Datum interpretation
Datum interpretation
(OP)
Attached print is a fiber glass reinforced resin molded part, my question is:
1.The pivot hole is datum B, section B-B defined the datum, but I can't image how to set up the datum, it's a datum point, datum line or datum area?
2.There is a position callout on the pivot hole, can we use the pinot hole itself as a 2nd reference datum in the DRF?
Thanks in advance for the replies.
SaesonLee
1.The pivot hole is datum B, section B-B defined the datum, but I can't image how to set up the datum, it's a datum point, datum line or datum area?
2.There is a position callout on the pivot hole, can we use the pinot hole itself as a 2nd reference datum in the DRF?
Thanks in advance for the replies.
SaesonLee





RE: Datum interpretation
2. Shortly saying in my opinion this is illegal callout.
RE: Datum interpretation
2.I guess the designer intended to control the pivot hole axis parallel to datum A. What is the best way to call out this ?
SeasonLee
RE: Datum interpretation
If the desire is to control the axis of the hole tighter than a diameter of .020 parallel to datum A, then just make the position FCF a composite and place a tighter tolerance than .020 with respect to datum A in the lower frame. This should solve the problem.
You still have the problem of constraining the hole in the vertical axis. You can use coordinate tolerancing to locate the 10-24 holes from the hole in question, but not the other way around and still use position with a cylindrical tolerance zone. As it stands the hole is located .025 from datum A, perpendicular to datum C but it is nothing says it has to be centered in the protruding feature. Wherever it falls, the 10-24 holes will follow.
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum interpretation
Frank
RE: Datum interpretation
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum interpretation
|POS|Ø.020|A|
SeasonLee
RE: Datum interpretation
Frank
RE: Datum interpretation
Do you think its legal on the pivot hole position callout ? The pivot hole is assigned as datum B, can we use the pivot hole itself to define the position ?
SeasonLee
RE: Datum interpretation
I think that the datum targets are trying to show a line on each end rather than a point especially since a point does not assist in the set up in any manner but the full diameter certainly does assist. The method on the drawing was not quite right though.
Fig. 4-52, page 89 of the 2009 version of the standard reflects the attached sketch on a cylindrical feature while fig. 4-32 of the 94 standard has the same approach for a line but on a flat surface.
Dave D.
www.qmsi.ca
RE: Datum interpretation
I suspect you are exactly correct. I tend to assume they spent the time doing this for some reason, that is how I would choose to interpert it, personally. I am not saying it is shown correctly. How about B1 & B2?
Frank
RE: Datum interpretation
Right on. It should read B1 & B2. Copy and paste sometimes messes up unless you read it again. Unfortunately, I did not.
Dave D.
www.qmsi.ca
RE: Datum interpretation
Frank
RE: Datum interpretation
Dave D.
www.qmsi.ca
RE: Datum interpretation
No, you can't have two datum targets labeled the same (e.g. B1).
There is nothing illegal with the position fcf on the thru hole relating back to datum B. It may not be terribly useful if the entire bore were the datum feature, but it is legal (and yes, a buggar to set up for inspection ... but doable). It is somewhat more useful given the datum targets establishing the datum axis.
People are often confused by a datum callout attached to a fcf which seems to reference itself. It doesn't. The datum is something theoretical (lots of discussions on this one already) and the datum feature is something real. Controlling something real to something theoretical is what GD&T does. There are lots of times that this is practical and necessary.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Datum interpretation
Dave D.
www.qmsi.ca
RE: Datum interpretation
SeasonLee
RE: Datum interpretation
http://www.tec-ease.com/gdt-tips-view.php?q=191
Before anyone responds with "how can this be inspected?", A good friend and an excellent machinist made a video illustrating the set up and inspection of this part. The video is on our Premium Site. The part is the Output Shaft on our Inter-Mec Air Clutch/Brake. It is very inspectable.
RE: Datum interpretation
A good example.
SeasonLee
RE: Datum interpretation
Just a guess...
Cant determine for sure or what material it is but if this were a cast or molded part, then I have seen this type of attempted dimensioning to deal with draft. Just the nature of the part makes me suspect this.
Agreed that the datums are not ideally defined.
RE: Datum interpretation
|POS|Ø.020|A|B| >>> |POS|Ø.020|A|
If this is legal, may I ask the difference between them?
Thanks
RE: Datum interpretation
I'd like to know "if" datum B is two internal target points / lines, and there is some physical gage
pin touching those points/lines; how does one get a gage pin or CMM probe into the full length
of the hole to inspect its location? Datum selection requires "accessibility".
If Datum A is the only referenced datum, then how do you set this part up to inspect the pivot hole?
It would only be mounted on flat datum feature A. It should be perpendicular to datum center plane C.
It also must have a datum reference to the location from the inserts.
A position control requires basic dimensions
5.2 (1994)
....Basic dimensions establish the true position from
specified datum features and between interrelated
features....
4.2.2.1 Mutually Perpendicular Planes. The
planes of the datum reference frame are simulated
in a mutually perpendicular relationship to provide
direction as well as the origin for related dimensions
and measurements...
Not lecturing you SeasonLee... just supporting my position....
Why not use the insert pattern as datum B and remove the confusion from the target points and datum B being hung off of the hole callout?
It appears that the pivot hole will be the hinge to close the cover and relative to the inserts and fasten the cover.
RE: Datum interpretation
Thanks for the input, as we discussed earlier that we all accepted the datum B should be a datum target line, and your concern is absolutely right on the datum setup, we can simulate the theoretical datum by CMM but I know some of the inspector will use physical pin gage instead (for easy work) to establish the DRF.
The pivot hole acts as the door hinge, I think the designer will not consider other features as datum B. There is only one basic dimension to define the pivot hole location and it will not help you on the position tolerance calculation with the referenced datum B.
Thanks
SeasonLee
RE: Datum interpretation
RE: Datum interpretation
Indeed, the datum target on the print is point, but there is only one point on both sides, how can they establish the datum axis, as what I know it should be at least three points on both sides of the pivot hole, just like Fig. 4-50 on 2009 standard, the three datum target points are equally spaced. Please let me know if I missed anything.
Thanks
SeasonLee
RE: Datum interpretation
Datum B is referenced secondary so only two points of contact are required. This certainly ignores the axis and form of the pivot hole. It may be a bad way to set up the DRF for this part but I don't think it's wrong per the standard.
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum interpretation
A DRF is a set of three mutually perpendicular theoretical datum planes, so we can use the 3-2-1 Rule to say only two contact points required to setup the secondary datum plane, but this secondary datum (pivot hole) is not a plane and it is not perpendicular to the primary datum plane, how can we use only two points to establish the secondary datum axis? I really want to learn how to build the datum axis.
Thanks
SeasonLee
RE: Datum interpretation
WOW... this thing just doesn't let you sleep or are you located in Asia?
I am not getting the goal here. The part is obviously not fully defined from what is shown
on the print I see. Could this possibly be a purchased part that is being modified?
I ask because it appears there are some modifications, possibly mount points for a PC board.
It really might be helpful to understand the objective. For instance which features are present
before the modifications; that is if this is a modified / altered part. Point being, is the pivot hole there already and only needs to be identified as a datum for reference; or does it need a positional control to locate it because it is a feature being created?
GDT ... says no problem for dropping datum B. It is a problem as I see it if the pivot hole is only
referenced by datum A. It has a positional tolerance control! Only 2 degrees of freedom restricted in one of the axis plus/minus direction.
Part can still rotate around that axis, while the other degrees of freedom are not addressed by defining
a complete DRF. This part needs a complete DRF IMO. There are not enough basic dimensions locating
the target points for datum B either as has been pointed out.
I believe you mentioned earlier that the designer wouldn't change the DRF. This drawing is obviously confusing. It appears the pivot hole is positioned to itself. Some have said that's okay, citing the example of a single axis from 2 coaxial cylindrical features for a runout control. I would take the position that the drawing you have doesn't meet that criteria. Different animal all together, having only one feature involved... the pivot hole.
Any way, if you can't appeal to the designer with the common sense, to define a DRF that makes sense then it appears to defeat the entire concept and purpose of ASME Y14.5 of defining the part functionality, clearly showing the relationships, and having one interpretation. (I know I'll get comments on that one (:
You might just try using three points 120 degree's apart; .060 in from the outside surface; however even that is bogus because those basic dimensions are supposed to originate from the DRF origin and that does not appear to be the case at all. They should come from the center plane of datum C.
Even so, that doesn't clear up all that is lacking as the drawing currently stands.
If it were me, I would propose a DRF and dimensioning scheme that would work for the objective and then try to reason with the powers to be.
Their dog just ain't goina' hunt in this case.
RE: Datum interpretation
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum interpretation
Thanks for your inputs, I am staying in California, the Pacific Time Zone is three hours late from East coast, just try to send the post before sleeping, this thing really make me not sleep well.
Sorry for I can't provide any functional information regarding this part since we are just an OEM, but glad to know your view point.
Thanks
SeasonLee
RE: Datum interpretation
Think of a shaft where we lock onto the OD as primary, and the bump the end of the shaft over to a flat plate for (potentially) one point of contact only.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Datum interpretation
It appears to me that the drawing is attempting to establish an axis as a datum with target points. If that conclusion is true, then I believe that it would require 3 points at each of location to accomplish this.
What do you say in regard to establishing an axis with target points? Wouldnt you need at least 3 equally spaced points in each location?
dtm
RE: Datum interpretation
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum interpretation
But now that I've roped myself into the discussion, I would simply say that datum B is a line created by the 2 target points B1 and B2. Although referencing B doesn't seem to add any value to the position tolerance, it's not wrong to have it up there.
Dtmbiz ... as it is now I would say that the 2 points simply form a line at the top edge of the hole, not an axis. If the goal was to establish an axis (which might make more sense?) then I would agree that another point is need to encircle the hole.
It's been said that the datum targets don't have enough basic dims to locate them -- I think the .025 in the right view serves to locate the targets, in addition to the TP of the hole's axis. So IMO that is OK.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Datum interpretation
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum interpretation
I was using the word line to get away from the notion of B being an axis, but I should have said that B is a plane.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Datum interpretation
Thanks
SeasonLee
RE: Datum interpretation
SeasonLee,
If this is a simple hinge hole for a lid and there is nothing really critical about how they interface, then it seems that getting rid of section B-B, and dropping the reference to datum B in the FCF but leaving it specified as datum B, would make this a much simpler part. This has been the direction that most of those here have already recommended and I'm certainly on board with it as well.
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum interpretation
The letters identify the datum features on the part. When you try to label the datum planes of the datum reference frame using these letters, once you get beyond a simple block, things fall apart. Think of a hole as the primary datum feature, A, on some other part. The axis of the hole establishes two datum planes that are perpendicular to each other and intersect at the axis of the hole. So, which one is datum A? There is no datum A, only datum feature A. The planes are the first and second planes of the datum reference frame. Sorry for the lecture. Here is a Tip I did on the subject. http://www.tec-ease.com/gdt-tips-view.php?q=257 and the video of this Tip: http://www.tec-ease.com/video/Aug_11.wmv
RE: Datum interpretation
One cent: when mentioning the axis of datum feature A, it is true that there are 2 planes formed from there. But I wouldn't say there is no datum A. Datum A is the axis derived from the datum feature simulator.
This comes from paragraph 1.3.13, which says that a datum is a point, axis, line, plane, etc. (I know what you're saying, in that the datum reference frame always consists of planes, but we don't want people to be left with the impression that the term "datum A" can never be used in conjunction with a theoretical axis.)
Second cent: Wouldn't it be better to say that datums exist only in a theoretical setting, and not "the datums exist in our processing equipment"? I think the standard would refer to the planes, axes, etc. on the processing equipment as "simulated datums."
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Datum interpretation
If the second plane passes through the two datum points (not the axis of the pivot hole), then I am imaging where is the datum point 0,0,0.
Thanks
SeasonLee
RE: Datum interpretation
Is datum axis A the "axis derived from the datum feature simulator" as John-Paul said or is that the simulated datum axis? Some of these arguments, to the layman, are a distinction without a difference. In the 1966 Standard we showed datum planes as existing in a milling machine and a lathe. The standard talked about using datum features to immobilize the part and align it to the datum reference frame found in this processing equipment. The figures were removed because they were too practical. I think they are great figures to explain what we are trying to do with datum features. Getting hung up on some of the theoretical terminology doesn't help folks understand the important concepts, in my opinion. Of course, if you want to get certified, you better know them.
RE: Datum interpretation
Sorry for late to get the mating part print, please take a look on the Cover print, it will help us to have a better understanding on its function and the designer intent, my questions:
1. As we discussed earlier the Case pivot hole position callout is
|POS|Ø.020|A|B| where datum B is the pivot hole itself (and assigned as secondary datum feature).
The Cover pivot hole position callout is
|POS|Ø.024|A|C| where datum C is the width between holes (and assigned as tertiary datum feature)
My first question is why not to use the pivot hole (datum B) itself on the position FCF?
2. The secondary datum feature is defined by|POS|Ø.024|A|C|, can we use tertiary datum C to define the secondary datum feature B? I think we can't, but I want to be sure.
3. Datum point B1 is located on a counterbore, B2 is on the through hole, so they are on a different feature, can we use these two datum points to establish second plane as we discussed on the Case earlier, I think we can't. I will think B1 / B2 should be datum target lines (a circular line) here.
4. For the Case, I will image the datum point o,o,o at the center point of the pivot hole as shown on the attached print, from the design viewpoint, the datum point for the Cover should be at the corresponding point, is it correct?
5. Again, still need to ask: can we simply plug a best fit gage pin to simulate the datum B?
Thanks again for the inputs
SeasonLee
RE: Datum interpretation
Thanks
RE: Datum interpretation
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum interpretation
Thanks
SeasonLee