example of axis parallel to primary datum?

[DonkeyPhysics]

Hi there,

I know for a fact that this forum has discussed the topic of using a primary datum that is parallel to an axis (i.e. to control the location, orientation of that axis). For the life of me, though, I can't seem to make that discussion appear in my search results. I've tried several searches in this forum, and several different searches of the wider internet (along with two reference books I have here), but I keep coming up empty.

At the moment, I don't have access to the ASME Y14.5 standard (any version). My company is in the process of ordering a copy of 2009, but... it's taking a long time because we're international and have weird purchasing rules.

Could somebody please point me to some examples of using a parallel plane as the primary datum plane for controlling position of a cylindrical body (not important if it's a positive or negative feature), specifically with a demonstration of its effects on composite vs multiple single segment position tolerances?

 

[powerhound]

Here's one:

http://www.tec-ease.com/gdt-tips-view.php?q=269

Here's another:

http://www.tec-ease.com/gdt-tips-view.php?q=270

and another:

http://www.tec-ease.com/gdt-tips-view.php?q=271

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[DonkeyPhysics]

Awesome, thanks! Tec-ease has some great stuff. Thanks for the help!

 

[DonkeyPhysics]

OK, so... to make sure I'm getting this, I re-created the model from their "Composite Gage Tip, Part III of III"
(

http://www.tec-ease.com/gdt-tips-view.php?q=271)

Within that model, I also extruded & sketched what I understand to represent the top 3 tolerance zones (see attached image).

The 4th line is 0 @ MMC to indicate perfect coaxiality of the 2 holes, and as such is not shown.

My understanding is that the single axis of the pattern of holes must both fall within the primary tolerance zone (modeled as an extruded cylinder concentric with the hole). Within that primary tolerance zone, the secondary zone represents a PAIR of cylinders within which the axis of each hole must float (modeled as a hole for one side, and sketch for the other, each floating freely within the primary zone). The tertiary zone would then represent another pair of cylinders, 1 for each member of the pattern, floating within the cylinders of the secondary zones (for their respective member of the pattern).

Obvoiusly, if we were dealing with more than two members for this particular pattern, I'd be talking about 3 or more secondary and tertiary cylinders. My understanding though, is that while I may have multiple, independently floating secondary cylinders, the respective tertiary cylinders would always float within the corresponding secondary cylinder (i.e. tertiary A would fall within secondary A, tertiary B would fall within secondary B, etc).

If this understanding is correct thus far... then I am confused by 2 things.

1) How is it possible that a cylindrical tertiary tolerance zone can be controled ONLY to A (and not automatically to B by nature of the geometry).

2) How does the perfect coaxiality (4th level) work if the higher-level zones are free to float relative to one another. Am I misunderstanding something? Should the secondary/tertiary zones for each pattern member not be allowed to float relative to each other within the primary zone?

 

[DonkeyPhysics]

OK, I've been thinking about this a bit more, and I've refined my understanding. Please see attached image (v2) and let me know if I'm on the right track here to at least answer my 2nd question from above.

My 1st question remains about the tertiary zone refining also to B by default, by nature of the diametral tolerance definition.

If I'm not mistaken about the tertiary tolerance zone... that would seem to make it entirely redundant once the 4th is brought in.

 

[pmarc]

DonkeyPhysics,
Look at attached picture. It shows how this 4-tier composite positional tolerance should be interpreted. In order to show tier 4 tolerance zone, I assumed that both actual holes are at their LMC = dia. 15.05. Then the tier 4 tolerance zone becomes dia. 0.1.

http://files.engineering.com/getfil...-93b1-b788a76e1f60&file=composite_pos_tol.JPG

1. Notice that there is actually only one tolerance zone for each tier within which both axes of holes must fall. So it is not that tolerance zones for each hole in one segment are separated.
2. Tier 3 (the one with no reference to datum feature B) controls parallelism of both holes to datum plane A, so it does not have to, or as a matter of fact can't use B. If B was used, tier 3 would simply override tier 2 requirement.
3. See point 1.

 

[pmarc]

Comment in point 3: "See point 1." is about: "How does the perfect coaxiality (4th level) work if the higher-level zones are free to float relative to one another. Am I misunderstanding something? Should the secondary/tertiary zones for each pattern member not be allowed to float relative to each other within the primary zone?"

 

[DonkeyPhysics]

excellent. I think I get it now. After everything, part of my problem was failing to consider the zone in 3D when I was making my little demo model. As for the bit about just having one single secondary/tertiary zone, etc for all members of a pattern... well... that was just something I didn't get before, so this was new information for me.

Thanks!

PS: I finally got access to read the standard, so I imagine that'll help too.

 

[Vimalmechs]

Hi all,

In between i got a doubt in one of the below mentioned tec ease tip.

http://www.tec-ease.com/gdt-tips-view.php?q=270

Please clarify me,
Here "Tier 2 FCF" locate the hole axis with DATUM A with in .25 parallel zone. IF the zone is parallel plane, then the tier 2 callout should be without diametric zone symbol . is it ?

OR am i misinterpreted the FCF ?

//files.engineering.com/getfile.aspx?folder=b2b9ab59-27ed-485e-8328-02e8b0d511be&file=Composite_tol_.jpg[link //files.engineering.com/getfile.aspx?folder=b2b9ab59-27ed-485e-8328-02e8b0d511be&file=Composite_tol_.jpg]

Vimal VV

 

[CheckerHater]

IF the zone is parallel plane, then the tier 2 callout should be without diametric zone symbol . is it ?

Both ways will result in the same tolerance zone.
When your cylindrical tolerance zone is allowed to move parallel to the datum A, it “sweeps” what appears as space between two parallel planes.
For more information see ASME Y14.5.1M-1994 Table A-1

 

[pmarc]

Vimalmechs,

First, tier 2 FCF does not locate wrt A - it controls parallelism to A.

Second, with no diameter symbol in tier 2, the maximum tilt of 0.1dia. cylinder defined in tier 3 (when holes are at their LMC) in horizontal direction is limited by tier 1 cylindrical tolerance zone. With diameter symbol present in tier 2, the tilt of tier 3 cylinder is tightened by tier 2 cylindrical tolerance zone.

 

[Vimalmechs]

Thanks CH and Pmarc,

Pmarc,
I have understood your reply text.

But i am confused by the reply "tier 2 FCF does not locate wrt A - it controls parallelism to A." . what’s the error in Locate ?.

as per me "As per the tier 2 frame, "axis of the holes must lie within .25(MMC)zone. ie. .25/2 near side and .25/2 far side LOCATED in relation to the DATUM A". My understanding is right. is it?

Vimal VV

 

[pmarc]

Location of holes is controlled by tier 1. Tier 2 tolerance zone may freely float within tier 1 tolerance zone while always staying parallel to datum plane A.

 

[Vimalmechs]

Thanks Pmarc.