Datum split
Datum split
(OP)
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?
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?





RE: Datum split
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
SolidWorks Legion
RE: Datum split
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,
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RE: Datum split
Dave D.
www.qmsi.ca
RE: Datum split
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,
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RE: Datum split
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.
www.qmsi.ca
RE: Datum split
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,
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RE: Datum split
Extension line
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Datum split
h
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08; CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion
RE: Datum split
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
RE: Datum split
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.
www.qmsi.ca
RE: Datum split
RE: Datum split
V
RE: Datum split
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,
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RE: Datum split
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 www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Datum split
I missed some fig references from the below.
KENAT,
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RE: Datum split
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." - Robert Hunter
RE: Datum split
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com