Implied Centerlines/Symmetry

[KENAT]

This topic has come up before most recently in thread1103-239885, but on a quick look in ASME Y14.5M-1994 & a search of this site I can't find the text/section in the standard that says explicitly when using datum feature center planes you get implied symmetry/centerlines.

It's shown in figure 5-4 amongst others but I'd like to refer to the actuall text that says when you show a pattern or similar centered to "datum center planes" you don't have to add an extra dimension centering it.

KENAT,

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[axym]

Oh no. Don't get me started on this!

Try section 4.5.3 (a) through (d) and see if that helps. One quote is "the simulated datum is the center plane of the true geometric counterpart of the datum feature".

I'm not sure if this is the issue you're referring to or not. You might be referring to an "implied basic zero", in which features that look like they're centered or symmetric are assumed to be nominally centered or symmetric unless there are dimensions showing otherwise. It's along the same lines as implied 90 degree angles. I don't know of any specific reference to implied basic zero in the standard though.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[btrueblood]

Whee!

 

[btrueblood]

Again, I got bruised when implied symmetry was brought up in different context in an earlier post/question. My interpretation these days is that if you "imply" things that are important, you guarantee they will be missed.

Since you can't "dual-dimension" (i.e. create ambiguity) when using basic dimensions (unless you screw up and have the numbers not add up), go ahead and add the basic dimensions that clarify the location of the centerlines.

 

[drawoh]

KENAT,

We are moving into a gray area where we have to determine how obvious it is that the part is symmetrical. A rectangular plate with four holes in it can obviously be symmetrical. If we add one more hole that is not symmetrical, then I would be suspicous of the symmetry.

As a checker, you ought to be able to criticize drawings for format and clarity, regardless of what the rules say. This is no different than people who place the diameter of a hole in one view, and the depth in a section view on the next page.

JHG

 

[KENAT]

Drawoh - you place more esteem in my position as 'checker' than my colleagues or management do (my directs OK but the rest wouldnt' know a good drawing if it jumped up & smothered their face).

If it's clear it's symmetrical, and is using appropriate GD&T then to me it's often clearer, or at least cleaner without a 'centering' dimension. This is shown in the standard in a few places but I was hoping there was something a bit more concrete.

KENAT,

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[ctopher]

Page 107 'Appendix A' has a little information:

http://mscweb.gsfc.nasa.gov/543web/files/GSFC-X-673-64-1F.pdf

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08; CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion

 

[KENAT]

Were you thinking section A or C Chris?

KENAT,

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[ctopher]

Section A

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08; CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion

 

[MechNorth]

Unfortunately Y14.5M-1994 doesn't explicitly describe the implied basic zero condition. It does show the idea in several graphics (Fig. 5-4 for example), however it's not immediately clear that this absence of a centering dimension isn't an "omission for clarity". Also, it has been an accepted drafting technique pretty much forever, however common practice doesn't typically stand up in court.
Apparently they have rectified this shortcoming in the '09 release, Section 1.4(k).

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fcsuper]

This is a gray area. I think that if it isn't stated in the standard or stated on the drawing, then the drawing is incomplete. In the example from that previous posting, the standard is specific to that particular case for the tooling holes, but the 2nd pattern that is driving from its datum may need at least a reference. I think at least a reference is enough so that it is no longer implied. But as you suggested, it should be OK to make the dim to the centerplane a basic.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[ringster]

Once upon a time, we did drawings the hard way. We used drafting machines and pencils. It took a long time to create some of them. To minimize the time required, if the part were in fact symmetrical, we noted 'cl of symm'.
It had the potential for saving a lot of time with repeated details.

These days, with computer generated documentation, you just hit the right key and you have mirrored the information.
Therefore, symm has less if any purpose.

When we invoke Y14.5 the CLs are the center of a feature rather than the centerline of symm of the part. There may be some exceptions, but in general that is the case.

 

[KENAT]

Thanks for the history lesson ringster;-).

The way I see it, if you are not using GD&T then you can't really use implied centerlines/symmetry or variation there on because it means you don't have a tolerance stated - your definition is incomplete.

If using GD&T where you're using position or surface profile then it's Ok because you are still fully tolerancing the part.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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What is Engineering anyway: faq1088-1484

 

[ringster]

What is an implied centerline?

That was history and trust me it was/has been done that way.

You might want to look at 1.8.8 of the 1982 Std.

 

[MintJulep]

Whatever you save by dimensioning a pattern of features from an implied center you lose by having to separately dimension left/right, top/bottom from the center. Otherwise you would seem to run the risk of tolerance accumulation - the very thing GD&T is trying to avoid.

 

[KENAT]

I'm thinking of the case where you have a feature or pattern of features effectively 'centered' on a part as per figure 5-4 in the ASME standard.

I'm not sure where your comments fit into that scenario Mint.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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What is Engineering anyway: faq1088-1484

 

[MintJulep]

Do you want the pattern centered on the "basic" part, or the physical part?

For simplicity, consider a simple rectangular plate with a hole pattern.

Imagine the left edge is datum A. Dimension the center as basic from datum A. Dimension the right edge as basic from datum A and some sort of feature control on the right edge.

Now add basic dimensions from the centerline to your symmetrical hole pattern with position controls for each hole.

In a large population of parts, your pattern will always be biased from the physical center of the actual part - except for the rare case when the right edge is dead-nuts. This bias will be dependent on the process capability of whatever operation creates the right edge.

Alternately, make the centerline datum A. Dimension and tolerance the left and right edges separately from there. Dimension and tolerance the holes from there.

In a large population the hole pattern will be evenly distributed around the physical center of the parts because the width variation should have a more normal distribution as the variations of left and right edges will tend to nullify any bias.

I think.

It may or may not matter for any given design intent.

It may be possible to achieve the same thing either way by paying sufficient attention to feature controls.

 

[KENAT]

I'm talking about the case where the overal width is the datum, that's not quite right terminology but I hope you understand.

Figure 5-4 shows the case I'm talking about, I tried using the correct terminology in the OP;-).

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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What is Engineering anyway: faq1088-1484

 

[drawoh]

MintJulep,

Tolerance is only an issue if you use ± tolerances. Basic dimensions and true position work from datums, regardless of how you apply the dimensions.

My favourite scenario is holes located accurately to an interfacing part, and a part with a sloppy outline. A dimension from one side or end to the hole pattern might be nominally correct, but it could be wrong if you prefer the symmetry on an otherwise, inaccurate part.

JHG

 

[MintJulep]

Drawoh,

But it's fully possible to use GD&T to define a part that doesn't meet the design intent.

If your controls on the position and profile of the edges of the piece allow a wide range of variation then the centerline of individual parts may be significantly different than the centerline of the basic or nominal part.

That may or may not matter - depending on design intent.

All I saying is make sure your dimensioning will get you what you need.