Datum split 1

[jerry1423]

I have a base plate where the bottom of it is datum -A-.

There is a groove along the entire length of the bottom of the base plate.

Can it be implied that datum -A- is the total bottom face, as if the groove is not there, or do I need to deal with it as two bottom faces?

 

[ctopher]

Whatever surfaces are implied or dimensioned with that datum indicating it, that is the surface (or plane) that the datum references.
The surfaces on both sides of the groove will be the datum -A-, unless there is some offset in your design.
The groove surface is separate and can have a separate datum referencing other features/dims.

A picture may be more useful.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08; CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
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[KENAT]

Assuming you're working to ASME Y14.5M-1994 look at section 4.5.7.1 & figure 4-20.

If the groove is relatively narrow you may get by without making the distinction since 4.5.7.1 does say separate identification if the 'groove' in your case is of "of sufficient width".

Another option is to show an extension line across the groove but this may not work for a narrow groove.

I've sometimes seen a note "2 SURFACES" placed next to the datum ID to clarify that the datum is derived from the 2 surfaces but I'm not sure this explicitly in the standard.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[dingy2]

Place a phantom line across the groove to both surfaces which indicates that it is now considered one surface. That's all.

Dave D.

http://www.qmsi.ca

 

[KENAT]

Dingy, I nearly suggested a phantom line and it's something I've done before but noticed that 14.5 says 'extension line'.

Per ASMM Y14.2M-1992 section 2.8 extension lines are 'solid lines' (though with short visible gap from the part out line ASME Y14.5M-1994 1.7.1.4).

A phantom would probably work, but if you'r a stickler may not be correct.

Also if it truely is a narrow groove then neither phantom or extension will work well unless you do a larger scale view or detail showing the groove.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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[dingy2]

Kinat:

I don't know where in 14.5 stating a extension line should be used and 1.7.1.4 reflects crossing dimension lines.

I would suggest that you go to page 172 and 173 fig. 6.20 and 6.21 where it does show a coplaner condition and a phantom line is used in both examples.

Dave D.

http://www.qmsi.ca

 

[KENAT]

Dingy, the reference I gave in my first post is where it states to use extension lines, 4.5.7.1.

Penultimate sentence "Where appropriate, an extension line may be used to indicate a continuation of one datum feature across slots or obstructions."

The examples you give with phantom lines are talking about using profile tolerance for coplanar surfaces, not explicitly about simulation of a single datum plane. Also, as has come up before, 1.1.4 means we can’t overly rely on just figures.

I was just trying to share something I’d noticed.


KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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[ewh]

Extension line

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[ctopher]

Some additional info on datums:

http://www.engr.sjsu.edu/mae/faculty/sullivan/04_GD&T_IntroTutorial.ppt#355,34,Features

that Require Datum Reference

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08; CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion

 

[aardvarkdw]

KENAT is correct that 4.5.7.1, which specifically addresses this situation with respect to datum definition, says to use an extension line. He is further correct that Y14.2M say that an extension line is solid.

However, Dave is correct that the standard "Shows" in figures 6.20 and 6.21 that a phantom line should be used.

My opinion in this is that you should defer to the section 1.1.4 that states that the figures are only intended as illustrations of the text. Therefore the text in 4.5.7.1 is correct and the figures are wrong. Though I would probably use a phantom line for clarity's sake regardless of how the text actually reads.

David

 

[dingy2]

Kenat:

4.5.7.1 does state an extension line and it does make sense. The figures I previously mentioned do not follow the standard but words do supersede drawings here. Going to make a couple a changes in my training book.

Got to give you a thanks on this one. Hats off to you!

Dave D.

http://www.qmsi.ca

 

[jerry1423]

Thank you to everybody for the quick replies ... phantom line it is.

 

[vc66]

Didn't we just conclude that it was an extension line? Or am I missing the sarcasm?

V

 

[KENAT]

Yeah, jerry assuming you were being sarcastic do me a favor and put a smiley next time, my head nearly just exploded;-).

aardvarkdw & dingy, I'd argue as I put above that figures 6.20 and 6.21 aren't explicitly talking about multi surface datums per se and so probably aren't directly relevant. They are dealing with using surface profile for co-planarity which is different though related. So I'm not sure we can say they are wrong.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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[MechNorth]

Kenat's last post addresses the difference here. An extension (solid) line shows a relationship (i.e. desired or implied coplanarity) but is not used when controlling the coplanarity relationship. To control the coplanarity, the phantom line is used as specified in 6.5.6.1, Fig. 6-20 & -21. In simplest terms, phantom lines are used to establish relationships between features such as coplanarity, and to show relationships to geometries not present, such as mating parts.

I don't have a copy of Y14.2 at hand; can someone pls post the stated uses of a solid line? Of a phantom line? My recollection (hazy) is that solid lines are used on mechanical engineering drawings to represent visible geometries, extension lines extending from geometry to notes and dimensions, and on leaders. My suspicion is that the use of an "extension line" is inappropriate to the intent and a "phantom line" would be correct. It wouldn't be the first time that an inconsistency has slipped through reviews into final production. Of course,it wouldn't be the first time that I was mistaken either.

I've seen enough problems in manufacturing to know that if a print can be misinterpreted, then it will be. Putting a datum callout on a leader to one surface, with a solid line extending between the implied coplanar surfaces, has resulted in the shop using the first surface as the datum surface. While the drawing may be technically correct, it's ambiguous. A surface profile must be used to establish the coplanarity of the surfaces, and the datum feature callout should be attached to either the FCF, or to the phantom line connecting the features.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[KENAT]

How's this:

2.8 Dimension, Extension and Leader lines

These are solid lines used to dimension drawings as described in the following paragraphs (see ANSI Y14.5M, Dimensioning and Tolerancing).

2.8.2 Extension Lines. Extension lines are used to indicate the point or line on the drawing to which the dimension applies (see fig. 1 and 2). They are also used to indicate the extension of a surface to a theoretical intersection (see Fig. 10).

I missed some fig references from the below.

2.11 Phantom Lines

Phantom lines are used to indicate:
(a) alternate positions of moving parts
(b) adjacent positions of related parts
(c) repeated detail
(d) filleted and rouned corners

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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[ewh]

Seems pretty clear to me...
This is more of a drawing standard issue than it is a GD&T issue.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[MechNorth]

Tks Kenat. Agree ewh.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com