Setting up Datums, and controlling the orientation of a datum simulator or measurement
Setting up Datums, and controlling the orientation of a datum simulator or measurement
(OP)
Hi all,
I have a fairly simple problem I think, but I am new to this and I think I am missing something basic.
I have a simple tube that I am trying to control the length of. Its ends are cut to a certain perpendicularity to its own axis (I have a good set of datums that allow me to define a datum axis along the length of the part). They are not necessarily cut parallel to each other.
How do I set up a feature control frame to measure length along the length of the axis? If I put a datum on one end and set up a position tolerance on the other, relative to that datum, then the datum simulator will end up taking up the angle of the lack of perpendicularity of the end it is on, and therefore the tolerance zone will do the same thing on the other side. That throws out the measurement.
Is there a way to reference the datum axis in order to 'measure along it'?
I definitely feel like I am missing something basic in the theory of setting up Datums for a part. I think I will be taking Y14.5 home with me over the weekend.
I have a fairly simple problem I think, but I am new to this and I think I am missing something basic.
I have a simple tube that I am trying to control the length of. Its ends are cut to a certain perpendicularity to its own axis (I have a good set of datums that allow me to define a datum axis along the length of the part). They are not necessarily cut parallel to each other.
How do I set up a feature control frame to measure length along the length of the axis? If I put a datum on one end and set up a position tolerance on the other, relative to that datum, then the datum simulator will end up taking up the angle of the lack of perpendicularity of the end it is on, and therefore the tolerance zone will do the same thing on the other side. That throws out the measurement.
Is there a way to reference the datum axis in order to 'measure along it'?
I definitely feel like I am missing something basic in the theory of setting up Datums for a part. I think I will be taking Y14.5 home with me over the weekend.





RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
What I want is roughly seen in the attached. All of those green arrows must be within the tolerance specified, with all measurements taken along the blue axis.
What is the graceful way to achieve that?
I've been reading for the last hour and it's making me doubt everything I thought I knew about dimensioning. This can't be that hard!
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
For a feature of size, you have perfect form at the maximum material boundary per Rule #1. So, in this case at MMB the cuts in the pipe are perfectly perpendicular and flat. As you deviate from the MMB, you can have error in perpendicularity, flatness, etc. No point to point measurement can violate the least material boundary for the dimension.
Hopefully someone will correct me if I'm wrong.
Please note the correction below.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
Another way might be:
Datum B could be assigned to one end of the tube with a perpendicular orientation control relatived to datum A.
A basic dimension from datum B to the opposite end.
A Profile geometric control could then be assigned to the opposite end of datum B, using datum A & B as a DRF (Datum Reference Frame)
Profile controls: form, orientation, size and location.
Something to consider....
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
To achieve a part which is, in fact, perfectly square at the MMB, you can do the following (see attached/below):
So, in this case at MMB the cuts in the pipe are perfectly perpendicular and flat. As you deviate from the MMB, you can have error in perpendicularity, flatness, etc. No point to point measurement can violate the least material boundary for the dimension.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
Don't quite believe you have captured the OP's concern with this latest post.
The latest post shows that you are holding the "center plane" with a perpendicular control to Datum A;
which will not control the end surfaces beyond the size limits. You have identified a feature of size
and not the end surfaces.
Your first post was fine. You could use a parallel control to relate both ends to each other, however
you would need to identify one or the other end as a Datum to reference.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
This can be achieved be a text note similar in meaning to the note given in para. 4.4 of Y14.5M-1994 (it would just involve single datum axis for that specific length dimension).
Much more "elegant" way would be to follow dtmbiz's suggestion (with a profile callout referencing to A|B).
But there is one thing to remember - these two options will not result in the same size of maximum material boundary (envelope) perpendicular to datum axis A, containing the pipe produced at maximum possible length and at maximum possible orientation error to datum axis A.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
Another thing worth considering is whether the datum could be referenced at MMB. If so, the part could be measured with a height gage after sliding it into a vertical tube-shaped datum simulator (getting back to the original question about measurement).
Anyhow, here is an illustration of the control that I specified above:
I suppose that I'd need some clarification if that's not what the OP is looking for.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
I have read your responses and understood what I could - I suppose the issue I am having is that the perpendicularity tolerance I am using is quite large compared to the position tolerance (roughly half of the position tolerance is available for perpendicularity, on each side), This is compounded by the fact that the pipe is quite long compared to its width, so if an off square end is brought up against a flat datum simulator, then the normal distance to the end from the datum simulator becomes very different than the distance along the pipes axis.
I have thought about this, and I can control it if I can specify that the datum simulator be perpendicular to the parts axis. This is all possible with the physical part but the question is, how can I actually specify it with GD&T? I have a theory that I haven't used before as I am new to this, please see below:
I have previously seen, and not understood, tolerances given with respect to Datums that they cannot be measured from, for example, a hole position referenced from a datum plane perpendicular to its axis. Having just done some reading of Y14.5, it appears that what these datums serve to do, is to require the latter referenced datums to be set up in a particular way to create 3 orthogonal planes for the reference frame.
Therefore, here is what I want to do:
1) Datum axis established along the pipe based on bearing finishes on the pipe. This is what we want to measure along.
2) Datum on the face of one side of the pipe
3) Other side gets a positional tolerance with the primary datum being the axis of the pipe, and the secondary datum being the other face of the pipe.
4) Both sides get there own perpendicularity tolerances relative to the axis.
In theory this means the datum feature simulator for the secondary datum needs to be perpendicular to the axis of the pipe (first datum), and then brought in to touch the edge of the pipe. Does this all sound like it should work?
Thanks.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
If you're only concerned about size relative to the axis of the cylinder and not the end condition, I think that you might consider using the zero perpendicularity error on the centerplane of the end cuts at max material condition. That way, your pipe will never be longer than the maximum size dimension or shorter than the minimum size dimension along its axis.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
You can base the profile of each end wrt to the axis and a datum plane touching the opposite end; there will be a basic dimension between them.
There isn't a control in 'Y14.5 for the orientation of a two point measurement, though you can create your own note to describe the measurement method you want.
The use of a perpendicular face in a typical position tolerance is because the primary controlling factor for position is the orientation of the feature; then the location relative to some other feature. Position is also not applicable to planar surfaces.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
3DDave, I agree that it seems a profile tolerance on one end with primary reference being the axis and secondary being the datum on the other end, should do the job. This scenario is almost exactly out of figure 8-13 of Y14.5. It is what I had attempted with position tolerance so all I need to do is change to a profile symbol.
Thank you very much everyone, I think with your combined responses, it has been resolved, and more importantly I have learned a hell of a lot. I have included a drawing of the part with all the unrelated stuff removed, please let me know if you see any issues.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
A couple of notes:
* Items in red updated in accordance with comments below
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
First, datum [C] implies that functionally left end of the part is somehow more important than right side.
Second, when you give real part to quality control guy, how will he know which side is datum [C]?
To me specifying Profile of a surface 1.000 wrt A-B and Total runout .300 wrt A-B on both sides should do the trick.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
I guess you might need Datum C to measure the length from; the two ends are independent features and get independent profile tolerances. I think it's needed.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
Basic dimensions need to be related (traced back) to Datum(s) (DRF)
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
As an alternative to additional runout controls, two perpendicularity callouts wrt A-B can be used.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
I guess that the figure you provided is illustrating the use of "all around" profile. Since they're treating the entire shape as a single profile with an all-around tolerance zone, only a single datum reference is required to establish perpendicularity. I don't think that's appropriate in the case of the pipe.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
Nice... good example to get one's attention.
My position would be that the shape example that you posted is just that, a shape that is related to itself via basic dimensions and back to Datum A, a planer surface feature. That shape would have a 90 degree implied basic angle to datum feature A (a planar feature, presuming there is another view to confirm that).
Not so without Datum C. Yes Datum A-B is an axis with the implied 90 degree basic to the end surfaces. However, in practicality the axis is a line and IMO not as reliable to serve solely as a stable or clear DRF without Datum C. Datum plane A could easily be qualified with a flatness control and stable for setup.
Datum A-B doesnt clearly show the relationship to the end surfaces regarding a physical DRF setup. Like for instance the end surfaces would have 2 parallel plane tolerance zones for each end.
Where would the DRF setup locate those tolerance zones? At the end point of the Datum A-B axis? A point location?
What is the physical stop to measure from? More than likely the datum simulators to establish Datum A-B would be hindering and/or preventing access to those points.
Where is the measurment taken from, a physical feature? Shouldnt it be taken from a datum simulator?
For simplicity and clearity I would have Datum C ( I do believe my Datum C would have been at the bottom of the C-bore; likely the mating surface).
Figures in the standard have a disclaimer not to be considered complete. Personally I believe another datum(s) for clarity as to where to position the shape in a physical DRF setup would be good to add.
Theoretically you have a point to be considered.
IMO, in practicality it would be much more clear and practicle to add Datum C. Avoids confusion.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
With Profile tolerance zone you know where the body of the part can and cannot be. That is usually enough to devise a gauge - there is no need for "physical stop to measure from"
And in practical world it would be more clear NOT to specify Profile - just toleranced length of the pipe combined with Total Runout requirement for the ends.
Also I don't believe the figures in the Standard are "incomplete" to the extent of concealing the truth.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
I do agree that in the practical world it would be clearer not to specify profile. In fact in reviewing the part we tightened up the perpendicularity tolerance (the ends will have to be milled or machined for the bore, depending on the process the suppliers chose, so cleaning up the ends for perpendicularity is not going to be an issue), which has thus reduced the importance of ensuring you measure along the axis of the pipe. As such that length tolerance has been replaced with a directly toleranced length of pipe.
This was in combination with speaking to some potential suppliers here about the clarity of the symbology (we have to be careful about scaring off shops, as we have an exceptionally weak manufacturing industry here so not all shops are used to GD&T, not that I'm in a particularly strong position myself), and that one tolerance was the one that was flagged as "not sure why that is there". I explained the reasoning and meaning, but for every one that raises it with me there will be another that didn't understand it but doesn't want to ask.
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
This is exactly what I mean by "practical" - when you specify nice finish on the ends of pipe, possible ambiguity in defining "length" becomes irrelevant.
But just like Nereth1 I am curious to see more opinions
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
- From the purposes ASME Y14.5 came into being over the years regarding dimensioning and tolerancing for commonality, practice, understanding, cost savings concepts, etc; for me has proven to be defined by who is wielding the checker pen and how much will it cost the company.
A man convinced against his will is a man that remains unconvinced....
Somebody famous said that
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
RE: Setting up Datums, and controlling the orientation of a datum simulator or measurement
As such, the actual justification was driven by "why risk it when it's so easy to get right" more than a serious investigation on how far we can push the tolerance. Our presses touch that surface during assembly and we don't particularly want to find out what happens if it isn't quite square. There's an arrangement of seals pressed in there as well and it seizes up quite easily when the wrong combinations of misalignment and mispositioning takes place.
Maybe not the perfect justification for tightening a budget, but a finite run of parts means finite resources to optimize the design.