GD&T overload

Tim Walsch

Registered
Hello guys,

I am struggling with the drawings of possible new parts. I do the programming of our CMM, surface tracer, measuring microscopes, ...
I received the question if we are capable to measure this. Our machineshop already committed to machine these parts, but I am not sure on the measuring side of the story.
It seems like the designer went completely overboard with the GD&T dimensions.
For me they don't even make sense. Maybe I am wrong because we never had parts that came close to something like this.
So I am happy to hear your specialist opinions.

Number 1 on picture:
Surface profile of a plane against ref A and B.
I guess the arrow is pointed to the plane on the bottom side. But as you can see there is no plane at the bottom side of the part because it has a rounded edge of 0,1 (see number 2).
I have no idea how to measure this.

Numbers 2 on picture:
CPK of 1,33 on Ø6,18, but the diameter is specified on the center of radius 0,1.
Same remark, how on earth would you do SPC on this dimension (also note when CPK 1,33 is not achieved, 100% inspection is needed)
And I don't know what to make from the positional tolerance to ref B for Ø6,18

Dimensions are in millimeters btw


Any thoughts on this?

Thanks!

Tim
 
Last edited:

John Predmore

Trusted Information Resource
I am confused by the depiction of the part in the drawing, before I get to dimensioning and tolerancing. I see a cross-hatched surface with lines extending to the right, which makes me think this is a shape like an extrusion. However, it is not drawn in orthographic projection, and it is not drawn as a isometric view. It is difficult to interpret which leader lines apply to what feature in which plane. In order to interpret rules of GD&T, one must know which set of rules (i.e. drafting standard) was used. In the USA, ASME Y14.5 is commonly used, but there are different revisions (1994, 2009, 2018) and that can change interpretation. However, the use of a comma before the decimal leads me to think this drawing is non-USA in origin, so maybe not Y14.5.

I have other questions, but lets start with which framework set of rules (drafting standard) apply
 

Tim Walsch

Registered
It is a EU drawing, not sure about the standard. I'll have look when I am back at work.

To give more detail, it is a cross section of a round part. Machined on a CNC lathe out of a cilindrical rod.

The lines that go to the right are the marks where a radius and a flat surface meet (also very fun to measure... But those are not my biggest concern).
So it is and inside bore with a small chamfer on top and a big complex chamfer at the bottom, with flat and curved sections.

I understand this drawing section alone is difficult to read. I'll make a bigger snip of the drawing next week, that will make more sense.
 

AuditFan

Retired
I've found that it's often the case, that the engineer drafting such details didn't actually speak with anyone who has to make it, let alone measure it. If they did make some prototypes, it would be interesting to know who those were made and measured. I'd encourage you to contact your customer and ask instead of us guessing. If they make samples, they should be able to help. If they didn't and this is the first time, you still have a legitimate reason to ask for their guidance, to avoid rejects and hence keep the cost down (!)
 

Matt's Quality Handle

Involved In Discussions
Is this some kind of sealing surface? It looks like they're very concerned with that high point on the lip. Perhaps a series of radial scans that go "over the hump." Then pick out the high points of each scan and collect them into a circular element. With that element, you could calculate the dimensions that they're asking for.

I think doing it this way, you'd be able to put a number on it, although I don't know how useful it would be.

Alternatively, for dimension 1, you could place the part down on your CMM table. Scan B, A, and the CMM table to simulate the feature, and profile should work that way.
 

Matt's Quality Handle

Involved In Discussions
Is this some kind of sealing surface? It looks like they're very concerned with that high point on the lip. Perhaps a series of radial scans that go "over the hump." Then pick out the high points of each scan and collect them into a circular element. With that element, you could calculate the dimensions that they're asking for.

I think doing it this way, you'd be able to put a number on it, although I don't know how useful it would be.

Alternatively, for dimension 1, you could place the part down on your CMM table. Scan B, A, and the CMM table to simulate the feature, and profile should work that way.

Actually, as I think about it more, placing the part down and scanning the table wouldn't give you high/low variation (flatness) around the surface.
 

Tim Walsch

Registered
Is this some kind of sealing surface? It looks like they're very concerned with that high point on the lip. Perhaps a series of radial scans that go "over the hump." Then pick out the high points of each scan and collect them into a circular element. With that element, you could calculate the dimensions that they're asking for.
Yes it is a sealing surface.
The method you are describing is exactly what I was thinking. That's is probably the only way to get a number out of it.
But the repeatability of this measurement will be questionable (I think). Need to test it with a small R&R study when the parts are manufactured.

The more I look at it, I think for the surface dimension a parallellity would be more in place.
The position of Ø6,18 against datum B is also weird one, can't figure out what they want to control what is not covered by the surface symbol.

I talked to the machining guys who will manufature the parts.
They are confident the CNC lathe will machine the parts perfectly since they load the .STEP file into the machine.
But they are also stuggling with the measurements.

I've found that it's often the case, that the engineer drafting such details didn't actually speak with anyone who has to make it, let alone measure it.
That is absolutely right.
I am trying to build a case to push back on this, but I am not in a direct contact with the customer so I need to convince our sales and design people to call the customer on this. That's why I want to gather as much info as I can.
 

Matt's Quality Handle

Involved In Discussions
The more I look at it, I think for the surface dimension a parallellity would be more in place.
The position of Ø6,18 against datum B is also weird one, can't figure out what they want to control what is not covered by the surface symbol.


That is absolutely right.
I am trying to build a case to push back on this, but I am not in a direct contact with the customer so I need to convince our sales and design people to call the customer on this. That's why I want to gather as much info as I can.

If anything, I'd question why the profile is of a surface, not a line. There may be a small flat in the .step file that's not showing up on the drawing

The position is attempting to control coaxiality of the sealing surface to A, perpendicular to B, so I can understand that. It's also controlling it tighter than the profile, so they're saying that you can have .04 total variation, but at most .03 of that can be in position.

I'd agree with your instincts to talk to the customer. I wouldn't call it pushback, but there is a need to ensure that what you're seeing and trying to do captures their design intent. What you and I talk about doesn't really matter, you've got to be on the same page as the customer.
 
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