Datums To Show Orientation

[ModulusCT]

Is there any precedence for using Datums on a drawing simply to show the orientation of a part?

First off, know that the man in charge where I work has deemed it appropriate to only use A size sheets on all drawings. Because of this, some additional sheets are often necessary to fully describe our parts. In some cases, a sheet will show only one view. For the part in question, this fact could cause confusion because it is nearly symmetrical.

My initial thought is that showing section view arrows on another sheet that reference the sheet and view in question is a simple way to describe the orientation of the part.

I also thought that showing hidden lines could clear the confusion, as I've stated before, the part is nearly symmetrical and doing this doesn't really solve anything.

Another idea would be to use Datums on every sheet (even though they're not needed functionally) to show orientation. Would this even be allowed under Y14.5?

Opinions?

Thanks,

Mod

 

[Belanger]

Datum features should be labeled as such only when they are to be used as a reference in a feature control frame elsewhere on the same drawing. An exception is when datums are given to establish a datum reference frame to check non-GD&T tolerances (size), but this requires a note saying that "dimensions are related to datum A (primary), datum B (secondary), and datum C (tertiary).

This is the letter-of-the-law interpretation. But let's see what the other folks have to say...

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

http://www.gdtseminars.com

 

[KENAT]

A variation of your first idea is most technically correct. However, they wouldn't be sections as such but "REMOVED VIEWS" per ASME Y14.3-2003 secton 1.7. Typically you would still show them similar to a cross section but the 'cutting plane' line doesn't go through the part but is off set to the side. There is another option in the standard but this is most typical.

Repeating datum identifiers (obviously to the same feature but you hopefully get my point) on different views is acceptable. I wouldn't normally do it unless they are actually being used in that view though.

Trouble is by mandating use of A sheets only, a lot of the standard practices on drawing layout issues will be difficult to apply since one of the fundamental rules is where possible you draw at 1:1 and only scale when necessary. Minimum text sizes are 3mm etc, and with a marginally standard title & rev block there isn't going to be much real estate.

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[fcsuper]

There are ways to specifying orientation in the standards that may be more helpful than trying to repurpose datum flags. As KENAT pointed to ASME Y14.3-2003, I would recommend looking into that standard or a summary of that standard from the Drafting Zone.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[KENAT]

http://files.engineering.com/getfil...-4609-9d02-c4fa132bb922&file=removed-view.tif

This is a simple example of how I'd probably do it, based on Figure 9 of 14.3 and text in section 1.7. I've been taught that when the removed view isn't on the same sheet add the sheet number next to the view label as I've shown. It's not explicitly in the standard that I see but is probably good practice.

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[ModulusCT]

Thanks, guys... What I initially supposed about an offset set of section view arrows referencing the view on another sheet seems valid. I'll check out Y14.3-2003.

I don't like seeing datum identifiers on a drawing unless they're being used in a FCF somewhere.

 

[ModulusCT]

Didn't see your post before my reply, Kenat. Yes, that's exactly what I was intending to do.

Thanks.

 

[fsincox]

I agree with KENAT's method for removed views. This is a very common practice with auto company, aircraft and even architectual drawings, I believe, complex drawings of complex parts. Datums can be repeated as required provided they are used for the purpose they are intended. You should not create datums only for view orientation, only, that is not their purpose. I was once told datums must reference a FCF (feature control frame). I have seen auto company drawings that had a table on the first sheet listing all the datums and their primary location in the drawing sheets, the one that not only calls out the datum but it's additional feature controls. AMSE-ANSI Y14.3 Multi and Sectional View Drawings is what you are looking for.

 

[MartinSr00]

As an aside, a requirement to use only A sheets, for example, is not the source of the problem. It's splitting different 1st or 3rd quandrant projected views across sheets. You could do that on any size paper. By using datums to clarify orientation, you're using the datum callouts kind of like a theatrical prop.

Better to reduce the scale on the overall drawing showing adjacent views sufficient to establish the orientation, and show enlarged detail views to show the part detail. Othewise show sectioned or explicit views with cutting planes.

Drawings on multiple sheets is established practice many places. And a 1:1 scale may sound like a good idea, but not very workable if you make tiny fasteners or are in the ship building business.

Though repeating datum callouts on removed views and the original view is allowable, it's bad practice when you're trying to control configurations. If a change is needed, and you don't make the same change twice, your drawing is no longer subject to one interpretation.

 

[ModulusCT]

The view that's being placed on the following sheet is a 2nd projection (the back). It's difficult to grasp the orientation of the part when both the front and back views aren't visible on the same sheet.

 

[KENAT]

MartinSr00, given the minimum font sizes per ASME stds (and the ability of many myopic folks to read them) and the related proportion of other drawing symbology, title block etc, then if you want a nice clear drawing to ASME standards then A size is an issue. Also, while I can't remember the specific reference, I'm pretty sure that 1:1 is the preffered scale in the standards and that larger or smaller is only used when necessary.

As to your comments about boats, obviously you aren't familiar with where the term lofting came from;-).

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[MartinSr00]

Fair enough on the A size -- it is pretty tiny.

The GD&T Standards, if I'm not mistaken, simply require that it be to scale with a few exceptions. I've often dealt with the machining of 80-inch ball valves and 120 inch diameter pump covers. 1:1 doesn't work. Of course if I had to do that on A sheets it would be quite a lot of them, and would be very difficult to do given the requirement for text readability.

At my work, we used almost exclusively B sheets. Given my 'druthers I would have liked to use something like an E sheet. We did it -- and the drawings were clear.

You are, however, prone to "page-switch-related" errors when manufacturing the parts. In the example described above, it seems that Mod is worried about this type of error.