New to GD&T

[Bobby.T]

I'm just starting to get my feet wet with GD&T and I'm hoping to get some general feedback on this print I'm working on. I start to get confused when the datums are anything but planar faces. The part interfaces on datum A and the two countersunk holes. I appreciate any input or advice.

 

[pylfrm]

Bobby.T,
[ul]
[li]What is the drawing interpretation standard?[/li]
[li]Do you intend for datum feature B to be the two conical surfaces, the two cylindrical surfaces, all four surfaces, or something else?[/li]
[li]Will flat head screws be used in the countersunk holes?[/li]
[li]Can you provide more information on the functional requirements?[/li]
[/ul]


pylfrm

 

[Bobby.T]

-ASME Y14.5
-I intend for the cylindrical surfaces to be the datum
-Flat head screws will be used

 

[pylfrm]

Bobby.T,

What version of ASME Y14.5?

Usual practice is for a flat head screw to make contact with the clamped part on the conical surface only. Unless you are doing something unusual, it seems more appropriate to use the conical surfaces as datum features for your part. Why do you intend to use the cylindrical surfaces?

Is that a "no" to the fourth question? Without some idea of what the various features on the part are supposed to do, it's hard to tell whether they might be toleranced appropriately.

As for general advice, first I'll point out that there is a difference between a datum feature and a datum.

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. This idea is discussed in thread1103-451664, from which I copied the previous two sentences.


pylfrm

 

[Bobby.T]

2009. You're right about contact happening on the conical surface. This just goes back to my confusion about choosing datums. I keep trying to imagine that there needs to be some physical measurement fixture and wasn't seeing that being very straightforward with the conical surface as datums. Would you say as a general rule that datums should be the mating surfaces? As for the function of the part, it's not very complex - it's kind of a bracket/mask that is only intended to contact the mating part on datum A surface. The rest of the surfaces are clearance. Thanks for the input.

 

[pylfrm]

I keep trying to imagine that there needs to be some physical measurement fixture and wasn't seeing that being very straightforward with the conical surface as datums.

I imagine a fixture would actually be more straightforward with the conical surfaces. Screws of sufficient accuracy might do the job just fine. With the cylindrical surfaces you'd theoretically need expanding pins, and that can be hard to arrange at a diameter of 1.4 mm.

Depending on material, manufacturing process, and order quantity, a locating fixture may not even be needed.

Would you say as a general rule that datums should be the mating surfaces?

The features that control the orientation and location of the part relative to whatever it interacts with are usually the best candidates for datum features.

The assembly process should be considered as well. In your case, the final orientation and location of the part in the assembly may depend on the order in which the screws are tightened.

As with many general rules, there can certainly be exceptions.

As for the function of the part, it's not very complex - it's kind of a bracket/mask that is only intended to contact the mating part on datum A surface. The rest of the surfaces are clearance. Thanks for the input.

In that case, RFS position tolerances may not be ideal. You might want to consider MMC position tolerances or profile tolerances.


pylfrm