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2 Single Segment FCF to make Rectangular Tolerance Zone?

2 Single Segment FCF to make Rectangular Tolerance Zone?

(OP)
I have seen a number of drawings where a hole ends up with a larger tolerance east-west than north-south, so rather than expand the diametrical tolerance to cover both, I see two reference linear dimensions with separate control frames. (See top example) I was wondering if possible to combine these into two single segment control feature controlf frames on the diameter dimension. (See bottom example). Is this allowed and is it clear?

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

To my interpretation, the first example has no interpretation possible under ASME or ISO.  The second example actually is a reference to a composite pos tol, which precludes its use in this manner.

To use positional tolerancing with an elliptical zone, you'll have to completely describe your pos tolerance zone within the content of the drawing.

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: 2 Single Segment FCF to make Rectangular Tolerance Zone?

The top one doesn't explicitly match any example in 14.5M-1994 that I'm familiar with but I'm inclined to say the intent is clear.  You've dimensioned it like you would a slot or similar.  It's probably how I would have attempted to dimension the case you state, I may have left the dia symbol on the ref dims, not sure.

I think the second example is not composite positional tol but (incorectly applied) "multiple single-segment' FCF, or at least looks like it so is definitely wrong for your intent.  (See 5.4.1.8 of ASME Y14.5M-1994 for disambiguation from composite).

So in summary, I'd say your second option is wrong and unclear.

Similar came up in a recent thread though I don't recall much detail being given.  There are a few very knowledgable members who seem to enjoy extrapolate from what the standard says, maybe they'll chime in.

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: 2 Single Segment FCF to make Rectangular Tolerance Zone?

2
The first drawing is called a "bi-directional" positional tolerances since the tolerances are not equal in both the X and Y axis. This is used a lot on slots where the length of the slot usually has more tolerance than the widthand is covered by ASME Y14.5M-94 section 5.9 page 135.

The second example reflect 2 single segment feature control frames on the same feature. They relate different tolerances to different datums. There are two (2) requirements here.

Dave D.
www.qmsi.ca

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Well done Dingy,

JasonSP, figure 5-41 page 139 referenced from within the reference Dave gave shows what you are trying to achieve, it doesn't have the reference dimensions shown but is otherwise similar to what you have.

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: 2 Single Segment FCF to make Rectangular Tolerance Zone?

(OP)
I was just looking through the book again and found Fig5-41 on page 139. So it's clear that the top example is both by the book and clear. And the bottom example is just incorrect. Thanks all!

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

I believe the first example to be acceptable and proper.  However a close examination indicates that 'physical identification' of the datum features might just be in order.

It would provide maximun latitude for locating the hole while cproviding a control for the edge distance.

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

The top example is good but the bottom example is not. To use single segment FCF you still have to abide by the same rule as in composite, the top segment must have a larger value than the bottom. The top segment should read "position within a diameter of 0.5 to A, B, and C - the bottom segment should read "position within a diameter of 0.2 to A and B." This will give you more tolerance in one direction than the other but the full zone won't be rectangular like the top example. It will be more like a small cylindrical zone positioned within a larger cylindrical zone and the small zone can move within the larger zone from left to right only, not up and down. Depending on your application, you can use the top example on your drawing, or change the bottom example to what I've recommended and you will have a legal callout.

Here are 2 links on the subject that really helped me when I was struggling with the concept.

http://www.tec-ease.com/tips/december-98.htm

http://www.tec-ease.com/tips/dec-08.htm

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

ringster,

  What do you mean? I see all the relative datums identified and the feature being controlled is shown with basic dimensions from their respective datums.

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Powerhound,

The standard contains only one sentence as I recall with regards to Physical Ident of Datum Features.  Given a symmetrical part, it becomes necessary to mark the part physically by 'some means' to assure that everyone is using the same datum reference frame.

Par 4.2 of the 1988  Standard.

Too often overlooked in my opinion.  
 

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

I agree that the second example is incorrect. It is trying to say the same as the first with a bi-directional tolerance.

One can have two single segment feature control frames referencing different datums but not in this case. It's just wrong.

Dave D.
www.qmsi.ca

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

I wouldn't go as far as to say that the second example is wrong, but it definitely doesn't match the intent.  The second example would amount to a bi-directional zone, but the directions would be skewed if there was any perpendicularity error between B and C.  The tolerance zone would be a parallelogram shape, with a different angle on every part.

In the first example, one could argue that in the FCF for the vertical direction the C reference should be omitted.  It doesn't do anything.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

A couple other points to consider are:
1) the Dia 30 size needs a tolerance somewhere for it to be a feature of size, and therefore locatable by a position control
2) the (30) ref dimension with each of the position controls is not necessary, and conflicts with the Dia 30 dimension as it purports a linear rather than cylindrical feature

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

ASME Y14.5M-1994, 5.9
"Where it is desired to specify a greater tolerance
in one direction than another, bidirectional positional
tolerancing may be applied. Bidirectional positional
tolerancing results in a noncylindrical tolerance zone
for locating round holes; therefore, the diameter symbol
is omitted from the feature control frame in these
applications."

I believe that figure 5-41 is actually incorrect per the text of section 5.9 as it shows diameter symbols in both FCFs when the text clearly states that they should be omitted.

Correct me if I am misreading this.

David

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

aardvark ... the Fig. 5-41 that I'm looking at doesn't have a diameter symbol in the tolerance section of the two position FCFs.  The feature is a cylindrical hole, so a diameter symbol belongs on the size callout.  The orientation of the axis of the hole is a refinement of the rectangular positional tolerance zone (i.e. floats within it), and it's ok to have a cylindrical orientation tolerance zone within a rectangular positional tolerance zone.  No problem with 5-41.

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Allow me to pull my foot out of my mouth.... I saw the position tolerance symbol and for some reason my mind read diameter. I even reread it before I hit the post button to make sure I was not seeing things... I'll just blame Lysdexia and leave it at that... sorry.

David

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

My copy of 14.5M-1994 doesn't have dia symbols on the FCF either.

I'm kind of wondering why we're still debating this though, seems to me the OP got the answer they needed from Dingy yesterday and the OP replied acknowledging it.

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: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Dingy,

Im not sure that the fact different tolerances are used has a bearing, or is necessary to apply the technique.  I believe they could have correctly been the same and resulted in a square zone rather than a rectangular.

I feel slighted that I did not get a STAR for mentioning Physical Datum Feature Identification.  :>(

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

As I said in my previous post, the bottom example is incorrect because the top value is smaller than the bottom value. You can't do it using composite position and you can't do it using single segment.  

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

C'mon, Kenat ... the horse just won't die!

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Whoa,  

This horse aint dead yet.  Why not apply the symbol for the origin of the dimension and a +/- tol to the location?  Forget datum features and PHYSICAL IDENT .

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Please don't take this thread there.  cry

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

The OP's illustration was primarily to show the 2 different approches the OP was considering to trying to show 'bi-directional' tolerance zone.

They were perfectly adequate for that purpose.  

Were they a complete engineering drawing fully complying to the standards, no.  However, when looking at illustrations on this site (as opposed to actual drawings/extracts of drawings) I wish we could treat them like figures in 14.5 and invoke paragraph 1.1.4. It even states "EXAMPLES ARE INCOMPLETE BY INTENT" in the top left.

For what the OP was asking about the second sketch was just plain wrong, it was trying to use terminology/symbology that means something completely different.  The GD&T 'format' issues of the zones being different sizes in the wrong order etc were completely irrelevant.

deadhorsedeadhorsedeadhorse

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: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Kenat, The Y14.5 Section 1.1.4 clause would be a great suggestion to include in a list of etiquette for this forum.  Maybe that should be a new thread?

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

I've brought it up before and it's typically the same posters that violate it, I doubt that formalizing it in a forum specific 'rules of use' or similar would stop them.

However, I did have the same thought.

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: 2 Single Segment FCF to make Rectangular Tolerance Zone?

KENAT,
I don't know if I understand your meaning when you say the GD&T format issues and things being in the wrong order are irrelevant. If I do understand your meaning correctly then are you saying that the appropriate response to the OP should have been a simple "No" with no further explanation and everyone else just move on to the next thread?

Stating "EXAMPLES ARE INCOMPLETE BY INTENT" is one thing and in that light, I wouldn't address issues of incompleteness. The tolerance values being out of order were not a matter of incompleteness, they were a matter of incorrectness and from reading the responses of other posters; some said the example was wrong but no one said why. I didn't want the OP to think that it would be okay to use single segment in that fashion for a different application. I guess it would be like a planar feature of size having a tolerance of +/-.005 and then applying a parallelism callout of .015 to one face relative to the other. That's not incomplete, it's incorrect and most people around here would point it out on a print that was submitted for our review even if it wasn't the subject of the post.

Approaching the issue of intentional incompleteness from another angle, I fully agree. I think it's ridiculous when someone asks a question about a positional callout on a hole in a square part with rounded corners and someone else responds with "Hey, there's no radius callout on the corners."

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Let's go back talk more about 2 single segment FCF callout here, if the second example is incorrect to have a 2 single segment FCF callout on the same feature, how about the 2 single segment orientation callout on datum C as shown, Datum surface C is to be perpendicular to datum A within 0.15 and also perpendicular to datum B within 0.12.

Correct me if I am wrong.

SeasonLee
 

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

powerhound,

As far as I know, there are no rules regarding the different segments in a multiple single segment callouts.  Each segment is an independent control.  The lower segment(s) of a multiple single segment callout do not need to be refinements of the upper segment like they are in composite FCF's.

The usual convention is to make the FCF with the largest tolerance value the upper segment, and go down from there.  But I don't know of any rules relating to that.

Where are you getting the idea that there are rules that apply to multiple single segment callouts?

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Powerhound, my point was that the OP was trying to create a bi-directional tolerance zone.  The second example has a different meaning which 2 of the first responders pointed out.  Once you've pointed out it has the wrong meaning I'm not sure I see the specific benefit of going into detail of what that meaning is and how it's even incorrect for the wrong meaning.  I did give a ref to the std to clear up the confusion with composite.

Maybe I'm just in a bad mood, I don't normally disagree with you Powerhound.

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: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Evan,
  Single segment is a variation of composite (or vice-versa). The difference between composite position and single segment is that single segment maintains basic relationship to the datums in the lower frame while composite does not. Everything else is the same. They are not independent controls. See 5.4.1.3 and also use those links I provided earlier in this thread.
   

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Powerhound,

5.4.1.3 directly follows the sections on composite FCF's, and shows an example that highlights the distinction between single-segment and composite.  This kind of implies that single-segment is a variation of composite, but I don't think it should be taken that way.  The Tec-Ease links show similar examples, specifically intended to emphasize the distinction between single-segment and composite.  But there are many other applications of single-segment in which the datum features in the lower segment are not subsets of those in the upper segment.

There is a note in Y14.5 which mentions the independence of single-segment feature control frames.  This is on page 95, deeply buried in the endless sections on composite positional tolerancing, so it's easy to miss.

Another case in which the explanation in Y14.5 is misleading.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Perhaps I am missing something, but do not composite tolerances apply to PATTERNS OF HOLES, rather than a single hole?  

Im confused as to how composite tolerancing has entered this thread.

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

SeasonLee,

Sorry we got sidetracked there with the composite FCF thing and didn't address your question.

I would say that the 2 single-segment orientation callout isn't illegal, but it doesn't match the functional intent.  Because C is going to be functioning as a tertiary datum feature in an ABC datum referencing scheme, its orientation tolerance should reference both A and B as follows:

PER|0.15|A|B

If C's orientation relative to B needs to be tighter for some other functional reason, a second single segment could be added as a refinement:

PER|0.12|B

Because A and B will not be exactly perpendicular to each other, referencing A and B separately will give a different control than referencing them together.  The tangent plane extracted when B is primary might not point in exactly the same direction as the "orientation constrained tangent plane" extracted when B is referenced as secondary to A.  A diagram is badly needed here.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

OK, back to the tangent for a moment;

When the two single-segment position FCFs follow the same DRF or progressive refinements thereof, they are a special variant of the composite positional control.  The second FCF refines inter-feature position, orientation of the features & pattern wrt the datums, and also restricts the location wrt the last datum in the DRF sequence.  Over-simplified, but accurate.

When the two single-segment position FCFs contain unique DRFs that are not a progressive refinement, they are separate controls, without any refinement functionality.  The most common example is on a casting where a hole is positioned wrt the cast datums (e.g. /Z/Y/X), and wrt the machined datums (e.g. /A/B/C).  Though this isn't on Tec-Ease's Tips section (yet), I can assure you that it is in our training material.

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

I believe that even Lowell would find the first example acceptable and would suggest a functional gage for it.
There is a very similar example in one of his texts.

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Jim,

I'm still not sure about the "special variant of composite" thing.  

In "regular" FCF's, which multiple single-segment FCF's are an example of, the datum features constrain all of the degrees of freedom that they can and may.  In other words, each datum feature constrains all of the DOF's that it is able to and that have not already been constrained by higher precedence datum features.

In the lower segment(s) of composite FCF's, the datum features are only allowed to constrain rotational degrees of freedom.  This was presumably devised to provide a means of applying the equivalent of orientation tolerances to patterns of features.

To me, multiple single-segment controls are just regular FCF's with no special rules applying to them.  Even if the lower segment has a subset of the upper segment's datum features.  In general, FCF's must represent a refinement of any other controls that indirectly limit the characteristic in question.

The composite FCF is the special variant, with special rules applying.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?



I have a copy of a a letter to the editor of Machine Design, dated 1990, by a committee member, condeming them for the article that appeared in the magazine.  The article was "The Problems with Geometric Dimensioning and Tolerancing".  The editor was taken to task for being ridiculous and full of opinions and misconceptions.


looking at this thread, and the problems in coming to a consensus on a single hole in a rectangular block,  it leaves one to wonder.  Maybe they were ahead of their time.

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Interesting thread!  I believe the original question has been answered by MechNorth and others.  The first example is legitimate.  The second example has a couple of problems.  However, putting the lower tolerance on top is not one of them.  The lack of the diameter symbol in the FCF without indicating direction is a problem.  The lack of a diameter symbol indicates that the tolerance is a width or bi-directional tolerance but there is no indication of direction.  The assumption that the datum reference clears this up is not supported in the Y14.5 standard.  
It was suggested that a drawing of 2 single segments might help clear things up.  Attached are two pages from our Fundamentals text that may help.  See the 2 position tolerances which are 2 single segments on the second page.  Putting the smaller position tolerance on top would not change the requirements of the drawing.  I was reluctant to sent this for fear that it will start more tangents but after receiving a couple of emails from your participants, I decided to pass it on.  I hope it helps.     

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Prof,

A review of one of the leading GD&T texts shows the datum identifier used in conjunction with the targets.  It seems to clarify the precise intent of the targets.  

In the illustration it is not clear that the points X are used as a width.  It would be clarified if the triangle (datum identifier)were associated with X1 and X2 as a width.

 

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Ringster, what else could the opposed points be taken as other than a centering datum?  In the texts you refer to, does it show the datum identifier symbol attached to the drawing centerline?  That tends to creat even more confusion as drawing users may think it's the centerplane based on the width of the opposed faces rather than the opposed points.  Could you post a pic so that I can see your proposal?  Tks.

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

On the first page of ProfDon's casting drawing, it says "datum targets were used to establish datum planes Z, Y and X".  What exactly is datum plane X?

Does this mean that the datum extracted from the two X datum target points is a plane?

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Given the DRF Z/Y/X, Z establishes the primary datum plane which arrests 3 dof (a translation & 2 rotations), and Y establishes the secondary datum plane perpendicular to the first datum plane and eliminates one translation and the final rotation.  All that's left is for datum X to constrain that last dof.  Given that all dof are eliminated, you have a fully constrained part; a fully constrained part has three mutually perpendicular datum planes.  What else could datum X be other than a plane?  Consider that the two datum target points establish a point in space which is centered between the two datum target points and is simultaneously located on the two preceding datum planes.  That center point thereby establishing the location of the third datum plane which is oriented with mutual perpendicularity to the two preceding planes.

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

I see the DRF in ProfDon's attachment as the primary datum establish by the 3 datum areas identified by the 3 Z's. The secondary datum is a datum line identified by single Y1 and being that it is a datum line, it restricts 2 degrees of freedom. The tertiary datum is the plane that runs between X1 and X2, perpendicular to Y1. Do I understand this correctly?

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Jim,

You stated that a fully constrained part has three mutually perpendicular datum planes.  Does that mean that a part that is not fully constrained does not have three mutually perpendicular datum planes?

The two datum target points establish a center point that establishes the location of the third datum plane.  So is the tertiary datum a point or is it a plane?

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Evan,
There has been considerable debate for a long time about your first question.

Per '94, 4.1; "... These elements exist within a framework of three mutually perpendicular intersecting planes known as the datum reference frame."  That does not distinguish between fully constrained & partially constrained, so therefore the argument is that once a single datum is referenced, three mutually perpendicular planes are established.  To me, that implies that everything is set in stone (so-to-speak) with a single datum (point, axis or line).  More correctly, only a discrete number of those three planes has been located in space (one plane for a planar datum ; two planes for a datum axis; three planes for a point datum, with a conical datum feature establishing an axis and a plane normal to it), and therefore only those located planes can be used as an origin of measurement.  The spatial location and orientation relationship of each datum plane is established by the DRF.  It's late, so I hope I've answered that somewhat coherently.

For your second question, the two opposed datum target points will establish a datum point which has its own three mutually perpendicular datum planes, one of which establishes the location of the third datum plane of the DRF, thereby eliminating the final dof.

A lot of this is tantamount to the chicken & the egg.  A datum point establishes three mutually perpendicular datum planes intersecting at that locale ... or does the intersection of the three mutually perpendicular datum planes establish a datum point?  A datum axis establishes two mutually perpendicular datum planes which intersect at the axis ... or does the intersection of two datum planes establish a datum axis?  Per 4.4.2, "A cylindrical datum feature is always associated with two theoretical planes intersecting at right angles on the datum axis. ... This axis serves as the origin of measurement from which other features of the part are located."  So which came first, the two planes or the axis ... is totally irrelevant beyond a theoretical debate.  We measure from the datum simulators.

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Jim,

I appreciate the input.  It's always interesting to get another practitioner's views on fundamental questions that I'm not sure about.  I credit (blame?) DesignBiz for this - the recent thread with the skewed A-B cylinders has made me question my whole belief system regarding datums.

So you're basically saying that if a FCF has at least one datum feature reference then it has a DRF.  I suppose that makes sense.  

When you state that "the two opposed datum target points will establish a datum point which has its own three mutually perpendicular planes, one of which establishes the location of the third datum plane", that's where I get lost.  I'm not saying it's wrong, but I'm not able to reproduce the logic of it.

Your comments about the chicken/egg nature of datum points and axes with associated theoretical planes were interesting.  If you can make sense of the theoretical planes, you're a better man than me.  I can see it for primary datum features, but for secondary and tertiary I just can't.

You're final comment that "we measure from the datum simulators" made the most sense to me.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

MechNorth,

You might take a look at pages 262 and 263 in Gro-Metrics III.  His illustrations show the datum identification attached as appropriate to the lines and planes.

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Evan,
Re the two datum target pts establishing a datum pt, try this mental exercise;
First, poing your two index fingers at each other, as if using them as a set of opposed datum target points.  Next, visualize a line between the tangent points (i.e. closest points to each other).  Accepting that opposed features establish a center value (plane, line, point) of some kind is the first step; this is no different from working with FOS (Features of Size).  So, at the center location between these two datum points, you have one of the following; a plane, an axis, or a point.  A plane requires the establishment of three points in space (minimum), so the two opposed points don't establish a plane directly.  Similarly, a line (axis) requires two points in space; again, not what you have at that center value.  All that is left is a single point as the center value.  That datum point has three mutually perpendicular planes associated with it, all meeting at the datum point.  The problem most of us face is what we do with the datum planes that we're not using from each of the datums.  In this example, Datum Z establishes one plane, Datum Y establishes one plane, but Datum X establishes three planes while only one is needed to complete the DRF.  So we use the one which is mutually perpendicular to Datums Z & Y as the third origin of measurement, and ignore the other two.


Here's something similar, though not quite the same.  
http://www.profileservices.ca/files/tidbits/thd1103_239765.pdf
Datum A establishes the first datum plane, Datum B gives you an axis with its two datum planes ... that's three datum planes and three origins of measurement already.  Datum C, however, adds a fourth datum plane ... which is not used as an origin of measurement, but only to lock down the orientation of the planes established by Datum B.  I included this graphic because it is often overlooked that Datum C is indeed a datum plane, but it happens (in this case) to be coplanar with one of the Datum B planes.

Hopefully that helps rather than muddles things.

 

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Ringster,
I don't have GeoMetrics III.  Pls post a quick hand-sketch that I can look at.  tks.  

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

RE: 2 Single Segment FCF to make Rectangular Tolerance Zone?

Jim,

I can see how the center point is extracted from the two datum target points.  The three mutually perpendicular planes associated with the center point still didn't make sense to me though.  Part of the problem for me is that the "center point between two datum target points" does not have the same constraint properties as a center point extracted from a spherical datum feature.

One way that I evaluate my understanding of an issue is to imagine teaching it and dealing with the questions that would arise.  Every class seems to have one or two students who just have to know where everything comes from and won't just take the instructor's word as gospel.  If one of those students asked me to explain the logic of imagining three theoretical planes and then ignoring two, I couldn't.  

So I read through the DRF chapter in one of the public review drafts of the new standard.  The description of cylindrical datum features has significant changes.  

1994: A cylindrical datum feature is always associated with two theoretical planes intersecting at right angles on the datum axis.

2009: A primary cylindrical datum feature is always associated with two theoretical planes intersecting at right angles on the datum axis.  Depending on planes established by higher precedence datums, secondary and tertiary datum axes may establish zero, one or two theoretical planes.

I couldn't find similar explanations for planar and spherical datum features, but the logic could be extended to apply to those as well.

This new description doesn't explain how secondary and tertiary datums actually establish their theoretical plane(s), if any.  But at least it releases us from the "two planes for a datum axis" and "three planes from a datum point" concept.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

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