Can symmetry be implied without GD&T?

[cwdaniel]

A part has a defined center line, features are symmetrical about that center line. Distance between the symmetrical features (tapped holes) is important. Is a dimension mandatory from the center line to a feature on one side?

Or is it allowable to leave that dimension out to imply symmetry and not a tolerance stack to one side of the center line?

 

[MechNorth]

Yes, the entire hole must be contained within the max & min size limitations, but that has nothing to do with the positional tolerance if not stated at MMC or at LMC; the entire tolerance is available RFS if you don't specify otherwise. If you have a zero-positional tolerance without modifier @ MMC or LMC, then you have to be dead-on for your position...i.e. zero positional error and you don't get to trade-off size for position.

MMC essentially allows greater "play" by trading off size & location tolerances; LMC is typically used to ensure minimal wall thickness.

0@RFS isn't an option because you can't physically achieve it.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[wwenzli]

I have a part with two holes located symmetrically about the part center line. A basic dimension defines the spacing between the two holes. The symmetry tolerance is 10 mils about the center line, RFS. Here's the question: Is the distance between the holes an exact number, or does it vary? Does the symmetry tolerance just dictate where the features can float on the part, or does it allow the features to float relative to each other?

 

[MechNorth]

Symmetry will limit the float on the part, not wrt the other feature. If you want to limit float wrt each other, use one of them as a datum, or use a composite positional tolerance.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[fcsuper]

If they frown on using GD&T, then there may be lack of understanding. Sure, if applied a certain way, it can increase cost. But, if applied correctly, it can reduce cost by allowing you to use the each tolerance you need.

Matt
CAD Engineer/ECN Analyst
Silicon Valley, CA

http://sw.fcsuper.com/index.php

 

[Runz]

I haven't read all the posts here and have noticed that the thread has gone in another direction, but I am wondering about this implied symmetry that people are talking about.

As far as I am aware, symmetry is not implied, unless it is specifically called out and references a datum.

The following must be present for symmetry to apply:
1. A feature control frame must be applied to a planar FOS that is symmetrical about a datum centerplane.

2. Datum references are required.

 

[ewh]

Runz,
I think that you are correct as far as Y14.5-1994, but previous versions of the standard did have a symbol which indicated symmetry, and datums were not necessary. Strictly speaking, this would not be implied but explicit, since the symbol directs the interpretation.
Much depends on which version of the standard is being followed.

 

[Runz]

I was questioning what "drawoh" wrote on Nov 8th (4th thread down).

It just seems that many drawings have Y14.5-1994 added to their title blocks, but then only add limited GD&T. They seem to think that other things that were implied in the past should hold true today.

 

[ewh]

I would also be interested to find out where in the standard "implied" symmetry is allowed. Still learning new things about it quite often.

 

[namdac]

YES,YES,YES. If the machinist can't figure it out from there, time to get a new machinist. Everyone has to quit reading into the words. After doing some research in an old Engineering Design Manual by French 1st Addition. Being from the old school of doing everything by hand this was performed by the designer to save time in having to draw all of it. This was a cost savings to the manufacturer.
Regards,
Namdac

 

[ewh]

That was then and this is now. If you state that you are following a standard, that is what the customer expects. Yes, 99% of the time you may get away with your shortcuts, but that last 1% can lose you a major customer or two. Losing your reputation for quality may be worth a little money saved for some, but that is a very short sighted view. Unfortunately, businesses in the US are getting more short sighted every day.[flush]
To create quality parts consistently, there should be NOTHING ambiguous about their definition (drawing).

 

[fcsuper]

ewh,

eh, not everyone makes drawings for customers. If someone does, I guess you are right about the responses. I wouldn't consider using shortcuts myself for customers.

However, as the customer myself when working with machine shops, it's not a big deal if I don't make it a big deal...and that's the reality. In that case, there's nothing short sided about it.



Matt
CAD Engineer/ECN Analyst
Silicon Valley, CA

http://sw.fcsuper.com/index.php

 

[ewh]

Different solutions for different situations. If you don't need to hold the drawing to the standard, then that is fine. It shouldn't be a problem, unless the drawing includes a statement that it is to be interpreted to the standard.

 

[ctopher]

Just my 2 cents.
I always make my drawings so that anyone can understand them. It keeps me up to the standards and there is never any question. Also, every drawing is unique to the part regardless if one part will be made or a million parts will be made. Being consistent is the way to go.

Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 01-18-07)

 

[drawoh]

Runz,

I could have sworn that this was called out explicitly, somewhere in the standard. I cannot find it.

Symmetric holes are shown on Figure 5-4. The secondary and tertiary datums are the width and the length, respectively, and they are called up at MMC, quite properly in my opinion. Using two edges would be legal I think, but not very logical.

Any time you see a bolt circle on a round feature, symmetry is assumed.

Implied symmetry is fine if there is no other possible interpretation of the drawing. If you are using 3D parametric CAD, your model should force symmetry, so that you cannot change something and mess everything up.

Plus/minus tolerances locating symmetrical holes, minus any specified standard, are ambiguous. Plus/minus tolerances are ambiguous anyway without a standard, especially if you dimension from edges that are hard to fabricate perpendicular.

ASME Y14.5M-1994's section on datums is so important.

JHG

 

[ringman]

drawoh,

A slip perhaps, but since when is symmetry assumed on a B.C.? I don't believe it ever was for that matter.

 

[Runz]

drawoh,

The only symmetry which is applied to a drawing is that which uses the symmetry callout and references datums.

You can assume symmetry all you want, but when your drawing states it is created per ASME Y14.5M-1994 and there is no symmetry callout and a part is not made symmetrical by an outside manufacturer, what do you do?

If he understands GD&T, you are out of luck.

I have numerous training documentation that states specifically that A.) A feature control frame must be applied to a planar FOS that is symmetrical about a datum centerplane and B.) A Datum reference is required.
If these two things do not apply, then neither does symmetry.

Making false assumptions is what causes much of the confusion!

R

 

[cwdaniel]

So if I dimension a slot by it's center, without a positional tolerance, I can't expect the slot to be symmetrical about that center?

All great input by the way.

 

[ringman]

cwdaniel,

Dimensioning to CL is not acceptable. But it will be symmetrical about the centerline.

 

[cwdaniel]

"Dimensioning to CL is not acceptable. But it will be symmetrical about the centerline."

Y14.5M-1994 shows examples of slots basic dimensioned to the CL and located with a positional tolerance.

If you don't use the positional tolerance you're not allowed to dimension to the CL?

 

[drawoh]

ringman,

When is symmetry called out explicitly on a pitch circle on a round part?

Please note that I am assuming that the nominal dimensions are evidently symmetric, and that the position tolerance controls the actual position of the holes in the exact way it would if the parts were dimensioned from datums on one side. There would be no symmetry beyond what the positional tolerance provides.

ASME Y14.5M-1994 Section 5.13 covers symmetry. On two of their examples, they draw centre lines and use positional tolerances to show that the feature is perpendicular to the primary datum, and symmetric within the secondary datum. The symmetry callout does not use the perpendicular face for control.

JHG