Fixture design for inspecting part with datum targets

[lowerp]

For a plastic molded part that has defined datum targets and requires inspection in the restrained condition; do the faces of the corresponding inspection fixture components which contacts the part, mirror the parts geometry?

 

[Nescius]

I'll ignore the issue of checking the part in a restrained state, as that's not something I have experience with.

The nature of the thing that touches the part* (I'm assuming a target area in your case) can be whatever you want it to be, including a complex surface.

Depending on your application, the implementation may be quite tidy...easy to show on a drawing, easy to interpret and check...or it may be nearly impossible to depict on paper, requiring a solid model and pages of notes.

Just for fun, let's say I want to use some simple, curved, rectangular areas to fixture on the perimeter of a hockey puck. Very tidy. Now, let's say I want to fixture the Earth using some datum target areas. I could define the surface of one my target areas as the topography of the Himalayas. Not tidy.

*For now, I'm using that phrase to avoid getting bogged down in terminology.

 

[Belanger]

The short answer is yes, the datum simulator mirror's the part's true geometry as defined by CAD or basic dimensions. A couple of paragraphs from the Y14.5 standard pertain to your question:

7.20.2(a) says that, unless otherwise specified, "the restraint load shall be applied over each datum target, normal to the surface at that location and the same size and shape as the datum target..." (emphasis added).

But even more to your question is paragraph 7.24.9, which states: "When a datum target area or datum target line is shown on a nonplanar surface, the shape of the datum target’s true geometric counterpart is the same as the basic shape of the surface."

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

 

[Nescius]

...paragraph 7.24.9, which states: "When a datum target area or datum target line is shown on a nonplanar surface, the shape of the datum target’s true geometric counterpart is the same as the basic shape of the surface."

Hi Belanger,

I don't have the 2018 standard. Does it allow for deviation from the language you cited? I would be very disappointed if the most recent standard mandates that a counterpart MUST mimic the surface it touches.

See Figure 4-47 in the 2009 standard for an example where straight fixture hardware would touch a nonplanar surface. Paragraph 4.24.5 sums it up: "Figure 4-47 is an example of a V-shaped datum feature simulator established from two datum target lines."

Going further, even though the language you cited mentions only lines and areas, does the most recent standard object to using spherical hardware to interact with something non-spherical, like a conical datum feature, as shown in Figure 4-49 of the 2009 standard?

 

[3DDave]

That figure is carried over. Consistency in such details isn't expected between what one paragraph writer expects is not what another one wants to show. Just have to roll with it.

 

[Nescius]

Many of us live in the land of gauge balls and gauge pins, where a CMM is simply not realistic. Likewise, a fixture with elements that mimic a complex surface may be orders of magnitude more expensive than an old-fashioned setup of precision primitive shapes.

Bluntly, that language from the 2018 standard seems to largely defeat the utility of datum targets altogether. Why in the world would I build das-uber-fixture to hold a rough casting?

 

[3DDave]

The language is for specific cases of datum target areas where a tooling ball would be inappropriate. It doesn't defeat anything. If that area is nominally flat, the tooling surface will also be flat. If the datum target area is a complex compound curved surface then it is reasonable to expect the matching tooling to be the same. If you don't need a complex compound curved surface as a datum target feature (I am unsure when one would,) don't use a complex compound curved surface as a datum target feature.

The language is specifically limited to "7.24.9 Datums Established From Complex or Irregular Surfaces" not points or lines that are in contact with those surfaces.

As to why you would use an expensive fixture? If the machining solution was sufficiently complex it is worthwhile to do so. We had some peculiarly shaped castings where all the approximately flat surfaces were intended to be machined away relative to all the complex curved surfaces. QA/QC took (wrested) full control of that from engineering, and because they analyzed the problem incorrectly it blew up in their face. Turns out you need to consider all the important surfaces not just the ones you like. Rather than fix the fixture they added a larger number of inspection hours to each part - but that was their manpower budget; they had already blown their budget for buying fixtures. Still, the fixture essentially imitated the effects of machining the casting so that a location and orientation solution could be found to ensure that there would be metal and not thin air where the end mills were sent. Pin holes and spot faces "recorded" the solution for the matching machining fixtures.

 

[Nescius]

If the datum target area is a complex compound curved surface then it is reasonable to expect the matching tooling to be the same.

I wish I could dig deeper, but I don't have/use the 2018 standard.

Of course, it sometimes would be reasonable to match the tooling to the basic shape of the part in the area of contact. However, mandating this is very limiting. A primitive thing can serve perfectly well as a locating stop against a non-primitive surface, even more so when it's serving as a secondary or tertiary restraint.

Sorry for the minor derailment of your thread, lowerp.

 

[pmarc]

Nescius,

The sentence from para. 7.20.2(a) that J-P (Belanger) quoted here is lacking one small but crucial element regarding the point you are making:

"When datum targets are specified, the restraint load shall be applied over each datum target, normal to the surface at that location and the same size and shape as the datum target UOS."

UOS stands for Unless Otherwise Specified. This gives you the option are looking for.

 

[Burunduk]

Nescius,
If you have access to the 2009 version of Y14.5, take a look at sub-para. 4.24.11. The wording is almost identical to what Belanger quoted above from the 2018 version. It says:
"When a datum target area or datum target line is shown on a non-planar surface, the shape of the datum target line simulator is the same as the shape of the surface. In Fig. 4-42, the datum target area simulators for A1 through A5 are the same as the contour of the part surface."

If it is not desired and/or needed to copy the shape of the curved/complex datum feature to the datum target simulator (fixture component), the designer and manufacturer should cooperate to replace datum target areas or circular lines by datum target points, or straight lines where applicable.

 

[Belanger]

pmarc -- read my post again. I was careful to state "unless otherwise specified" immediately before my opening quotation mark.

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

 

[pmarc]

Right, J-P. It's just that it looked to me, based on subsequent comments, that it wasn't clear to Nescius that the UOS is actually stated in the text of the quoted paragraph.

If I misinterpreted this, then I apologize.

 

[3DDave]

Of course, it sometimes would be reasonable to match the tooling to the basic shape of the part in the area of contact. However, mandating this is very limiting. A primitive thing can serve perfectly well as a locating stop against a non-primitive surface, even more so when it's serving as a secondary or tertiary restraint.

The interpretation is mandated, not the application. If the drawing says to do it, the inspector is mandated to do it. If doing what the drawing requires is limiting that's a management problem. If the inspector it taking shortcuts that don't represent what the drawing requires that's a time to look for a new job.

 

[Nescius]

pmarc and J-P,

I'm referring to paragraph 7.24.9, not 7.20.2. Does the 2018 standard indicate "UOS" in both instances?

Burunduk points out that similar language was actually in 2009 all along, although it references a depiction of square contact patches on a car hood in a section named "Circular [line] and Cylindrical [area] Targets". It feels a bit out of place.

I wrongly assumed that the 7.24.9 language was new in 2018 because the 2009 standard has depictions and language that are in direct conflict with it. I knew this from memory, as I was neck-deep in an issue involving datum target lines circa 2017.

Instead of demanding to see the manager of ASME , I'm satisfied to take 3DDave's advice and "roll with it".

 

[lowerp]

Thanks all for the timely responses, much appreciated.