ASME Y14.5m application and general drafting stds 3

[DesignBiz]

1. The attached tube dwg is the current template for tube dwgs. I find it obviously in error in many respects, in particularly the use of A-B as a single datum. (Ref ASME Y14.5m 1994 4.5.7.1 and 4.5.7.2) In this case the ends of the tube are A and B and these features of size are at compound angles to each other. The in-house consultant rationalizes this callout somehow in his thinking and claims it is easily understood according to ASME Y14.5m 1994. I couldn't disagree more.

2. The profile callout via default note completely defys ASME Y14.5m 1994 in my estimation. (ref ASME Y14.5m 1994 6.5.1 paragraph (a) regarding required view or section for a profile callout.

3. Basic drafting standards seem to be a foreign concept.

Comments invited.

D-Biz
Sr. Designer (Auto-Aero Mechanical)
NX4 / Team Center user
25+ yrs experience

 

[MechNorth]

Evan,
The designer arbitrarily selects, but documents that selection on the drawing as shown, and relates it back to the basic CAD geometries by means of basic dimensions. That is the only arbitrary choice made; after that, the inspector must fixture, or establish in the CMM, the origins of measurement specified on the drawing, and then inspect back to those origins. While I use the term arbitrary, it usually / should include discussion with the inspection department so that the selection can facilitate the inspection wherever possible. Most CMM operators have fixturing components (rails, parallels, 1-2-3 blocks, calibration balls, etc.) somewhat permanently mounted to their CMM beds, and hopefully the designer makes use of those in the selection.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

Sorry to resurrect this thread after a few days of inactivity, but there were some things that were left unresolved (in my mind anyway). After e-reading the last few posts, I’m not sure that we’re all talking about the same thing anymore.

Norm,

The reference to simultaneous requirements was a bit of a surprise. I assume that you’re using the term “simultaneous requirements” in a generic sense and not referring to section 4.5.12 and its effects. These effects become important if DOF’s are left open or if datum features are referenced at MMC (or LMC) and that isn’t the case here (assuming we’re still talking about the A-B reference which is RFS). I will agree that all FCF’s referencing A-B should be inspected with a consistent DRF origin.

Jim,

I agree with your description of the arbitrarily-selected DOF origin and your example drawing illustrates it well. I’m not so sure about the statements regarding inspection though. As to whether or not the inspector must inspect back to the origins of measurement specified on the drawing, I think it depends. If the inspector is going to be reporting X,Y,Z coordinates for actual points on a surface or actual axis endpoints, then the origins of measurement would be relevant. But if the inspector is only going to report deviations relative to true profile or actual values of position, then the origins of measurement are not relevant. The numbers would be the same regardless of what origins of measurement were used.

I’m also not sure how the designer would use information about fixturing components on the CMM bed when selecting the DRF origin. Can you give an example? The CMM operator would still need to create a coordinate system on the datum features, and transforming to a surrogate measurement origin wouldn’t involve the fixturing components.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[MechNorth]

Interesting point, Evan, and quite correct. The origins of measurement become irrelevant if you're mapping deviations from the surface. My efforts to convert the masses to using whisker diagrams and such have largely fallen on deaf ears as most of the suppliers that I've worked with have client mandates to provide +/- size and position errors. I've had a couple clients that had halleleujah moments and passed the blessing on to their suppliers, but most are bound by higher-ups that don't understand the idea. C'est la vie.

Most CMMs that I've seen in use have parallels, angle blocks & such affixed to the CMM bed for dealing with small to moderate-sized parts; they are typically used as datum simulators. Their use makes it quicker and easier to repeatably establish the datums, and actually follow the standard by using simulators rather than probing directly on the feature surface. Providing that the fixturing used to hold the workpiece is repeatably coordinated with the on-bed blocks, you have a direct and repeatable relationship established by basic dimensions.

I've also seen tooling balls mounted to workpieces (typically larger), and subsequently used as datum simulators of sorts to establish the datums. Also seen them used to chain back to an "origin" when using movable arm-type CMMs on larger workpieces. Not perfect, but often more repeatable than using the workpiece surfaces directly.

I'm tired, but I think that's a relatively complete answer.(?)

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com