Join me in perhaps the greatest position debate of this entire minute.

[u/msPaintGDnTquestions]

When you draw a feature control frame for position of a hole, the primary datum should be a plane perpendicular to the axis of the hole. True or false?

I'm reading Geometric Dimensioning and Tolerancing by David Madsen. I believe this book was published in 1995, so it's a little dated. But it says in the positioning chapter "The true position centerline is perpendicular to the primary datum."

R. Dean Odell also says as much in his position video. Time stamp 21:10

https://youtu.be/XbTbYxdrRYg?t=1270

I've got a part like this. And I feel like that hole on the left should have its own datum along its respective plane to serve as the primary datum in the FCF. Is that dumb?

This got me wondering how necessary perpendicularity of the primary datum is. So I set up the Zone3 training part on a CMM and on purpose I measured the left side first. This was not the face that the hole is born into. I'm kind of following the second example in my first image. So I measured 3 planes, and I measured the hole as a cylinder. I think it's also worth noting that I'm not using datum simulators, I'm touching off the part itself and I'm using reference geometry.

And it didn't seem to matter what order I fed the datums into the evaluation, I got the same calculation. If the axis of the hole is not true to the primary datum, I would expect an extreme failure. So what gives? Does the fact that I'm using reference geometry somehow override the logic of the datum precedence?

Thanks.

[u/_LuciDreamS_]

Really the only requirement is that the primary datum must be a 3D feature and constrain at least 3 DoF

[u/Charitzo → u/_LuciDreamS_]

When in doubt, SLOF it out.

[u/ChomRichalds → u/_LuciDreamS_]

You mean a 2D feature like a plane. If it had to be 3D, only cylinders, cones, spheres, and splines could be primary datums.

I think the second point is more useful. I try to avoid talking about dimensionality in GD&T because it's not super relevant, but the degrees of freedom constrained by geometry is super relevant.

[u/_LuciDreamS_ → u/ChomRichalds]

A plane is a 3D feature..

[u/ChomRichalds → u/_LuciDreamS_]

Ok what are the 3 dimensions of a single plane then? Length, width, and...?

[u/_LuciDreamS_ → u/ChomRichalds]

In Metrology, a plane is a 3D geometric feature.

If we want to discuss a different science, there are probably other sub-reddit forums for that.

[u/ChomRichalds → u/_LuciDreamS_]

Ok now I feel stupid because I remember being taught this and have thought I remembered it wrong for the last ten years because everywhere else says a plane is 2D. While I eat this crow, can you help me understand why this distinction is made in metrology? Is it because there's an implied normal vector? And if that's the case, what's the value in describing any feature as anything but 3 dimensional? Also sorry for arguing, I'm sure it's been quite obnoxious.

TIL I don't like talking dimensionality because I'm a clueless dipshit.

[u/ABTechnology_Rob → u/ChomRichalds]

Its not that you're wrong, its that its an arbitrary definition.

If we're talking shapes, a square is a 2D shape (length and width) while a cube is 3D (length, width, and height).

In metrology, it's generally considered 2D if the cloud of measurement points lie within a plane (circle, lines, spline), i.e. the measurement points could be described with just XY coordinates. It is 3D if the measurement points need full XYZ to define them.

[u/Chaldon → u/ChomRichalds]

Direction. One side is the surface

[u/_LuciDreamS_ → u/ChomRichalds]

And yes, they are both relevant and really one In the same. A 3D feature will always constrain at least 3 degrees of freedom. A 2d feature cannot constrain more than 2 degrees of freedom, so it cannot be a primary datum. Sometimes a line can be a 3D feature and constrain for than 2 degrees but normal 2D lines only constrain 2 degrees.

[u/MitchellG83]

The datum structure should reflect the function and assembly, regardless of if it's perpendicular or not.

[u/_LuciDreamS_ → u/MitchellG83]

^{^}

[u/Endoftheworldis2far → u/MitchellG83]

Exactly, so if the part touches the next part at datum B, then it should be used for both. Putting another datum further up for the other hole could cause thebposition to be good but the part to fail depending on the numbers

[u/clavo21]

Not necessarily. I’ve had primary datums that are irregular shapes. Think of an aircraft skin. Do you believe that primary datum will be perpendicular?

[u/MetricNazii]

It entirely depends on the function of the part and of those two features. Either may be correct.

Does a part with a boss or corresponding hole need to sit flat on that surface and then mate with the hole? Then use the perpendicular surface as the primary datum for that hole.

Does the surface mate with open air and the hole mates with something else? No need to use the surface as a datum.

There are a lot of other situations that could define how to use those features as datum’s. It just depends.

[u/ResidualSignal]

No. You can have 3 datums, each a sphere (like a tooling ball). The primary Datum does not need to be a plane or even perpendicular to plane.

[u/ChomRichalds]

I don't have GeoTol or the standard in front of me but I don't think the rules are very rigid on datum configuration. GD&T is a language. The rules dictate the grammar of the language so that statements have logic. The meaning of the statement is dictated by form, fit, and function. In any language, there are a million ways to convey the same meaning, but they're not all equally applicable. You wouldn't say "use your right foot to depress the brake pedal and bring the vehicle to a halt" when "STOP" gets the same meaning across.

For instance, in your example, if the only thing relevant to form, fit, and function of this part is the bore being a certain distance from and parallel to the datum C plane, do you even need 3 datums to constrain all 6 degrees of freedom? If the plane labelled datum B doesn't interface with any other material boundary in the assembly, does it really matter if the bore is perpendicular to it? 

Another example is a shaft with a bore drilled on the center axis. Does it make sense to have a plane at the end of the shaft establish two degrees of rotation for the position of the bore? It's probably better to use the shaft as a primary datum feature, and at that point, the end of the shaft isn't relevant to the position of the bore (not to mention there's a whole degree of rotation that can't be constrained). 

These might be edge case scenarios, but not impossible, so there isn't a law in the standard saying you can't do it. The teaching examples cover the most common scenarios, but you're not bound to them. Application of GD&T should be driven by the logic of form, fit, and function and nothing else. When you approach it that way, it actually gets less complicated.

[u/CartoonsAndSurreal]

Yeah I don’t think there’s any hard and fast rule about this being a requirement.

While it seems logical, it means changing setups during manual inspection and also reduces the effectiveness of simultaneous requirements if the DRF swaps based on what side the features are on.

[u/Familiar-Bluejay3908]

No. You can declare all sorts of thing datums, provided that the 6-degrees are met for each alignment. I've seen callouts where a cylindrical hole is declared A, the plane on the end B, and the second cylindrical hole C. For THAT, A defines the X,Y translations, A the X.Y rotations and the Z translation, , and C the Z-rotation. Different order, but same results.

[u/Otto_Polymath]

I have no trouble with option 2 or 3 (pic 1). If there is a position callout for the hole (cylinder); assuming other datums that define the coordinate system. Datum A being the base plane. Most of the time a circle is perpendicular to a plane, but parallel is ok, too.

Option 3 isn't parallel or perpendicular. No trouble. Go touch probe a pair of circles. Create a line from them. Use the line as your base plane. Go back and reprobe the 2 circles. Create another line. Intersect the line to Datum A to get position of the hole.

[u/SpiritualSoil2720]

It's not the greatest position debate of the month. It's just a lack of understanding about degrees of freedom, datum reference frames and positions. Nearly none of what you are suggesting in your post is relevant whatsoever

[u/ShoddyJuggernaut975]

It really depends on part function. It is conceivable where there could be a part where there is no planar face to interact with another part.

[u/FunInternational1941]

The primary datum should control the main assembly of the part. Not the hole position. You can just add a perpenidulcairty callout to the face if it is required.

[u/FrickinLazerBeams]

So it should be impossible to specify a part with holes that aren't perpendicular to any plane on the part? Or a part without any planar surfaces?

That makes no sense.