2nd opinion on dimensioning scheme 1

[prdave00]

Does anyone see anything inherently wrong or ambiguous with the attached drawing per ASME Y14.5M-1994 (and/or Y14.5-2009)? I've been staring at my drawing of a part with similar features and have been battling in my head first and foremost with the Ø1.25 feature located using a positional tolerance versus locating the conical taper feature (datum B) using run out.

In use a cylindrical shaft will fit over the Ø5 feature until it bottoms out against the feature located from the virtual sharp by the .25 linear dimension and then secured using an adhesive. This shaft will slide within a tube that has a female 100 degree conical taper. The conical taper will limit the translation within the tube. This is why I've selected datum features A and B as shown. There will also be an accompanying assembly drawing where the OD of the fitted shaft will be datum feature A' and the conical feature will be datum feature B'. I want the similar features to serve as the datum features in both the component and assembly drawings.

 

[fsincox]

The profiled surface currently dimensioned with the basic .925 does not tie back to datum "B" although you have it referenced as such by your FCF statement. How were you planning to dimension the other suface currently tied by the basic .925? I will assume this is not complete on purpose?
Frank

 

[prdave00]

Frank:

Big oops on my part. The dimension to the "bottom" of the profiled surface should be from the vertex of the conical taper. The magnitude would then be .875 & not .925.

This wasn't intended to be a complete drawing. I tried to provide enough detail as possible to address my main question.

 

[powerhound]

I think the bigger issue is the use of both datums A and B on the profile callout. They both constrain the axial degree of freedom but only B constrains translation along that axis. The profile callout is overconstrained. Dropping the A datum and just using B should give you what you want. If you want to drop B then you will need a secondary datum to constrain translation along the A axis.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[fsincox]

powerhound,
The issue is not can it do it, but should it do it, that is determined by functional requirements we have not been given. One of the changes in the 2009 standard it to allow specification of datums that can actually restrain more degrees of freedom but are referenced to do less, obviously to mirror some intended functional requirement.
Frank

 

[powerhound]

Frank,
I agree with your statement in general, but in the absence of any indication of datum precedence violation (u, v, or w in the FCF), I would read this print as it is, not necessarily as it should or could be. As it is, I still believe the profile callout is overconstrained.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[prdave00]

Frank & Powerhound:

Thanks for your feedback. I agreed with Powerhound that I over constrained the datum structure since Datum A would already arrest all but the translation along the long axis. When I sent the original email I threw in the 2009 standard reference as I was considering using a customized FCF whereby datum feature B would be the secondary datum (A being primary) and arrest this last degree of freedom (there is no need to clock, thus no tertiary datum to constrain rotation about the long axis). After Powerhound's response I reconsidered leaving datum feature B as my primary without the need for a secondary.

However, now I'm back to looking at a customized FCF since datum feature B really isn't big enough to physically arrest all the motions. Datum feature A does a better job since (in my actual part its longer).

I'm not sure what other functional requirements beyond what I supplied in the OP would help elicit some better feedback.

 

[MechNorth]

Holdup. the datum reference frame in the profile control is not over-constraining. Datum A-primary establishes the datum axis as primary, thereby constraining motion along the two axes normal to it; it also controls rotation about those other two axes. In all, 4dof controlled by this datum feature. Invoking B as the secondary datum adds the translational constraint along the primary datum axis. Because the secondary datum is controlled wrt the primary datum, any other dof that would be eliminated if the conical feature were primary datum feature becomes irrelevant.
If I understand the functionality correctly, the assembled/adhered tube abutts the face which is normal to datum feature A, which makes it a better secondary datum feature than the conical feature. Don't get me wrong, I love cones, but there is a significant issue using them as datum features in '94. In '94, you get the same constraints as for a cylindrical FOS, but you also get a point location/locking down the translation along the datum axis. The issue is that the point is rather difficult to reproduce with even the slightest variation on the taper draft, and therefore less than effective as a datum point (i.e. origin of measurement).
Also, the conical surface isn't located in a useful way; a surface profile is the preferred and more replicable way to control a conical surface, eliminating the need for a runout control in most cases.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[SeasonLee]

Profile tolerance zone coverage is an another issue.

The tolerance zone for a profile control extends the length and width of a surface. However, if an edge, corner or another tangent surface begins, the profile tolerance zone ends. The leader line of the OP profile control is directed to the arc R.100, that means the profile tolerance zone covers the arc surface up to the tangent point at each end of the arc. Pls note as soon as the surface changes geometry, the profile tolerance zone ends. This is the default condition for profile.

There are three other ways to extend the coverage of a profile control:
• The between symbol
• The all-around symbol
• The all-over symbol

For the OP, the between symbol could be your candidate, you need clearly specify the two tangent points.

SeasonLee

 

[MechNorth]

SL,
I agree, but... an issue arises with finding the tangency point between two surfaces in practice. As a result, I tend to use the between symbol perhaps more frequently than absolutely necessary to eliminate ambiguity.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[SeasonLee]

Because of the unique geometry of datum feature B (cone), it removes 3 translations and 2 rotational dof. If the designer feels the taper on the plug is too shallow to adaquetly constrain this translation along the axis, then the designer can use a large back face to arrest this translation, this is exactly same as Fig.4-45 of 2009 standard. So, I will recommend to choose the cone as primary datum and specify a face as secondary datum.

SeasonLee

 

[pmarc]

I am with Jim on this basically because according to prdave's description of assembly function it seems much more reasonable to use .5 cylinder as primary datum feature and the face perpendicular to it as secondary. Cone could then be controlled by profile of surface/line callout and all other features could be very easily tied to defined datum reference frame.

 

[prdave00]

Thanks for everyone's input and thank you Jim for correcting my statement regarding over constraining. I think I confused the issue in my head that I was violating datum precedence in some way even though I think I would be wrong on both accounts.

Does it matter that the flat won't be accessible after assembly when the cylinder shaft is abutted against the flat? This is why I was trying to make the tapered cone the secondary datum as it would be present both before and after assembly. Also the flat surface doesn't have to be tightly controlled since I only care that it stops the cylinder after contacting one point on the surface. Any thoughts? Is this where I compromise on choosing a functional datum feature over a convenient one (at least with the component before assembly)?

Also why does there seem to be an aversion to run out being used to control the location and form of the conical surface? For me the inspection set up is more flexible that way.

I do think I'll add the in between notation for the profiled surfaces. I'm sold on the fact that it would remove ambiguity.

 

[MechNorth]

Runout doesn't locate, it only makes sure that all radial points (i.e. the difference between the highest & lowest radial points) at any cross section are within the runout tolerance. You can get the same runout for a 25mm diameter as for a 0.25mm, but which surface would be at the right radial location? On a cone, you can't use total runout, and circular runout is only applicable at each individual cross-section, so you don't know what's happening to the form as you traverse the cone longitudinally.
Checking profile controls on a cone isn't a big deal really, it just takes a bit more setup. Once you have the datum simulated (i.e. part fixtured) and the axis established, you set up an indicator stand with parallel bars to run parallel to the nominal conical face (at a basic angle and offset from the axis), then slide the indicator along the bars as you rotate the part. I've seen setups doing this before. Alternately, incline the datum simulator which establishes the axis at half the basic conical included angle wrt the surface plate. Don't let surface profiles or cones intimidate you, they're both massively useful once you get the hang of them.
If you're thinking about using the cone as a datum feature, make it the primary feature then and get the axis & point all in one. Just keep in mind that the datum point may be hard to establish repeatably.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

I would not advocate choosing datums based on type of feature but by how the part actually installs (function).
Frank

 

[pmarc]

Jim, I am a little bit surprised by your statement that "runout doesn't locate, it only makes sure that all radial points [...] at any cross section are within the runout tolerance".

If so, my question is: what is the location of runout tolerance zone relative to specified datum axis? Let's say we are talking about circular runout. Aren't two circles defining the tolerance zone coaxial with datum axis and therefore tied in radial direction?

 

[SeasonLee]

Jim

Thanks for you caught the point : “Just keep in mind that the datum point may be hard to establish repeatably“. To specify a back face as a secondary datum will make the datum point repeatedly and easier for inspection.

SeasonLee

 

[Belanger]

I too was surprised to read that "runout doesn't location," but I think Jim was simply saying that runout doesn't control the rim's "location," or radial distance of the rim of a diameter from its center.

That's just a fancy way of saying size. When we think of location, we are usually implying a feature-of-size location, which boils down to an axis or centerplane's location. In that respect, runout certainly controls location.

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

 

[powerhound]

Once again humbled by the experts. As soon as Jim said "Holdup" I knew I was about to get schooled. I thought about the DRF setup a little too hastily. I usually actually try to think about these things before I say anything but this one seemed really open and shut. There are times I am asked a question where I work and I will hesitate for a few seconds to fully ponder it, and the guys will think I don't have an answer. Sometimes I am tempted to blurt out the first thing that comes to mind, as I did in this case, and I will usually be off a little.

Thanks for the reigning in Jim,

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[MechNorth]

Pmarc, pls see J-P's follow up. Tks J-P, you are correct.
Powerhound, sorry if I seemed harsh; not intended. I'm also trying to fire off reponses quickly as I prepare for some upcoming training.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com