PLTZFvsFLTZF

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Greg

Swapnil

Murugaperumal

Greg

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1 day ago

Relative sizes of PLTZF and FLTZF tolerances?The standard seems to indicate that the PLTZF tolerance must always be larger than that of theFLTZF. Is this truly a mandated restriction? Why?

I'm asking because I have a pattern that is used for a machine-vision application. The visionsystem captures the positions of the features and computes the center point. As long as thatcenter point (the average of all the positions) is precisely where it needs to be, the individualfeatures can actually wander a fair amount. So it's the overall pattern that I care about. Must Imore tightly tolerance the individual features just to meet "the letter of the law"? Is there apractial reason why I must?

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1 day ago

Swapnil G. Hello Greg,

Yes, PLTZF is always larger tolerance than FRTZF. FRTZF is always refinement of thePLTZF.

Here's to simplify the things: if you add regular single segment frame of position tolerance,it will control location directly and orientation for free of cost. Exa. if you added a position

tolerance of dia 1mm, location & orientation will get controled within dia 1mm tolerancecylinder.

But sometimes, we are not ok with same orientation value or inter-relationship between

features (maybe simply the max & min distance between produced imperfect holes).

One will use composite tolerance (PLTZF & FRTZF) only when its okay to have liberallocation tolerance but tighter orientation and inter-relation between pattern features.

Let's say the part you have has pattern of holes. Meaning of adding composite tolerance is

they can go off-location more, but still the mating part will assemble in those holesbecause we controlled inter-relationship with features with each other more tightly.

If you can send some picture, it will be very easy to explain / mark-up.

Hope this is of some help to you.

[email protected]

1 day ago

Murugaperumal R. Hello Greg

PLTZF tolerance must be larger than FLTZF. It is a functional requirement of a part havingpattern of holes can be positioned using more tolerances with respect to the datum andtight tolerance within the feature. Take an example of positioning a name plate having 4pattern of holes to mount it in a board. The requirement of the name plate is to bepositioned with respect to the datum and it should match with the four holes in the board.

So the pattern of holes can have more tolerances with respect to the datum and tighttolerance with in the feature to match with in four holes in board

RegardsMP.

Greg F. Greg,

If you were to have larger tolerances in the lower segment/FRTZF, those larger toleranceswould not be meaningful, because the individual features must still satisfy the tolerancesin the upper segment/PLTZF. My first concern would be whether the dimensioning andtolerancing scheme being applied was truly based upon the functional needs of theproduct, or whether it was being based upon an inspection scheme. I suspect that it may

be the latter? If what you are describing truly represents the functional requirements, a

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3 30-03-2013 07:03


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Mark

Greg

21 hours ago

Mark F. Greg F is right. The first couple of responses describe how a Composite positionworks, and Greg H, you were misunderstanding what the PLTZF does, so the explanation

of how a Composite Position tolerance works was necessary. But as Greg F noted, if whatGreg H described as the truly functional requirements actually are the truly functionalrequirements, then Composite is the wrong tool for the job.

So first, just to clarify and consolodate the other responders' explanations of how thePLTZF and FRTZF work, you need to understand that the PLTZF does NOT orient/locatesome imaginary construct of the "center of the pattern" of features. It is simply a positionalrequirement for each of the features within the pattern to a datum reference frame -- justlike any single-line Position feature control frame would do. The FRTZF simply refines thelocation of the features within the pattern to each other as well as orientation to any datum

references. The COMBINED EFFECT of the PLTZF and the FRTZF is to locate thepattern "loosely" (i.e. more loosely than the FRTZF) and to orient/locate the features toeach other more "tightly." So you could think of the PLTZF/FRTZF (i.e. the composite FCF)having located the "center of the pattern," but I would advise against looking at it that way

because that is not how it is defined.

With that, now let's look at how to really specify your true design requirement where you

have a very loose tolerance for the feature-to-feature orientation/location within yourpattern of features, but you want to keep the "center of the pattern" located to some otherdatum features more loosely. If I'm understanding what you are describing correctly, it

seems to me that you should be using the pattern of features itself as the datum feature,and then locating other features of the part relative to the datum reference frame defined

by that pattern of features. So then you could have just a single-line feature control framefor the loosely-toleranced position of the features to each other, attach your datum feature

symbol to that position feature control frame (thus defining the pattern of features as adatum feature), and then relate whatever other features need to be relative to the datumreference frame you have just defined.

Greg H. Thanks, all.

Greg, you're on the right track with your supposition that this is more of an inspectionscheme than a typical assembly issue, In this case, however, the requirements of eachare not mutually exclusive.

We have a vision guidance system used for assembly that looks at one of the locatedarrays and uses an algorithm to compute a derived center point. That derived center pointis then used as the "go to" target for the motion system to achieve a properly aligned

placement of the part to be assembled.

So, my abuse of the definition is actually based on functional reality. I've had the strong

feeling, however, that I was perhaps headed down the wrong road considering use ofeither a composite positional tolerance or two single-segment feature control frames, soI'm glad I asked.

I wish LinkedIn allowed me to post a picture, but I think perhaps I can draw an adequate"word picture" using a familiar reference to do the job:

Picture the front grill of an old-school Willys Jeep: A headlight on either side of sevenvertical slots. (Google image search "Jeep grill" if you haven't looked a Jeep in the facelately.) Bottom center is a license plate with some sort of graphic in the center of the

numbers. That's essentially analogous to the geometry I'm wrestling with, except in mycase it's a bit of a mutant Jeep with four equal-sized headlights forming a 2X2 array andthe seven grill slots bisected horizontally such that they form a 7x2 array.

My intended primary datum is the rear surface of the grill (the side facing the radiator),

restricting the translation DOF along the longitudinal axis of the Jeep and rotation aboutthe vertical and transverse axes. The secondary datum is the graphic at the center of the

license plate, restricting translation along the transverse and vertical axes. The tertiarydatum is established by the height dimension of the license plate, restricting rotation aboutthe longitudinal axis.

The position of the grill slot array as a whole can be loosely toleranced with respect to thelicense plate graphic, but each slot relative to the others must have much tighter positionaltolerances. So that would seem to be a great application for either a composite positionaltolerance frame or a two single segment frame. But which? (Incidentally, I don't carenearly as much where the slots fall along the vertical axis relative to either the licenseplate graphic or each other as I do about their slot to slot position along the transverse

axis.)

The center of the headlight array must be very precisely located relative to the center of

the grill slot array along the transverse axis, but that's really all that matters about thatarray.So it seems to me that Mark's suggestion of using the grill slot array as a datum forthe head light array is indeed the way to go, but at that point, I have a very imperfect grasp

on how things should work.

Using the rear surface of my grill as the primary datum again, I again restrict the same onetranslation and two rotations as before. But does using the grill slot array as a datum

restrict the remaining three by itself?

And then what position control would be best for the position of the head light


3 30-03-2013 07:03


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Mark

Greg

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19 hours ago

Mark F. Interesting issue, Greg, and I agree with you that I wish L inkedIn would allow forattachments in this group. You did a creative job of painting the word picture, but I s till feellike I need a drawing to ponder your true des ign requirements. If you would, please emailme anything you can to help me see your issue, and then I will try to comment back to thegroup. [email protected]. Thanks.

15 hours ago

Greg H. Thanks, Mark. I'll take you up on it. Hopefully no "lurkers" are waiting with baitedbreath to also see the answer.

I'm sending you a PDF format file called "The Mutant Jeep.pdf" via email that contains afinal page that contains only a non-annotated representation of my "Jeep grill" just in caseyou might want to doodle on it.


3 30 03 2013 07:03

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