ASME Y14.5-2009 - Total runout example 1

[pmarc]

Looking for one information related to total runout, I found in Y14.5-2009 standard, section 9, figure 9-7 a callout that looks really weird to me. Datum feature D is controlled with total runout 0.05 relative to C primary and D secondary. Is it only a typing mistake or something more stands behind this specification?

 

[MechNorth]

Not a typo, and I'm glad to see it finally in the standard. A lot of people get the whole chicken & egg thing happening mentally on this one; they think that they are controlling the datum back to itself, but they aren't. The thing to remember is that a simulator of perfect form (i.e. cylinder in this case) encompasses the datum feature, establishing a datum axis off of that simulator, THEN you check the actual feature, inclusive of its errors. Chucking/simulation of the setup can be challenging because you have to establish a substitute datum simulation on a second feature if you're doing open setup, but otherwise it's not a problem.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[dingy2]

pmarc:

Yes it looks weird but it is valid. The 94 standard has an identical example in fig. 6-52 with the exception that the feature control frame reflected circular runout rather than total runout.

The feature that will become datum D has very little length and I really don't know how one could confirm the form. There just isn't enough length by itself for roundness or cylindricity so we do need datum A for set up. We then have a choice of circular or total runout.

I agree with Jim that chicken and egg thoughts do occur in this example. Should one have datum D before referencing it? This is a bit of an exception.

Dave D.

http://www.qmsi.ca

 

[joedal]

I agree with all that Jim and Dave said. But I have a bit of trouble with a drawing or perhaps a "figure" that, in itself may be correct, but it may not be very practical. The ASME example (Fig 9-7) is correct and it certainly has an accurate interpretation. But, how practical from a measurement perspective is it to have a secondary datum axis of rotation (perp to a primary planar datum) and require a total runout measurement to be taken on that same cylindrical feature used to define secondary datum axis? Yes, the datum does come before the feature measurement (egg-chicken thing), but this would drive the inspection people crazy I would think. How do you use a perfect feature counterpart simulator for datum D and actually measure total runout on that cylinder at the same time? Even with a CMM it would make the inspector have to think long and hard about this one. I believe that GD&T is a very powerful language but we sometimes confuse users by the somewhat convoluted application of the language at times. On this part there may be a more conventional approach to controlling the form and orientation error to a set of datums.

 

[MechNorth]

I don't think it's convoluted so much as it is higher capability. You would establish the datum axis based on simulating the datum feature, then with a second collet or set of jaws on the same simulator axis, clamp up on the workpiece in another area and release the first simulator. It's a substitution which will introduce some degree of error ... acceptable or not is up to the user. In a one-off shop, that WOULD drive people nuts, but in a production facility, they would develop a fixture to do exactly that. If I was advising a company with little capability, I might suggest alternative datum features if possible, such as tooling centres; there are considerations of manufacturing process to be looked at too, but you get the idea.

I find that many inspectors aren't trained as such; they're typically veteran machinists who have been booted up the line. I haven't heard of a professional metrology training program or certification yet ... shame, really, because it would be a major step for North American industry.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[dingy2]

Actually, the total runout is not that difficult to confirm as long as on has a divider head located horizontally on a granite table. Total runout is not a measurement that should be performed on a CMM.

Chuck at the end of diameter datum D and then using a dial indicator on the face which is datum A, true up the face. One would tap the face with a hammer until one had as close to zero TIR on the indicator as practical understanding that the face may not be perfectly flat.

Now datum D at our contact point is perpendicular to datum A.

One would then take the dial indicator (on a stand of course) to the highest point on the diameter (centre) of datum D and rotate recording the highest and lowest reading. Move the dial indicator along the length and, again, move it to the highest reading (centre) and rotate the part recording the highest and lowest reading. DO NOT RE-ZERO the indicator. One can take a third reading if desired.

The total runout is the range from the smallest reading taken from all readings to the largest reading. This will include circularity, taper, straightness and maybe a bit and angularity if the OD was not truly perpendicular to datum A.

It is not that hard to do.

Dave D.

http://www.qmsi.ca

 

[MechNorth]

Interesting method. In truth, I wasn't thinking specifically about Fig. 9-7; I typically run into Fig. 9-6 instead. My bad.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[joedal]

These are certainly good approaches to measurement of that specific total runout requirement (Fig 9-7). Good stuff! Anyway, it really gets to the point made by Jim that a lot of inspectors just don't have the "insight" to be able to figure that out, without some help from a GD&T expert or an experienced person who's been there done that.

 

[axym]

I agree that the feature control frames in figure 9-7 and (9-6) look weird. Personally, I think that referencing the considered feature as a datum feature is a flawed concept. Inspecting the feature presents huge practical difficulties, and the self-referencing requirement isn't functionally necessary in the first place (IMHO). It's one of those "traditional" Y14.5 chestnuts that doesn't really work theoretically and gets fudged in practice.

If we decode the Total Runout FCF for feature D in Figure 9-7, here's what we get. The entire surface of D must lie within a tubular tolerance zone of wall thickness 0.05. The tolerance zone is perpendicular to Datum C. The zone is also coaxial to Datum D, which is the axis of the related actual mating envelope of feature D.

To me, the requirement for the zone to be coaxial to D is artificial. It's only there because of the limitations of the Y14.5 tool set. The real functional requirements are that the feature be nice and cylindrical and perpendicular, so that it acts as a good stable secondary datum feature. So the Total Runout zone just needs to be perpendicular to Datum C, it doesn't need to be "self-centered". It needs to have Total Runout within .005 to some axis, but we can pick the axis. But there isn't a way of saying that in Y14.5 - a Total Runout FCF must reference a well-defined datum axis. So they came up with the idea of making the feature a datum feature for its own runout control. But this adds in an extra constraint that is highly impractical, not necessary, and (I would say) often ignored.

Figures 9-7 and 9-6 are nothing new, they are literally as old as I am. They are based on figures that were in USASI Y14.5-1966, and are essentially unchanged. So the concept of self-referencing runout has been around a long time, and has survived mainly because it has been around a long time. But it doesn't stand up to close scrutiny, and will go away if and when rigor is added to Section 9.

Luckily we don't have the same problem with other characteristics. Take the common example of a cylindrical hole that is nominally perpendicular to a primary planar face A. The hole is going to function as a secondary datum feature, so we just need to control its orientation to A and label it as B. The control would typically be Perpendicularity to A. Or perhaps Position to A, which is equivalent in this case. But nobody would specify Position to A and B! It's not necessary to reference the secondary datum feature to itself! Yet that is exactly what we have in Figure 6-7.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

Evan,
The issue with positon tolerance on a cylindrical hole that is nominally perpendicular to a primary planar face A was exactly the next thing I was going to ask.

 

[Belanger]

Despite having seen it on many drawings, it is not correct to use a position tolerance on a feature if the only relationship being established is orientation. This is because the position symbol by definition is meant to control location, per paragraphs 7.1 and 7.2 of Y14.5-2009. (Of course, orientation usually comes along for the ride, but it is never to be the only quality.)

Other than that, I agree with Evan's post!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[axym]

John-Paul,

I would say that using a Position tolerance for an orientation-only relationship is "not necessary" or "not preferred". "Not correct" is a bit too strong for me though. But I agree that using an orientation tolerance is more clear.

I suppose that this is similar to using a Surface Profile tolerance on a single planar surface, with no datum features. Would the existence of the Flatness tool make that not correct as well?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Well, profile's main job is to control form -- even without datum references it does that. Of course we often use it to control orientation, location, or size as well, but form is always part of the equation (unless showing the T modifier, but let's leave that aside for now). So I agree that profile with no datums on a flat surface is equivalent to flatness. And perhaps flatness is better because it's more direct. But profile would be acceptable because its fundamental definition is still being met.

But with position, the very definition is to control "the location of one or more features of size relative to one another or to one or more datums." (Para. 7.2 of the new standard; similar concept in para. 5.2 of 1994).

So to use position to control only orientation is not meeting the fundamental purpose of position, which is location. Sorry if I got us off topic; I didn't mean to stir the pot

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[dingy2]

J-P:

You are correct about positional tolerances controlling location, orientation of non-round features of size and perpendicularity. If one did not place a positional tolerance on a feature of size, how does one arrive at a tolerance of location?? Could it possibly be a +/- tolerance?

Dave D.

http://www.qmsi.ca

 

[Belanger]

If one does not place a positional tolerance on a feature of size, then yes, we look for +/- tolerancing. But the designer should also add a note on the drawing that says "all coordinate tolerances are to be measured from datums A (primary), B (secondary), and C (tertiary)."

To leave off this note would yield an ambiguous tolerance for that feature's location, since we can't make the implied datum features (the two edges) perfect every time. Even if the location is not that important, it must have a location tolerance -- perhaps a very generous number -- and the origins for those locating dimensions must be clear.

Hey, are you trying to stir the pot now?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[axym]

Keep in mind that the feature of size does not necessarily need a tolerance of location. This occurs when the feature is a secondary datum feature and the location of the edges is toleranced relative to it.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Yes, good point!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[rgb210]

It is important to remember in this case that runout is a surface control. Datum "D" is an axis perpendicular to datum "C" centered about datum feature "D". Runout controls the surface of the feature relative to the axis, so it is not actually referencing itself.

Robert Bohot
GDTP-S

 

[MechNorth]

Thank you, Robert. Exactly.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

Evan,
As I understand you are on the committee, did you express your concerns there? Are you not in favor of compound datum applications, in general (other than the traditional "A-B" centerline? I think is ironic that people will say any given example from the standard should not be "done like this" when it is just an example in the standard, probably given just to show that concept in particular, I doubt those parts even really exist, they are just simplified concepts, are they not? Is that a particular application the committee knows? My guess is they are laying the groundwork for the complex datum frameworks that do actually occur in real life.
Frank