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





RE: Unequal bilateral positioning
RE: Unequal bilateral positioning
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
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
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Unequal bilateral positioning
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,
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RE: Unequal bilateral positioning
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,
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RE: Unequal bilateral positioning
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Unequal bilateral positioning
It may fall anywhere within the zone after it has been properly defined, I do believe.
RE: Unequal bilateral positioning
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.
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Unequal bilateral positioning
KENAT,
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RE: Unequal bilateral positioning
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.
RE: Unequal bilateral positioning
KENAT,
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RE: Unequal bilateral positioning
If this drawing has been prepared IAW 1994 version you cannot simply make all interpretations of the 200X versions apply.
RE: Unequal bilateral positioning
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
RE: Unequal bilateral positioning
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Unequal bilateral positioning
RE: Unequal bilateral positioning
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
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.
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
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
Do you advocate the use of positional tolerance or profile
for rivet patterns?
RE: Unequal bilateral positioning
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
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 standard
KENAT,
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RE: Unequal bilateral positioning
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Unequal bilateral positioning
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
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Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08; CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
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RE: Unequal bilateral positioning
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
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08; CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion
RE: Unequal bilateral positioning
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 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
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 forum
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Unequal bilateral positioning
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
RE: Unequal bilateral positioning
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Unequal bilateral positioning
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.
RE: Unequal bilateral positioning
Typically edge distances will be of more concern than the distance from the actual TP for the hole.
RE: Unequal bilateral positioning
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
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.
RE: Unequal bilateral positioning
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
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
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.
RE: Unequal bilateral positioning
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08; CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion