b634-fa3eceb7c750&file=center_line.jpg

http://files.engineering.com/getfil...1-8f89-e7fdc8b3dcec&file=line_of_symmetry.jpg

http://files.engineering.com/getfil...b97a-b957dc009662&file=implied_annotation.jpg

 

[fcsuper]

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
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[MechNorth]

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
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[fcsuper]

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
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