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Centerline used for local symmetry

Centerline used for local symmetry


I'm quite new to GD&T topic so please excuse my lack of fundamental knowledge.

Heres my question. Is it legal in terms of ISO standard to use center line as a symmetry line like shown on attached picture? Obviously the part itself is not symmetrical, but the upper part of it is. So I am curious whether you can simplyfy the drawing like this, or you have to dimension all those features from side also.

I know that whole drawing have some mistakes, but it's not the root of my question, This drawing was made by somebody else, I'm just checking it before release and I'am not sure about legality of that simplification.

Thanks for clarifying this out. You can also provide some standards which are covering this topic if you know some.

PS: Excuse also my english, I'm not a native speaker :)

RE: Centerline used for local symmetry


Yes it is acceptable. The entire part does not need to be symmetrical. However ,there there are no datum (features) referenced in the feature control frame (FCF) that specify which part feature is to be used to establish the datum center-plane of the vertical leg (about which the features are symmetrical) So the feature used to establish the center-plane is not known. This must be defined. Also, be careful with the concept of symmetrical relationships verses the application of the symmetry control, which invokes Derived Median Planes and other "weird" inspection requirements. Position control with Material Condition Modifiers (MMC LMS RFS) can usually be applied to control the symmetrical relationships.

Certified Sr. GD&T Professional

RE: Centerline used for local symmetry

Okay, so lets say that dimension 50 should define the symmetry plane of vertical leg and I'am ok with ISO 2768 default tolerances for symmetry (0.5), so I will not call that mid-plane in any FCF. What is the propper way of specifying that dimension 50 should define the mid-plane?

RE: Centerline used for local symmetry


I am not an "ISO" guy. I am a Y14.5 guys. I do not have software to add comments to your drawing, so let me try a few words describe what I think you are trying to accomplish.

Realize the function of the part drives the dimensioning schema. I have on idea of how the part "works" , so I am shooting in the dark with my response, but here goes...

I would make the "flat" surface defined buy the periphery of the part in the plan view shown a datum. This would the primary datum for all features. Call it Datum A - a plane

The 50 width is a feature of size (FOS) and should have a datum feature symbol attach directly in line with the "arrowed" dimension line. Call it Datum B - a center plane. Doing this means the center-plane between the to parallel sides of the feature is the plane about which other features will be symmetrical.

So every feature that is to be located symmetrical must have this datum in the feature control frame (FCF). This would include the 35 dim, the 15 dim.

Additionally the 5 dim from the top edge locating the holes must be BASIC and the top edge must also be a datum feature placed in the FCF for the two hole. Call this Datum C - plane.

The datum sequence for the 50 and 15 widths would be A | B

The datum sequence for the two holes would be A | B | C

Material condition modifiers could be beneficial, but require a knowledge of part function to apply correctly

Hope this helps.

Certified Sr. GD&T Professional

RE: Centerline used for local symmetry

Thanks for your explanation. The problem with such solution is, that it makes this part more expensive. We are making special machines and calculating process of our parts manufacturer is that he just counts tolerances and FCFs, roughness etc. and more of them is in drawing the higher the price is. And if you make 1000+ parts in one machine, the difference is huge.
So the aim is to keep dimensioning and tolerancing as simple as possible, yet still functional.
The part itself is just simple pusher made out of sheet metal mounted on pneumatic piston with plastic prism mouted on those 2 M4 holes. So the inner hole is just for lessen the overall weight. Prism in the front has the groove which fit the 35 dim and also grooves for screws so it can be adjusted in the direction of vertical leg axis.
Please look at the picture, do you think this can make the trick?
Thanks a lot for your patience, that conflict between simplicity and completness in drawing is driving me crazy, especially when ISO does not specifying some things specifically.

RE: Centerline used for local symmetry

Quote (Tomas)

We are making special machines and calculating process of our parts manufacturer is that he just counts tolerances and FCFs, roughness etc. and more of them is in drawing the higher the price is.

'nuff said. cry

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: Centerline used for local symmetry


The new drawing is not with GDT conventions - you cannot mix BASIC and +/- coordinate dimensions with position. If you want to apply position for the two holes: along with the 25mm dimension, the 5mm dimension must be BASIC and the top edge must be a datum feature.

Also: like Checkerhater, I am crying too. In my experience the opposite is true - the lack of info causes higher costs. I believe the lack of a supplier's knowledge about how to read and interpret drawings "scares" them and causes them to see higher risk. They then raise their price to cover the cost of possible "rework/replacement" due to their "fear" of making unacceptable parts. The lack of knowledge(and training)is the problem, not too much info on the drawing. Few organizations measure the increase in cost caused by missing or misinterpreted part definition - incomplete part definition. Artificially high supplier costs aren't the only cost of "poor quality". For example: every time there is a "argument" about what the drawing means is non-productive time. In can only hope your competition has the same problems to level the playing field.

Certified Sr. GD&T Professional

RE: Centerline used for local symmetry

Well, more and more I'm digging into ISO standard I'm more and more confused. How should I dimension those two holes, using positional tolerance, but with that 5 dimension loose? In this part I really do not care about how far from upper edge this holes will be... I found this (pic. iso_1). It is not recommended to do it like this anymore, you should do it like this (pic. iso_2). But it looks to me much more complicated and less versatile. What should I do if I really don't care about position of pattern against A abd B plane, but it is necessary to constraint position of holes against each other?

Also like you two, I'm crying too. To be honest, we don't have any competition. We are "company in company", making special machines for our production division. And the problem with suppliers lack of basic knowledge results from huge ammount of production we have. Deadlines for machines are always so tight, we just have no time for sending wrong parts for rework. Also they are overwhelmed with work and have no time to precisely examine the drawings and establish the price correctly. It is something I have no chance to change, I just have to live with it...

RE: Centerline used for local symmetry


Your first drawing is "composite positional tolerancing". The upper FCF controls the location for the 4 holes (pattern) as a group and the lower FCF only control the distance between the holes. Not knowing the design requirements I cannot determine if this is applicable. Y14.5-2009 para 7.5.1

The second drawing: the note at the bottom tells it all. Position without datums and basic dims was abandoned over 20 years ago - there are "new" exceptions to position without datums with coaxial features.

Given your response - I suggest you look at bi-directional positional tolerancing. Y14.5-2009 para 7.4.4G

As I said earlier, I am not an ISO guy and don't pretend to grasp the ISO GPS world. I live in USA and have "followed" ASME 14.5 since 1980. I know that one of the major problems with ISO GPS standards is that they are VERY disjointed. I read that there are over 30 ISO standards that "combine" to equal the "coordinated" concepts in the one ASME standard - Y14.5-2009. This makes application and interpretation (and learning) that much more difficult. Frankly I don't know how anyone can keep track of the "current" ISO GPS revisions, which as far as I know ,are not coordinated. A change in one can impact interpretations another standard - go figure. appears your lack of basic GDT knowledge is getting in the way of our communication. Unfortunately my time constraints and poor typing skills make it impossible to perform "training" on this forum. I highly recommend you seek out some basic training and analyze as much written material as you can get your hands on. Sorry. Maybe someone else following this thread can step-up.

Certified Sr. GD&T Professional

RE: Centerline used for local symmetry

If you are considering going deeper into world of GD&T (or rather GPS in ISO realm) I would suggest doing some reading.
For dryer, but more informative "Cheat sheet" approach, The ISO GPS Ultimate Pocket Guide by Alex Krulikowski. Covers latest changes (up to ISO 1101-2012):
If you are into deeper "philosophical" approach, the Handbook by Henrik S. Nielsen (this one also takes the latest changes into consideration):

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: Centerline used for local symmetry

Yeah sure, I understand. Thanks for your patience anyway.

RE: Centerline used for local symmetry

Thanks, I'll check it out and see if it will shed some light into my confusion about some ISO GPS topics.

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