What is a "datum line"? 6

[Burunduk]

Hello.
I have a theoretical question about datums. According to ASME Y14.5-2009 a datum is:

"a theoretically exact point, axis, line, plane, or combination thereof derived from the theoretical datum feature simulator."

I have encountered all of the types of datums listed, except "line". Didn't find an example in the standard either. Perhaps someone on this forum can help - what is a datum line? What is the corresponding datum feature and datum feature simulator from which it is derived?

Thank you

 

[chez311]

My initial answer was because datum target lines were the first thing that came to mind when the question was asked about "datum lines" as it is a theoretically perfect line which is derived from the datum feature simulator. I realize that there are other aspects of datum targets that might not allow them to fit neatly into the definition in Y14.5-2009 1.3.13 (target areas, or "other geometry" allowed by fig. 4-49 in 2009 and later included in the definition in Y14.5-2018 6.3.3.1) - still theoretically exact but not necessarily a point, axis, line, plane or combination thereof. Also per recent discussion I have had some of my notions that perhaps much of this analysis of theoretical geometry/datums is quite arbitrary cemented in my mind. For these reasons I maybe shouldn't have opened my mouth instead of trying to be clever.

Looking at figure 4-47 – the targets B1 and B2 define the limited contact with the datum feature’s surface (a cylinder) by lines, but the (perfect) datum is the axis at the center of the boss (as determined by partial contact with the surface at the targets).

You are saying that the perfect datum is the axis at the center of the boss because that is where the DRF/coordinate system is shown in Y14.5-2009 4-47? This could be located/oriented anywhere one desires with basic dimensions per 4-28, its really quite arbitrary. Whats most important is the relationship between the part and the datum feature simulators, and accompanying constraint of DOF - not whether the DRF is coincident with a theoretically perfect axis or not. To pull from another thread (

https://www.eng-tips.com/viewthread.cfm?qid=449676)

as quoted by Evan "In other words, the placement of the "datum planes" and DRF origin are completely arbitrary. The only thing that really matters is the mutual relationship between the datum feature simulators."

 

[Burunduk]

greenimi,
I can offer some help with fig. 4-54. Is there anything particular that you need clarified?

 

[chez311]

I admitted before on this forum, I do not understand fig 4-54. For my life I do not know the fine details of this figure. I see that a datum reference symbol (axes X, Y and Z) are shown and in fig 4-53 are not.

Inclusion or removal as well as the placement of the axes is entirely arbitrary per my previous post, and do not provide any added definition - except maybe to help provide a more "clean" and readable reference for dimensions.

I think its best to consider the datum feature simulators for Y14.5-2009 fig 4-54. Overall, they would not look that dissimilar to 4-48 except A1,A2, and A3 would be points most likely defined by contact with a rounded pin instead of areas of flat pins, and would be at different heights because of the contorted geometry of 4-54. B1 and B2 are target lines defined by contact of the part on the edge shown with the sides of 2x round pins. C1 would be another target line defined by contact with the side of a pin on the edge shown.

 

[Burunduk]

chez311,
I think that your contribution to the discussion by bringing up datum target lines is very important, even though my personal opinion is that it is probably not what the committee members meant by listing "line" in the definition of the datum concept.

By now I think that a datum line can only be established as a component of a combined datum ("combination thereof" per 1.3.13), but never as a stand-alone single datum derived from any kind of datum feature/simulator.

 

[Burunduk]

mkcski,
I was incorrect in my post from 7 May 19 03:24 saying that simulating a datum axis derived from a cylindrical feature by datum target simulators requires contact at opposed points. But I still think that datum B defined by datum targets B1 and B2 in fig. 4-47 is not a datum axis. For simulation of a secondary datum axis from a cylindrical datum feature defined with datum targets, the required scheme is described in para. 4.24.12 and fig. 4-53.

From para. 4.24.12:
"At RMB, a typical centering method used to establish the datum axis has a set of three equally spaced contacting datum feature simulators capable of moving radially at an equal rate from a common axis that is perpendicular to the primary datum plane."

This isn't in line with datum targets B1, and B2 from fig. 4-47.

 

[greenimi]

I can offer some help with fig. 4-54. Is there anything particular that you need clarified?

Thank you.

"Where a datum reference frame has been properly established but its planes are unclear,"

Not sure I understand this concept correctly. DRF are 3 mutual perpendicular planes. In what cases they could be considered unclear and where the origin of measurements (0, 0, 0,) is in fig 4-54?

What is the meaning of this note : " DATUM FEATURES B AND C ARE INVOKED WHERE ONLY DATUM FEATURE A IS REFERENCED TO RELATE THE TARGETS THAT ESTABLISH DATUM A"

Never seen applied on any drawings hence my dilemma. How datum feature B and C are to be depicted in fig 4-54? Or maybe 4.24.14 (where this note is coped) is not applicable to figure 4-54?

 

[chez311]

greenimi - if I can chime in:

"Where a datum reference frame has been properly established but its planes are unclear,"

Not sure I understand this concept correctly. DRF are 3 mutual perpendicular planes. In what cases they could be considered unclear and where the origin of measurements (0, 0, 0,) is in fig 4-54?

The rest of the quote you referenced adds some clarity "the datum reference frame coordinate axes may be labeled to appropriate extension or center lines as needed" ie: they can be placed anywhere you like. As I said before, the placement/orientation of the origin and axes is entirely arbitrary. With nice, neat parts with flat, perpendicular surfaces or holes/bosses utilized as datum features the "default" location of said origin can be easily envisioned. When the parts are not so "nice", as in Y14.5-2009 4-28 and 4-54 (or really in any case with any shape part or DRF - as I said its arbitrary) an obvious "default" may not be so easily envisioned and your origin/axes can be located/oriented however you please with basic dimensions. This is entirely separate from how the part/features interact with the datum features simulators and there is no actual need or requirement to place said origin/axes. One can leave them "unclear" it does not affect this relationship*.

What is the meaning of this note : " DATUM FEATURES B AND C ARE INVOKED WHERE ONLY DATUM FEATURE A IS REFERENCED TO RELATE THE TARGETS THAT ESTABLISH DATUM A"

Never seen applied on any drawings hence my dilemma. How datum feature B and C are to be depicted in fig 4-54? Or maybe 4.24.14 (where this note is coped) is not applicable to figure 4-54?

I do not believe this note applies to 4-54. See the relative section in Y14.5-2018 7.24.10, the figure 9-16 was added for this purpose.

*As I said the only advantage is possibly a cleaner visual reference for basic dimensions, or if one must call out a specific direction for translational/rotational DOF such as in a customized DRF.

 

[Burunduk]

greenimi,
Regarding your question about fig. 4-54. I think chez311 already covered it quite well. I don't know if this will add anything to what was said by him above but: imagine the same part not being twisted but a simple flat rectangular plate. With the same datum targets, there would be no problem at all to establish a 3 perpendicular planes DRF. Plane A tangent to the surface at the back of the part, plane B tangent to the thin face at the top and plane C tangent to the side face. You wouldn't need the axes of the DRF labeled. With the twisted shape of the part, the planes are not clear. That's why an origin location was randomly* chosen and the axes labeled.

Edit: * - not completely randomly as it was a good practice to set it coincident with one of the datum targets - C1 in this case.

Regarding para. 4.24.14 I have no clue what this entire paragraph means, not just the note you quoted.
chez311, can I ask you to post the example figure that was added at the 2018 standard?

 

[chez311]

With the same datum targets, there would be no problem at all to establish a 3 perpendicular planes DRF. Plane A tangent to the surface at the back of the part, plane B tangent to the thin face at the top and plane C tangent to the side face. You wouldn't need the axes of the DRF labeled.

Just to continue to beat a dead horse, the only time one "needs" to define and label the axes is if a particular DOF is being referenced, say in a customized DRF. Other than that, the location/orientation of your 3 mutually perpendicular planes is entirely arbitrary and can be anything one could imagine (to borrow from pmarc on another thread - it could be anywhere even on the moon and still be valid) - even for a "nice" well-behaved flat rectangular plate as in your first example where most people would have "no problem at all" agreeing upon a location and orientation which might seem to be the default. 3 perpendicular planes and an origin can always be established, it is the constraint of the part relative to the datum feature simulators which is most critical.

Regarding para. 4.24.14 I have no clue what this entire paragraph means, not just the note you quoted.
chez311, can I ask you to post the example figure that was added at the 2018 standard?

Below is the figure. The text in the accompanying referenced section 7.24.10 in Y14.5-2018 remains unchanged, with the exception of the reference to the below figure.

Essentially because datum target A is the only reference in the FCF for perpendicularity, the target areas which make up A must be located/oriented relative to each other as well as to the surface which they need to contact and thus B and C must be invoked to do so - however the actual tolerance zone does not need to be oriented (or located - if the control was something other than an orientation control) relative to B or C. Unlike datum targets where they must make contact in a specified location, if A were a standard planar datum feature this would not be necessary - ie: the datum feature simulator would be a flat surface plate and it could be placed anywhere on that flat plate and make the same amount of contact, this is not so with 3x datum target areas.

 

[Burunduk]

chez311,
For simple parts with planar or cylindrical datum features the standard defines clearly how to decide the location and orientation of the DRF axis system, so if it is not labeled, it still should be established the same way by all drawing users (and makers). Perhaps for measurement purposes, a DRF axis system different from the default can be chosen as long as an exact location and orientation are maintained between the DRF axis system and the datums derived from datum features. If from some reason, for parts with simple datum features, the drawing maker decides to establish a DRF axis system in a non-default location and/or non-default orientation and communicate this to the drawing users, then he should show the X, Y, Z axes and locate and orient them by basic dimensions from the datum features (But I don't see what benefit could be gained by that). What sets parts such as the one in fig. 4-54 apart, and that's the point which I believe is dealing with greenimi's question "In what cases they could be considered unclear...?" Is that for non-planar and non-cylindrical datum features as in this figure the standard doesn't provide any instructions on how the datum reference frame should be established, except for showing the option of labeling the axes, locating and orienting them basically do the datum features and showing them on the drawing. In other words, there is no "default", and if the labeling and dimensioning are not done, every drawing user is free to decide his own DRF, which is not a bad thing either, but again different from simple parts.

Thank very much for providing the example figure from the 2018 standard. From some reason, I simply couldn't wrap my head around the definition from para. 4.24.14 (2009 standard) prior to seeing that example. Thank you for your explanation too, now it's perfectly clear

 

[greenimi]

Thank you guys for helping me to understand the details of these figures (from 2009) and some new ones from 2018.

Like someone said here on the forum recently :"....the more I do this work, the less it seems I actually know"

Or in other words : The more I learn, the more I learn IS to learn.

 

[chez311]

In other words, there is no "default", and if the labeling and dimensioning are not done, every drawing user is free to decide his own DRF, which is not a bad thing either, but again different from simple parts.

Just because more complex parts don't fit into the nice, neat box that Y14.5 defines for "well-behaved" simple parts doesn't mean the rules which govern them are any different. Its amazing how quickly things can become unclear - take an asymmetrical bolt pattern defined as a datum feature which does not have a clearly defined center. Where is the origin - is it at the center of one of the bolt holes, the locus of all the centers, or somewhere else? The only conclusion which makes sense to me is that it does not matter, the same logic applies even to the simplest of parts.

I won't belabor the point further, as I think theres not necessarily much harm in taking the standard at face value on a topic which it invests quite a bit of time in fleshing out (datums and datum reference frames) - however it has been helpful for me to take this with a healthy dose of skepticism to understand that it is not nearly as important as it might seem.

I'll leave you with a quote from pylfrm on a recent thread, which may be worth a read if the topic interests you. Hopefully pylfrm does not mind the quote being utilized here as I believe it applies. The thread of interest is (

https://www.eng-tips.com/viewthread.cfm?qid=451664).

https://www.eng-tips.com/viewthread.cfm?qid=451664[/URL]]I think the whole concept of datums and datum reference frames as defined by ASME Y14.5-2009 is an unnecessary distraction, so I generally ignore it. All that really matters is the relationship between the actual part and the theoretical geometry, and the constraint of that relationship by contact between datum features and datum feature simulators.

 

[chez311]

greenimi/Burunduk,

Glad I could assist in improving some understanding of these topics. 9-16 was definitely a worthy addition to the 2018 standard as without context the associated section and note are difficult to comprehend.

 

[Burunduk]

The only conclusion which makes sense to me is that it does not matter, the same logic applies even to the simplest of parts.

That's an interesting point of view.
However, I think that the question of location and orientation of the datum reference frame coordinate system should be approached slightly differently. The question that should be asked is: What is the datum reference frame for? The common answers are:
- The origin of dimensions (from design aspect).
- Reference for measurements (from inspection aspect).
If so, the location and orientation of the datum reference frame should be as adequate as possible to serve those purposes. That's why for example, for a part with 3 perpendicular datum features, the standard guides us to establish the origin at the intersection of the corresponding datum planes, and the same planes are the 3 planes constructing the DRF. You are going to measure your considered features to the plane simulators that represent those datums anyway. Why set the coordinate system anywhere else? The same logic applies where there is a datum axis:
"A primary cylindrical datum feature is always associated with two theoretical planes intersecting at right angles on the datum axis. Depending on the number of planes established by higher precedence datums, secondary and tertiary datum axes may establish zero, one, or two theoretical planes." (Para. 4.10.3).

It is only for parts with complex datum features and cases like your asymmetrical bolt pattern, or for cases such as fig. 4-47, 4-48 (more than one datum plane for the same stepped datum feature) that the arbitrary datum reference frame approach should take preference over the "default" DRF establishment. That is simply because for simple parts the "default" exists (paragraphs such as 4.10.1, 4.10.2, 4.10.3), and more importantly - serves the purpose better. I realize that some promote the approach that for any part, even with the simplest datum features as in figures 4-2, or 4-8 the DRF can even be set on the moon - as you mentioned. But that would be a lot like getting from the USA to Canada by flying to the moon and back

 

[Burunduk]

I hope it is not too late to ask - does anybody else share my conclusion or can confirm that the "line" mentioned in the definition of the datum concept is the "line in plane" as presented in this figure?

 

[Yuyu28]

Sorry for reviving this old (but interesting thread). I wonder if the use of datum target lines as in Fig. 9-16 applies to an example like the following:

To position the holes horizontally, I need some datum plane perpendicular to the plane of the view. I see two alternatives:

a) using one of the other planar surfaces with the basic angle as shown. However, according to Y14.5 2004 (I don't have the latest versions...), the datum plane C would be in the intersection of datum feature C with datum feature B, which does not seem very practical for measuring purposes. Also, the basic dimension is not a nice round number (99.03).

b) using the right edge of the part as a datum target C just for measuring purposes (not referenced in the tolerance frame). IT gives a round basic dimension and facilitates measuring.

If option b) is even possible, which one would be better?

 

[chez311]

Yuyu28,

Since you are not really continuing the discussion above and instead utilizing this thread as reference, I would recommend starting your own thread while utilizing a link to this discussion.

PS - there is no Y14.5-2004, do you perhaps mean Y14.5-1994 reaffirmed in 2004?

 

[chez311]

Well since I've already got this response lined up and have not seen posted a follow up thread, and assuming you mean Y14.5-1994 see the below:

To position the holes horizontally, I need some datum plane perpendicular to the plane of the view

All you really need is a datum feature which constrains translation in the direction of interest. In this case the angled planar face with the datum feature |C| symbol applied would accomplish this - it is not necessary to specifically notate where the creation of the theoretical datum plane actually occurs - this is a result of the geometry and in reality is quite arbitrary. See Y14.5-1994 Fig 4-4. Your measurement equipment and gauging cannot touch theoretical features - it would contact and measure the planar face of datum feature C and that would be perfectly acceptable. That is, if it actually makes sense to utilize this face as a datum feature (actually involved in assembly/function of the part).

Also, the basic dimension is not a nice round number (99.03)

This does not matter.

a) using one of the other planar surfaces with the basic angle as shown
b) using the right edge of the part as a datum target C just for measuring purposes (not referenced in the tolerance frame)

The choice of datum features should mimic function. It is only a rare case where one would be chosen otherwise - yours does not seem to be that case. The only case "for measurement purposes" I can see is something similar to Y14.5-2018 fig 9-16 which I have referenced in this thread, and yours I do not think falls under that category either (the additional datum targets are included to establish a repeatable datum A when only A is referenced but do not actually provide any additional constraint to the tolerance zone).

 

[Burunduk]

I agree with chez311 that the method shown in fig. 4-4 of ASME Y14.5-1994 or 4-7 in the 2009 version is the standardized and recommended solution, which would mean for the part shown Yuyu28's post not utilizing datum target line C1 (by the way, this datum target line is problematic if it means to be coincident with the edge of the part for measurement purposes. The real edge will not be the same as the theoretical one defined by the basic dimensions, so actually measuring from the real edge will not count as following the drawing specification).

However, unfortunately, based on my experience I can envision a case that the vendor or manufacturing department might insist on changing the drawing so that the right edge is utilized as the measurement origin, because the easiest solution for them technically for inspection might be supporting the part by the edge against a flat face in a fixture which is perpendicular to datum plane B and measuring from it. The downside of it is that the edge is probably not a functional feature in assembly and not very reliable for DOF constraints.

Your measurement equipment and gauging cannot touch theoretical features - it would contact and measure the planar face of datum feature C and that would be perfectly acceptable.

I agree, but the same argument (that measurement equipment cannot touch theoretical features) can be used against utilizing the datum plane from which basic dimension 99.03 is given, which is comparable with the "Third datum plane" from fig. 4-4 in the 1994 standard. Nevertheless, the fixture that is needed to construct for inspection of the part in fig.4-4, making the plane in question not theoretical, might be simpler than the one that is needed to utilize the vertical plane at the intersection of B and C at the part in question.
chez311 and others - how would you support the standardized scheme against such arguments?

 

[Yuyu28]

Thanks for all the answers, and apologies for not opening a new thread; I'm new to the forum so I'm not familiar with the customs.

My perspective with this was like Burunduk's that, in practice, it might be easier to measure from the edge even if it's not perfect. But I guess that's a decision for the manufacturer and I should just keep to the standard.

P.S. Yes, I meant Y14.5-1994 revised 2004.