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Can symmetry be implied without GD&T?

Can symmetry be implied without GD&T?

Can symmetry be implied without GD&T?

(OP)
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?

RE: Can symmetry be implied without GD&T?

It's been common for many years to just dimension between the two points and put "=    =" under the dimension line (I believe there was a thread on this a while back).  It's not part of any current standard that I've heard of, so it may not be recognized by the shop.  A second consideration is that a centerline doesn't exist on a physical part, so how are you going to check those positions without defining that centerline, which essentially is the domain of GD&T.  

In other words ... do the GD&T!

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

RE: Can symmetry be implied without GD&T?

(OP)
The centerline is extended from a hole in the center of the part so there is an actual feature to use as a datum. Trouble is they frown on the use of GD&T unless absolutely necessary here. Otherwise I'd use it.

RE: Can symmetry be implied without GD&T?

cwdaniel,

   If your part looks symmetrical and the drawing is specified as being done to ASME Y14.5M-1994, then symmetry is implied.  This is true, GD&T or no GD&T.  

   Without ASME Y14.5M-1994, there is no standard to interpret your drawing by, and you should make things explicit.

                        JHG

RE: Can symmetry be implied without GD&T?

If you are concerned about reducing dimensions on a symmetrical part, you can place a centerline on the part with "=" at each end as shown in ASME Y14.2M-1992 (referenced by ASME Y14.5M-1994). This is just a visual way of saying that the dimensions and tolerances apply idenically and individually to both sides of the part. You will want to verify that your tolerance stack ups are still valid if you do this. The better method is to, in Jim's words, "...do the GD&T!".

David

RE: Can symmetry be implied without GD&T?

Keep in mind that there is a difference between dimensioning (the goal) and tolerancing (how much you can blow it by).  With dimensioning, a symmetrical relationship my be implied.  But, features shown symmetrical must be controlled with geometric tolerances according to Y14.5.  In other words, listen to Jim.  By the way, GD&T doesn't make parts expensive--tight tolerances do.

RE: Can symmetry be implied without GD&T?

ditto!
What the others wrote.bigsmile

Chris
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)

RE: Can symmetry be implied without GD&T?

cwdaniel

Quote:

The centerline is extended from a hole in the center of the part so there is an actual feature to use as a datum.

Some of the above are sharper on GD&T than I but to me if you don't use it then you haven't really told them to use the hole as the datum.  They may be just as valid using the outside dimensions or some other symmetrical feature.

I've seen implied symmetry cause problems before so I'd always make it explicit.

Quote:

By the way, GD&T doesn't make parts expensive--tight tolerances do.
  If only we could convince everyone of this!

RE: Can symmetry be implied without GD&T?

KENAT,

   You are right.  The datum must be called up explicitly on the drawing.  It can be the hole.  It can be the two outside edges.  In either case, your datum is a feature of size.  Unless it is very accurate, you should call it up at MMC.  This may be an issue with the functionality of the part.

   I have an issue with the idea that GD&T is somehow more complicated and expensive than no GD&T.  A recommended procedure on a part like this is to apply a composite positional tolerance.  One true position attaches the pattern to the face sloppily.  The other true position locates the holes accurately with respect to each other.  This is an ideal solution when you have to match the holes in the mating part, but you are not concerned about the overall location.  

   In effect, the symmetry is nominal, not actual.

   I am not sure what happens when you do not apply basic dimensions.  The ASME standard goes very quickly into positional tolerances and true position.  I would regard non-basic dimensions as ambiguous.

                      JHG

RE: Can symmetry be implied without GD&T?

All that making a dimension basic does is tell the person reading the print that that dimension does not follow the standard tolerences set up in the tolerance block and that the tolerance to apply is shown elsewhere. It tells them to assume the dimension to be theoretially perfect.

David

RE: Can symmetry be implied without GD&T?

aardvarkdw,

   I am assume that when you do not make the dimension basic, you also do not apply the positional tolerance.

   Let's assume our part is a rectangular plate, let's continue to interpret the drawing through ASME Y14.5M-1994, and let's assume the tolerrance block in millimeters says X=+/-0.5, X.X=+-0.2 X.XX=+/-0.1, and angles are +/-1deg.

   The sides of the plate are allowed be one degree out of perpendicular to the base.  Are orthogonally dimensioned holes measured from the angled side, or from the corner at the base?

   If both sides are one degree out of perpendicular in the same direction, how do you determine where symmetry applies?

   If you ask yourselves these questions, the GD&T positional tolerance becomes simple.

                        JHG

RE: Can symmetry be implied without GD&T?

Drawoh,

I agree that using the GD&T is the best way to go. I was just trying to clarify your question. You said,"I am not sure what happens when you do not apply basic dimensions.  The ASME standard goes very quickly into positional tolerances and true position.  I would regard non-basic dimensions as ambiguous.".

For instance you would not make a hole diameter a basic dimension if your only feature control frame is a positional tolerance to locate the hole. That would be ambiguous because now you don't have a tolerance on the hole diameter. This is true for any feature of size. Only the position tolerances effected by the positional FCF would be basic, all others would not be basic.

David

RE: Can symmetry be implied without GD&T?

Actually, aardvark, you can't use a positional on a non-toleranced feature, so there can be no ambiguity on that point.  There must be a tolerance of size applied, and the general tolerance block could provide that ... hate it, but it is an (unpleasant) option.  A conflict would arise where a note appears on the drawing to the effect of "ALL NON-TOLERANCED DIMENSIONS ARE BASIC", along with traditional general linear-tolerance catch-all (based on # of decimal places) that reads "UNLESS OTHERWISE STATED".  Seen that a number of times, and it's hard for people to recognize that there is a conflict.

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

RE: Can symmetry be implied without GD&T?

... but you can use a positional tolerance of 0.000 at max matl on the hole.

RE: Can symmetry be implied without GD&T?

Jim,

My point is that you can't indiscriminatly make all dimensions basic. If they are all basic they must all have a FCF that they refer to. If they don't they can't be basic and therefore must either have a linear tolerance attached to the dimension or in a generic tolerance block. By making a dimension basic you are removing all linear tolerances from the dimension and saying that you have toleranced it in a FCF.

David

RE: Can symmetry be implied without GD&T?

Exactly right.  Tks for clarifying the statement, Dave.

EWH, you can use 0@MMC positional tolerance on a FOS that is linearly toleranced, but not on a feature that has a basic size dimension.  That assumes that you can live with the trade-off of size vs position.

Jim

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

RE: Can symmetry be implied without GD&T?

   When I said you should apply basic dimensions everywhere, I was refering to hole co-ordinates.  Holes mostly are features of size.  I should have been more clear.  The discussion was about hole positions.

   You can use basic everywhere if you use surface profile tolerances to control the holes.  Usually, this is not convenient.  I have done it once in a while.

   I have been using the zero true position tolerances lately, primarly because they are an exact description of what I require from clearance holes.

Eg.

   M6X1 hole located at 0.2DIA of true position.

   The clearance hole is 6.2DIA +0.8/-0 at 0 DIA MMC.

   There is a 6.2mm diameter potentially occupied by the screw, which must be kept clear.  The allowable positional error increases as the hole gets bigger.  The 6.8mm diameter hole may be off by 0.4mm (0.8mmDIA TP).  This is all extremely simple to calculate.  You do have to add some clearance for perpendicularity through thick parts.

   Note also that 0 true position is valid, fabricatable and inspectable, even if you do not specify MMC.  

                          JHG

RE: Can symmetry be implied without GD&T?

OK, drawoh, I'll bite...how is 0-Position (without MMC) valid, fabricatable and inspectable?

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

RE: Can symmetry be implied without GD&T?

I am confused by this also.  If the condition is RMC, then there are no tolerances to take into account.  I can see how tolerance would be available at LMC, but have never come across a situation where that would be desireable.

RE: Can symmetry be implied without GD&T?

   The profile of the hole must be contained within the MMC outline and the LMC outline.  Thus, the maximum position error is allowed when the hole is half way between MMC and LMC.  

   You control the MMC that allows clearance around a fastener or other features.  You control LMC, for whatever reason you would want to control LMC.  Perhaps you have a single big hole through a piece with a slightly bigger OD.

   I wasn't saying you have to do it.  It is an option.

                         JHG

RE: Can symmetry be implied without GD&T?

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
www.profileservices.ca

RE: Can symmetry be implied without GD&T?

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?

RE: Can symmetry be implied without GD&T?

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
www.profileservices.ca

RE: Can symmetry be implied without GD&T?

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

RE: Can symmetry be implied without GD&T?

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.
 

RE: Can symmetry be implied without GD&T?

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.

RE: Can symmetry be implied without GD&T?

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.

RE: Can symmetry be implied without GD&T?

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

RE: Can symmetry be implied without GD&T?

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

RE: Can symmetry be implied without GD&T?

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).banghead

RE: Can symmetry be implied without GD&T?

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

RE: Can symmetry be implied without GD&T?

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.

RE: Can symmetry be implied without GD&T?

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)

RE: Can symmetry be implied without GD&T?

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

RE: Can symmetry be implied without GD&T?

drawoh,

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

RE: Can symmetry be implied without GD&T?

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

RE: Can symmetry be implied without GD&T?

(OP)
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.

RE: Can symmetry be implied without GD&T?

cwdaniel,

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

RE: Can symmetry be implied without GD&T?

(OP)
"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?

RE: Can symmetry be implied without GD&T?

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

   

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