Datum Shift Explanation and Advantages
Datum Shift Explanation and Advantages
(OP)
Hi engineers,
I have a question regarding about specifying datum feature at RMB. What are the advantages of having a datum shift since we are not getting any more or less bonus tolerance? Wouldn't it complicate the inspection process since the part can now move (shift) around?
There is an example from the book that I can't wrap my head around it, and I hope someone here can help me out. On the attachment, for option b and c, I don't understand what the y mean by "to ensure that datum precedence is not violated..." and how they choose the 0.4 and 0.2 for each option.
Any help is appreciated.
I have a question regarding about specifying datum feature at RMB. What are the advantages of having a datum shift since we are not getting any more or less bonus tolerance? Wouldn't it complicate the inspection process since the part can now move (shift) around?
There is an example from the book that I can't wrap my head around it, and I hope someone here can help me out. On the attachment, for option b and c, I don't understand what the y mean by "to ensure that datum precedence is not violated..." and how they choose the 0.4 and 0.2 for each option.
Any help is appreciated.





RE: Datum Shift Explanation and Advantages
Second: if the part in its intended function can shift around (at least before being subsequently bolted down), then our tolerancing should imitate that, so that we can get as much flexibility in the tolerance as possible.
You say there is no bonus tolerance, which is technically correct, but there could be a resulting "datum shift." Although this doesn't make the tolerance zone itself get larger, it does allow the zone(s) to move around, thus giving more tolerance in a way.
As for the picture you give, each case has this "datum shift," but depending on the datums referenced in the callout where there is an asterisk, we might have different values for the datum shift. For each grouping of datums referenced at the end of the feature control frame, we can only take the datum shift from the callout that shares those same datum references (minus the D at the end).
In other words, if the asterisked callout only references D, then the part is not held against A for that tolerance, therefore the two feature control frames already attached to D are not carried over to the asterisked callout. The numbers they give actually represent the fixed-gage that could be used, but that will affect how much looseness a real part feels when placed on that gage.
Perhaps a little tricky, I realize, but this is certainly one of the slightly more advanced topics in GD&T!
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datum Shift Explanation and Advantages
Belanger already explained that datum shift has advantages when functional gauge is used. With CMM - not so much. CMM systems (and CMM people) don't like the idea of datum feature able to move. So datum shift is often ignored even if it may result in theoretically good parts being rejected.
RE: Datum Shift Explanation and Advantages
I think Dingy is the one who pointed out that if your datum is a feature of size at MMC, you can fabricate a fixture that will locate on it. If your datum is RFS, you are going to have to figure out where it is. RFS is convenient for CMMs. MMC is convenient to mechanical fixtures.
Never forget that your part has to work. Presumably, your FOS datum picks up a feature when your part is installed. At MMC, your part may fit exactly, allowing zero movement. Your bolt holes have to be positioned accurately enough that your bolts go through. If your feature is not at MMC, you have some wiggle room to position your sufficiently accurate bolt pattern over the mount holes.
If your part must be located precisely, an FOS is the wrong locating point, and the wrong datum.
--
JHG
RE: Datum Shift Explanation and Advantages
Thanks so much for all the response. They are all really helpful.
The Q.A department at my company always checks the parts with CMM machines, and the part is always stationary during the inspection process. That's why I really didn't understand the idea behind datum shift.
I have a follow up question. Let's use the picture from my first post as example. Assume that I am making a functional gage for that piece to inspect the 3.5 dia hole. Now that the part is allow to shift around both B and D datum, how would I establish datum reference frame? Should I still use the datum reference frame established from the gage?
Since the part is allowed to shift, is it possible that it shifts too much that the 3.5 dia hole is out of the 0.3 diametrical tolerance?
RE: Datum Shift Explanation and Advantages
If there is not any shift (looseness), then the part's tolerances appear to be tighter because you don't have the ability to shift it around.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datum Shift Explanation and Advantages
Yes, it is possible, but as long as you find at least one position of the part in the gage in which axis of the 3.5 dia hole is within 0.3 dia position tolerance zone, the feature is good.
RE: Datum Shift Explanation and Advantages
It seems to me that the third case 'c' in the picture attached to the original post is incorrect.
The explanation below the figure fails to note that perpendicularity constrains the diametral allowance for position.
I think the shape of the datum D simulator is not a diameter, but an obround that is 7.3 X 7.5
I'm a torn on the concept of whether the datum B simulator is in exactly the same location for validating datum feature D or is allowed to find a new location. If the former, there should be definition that identifies such sub-FCF groups (in this case A}B(m)) as a simultaneous requirement. If the latter, then the obround length of the datum D simulator should include the amount of possible difference between the two conditions.
RE: Datum Shift Explanation and Advantages
Pmarc,
Your latest comment *IS IT* or *IS NOT* in the same line of thinking of the candidate datum set defined by Y14.5.1 (math standard)?
Somewhere Mark Foster (Applied Geometrics) said:
"I would contend that your choice of datum planes is only one of an infinite number of candidate datum planes (to use Y14.5.1 terminology) and that you have a "rocker." In instances where we have candidate datum planes, the standard allows us to "optimize" and choose *the* datum plane that best suits our specification/application. I could have a mobile coordinate system for any reason, such as a datum feature of size referenced at MMB, and find an infinite number of candidate locations of the datum reference frame where the measurements would be out of spec. The point of optimization is to find the place that works, not to demonstrate the plethora of places that don't work."
RE: Datum Shift Explanation and Advantages
And the datum feature simulator would indeed be round. The apparent tolerance carved out within different scenarios may be a different shape over the span of many parts, but the key is to focus on the datums being referenced for each option (a) through (c). However, feel free to elaborate.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datum Shift Explanation and Advantages
Relative to A, it cannot tilt more than the perpendicularity tolerance allows.
It's very similar to the case where the [28] basic dimension is replaced with a directly toleranced dimension, but retaining the perpendicularity. The set of possible geometries is an obround.
RE: Datum Shift Explanation and Advantages
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datum Shift Explanation and Advantages
RE: Datum Shift Explanation and Advantages
First, you should realize that the picture is from the standard (page 61).
Second, we are talking about the datum feature simulator for datum D, and there are three options being discussed in that figure.
Third, the text right below the figure clearly states in each option that the datum's MMB is a diameter, so any datum feature simulator would be built to simulate that diameter. The datum feature simulator doesn't simulate the shape of the tolerance zone on the 3.5 mm hole, which is which I think you're trying to visualize.
It's all explained on the lower left side of the graphic given in the OP, under 4.11.6.1 (a), (b), and (c), as well as other paragraphs that relate to the MMB concept.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datum Shift Explanation and Advantages
I am referring to the shape taken by the 7mm diameter when it is allowed the movement afforded in the same DRF as 'c'.
The suggested interpretation does not match the set of allowable places 'D' could get to. If one took all the possible parts that met the requirements applied to D and transformed them to no longer include C, then it is an obround. Picture the view as if standing at the center of B; would D ever appear to be more than it's MMC and perpendicularity tolerance wide? Without a reference to C, there is nothing that fixes the direction of the view and so nothing that causes adding a location tolerance to the width.
It looks to me like an overly complicated case that was incorrectly analyzed. As I've mentioned before, one of the greater failings of previous versions of the standard is that they only show individual parts and how to inspect/interpret them and don't show the effects of tolerance schemes on the relation of features on multiple parts. I presume the same case applies here, where no mating part is shown to clarify the need for the 'c' case.
RE: Datum Shift Explanation and Advantages
Short answer is - yes, it is in the same line of thinking. From inspection point of view, at least one position of the part in the gage in which considered feature meets its tolerance is sufficient to say that the feature is good.
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3DDave,
In my opinion the standard is correct in (c), although I would really like to understand why you think it is otherwise. Unfortunately I am afraid I am not following this:
Assuming I am picturing it correctly, I disagree that when standing at the center of B and looking at D, the D would appear *just* as a 7.3 width. Material of feature D would indeed occupy max 7.3 width, but the width would additionally be allowed to float 0.1 left and 0.1 right from true position centered at the datum axis B, thus producing 7.5 wide virtual condition. And since the true position of feature D is fixed at basic 28 from datum axis B, this 7.5 width would actual be dia 7.5 cylinder.
RE: Datum Shift Explanation and Advantages
For every possible compliant solution to locating D, draw a line from the center of B to the center of D where their axes intersect A. Then drop a vertical from the top-center of cylinder D to A. The maximum distance between these two points from D projected onto A is the radial amount of perpendicularity. The maximum allowable perpendicularity is dia 0.2. That is all that can be added to the MMC diameter in a direction that is perpendicular to the line between the center of B and the center of D along the surface A.
Along that same line, the intersection of the D axis with the line can be closer or farther away by the amount of the position tolerance and at MMC at the nearest and farthest from B the axis of D must be perfectly perpendicular to A.
As a contrast.
Suppose the original limitation on D did not include a reference to C(m). This would mean that D could be anywhere in a 360 degree location around B and still be acceptable. To make a mating part that would align with A|B(m)|C(m) and avoid touching D would require a circular slot in order to clear all the possible locations for D. The annular width of the slot is 7.5mm
However, if a mating part was to align with A|B(m)|D(m) it would only have to clear the radial variation and the tangential tilt of D. This is the case no matter where D ends up around the circle.
The length of the slot is the same as the width of the annulus and the width of the slot is the MMC + perpendicularity at MMC, hence an oblong of 7.5mm X 7.3mm.
RE: Datum Shift Explanation and Advantages
Just curious: what you are talking about/debating has anything to do with "tertiary datum problem"? (solution could be adding a translation modifier)
Discussed here:
http://www.eng-tips.com/viewthread.cfm?qid=363878
Thank you
RE: Datum Shift Explanation and Advantages
It's an interesting overlap, but this case concerns the shape of the datum simulator while the tertiary datum problem focuses on the location of the simulator. I'd have to think more to see if they are simple transformations of one to the other.
In both cases, the lack of supporting practical applications suggests that what is presented in the standard is either an edge case or just a solution to a non-existent problem.
RE: Datum Shift Explanation and Advantages
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum Shift Explanation and Advantages
Speaking about the shape of the datum feature simulator IN RELATION with the tertiary datum issue here is what has been changed on Y14.43 (the gage standard)
Y14.43-2003 (Dimensioning and Tolerancing Principles for Gages and Fixtures) –used in connection or used to support Y14.5-1994 standard
- Fig. 1, page 10 a diamond pin construction is shown as a tertiary datum feature simulator. The same diamond pin is shown on page 68 Fig. B10. (same 2003 standard)
Y14.43-2011 (Dimensioning and Tolerancing Principles for Gages and Fixtures) –used in connection or used to support Y14.5-2009 standard
- Does not have the diamond pin construction/option shown. Also Fig B-10 (again in 2011) does show a fixed cylindrical pin instead on the diamond pin shown on B10 (2003).
Why was changed (datum feature E simulator from a diamond pin to a fixed cylindrical pin since the part design intent/functionality of the workpiece shown hasn’t changed?
RE: Datum Shift Explanation and Advantages
Since the committees don't publish a document on what problems they were trying to solve or who voted for the specific descriptions, there's no way for me to even guess what they were thinking or suggest who to contact. In addition, the ASME generally has a no-opinion policy regarding their standards.
In the thread: http://www.eng-tips.com/viewthread.cfm?qid=215333 a statement is attributed to Meadows concerning "a long drawn out fight" that led to the (apparently not unanimous) adoption of the translation modifier. If true, then that sounds like a good way to build errors into what is essentially a mathematics book.
The usual fights in mathematics are over discovery credit and notation, not the truth that mathematics represents. Usually, before being widely published as a solution in mathematics, both the problem and potential solution are subjected to a lot of peer review and validations by mathematicians; not by technologists. Not that mathematicians are unwilling to stab one-another in the back, but they do it to control credit and prestige, not to silence critics of their results.
The use of the diamond pin shape was not to represent an idealization of the shape of the hole as a tertiary datum to function as an angular control. It was to alter a round pin to eliminate as much as possible the affect on the angular control that the distance between the holes created. That is, for small variations in distance, the contact using a diamond pin is nearly perpendicular to the surface of the hole. If a round pin is used the contact could be tangential.
In the OP example, if D were diamond shaped the datum simulator for it would be - that's right - a hole 7.3mm in diameter; the same dimension as the width of the slot I previously proposed. Since D is not diamond shaped, then the simulator has to be an obround slot.
RE: Datum Shift Explanation and Advantages
FWIW, I think you're on the right track with the obround simulator. I had to look at this same example in a fair amount of detail a couple of years ago, and came to a very similar conclusion. When I traced out the places where the datum feature could be, and applied the rotational degree of freedom that is open, I got a volume that is obround when viewed from the top. I went even further, and saw that it was like a thick-waisted hourglass shape when viewed from the "radial" direction. The simulator is cylindrical only because there is another section in Y14.5 that states that datum feature simulators shall be the inverse shape of the datum feature, unless otherwise specified.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Datum Shift Explanation and Advantages
Look at the attached picture and the description below:
http://files.engineering.com/getfile.aspx?folder=0...
The picture shows 4 extreme cases:
- In each case a man is standing exactly at the datum axis B and is looking at all possible locations (as defined in fig. 4-16) of datum feature D actual axis.
- In each case the yellow circle is dia 0.4 positional tolerance zone defined by |pos|dia 0.4(M)|A|B(M)|C(M)| callout. No matter what, the axis of the datum feature D at MMC must fall into that zone.
- Each reddish circle simulates dia 0.2 cylinder within which the axis of datum feature D at MMC will be assuming that this feature has been produced with maximum possible perpendicularity error to A.
- In each case two green solid lines - horizontal and vertical - are 2 out of 3 datum planes of |A|B(M)|C(M)| datum reference frame. These planes are always fixed.
- In each case two reddish dashed lines perpendicular to each other are 2 out of 3 datum planes of |A|B(M)| datum reference frame. One of these planes always passes through the center of the reddish circle, so that when the man is looking along that plane he can only see 0.2 actual perpendendicularity error.
Now, my question to you is: Does the allowable movement of the reddish circle relative to |A|B(M)| really constitute an abround shape or perhaps a circle?
RE: Datum Shift Explanation and Advantages
Obround, as a circle is not required to encompass all the potential pin positions in the |A|B(M)| frame of reference.
RE: Datum Shift Explanation and Advantages
First you check datum feature D against |perp|dia 0.2(M)|A| & |pos|dia 0.4(M)|A|B(M)|C(M)| callouts. As I tried to show it, these two requirements allow axis of D to be within dia 0.4 cylinder oriented and located relative to |A|B(M)|C(M)| datum reference frame (i.e perpendicularity callout is irrelevant when one tries to find maximum possible volume that encompasses axis of D in |A|B(M)|C(M)| DRF).
Then you get rid of C, and search for MMB of D relative to |A|B(M)| in order to properly establish |A|B(M)|D(M)|. What makes |A|B(M)| & |A|B(M)|C(M)| different? It is only the fact that one rotational degree of freedom is left unconstrained in |A|B(M)|. But the volume within which the axis of D can be does not change - it is still the same dia 0.4 cylinder located at basic 29 from datum axis B. It just that the dia 0.4 cylinder can freely orbit the datum axis B.
Evan,
What do you think?
RE: Datum Shift Explanation and Advantages
In the attached image it is plain the scheme I describe is a workable solution that takes into account the perpendicularity refinement, which the original explanation does not. E is not part of A|B(m)|D(m); it should not influence the outcome.
RE: Datum Shift Explanation and Advantages
RE: Datum Shift Explanation and Advantages
Is that diameter based on the position tolerance that includes E?
pmarc, in your diagram, the datum planes do not align between B and D, instead they maintain an alignment with a feature that is not part of A|B(m)|D(m). The observer is not following the datum sequence as is specified in the FCF.
RE: Datum Shift Explanation and Advantages
Are we still talking about fig. 4-16 case c? Where is E on that figure?
My diagram does not show B and D aligned, becasue this is not the point of this diagram. The diagram is to show where in space (in what volume) relative to A|B(M) datum reference frame datum feature D can be. This is needed to find size and shape of MMB of datum feature D relative to higher order precedence datums, that is A|B(M). After that MMB is found, we can construct datum reference frame A|B(M)|D(M)| that is aligned with datum feature simulators B and D.
RE: Datum Shift Explanation and Advantages
Here is what I got for the MMB of datum feature D relative to A|B(M) when I looked at this a couple of years ago. I believe this is like 3DDave's idea, taken a bit further. It's supposed to be the shape that would always fit over all the places that D is allowed to go. The observer can "center" their reference frame in the tangential direction, because the last rotational DOF is not constrained. So we only see positional error in the radial direction. In the tangential direction, we only see the perpendicularity error.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Datum Shift Explanation and Advantages
Anyway, B & D need to be aligned because D is the orientation control for case 'c'.
The diameter you keep looking for is only a diameter in [A|B(m)|C(m)] When C(m) is removed, then the DRF is allowed to turn. There is no angular limitation. One can freely turn the part so that the pin that D is based on can be anywhere in a 360 position on that part.
If a DRF is set to follow [A|B(m)|D(m)] then the refinement applied to D relative to A needs to be considered. In order to get the answer supplied in the standard, the refinement has to be ignored. It's a guess, but there aren't rules in the standard that specifically say to ignore refinements, meaning the example is wrong.
Again,
What would the diameter be of the datum simulator for A|D(m)?
Is that diameter based on the position tolerance that includes C?
RE: Datum Shift Explanation and Advantages
You are missing the fact that in A|B(M) datum reference frame the feature D can still be within a cylindrical volume.
Again, look at the part from the top. In A|B(M)|C(M) axes of D can be within a dia 0.4 cylinder that is perpendicular to datum A, located basic 29 from datum B and rotationally aligned to datum C.
In A|B(M) that volume does not change. What is changing is the fact that the datum planes of A|B(M) DRF are now free to rotate about B, but that has no influence on shape and size of that volume. If all axes are allowed to be inside dia 0.4 cylinder and nowwhere else, this cylinder will not magically change its shape and size in A|B(M). I guess, you can think about it that way, because A|B(M) is a subset of A|B(M)C(M).
... Sorry, but I am afraid, I am not able to explain it better.
RE: Datum Shift Explanation and Advantages
In it's own frame of reference the feature only changes size; relative to A, the apparent diameter increases due to perpendicularity allowance; relative to another fixed feature, it increases due to position; but when it is used to locate itself it is only constrained by the intersection of the sets of constraints applied to it.
This is exactly as pictured in the file attached by Evan Janeshewski.
When viewed from B, the maximum apparent width of the solid piece of material that D is formed from can only be seen to be derived from the diameter of D and the tilt of D allowed by the perpendicularity tolerance.
RE: Datum Shift Explanation and Advantages
This is a very interesting discussion. Perhaps one of the more subtle and deep issues I've come across with GD&T.
pmarc,
It is true that D can exist anywhere within a cylindrical volume relative to A|B(M)|C(M). But the effect that Dave and I are seeing is that D cannot exist everywhere in that volume at the same time. Without the rotational constraint to C, the observer at B cannot distinguish the different tangential directions that D can exist at - he/she can keep turning to look straight at D. So the tangential variation of D gets becomes goes away, and the MMB becomes the quasi-obround subset of the cylindrical volume.
I admit that this gets very metaphysical, but there are practical consequences. A simulator made with the cylindrical MMB allows more datum feature shift than it really should - even when the D feature has extreme tangential tilt within its tolerances, there will still be significant datum feature shift. The only time that the D feature will fully constrain clocking is when it is at or near the radial limits.
If the simulator were made with the quasi-obround shape, then there would be no datum feature shift when the D feature has extreme tangential tilt.
3DDave,
Does these descriptions seem correct?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Datum Shift Explanation and Advantages
The description does seem correct.
I did think of an alternative for this case: [A|D(m)|B(m)] It makes more sense if an orientation and location sensitive feature is being made, such as broaching a square hole into the pin. Does adding B(m) cause the simulator for D(m) to change size? I'm thinking the reverse, that the B(m) simulator is now a diamond pin, or a sliding pin aligned with D.
The version in the standard, besides having an incorrect explanation, makes no sense as a real application. Why not use A|B(m)|C(m) for the hole being drilled in D and give it a large position tolerance? It looks tagged on with only a glance at the explanation.
RE: Datum Shift Explanation and Advantages
December 2014
RE: Datum Shift Explanation and Advantages
Subject: Calculating the Correct Maximum Material Boundary (MMB)
Hello Jim,
I got a question, if you don't mind. It is about size/shape of maximum material boundary D in case (c) in fig. 4-16 from Y14.5-2009. I am in a middle of a discussion with a person saying that in this case the shape of MMB is not a cylinder of dia. 7.5, but an obround geometry of 7.3x7.5.
His argument is something like this:
"The suggested interpretation does not match the set of allowable places 'D' could get to. If one took all the possible parts that met the requirements applied to D and transformed them to no longer include C, then it is an obround. Picture the view as if standing at the center of B; would D ever appear to be more than its MMC and perpendicularity tolerance wide? Without a reference to C, there is nothing that fixes the direction of the view and so nothing that causes adding a location tolerance to the width."
As you are a member of Y14 committee and most likely are well aware about the logic hidden behind the case (c), could you help me with understanding this?
Thank you very much.
Pawel
Pawel,
The rules in the 2009 Y14.5 standard are very clear that every datum feature of size is simulated at its basic location (whether it is secondary or tertiary) and at its virtual condition. The question is really a gaging question. How would you represent B and D in this situation? The answer is that both would be represented at their basic location from each other and at their applicable virtual condition and by a gage hole or pin that is the same shape as the datum feature.
One could argue that it was not defined that way in the 1994 version of Y4.5, but it is certainly true in the 2009 revision. We had to change some of our gages in the Y14.43 standard (2011 revision) because of this rule change. It was also the reason that a datum feature translation symbol was added to the 2009 standard to allow the equivalent of datum feature D’s simulator to translate along a line toward and away from datum B. But without that translation symbol the gage pin and hole are stationary, basically located from each other and cylindrical in shape.
With the current rules, the 4-16 illustration is correct.
James Meadows
Chairman ASME Y14.43-2011 Dimensioning and Tolerancing Principles for Gages and Fixtures
RE: Datum Shift Explanation and Advantages
As described existing tolerance analysis software will produce incorrect results. This is not a case of a previous indistinct interpretation getting clarified, this is a subversion of the concept of virtual condition.
Thanks gauge committee. 50 years of precedent down the drain.
RE: Datum Shift Explanation and Advantages
"It was also the reason that a datum feature translation symbol was added to the 2009 standard to allow the equivalent of datum feature D’s simulator to translate along a line toward and away from datum B."
What rule requires that the diameter of the datum simulator will now only be 7.3 and it's travel limited to +/- .2 from the basic position if the translation modifier was used?
In light of this interpretation, what shape is the gage for Fig. 7-29 Bidirectional Positional Tolerancing, Polar Coordinate Method? That is, what is the shape of the maximum material boundary for the hole? Would this gage be a different shape if the hole was then used as a datum? If not, why not?