Any Rules on Implied Symmetry?

[curiousmechanical]

Hello,

This has been bothering me for a while.

Please see the attached .pdf for reference.

I come across a lot of old drawings at work that use a simple dimensioning style that takes advantage of symmetry. I also see this style used in many catalogs.

I like this style because it gives the views a clean look (also fewer dimensions) and ensures that mating parts will match concentrically (which I sometimes require).

However, I have not seen any mention of this dimensioning style in drafting textbooks or ASME Y14.5M-1994 (I only have the old version). Are there any rules about this? Is it considered good or poor practice?

If it is legal, how much off-center can the feature be (say the two holes in my example)? How do you tolerance this sort of thing?

If it is not legal, how do I dimension parts that I want to be concentric? For example, we order a lot of flame cut plates and they come in oversized most of the time. If I dimension a part from the corner, the bolt pattern will be way off-center. This could result in a sloppy appearance (material hanging off two sides on a mated joint).

I've seen others dimension to centerlines, but that seems to be illegal.

"NOTE: The following shall not be used as a dimension line: a center line,..." - ASME Y14.5M-1994

I am really looking forward to your responses.

This will solve a lot of debates at work!

Thank you.

 

[KENAT]

As shown, your example is not legal. You've already noticed the crux of the problem in your question of 'how much off-center can the feature be'.

Take a look at section 2.7.3 of ASME Y14.5M-1994 as to why you can't simplistically do it.

Now, if you are using GD&T, specifically position and profile, then you can dimension in your 'implied symmetry' style - see figure 5-4 and some of the related text.

Essentially if you use say width of a feature as a datum feature, then you can then 'imply' centering of other features on that feature.

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[PaulJackson]

Curiousmechanical,

You can however do so if the ISO 2768 Standards are applied. The tolerances "unless otherwise specified" will depend upon what class you specify H,K,L for geometric relationships and f,m,c,v "fine, medium, coarse, very coarse" for dimensions and these classes have tabular tolerance values according to the value of the dimension. Datum features need not be specified either... but are rather determined for presedence by the longest features.

Much of the rest of the world operates as so.

Not defending, not recommending, just informing.

Paul

 

[TheTick]

Another good question is "What is the center?". Features could be symmetrical about the center of one key feature but not another.

Another reason why datum symbols never belong on centermarks!

 

[KENAT]

Paul - quite pushing that horrible Iso std;-).

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[drawoh]

curiousmechanical,

Symmetric dimensioning is good practise when combined with datums and GD&T as per ASME Y14.5. If this this is an old drawing with ± tolerance controlled by notes on the tolerance block, your drawing is ambiguous.

JHG

 

[curiousmechanical]

KENAT

Thank you for the excellent lead.

I have always (past 5 yrs) worked for companies that design custom equipment (usually one-offs). I’m not sure how we get away with it, but we seem to have very primitive drafting standards. We mostly stick to direct tolerancing (bilateral & limits); with the occasional flatness, parallelism, etc. notes. I get the impression that GDT would make our production manager blow a gasket. I think people get nervous when they see the symbols (I know I do) and assume the part is going to cost a fortune.

In any event, I need to get onboard with this GDT stuff, even if it’s only for my own knowledge. Right now, I’m clearly not up to speed because I am not quite following what you are saying.

I see the examples that you are talking about (for others - see attached .pdf). The examples seem to always use holes. I semi-understand basic dimensioning, true positioning, etc. with holes, but what do I do with that centered cutout in my part? It has a rectangular profile.

Also, in Fig. 5-4, what tells me that the 4-hole pattern is centered on the part? That is really confusing me.

It’s OK if I don’t fully understand right now (the general idea will do); I plan to save this conversation and revisit the details later; after I hit the books on GDT.

PaulJackson

Thank you for the info. Now I really feel overwhelmed! I will put this on file for further investigation.

TheTick & drawoh

Good points.

Thank you all!

 

[MechEng2005]

Last attachment (Fig. 5-3) isn't working for me.

 

[curiousmechanical]

MechEng2005

Me either...

Thank you for me letting me know.

Sorry guys. I don't think it liked "&" in the file name.

Please see attched for attempt no.2.

 

[PaulJackson]

Or... How about this (see attached)

 

[powerhound]

Paul,

I don't think your example is correct. You seem to show a flatness spec on a feature of size but I don't think you can do that with 94. I know you can with 09 in the same way you apply straightness to a FOS. Was your example actually to illustrate what you can do with ISO?

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[PaulJackson]

Was your example actually to illustrate what you can do with ISO?

No powerhound, I wasn't illustrating the ISO 2768... all you need for that is the the necessary views, nominal sizes, and tolerance class declaration.

I did make a mistake though (I always see them just after I press submit post), the four fastener hole sizes should have been 8 +/- 0.3.

You are correct... if this was drafted in accordance with ASME 94 the flatness note would be incorrect. I was perfectly comfortable controlling the median plane of a feature like this with a feature size straightness (RFS) callout... then the 2009 standard excluded the 94's extension of the control to median planes (6.4.1.1.3) but rather expanded the control of flatness to include median planes (5.4.2.1)... so I guess from now on according to ASME... form controls applied to "noncylindrical features of size" is the responsibility of flatness.

The only reason I offered the alternative was show how different functional datum features can alter the implied details. How about this one - the bolt clearance holes establish the coordinate X0,Y0 reference and stop rotation.

 

[pmarc]

Paul,

Two questions:
1. Your first sketch shows pattern of 4 holes as tertiary datum. Don't you think this is DOF overconstraining?
2. Second one specifies pattern of 4 holes as secondary datum at RMB (using 2009 wording). Wouldn't be better to apply MMB on this pattern, especially that you assume the holes are for clearance? IMO this would help to establish really nice origin for measurement of tolerances that are referenced to A, B DRF.

 

[PaulJackson]

Your first sketch shows pattern of 4 holes as tertiary datum. Don't you think this is DOF overconstraining?

No, not if these holes (as a pattern) are functionally stopping rotation in the assembly. If mates with a pilot diameter in the assembly it can only stop translation not rotation.

Second one specifies pattern of 4 holes as secondary datum at RMB (using 2009 wording). Wouldn't be better to apply MMB on this pattern, especially that you assume the holes are for clearance?

Sometimes yes, sometimes no! It depends on function. I always tell designers that you don't apply "variable feature tolerance allowances or variable DRF mobility allowances to feature controls if the ideal function is diminished or worsened by the increase in permissible deviation. Critical feature relationships are typically verified for their assembled location, orientation, clearance, or interference of where both constant and variable tolerances for size, form, location, are considered. You don't have to apply the variable allowances to the designs just because the geometric tolerance is applied to a feature of size... or the datum feature is a feature of size.

In the second example (since the clearances holes functionally establish the location and rotation of the spacer/gasket/whatever) I reduced tolerance and clearance of the holes to minimize the ID's translation and outer profiles rotation so that the part maintains maximum perimeter contact with the mating part surfaces and minimum risk of overlap on the mating part's toleranced boundaries. If I had added MMC modifiers to my datum feature pattern I would have had to add it to my stack calculations and possibly reduce my size tolerances to maintain the same predicted critical relationship results.

This whole justification is factitious and conjured to demonstrate the process I use in selecting whether or not to apply variable tolerance or variable datum reference mobility allowances. I will admit that I sometimes recommend their use for reasons other than function:
• To enable attribute gauging (most label it functional gauging) when it is best the best alternative to poor choices for the gauge strategy.
• When policies demand that critical characteristics are gauged 100% and there isn’t an alternative way to perform 100% non-contact automated inspection.
• Other reasons that I can’t think of just now.

I don’t have any problem with attribute gages being used when the modifiers are applied functionally. In that case I would agree that they are functional gages. They are however of no good use for process control… but that is another story.

Paul

 

[KENAT]

curiousmechanical, did you ever get it worked out?

In 5.4 as I understand it, it's centered because firstly it's shown centered, and then the FCF references the datums which are effectively the center planes make it clear what it is centered on.

There are some that argue you need the centering dimensions, it's come up on here before and I'm not sure if anyone changed their opinion even after lots of discussion.

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[ptruitt]

What about this?

 

[ptruitt]

The guy in the tool room called and asked if the four holes could float so he could do a two-stage blanking operation. I said: "Okay. You owe me one." So I rev'd the drawing:

 

[KENAT]

I wouldn't have used profile for the holes, preference per my understanding of 14.5M-94 2.1.1.1 would be to use positional tol.

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[ptruitt]

My colleagues at work would agree with you, Kenat. The profile callout seems more straightforward to me. Maybe position works better for CMM operators. I need to study this more.

 

[PaulJackson]

ptruitt,

I prefer to use profile when controlling part feature contours that have been functionally designed to have uniform tolerance and enable simultaneous feature contour processing methods i.e. blanking, casting, and molding... no mater whether they may be labeled features of size. When refinement operations are added that can independently control size location, orientation, or form… then I would add an appropriate additional control.

Paul