Does this drawing follow best practices? Will the datum orientations make it hard to inspect? 2

[cratechallenge1]

Hello,

It's been a few months since I've made a detailed drawing and I can't tell if I'm forgetting my fundamentals at the moment.

Do the dimensions and GD&T in this view follow good drawing practices? Primarily, I can't remember if assigning datum A and datum C to recessed areas (and dimensioned to each other) will be difficult to verify during machining/inspection. Although the dimensions represent the intent of the design, I want to make sure this is a friendly part for the machinist and CMM operator to easily locate the datums for their operations.

Thanks!
CC1

 

[greenimi]

No, it does not make sense.
Datum feature A used primary....oriented to B. Which one is first? A or B?

 

[cratechallenge1]

@greenimi good call, meant to put the cylinder as the primary, so would be B then A.

 

[3DDave]

The letters don't matter. They can be used in any order.

 

[drawoh]

3DDave,

Yes, it does matter. In his positional tolerance, he is showing A then B.

--
JHG

 

[drawoh]

cratechallenge1,

You can edit your drawing to make your diameter the primary datum, either by calling it datum feature[ ]A, or by swapping the datums on your positional tolerance. Other than that I don't understand your datum feature[ ]C, I don't see anything wrong, but I see some iffy practise.

I strongly prefer my datum features of size to be accurate. If you do not control the size and roundness of your [⌀]6.950[ ]feature, you will have all sorts of fixturing problems. You can always call up datum[ ]A at MMC, but that does not necessarily mean you have a working part. Your primary datum controls two positions and two axes of rotation. Your secondary datum feature's furthest point to the left controls the third positional DOF.

Your datum feature[ ]C should control the third rotational DOF. Could it be one of those holes?.

How are you going to inspect your [⌀]4.137 feature? You have a tight diameter and position, and no access to it.

--
JHG

 

[3DDave]

drawoh - I wasn't referring to the order in the block - I was referring to the idea that the primary is always identified "A".

The stubby cylinder is a terrible primary datum feature.

I originally wasn't going to comment because the goal of the OP is to make the inspector and machinist jobs easy, with no regard to the functional limitations that should control what variations are specified as tolerable. It's so far from the entire point of Dimensioning and Tolerancing that it seems useless to suggest improvements.

 

[cratechallenge1]

@3DDave, could you please suggest how I might go about making this into a functional drawing then? My intent above all is to make something that complies with good dimensioning and GD&T practice, and I am open to constructive feedback on how to improve this.

 

[3DDave]

I cannot - because I don't know what the part is intended to do, nor what the limits on how it functions are determined. The point of Dimensioning and Tolerancing (there's no such thing as "GD&T" outside the marketing of training materials and services) is to document the limitations that have been determined. If you don't know those limitations, then you cannot document them.

 

[greenimi]

cratechallenge1,

Could you, please, let us know how this part get assembled or get together with its mating components?
An assembly drawing would be good or even a simplified version of such.

 

[Bill B.]

There is nothing inherently wrong with the drawing you provided except for the placement of your feature control frame for the position of the 4.137 diameter. Whether datum feature A or B should be the primary datum depends on function of the part. If the orientation to datum feature A is most important then A,B would make sense. Again, depending on how the part functions it may be possible to apply a MMB modifier to datum B, potentially allowing additional radial displacement of the 4.137 diameter.
@greenimi, I see nothing wrong with holding datum feature A perpendicular to datum feature B, the two features are controlled to each other. It’s quite possible that there is a FCF on this drawing that has a B,A datum reference frame, say the upper left hole with the 2.00 basic dimension. How would we reconcile the question, what comes first?
@drawoh, Datum C looks like it will be used when positioning the 9x hole pattern given the .787 basic. I agree at some point the last rotational degree of freedom will need controlled, but it’s not needed in this view. I’m confused by your comment that you don’t have access to the 4.137 diameter. You could easily set the part with the right side face down on a cmm and have full access to datums A, B and the 4.137 diameter. A 3mm ball would fit nicely.

 

[greenimi]

@greenimi, I see nothing wrong with holding datum feature A perpendicular to datum feature B, the two features are controlled to each other. It’s quite possible that there is a FCF on this drawing that has a B,A datum reference frame, say the upper left hole with the 2.00 basic dimension. How would we reconcile the question, what comes first?

Per the GD&T and Y14.5 theory, the features are not defined based on their geometry (this feature should be perpendicular with that feature, etc. ) but based on their design intent (this features goes together with that feature and their mating relationship is as such). Also the feature precedence is primordial in how to correctly define a drawing, meaning the degrees of freedom stopped or arrested by each feature is the main requirement and philosophy.

 

[cratechallenge1]

Hello all, thank you for the constructive responses. I've attached a simplified version of the assembly with the mating parts shown. The 4.137" feature controls the location of the tube ID on the left, as the goal is to align the ID of the tube/cylinder as much as possible. The perpendicularity callouts (surfaces A and C) are to control the surfaces of the (face seal) O-rings so that when the parts are assembled there is as little of an extrusion gap (due to perpendicularity of the surfaces) as possible.
A section view of the mating parts is attached in the link.

Thanks,
CC1

 

[drawoh]

Bill B.,

That's great if you have a CMM. you are not getting calipers in there, or a micrometer. How accurate is your CMM?

--
JHG

 

[cratechallenge1]

Also thanks for bringing up the caliper note, I don't know why this hadn't occurred to me before. As far as the feature that is inaccessible with calipers/micrometer, I think I can make the entire A surface to be one plane (trim the outside to match) and just add a separate spacer to account for the flange offset.

 

[3DDave]

Any surface that controls this part to another part is a good candidate for a datum feature.

I'd make the 4.137 a secondary datum feature with the current [A] datum feature the primary one to control the positions of all other features. Datum feature A would get a flatness geometric characteristic in a related FCF (feature control frame). Then the 4.137 diameter would get a perpendicularity geometric characteristic in its related FCF. The far (currently [C]) surface could get a profile and parallelism pair - the larger tolerance on profile to set the offset between the mating parts and parallelism to control the gap for that o-ring.

The mounting flange gaps could likely be larger; they would be controlled by a profile tolerance using each of the nearest mating faces as their relevant datum feature.

With the close fit on the 4.137 diameter I don't know why there is a close fit on the 4.467 diameter. Does it control or seal anything? It might be nice to include some chamfers so that a little dent on the edge doesn't render the part useless.

The biggest question is that the original had multiple separate external diameters that aren't shown in the assembly diagram. What happened to that?

 

[cratechallenge1]

Thanks for the feedback everyone, and thanks @3DDave for the thorough guidance.

A few more questions:
I was previously under the impression the OD of the part had to be the feature which datum B lies off of, due to the lathe grabbing this surface of the part during machining.

My intent was for datum B to be centerline axis of the part (and defined on the OD since the cylinder will be fixtured on a lathe during machining), however I now realize that if I am not specifying a straightness or runout on the OD then the centerline axis may not be straight (correct me if this is a wrong assumption).

If I were to make datum B the 4.137 surface, how might this affect the features on the other side of the part, since the machinist can no longer touch off on that datum? Or am I thinking about this the wrong way?

I forgot to mention that the tightest controlled surface here would be the ID, which is 3.063 +/-.002 (dimensioned in a different view) so I might have the 4.137 surface controlled back to this.


How large of a gap would you recommend for the mounting flanges? As for the 4.467 you're right, there is no reason for it to be tight so I will widen it some more.
Also I should have clarified, the previous section view was unintentionally deceiving as the "external diameters" were actually just local flats/spotfaces for mounting plugs.

 

[3DDave]

It's a matter of money and importance. The machinist can make an expanding collet to fixture and grip, though in this case that isn't much. They could also make a form tool that cuts all the features on each end in a single operation, eliminating most of the tolerance buildup that is possible.

A possible approach is to simply replace all the form and location tolerances with total runout, which is more applicable to turned parts. Make the outer surface a precise diameter and it's very convenient for the machinist and the inspector at the expense of a possibly additional source of tolerance build-up by adding a non-functional datum feature to the tolerance path, particularly if there is a change in setup.

Is there some precise relationship between the two other parts that is maintained by external mounting?

 

[drawoh]

cratechallenge1

You want the machine shop to fixture off your datum features. You want to use your datum features to locate your part upon assembly.

--
JHG

 

[jassco]

Hi, cratechallenge1:

Your datum feature "B" should be primary. Also you need to add CF to diameter dimension of 6.950 so that OD across the entire part serves as datum feature "B".

Your datum feature "A" should be secondary which is perpendicular to datum feature "B".

To control position of feature of dimension 4.137, you use position tolerance in reference to datum feature "B" first and then to "A".

You don't need a tertiary datum feature unless you want to control positions of the cross holes. Your datum feature "C" does not function as a tertiary datum feature.

Best regards,

Alex