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Unequal bilateral positioning

Unequal bilateral positioning

Unequal bilateral positioning

(OP)
I have a hole that is called out with vertical positioning +-.005 and horizontal positioning +.006,-.001.  Is there a way to use a positional tolerance (or any GD&T method) to allow more movement in one horizontal direction than the other? Or does the hole need to be centered in the tolerance zone?

Simply put, the hole can move left .001 and right .006. How do I show that with GD&T?

RE: Unequal bilateral positioning

(OP)
Maybe bilateral wasn't the right word in the subject, bidirectional maybe.

RE: Unequal bilateral positioning

JLang,

What version of the Standard is being applied to your documentation?  What is the purpose of the hole and what is the interface?  It might be a case where positional tolerancing is of little or no benefit.

RE: Unequal bilateral positioning

(OP)
You know, a better question would be, does Y14.5-1994 allow me to leave it as a plus/minus dimension without applying an FCF?

I'd look myself, but I don't have a copy because my company decided to wait a little bit and get the 2009 version.

RE: Unequal bilateral positioning

There is no requirement in the standard that mandates the use of GD&T.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Unequal bilateral positioning

Quote (ASME Y14.5M-1994 ):

2.1.1.1 Positional Tolerancing Method.  Preferably, tolerances on dimensions that locate features of size are specified by the positional tolerancing method described in Section 5.  In certain cases, such as locating irregular-shaped features, the profile tolerancing method described in Section 6 may be used.

Some might interpret the above as a mandating use of either position or profile but many would argue otherwise.

Was the +.006 -.001 calculated from first priniciples/function or are you just trying to translate an existing +- tol of unknown validity into GD&T?

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Unequal bilateral positioning

In terms of your actual initial question, I wonder if you could use position on the whole without the dia symbol.  Essentially apply FCF like you would for a slot, one in 'X' and a separate one in 'Y'.  I believe you'd have to nominally center the hole in both axis rather it being off set, however from a pass fail criteria this makes no difference.  

From a design intent point of view some might argue the unequal +- better captures intent, implying the hole is preferably in the left of the tol zone.  However, inspection wise, and hence presumably function wise the scheme would be equivalent except with position you could take advantage of the MMC principle to gain some tolerance.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Unequal bilateral positioning

To me, this is like the "TYP" issue; there is a preferred method, but the preferred method is not mandated as a requirement.  The give-away is the use of "Preferably" in that quote.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Unequal bilateral positioning

In response more directly to the initial question.  The hole does not have to be 'centered in the tolerance zone'.

It may fall anywhere within the zone after it has been properly defined, I do believe.

RE: Unequal bilateral positioning

Sorry to keep vering off topic, but I so rarely disagree with KENAT, that I feel I have to explain myself further...poke
Positional tolerancing IS NOT a requirement of ASME Y14.5-1994.  There are requirements pertaining to the use of positional tolerances, but direct tolerancing is allowed.

Quote:

2.1.1(a) as direct limits or as tolerance values applied directly to a dimension.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Unequal bilateral positioning

ewh, while I do still use +- sometimes, I believe the section could be interpreted as the 'preferably' meaning to preferably use positional instead of profile but that one of those 2 should be used.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Unequal bilateral positioning

ringster,

   The hole does not have to be centred within the tolerance zone, but a GD&T positional tolerance zone is centred about the nominal hole position.

   ASME Y14.5M-1994 explains what ± tolerances mean.  The OP's description, however weird it is, can be interpreted by a fabricator and an inspector.  This is the fundamental purpose of the standard.

   ASME Y14.5M-1994 is entitled Dimensioning and Tolerancing.  The term GD&T does not appear on the cover, anywhere.

               JHG

RE: Unequal bilateral positioning

Should have added 'for features of size'.  I certainly am not one of those who thinks almost all tolerancing should be done by surfact profile.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Unequal bilateral positioning

JLang,

If this drawing has been prepared IAW 1994 version you cannot simply make all interpretations of the 200X versions apply.

 

RE: Unequal bilateral positioning

(OP)
The idea is to translate the existing +- to GD&T. However in this case, it really isn't necessary so if it's not required by the standard I will leave it as is.

The X,Y positional tolerancing as KENAT described would be fine, but that would require changing the dimension value so it lands in the center of the tolerance zone. Very simple, but in SolidWorks, editing a dimension prevents it from being "basic", so it can only be done by a tedious process I'd rather avoid.

Thanks for the responses, this forum never fails!

RE: Unequal bilateral positioning

I guess I am at somewhat of a loss as to the net gain of this discussion.  Anyone else?

RE: Unequal bilateral positioning

No net gain here nosmiley

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Unequal bilateral positioning

(OP)
I don't know, I seem to have had my question answered so I'm happy.  

RE: Unequal bilateral positioning

Per 2.1.1.1 Positional Tolerancing Method. Preferably, tolerances on dimensions that locate featues of size are specified by the positional tolerancing method described in Section 5.  In certain cases, such as locating irregular-shaped features, the profile tolerancing method describe in Section 6 may be used.

This lays out that the preferred method of locating features of size is the positional tolerance, and that profile of a surface may be used as the alternative.  Where does it say anything about using +/- conventional tolerances to locate anything?  If someone feels that 2.1.1(a) applies to the location of a feature of size, please back it up somewhere else in the standard with text or graphic.

This issue keeps coming up banghead because the use of Preferably, Must, Shall, Should, May and other such terms is not widely understood within the context of legal documents.  The terms do not have the same meaning as in conversational english.  In this case, "preferably" is an indicator of the primary methodology, and establishes such methodology in a discrete statement.  Acceptable alternatives are subsequently indicated in follow-up statements.  It doesn't give you latitude to use any other method than what is listed.

Also consider that a "centerline" or "center" of a feature isn't physically present and reproducible without going back to the feature itself.  Grab the axis of a hole!  How are you going to find that hole's center repeatably?  ASME addresses how to find the position of the center (axis or plane(s)) for features of size, but doesn't give any indication of methodology for verifying a +/- location.  What about tying the feature's position to the datums?  To invoke a datum reference you typically use a FCF.  But +/- tolerances on location dimensions to the side of the workpiece are point-to-point, not center to datum.  There's not one graphic or text anywhere in the book supporting the use of +/- location tolerances for features of size.
Off the soapbox now. soapbox


So, to the OP, if you're invoking Y14.5M-1994 or its ancestors, then you are bound to one of those two positioning methods.  As you're not looking for a cylindrical tolerance zone, you can use separate "vertical" and "horizontal" position controls attached to the feature of size dimensions in each direction (vert & horiz); if the feature is cylindrical, then you can put them inline with "blank value" horizontal & vertical size dimensions respectively.  In this case a position control of .010 vertically and .007 horizontally with the BASIC horizontal location shifted to the right (+ve) by .0025.  You can't use unequal tolerance zones for position (i.e. +.006/-.001) controls.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Unequal bilateral positioning

Jim,

Do you advocate the use of positional tolerance or profile
for rivet patterns?

RE: Unequal bilateral positioning

Kenat, yes ... but I really do like surface profile!  I find too many designers believe that they need to control the position and size separately, but then use MMC; I can usually achieve the same thing with a good profile control.2thumbsup

Ringster, I've done very limited riveting, and when I did it was with pre-drilled holes.   The rivet body didn't completely fill the hole as we were using it for axial clamping force rather than shear strength. In that case we used position though I would have preferred profile as we really were only concerned with boundaries.

In the case of riveting sheet metal without predrilling, I have heard from others that the positions are not typically critical.  I'd do some studies on how the head deforms and see if I could use a profile control based on the outside of the crown.  This, of course, is based on my limited exposure to rivets.  What would you suggest?
 

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Unequal bilateral positioning

Ringster, I believe you, or was it Ringman (one and the same?) have brought this up several times before.

You don't have to use the dia symbol in FCF or invoke MMC with position.  You could use position without and it would effectively be the same as +- coordinate dims if you felt this was appropriate, and since the standard says to preferably use position...

Well Mech, for features of size position is preferable per the standardwinky smile.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Unequal bilateral positioning

Yup, Kenat ... so far.  I use the "under certain circumstances" exception quite a bit.  noevil

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Unequal bilateral positioning

ringster,

  What "net gain" are you trying to achieve with that question?
  How could profile possibly apply to a hole pattern anyway?

   

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Unequal bilateral positioning

RE: Unequal bilateral positioning

Hmmm, all the links are really basic GD&T 101. Is this in response to my question about how to apply "profile" to a hole pattern?

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Unequal bilateral positioning

Powerhound,

That was a good question that you posed.  My response would be that one must be selective when applying GD and T to a drawing.  I is my opinion that it should not be applied to rivet patterns for one thing, inasmuch as most if not all rivet patterns are not intended to be interchangeable.


I have heard the statement that GD and T is like a box of tools and that where one individual might use one another might selelct a different.  

Back to the basics from years ago,  UNIVERSAL INTERPRETATION, INTERCHANGEABILITY AND 'SHORT HAND' FOR NOTATIONS.

It rather seems that we have departed from these basic principles.

 

RE: Unequal bilateral positioning

I'm still stuck on how "preferably" means "mandatory".  I respectfully disagree that GD&T MUST ABSOLUTELY EVERY SINGLE TIME WITH NO EXCEPTIONS be used on a hole location or the drawing is not valid per the standard.
  I am not saying that I don't beleive in using GD&T, just that I'm not going to waste the companies time and money sending a drawing back for correction because a rivet hole was located using +/- rather than true position or profile.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Unequal bilateral positioning

Ringster, sorry, it looks like I misread your question.  I took it to mean individual rivets as opposed to the pattern of rivets.  

Again, though, it always comes down to what you are trying to achieve; pre-drilled holes or blind riveting, interchangeability or not.  If you have two parts that both have predrilled holes, shouldn't those two sets of holes match somewhat, and how would you verify that?  If there is no predrill, but the relationship overall of the pattern and the rivets within the pattern is important, how would you check it?  It seems like that's the only guidance I can seek.  I think everyone here understands the basics of composite positional control.  As for how a profile control can be used to control a pattern of features...if the profile control applies to more than one feature, as indicated by a note such as "16X" or "16 HOLES MARKED 'M'", then those featues are part of a pattern.  Similarly, Principle of Simultaneous Requirements could group a number of features together as a pattern.  Of course, this doesn't give you the flexibility of separate PLTZF and FRTZF tolerance zones.

EWH, we all pick the battles that we wage within our companies (and here on this forumpeace).  Hopefully we make a stance on those issues that are comparatively more important to our longterm success and value.  I've had shops tell me that they weren't concerned over the GD&T and regular tolerances on the drawing because they just did their best and sent the parts on.  Most parts were accepted and occasionally they would have to rework something, but overall it worked well enough for them.  Not a battle worth pursuing.  What I particularly like about this forum is the opportunity it gives us to try to understand each other's perspectives.  We don't always agree, but at least we start to see where each other is coming from.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Unequal bilateral positioning

You can create your own custom positional tolerance zone that is not round, BUT you will have to fully define that zone and how it applies on the drawing to your particular feature.  In my opinion, it's not worth the trouble; just still to the rectangular tol zone.

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

RE: Unequal bilateral positioning

Does anyone's experience go back to 1973, when the PLTZ was allowed to be rectangular?   

RE: Unequal bilateral positioning

PLTZF & FRTZF can still be rectangular, it just is a set of callouts (for horizontal & vertical) rather than a single callout as you'd get for a cylindrical zone.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Unequal bilateral positioning

ringster,

   Why do GD&T positional tolerances not apply to rivets?

   When I call up rivets on a fabrication drawing, my assumption is that the fabricator is responsible for and has the resources to make everything line up.  When I inspect the part, I expect to see properly installed rivets.  My primary requirement is that they should not look sloppy, so I specify my true position accordingly.

   My vendor can fabricate rivet holes separately, accurately enough that they line up.  They can drill after assembly.  I do not care.   

               JHG

RE: Unequal bilateral positioning

IT IS MY OPINION, that due to the patterns not requiring interchangeability, there is really no need to positional tolerance them.  There are always some exceptions to the rule, or in this case opinion, but in general, NOT REQUIRED.

Typically edge distances will be of more concern than the distance from the actual TP for the hole.

RE: Unequal bilateral positioning

Drawoh,

Positional tolerances & Rivets...

My favorite was when someone handed me an already released drawing of a rivet with a shoulder containing a concentricity callout relating the shoulder diameter to the body of the rivet (the part that deforms).

For those who are not laughing...remember the function of a rivet is to join surfaces...tolerancing should be done to ensure rivets can do that job.

Michael
 

RE: Unequal bilateral positioning

ringster,

   Any positional tolerances on my rivet drawing will be for the rivets, not the holes.  Rivet holes and rivet bodies are are uninspectable once the rivets are installed, at least without an x-ray or a hacksaw.

               JHG

RE: Unequal bilateral positioning

Drawoh

This is an interesting concept that you offer.  Could you expand on the method you might use and justify the use of positional tolerancing on the rivets?
 

RE: Unequal bilateral positioning

drawoh,

   If I send something out to be fabricated with rivets, I want to control the location of the rivets.  I need them some distance apart.  I need them some distance from edges.  I need it to look like the fabricator had less than six beers at lunch.   

   Why should I not use positional tolerances to do this?

   As KENAT notes, I apply MMC when I need it.  This case, I do not.   

               JHG

RE: Unequal bilateral positioning

GD&T is used for all holes and removable fasteners, not permanent fasteners.

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

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