Datum Location

[TDATMOOG]

Does anyone know the proper way to add a datum point to an imaginary point?

 

[powerhound]

ringster,

Yes they do, so what? The point comprised of those planes is useless. A point out in space is NOT a valid datum, no matter how bad you want it to be...period.
Why don't you make a drawing using this concept you are vying for and post it here so we can get a better idea of what you're trying to say. I don't mean to draw 3 planes with an arrow showing where they intersect. Draw a part with a slot in it and then apply this datum scheme so we can all see what you're getting at.

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
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[ctopher]

More info:

http://gdtseminars.com/blog/2008/03/06/dimensions-with-datum-targets/

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

 

[DesignBiz]

Ringster,

Its okay to agrue... I prefer to call it debating and/or questioning or challanging other's statements. I find this to be very useful to get 'stuff' right. I do appreciate this site where my thoughts and understanding regarding variuos subjects can be affirmed or corrected. Makes it interersting to me. After all, this whole engineering enviroment should be based on accurate communication.


Agreed regarding 2 intersecting planes resulting in a line.
However I was trying to "point" out (no pun intended... on second thought that on a drawing the centerline of a hole has 2 lines representing the two planes and their intersection is a centerpoint. In this case I would consider this centerpoint to be a datum point. Technically the centerpoint would represent an axis and we would be looking normal to the axis. This was suppose to be relative to fig 1-35 (b) page 15 of the standard which shows a single center of the slot.

Maybe that line of thought was more confusing than helpful.




DesignBiz

"Quality is in the details"

 

[ringster]

Let me clarify my position. I did not propose the need for the identification of the point at the center of the slot. PAT had originally requested a method for identifying that point. It is my opinion that if he has established the three mutually perpendicular planes, he in essence has determined, (established,identified) that imaginary point.

 

[DesignBiz]

Ringster,

I believe I understand your position and it is clear to me that you have been describing the datum point as defined in the standard. My orginal comment was in regard to the origninal question. I took the question to be relative to a drawing. I have re-read this string and it is unclear to me as to what the question pertains; the slot feature center point or a representative point on a drawing view.

It would be beneficial to know the objective of the question.

I do see concerning comments through out this string, mixing the terms datums and features. As MechNorth points out datums are theoretical. They can be established on the physical part by feature simulators.

Drawoh comments, "Your datum must be a real feature."
A "datum" according to the standard is theoretical. A "datum feature" is an actual feature of the part.

Can be very confusing when terminology is not used correctly. My appologies in advance Drawoh. Just used your comment as an example. I do this at times myself and need to practice using the precise term required.



DesignBiz

"Quality is in the details"

 

[TDATMOOG]

OK. It sounds like that using a slot to define a datum is unconventional. To define a datum with a circle is conventional. A slot will have two center points whereas a circle will have one. The datum that I am defining on the slot is the midpoint between the two slot center points. This point can be located using CMM and I plan on defining that point on the drawing by pointing to it with a leader. These postings have been helpful. Thanks to all who have contributed.

 

[ringster]

Design,

I am totally in agreement with you on the proper terminology when discussing GD and T.

A pet peeve of mine is ORIENTATION vs ALIGNMENT. (not related to this thread)

 

[ringster]

Pat,

A course on the applications of GD and T might be of benefit to you. Some of your terminology is not quite proper.

 

[KENAT]

PATATMOOG, as to whether it is justified by function etc I'll leave that for you to decide.

However, if you want the center of a slot to be the 'X' & 'y' datum then picking up on drawoh's first paragraph the attatched should achieve this.

http://files.engineering.com/getfil...4c-4a1c-b836-60a8fce98128&file=SLOT-DATUM.tif

KENAT,

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

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

 

[drawoh]

KENAT,

That is what I was visualizing.

Note that you need four datums to fix your part, since datums[ ]B and[ ]C are needed to locate in[ ]X and[ ]Y. Datum[ ]C does not provide much clocking unless it is long. PATATMOOG could consider using one side of the slot for clocking purposes, but it should still be long.

Note also that your view shows that datum[ ]B is the profile of the radii at the ends of the slot. This is necessary for the mechanical fixturing I described, above. PATATMOOG feels he can program a CMM to find the two centres. The end radii could be datums[ ]B and[ ]C, but I can visualize no inspection method other than CMM. The central position between the two radii would be irrelevant.

Either way, you need a slot with an accurate length. PATATMOOG's concept requires an accurate end profile. A slot's usual design intent is to allow sloppiness.

JHG

 

[MechNorth]

A bit of an adjustment to a couple of posts above;

First, the centerlines on a drawing are only a convention; they actually mean nothing as far as datums are concerned. The graphic that Kenat posted would have been better if the "means this" graphic had been 3-D, with planes shown instead of centerlines. This is misleading, and a problem I occasionally come across and have to correct in training sessions. Might seem trivial, but it is another important distinction.

As for whether a slot should be used as a datum feature ... that depends. The width of the slot can be used as a datum feature of size to generate a single datum plane (represented as Datum-C on Kenat's dwg); whether this is primary, secondary or tertiary is dependant on the functionality intended. Using the length across the tangents of the radii isn't that common (from my experience) and may be hard to repeatably setup for inspection.

PatAtMoog, if you can, pls provide some further details and a good graphic to illustrate the intent. Tks.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[KENAT]

MechNorth - they aren't centerlines. If you look they are phantom lines, deliberately to try and avoid this confusion. The lines in the left hand view are centerlines.

I don't have the time to create 3D images when I believe a 2D one is perfectly adequate, if you do then I'm sure it will be appreciated by the OP.


KENAT,

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

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

 

[MechNorth]

Kenat, I hope you don't really believe that was my intention, because it wasn't. I train people and as I said, it is a common misconception that centerlines represent datum planes, even establishing the orientation of datum planes. In this case, I didn't see the distinction between the two line types on first view. It was as simple as that. The sarcasm wasn't warranted.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[KENAT]

There is no sarcasm, I'd hope the OP would find it useful.

A bit of frustration was released in my last paragraph (which I RF & is now deleted), so appologies.

KENAT,

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

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

 

[MechNorth]

Not a problem, Kenat. I understand the frustration too.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ringster]

Mechnorth,

Have you seen the Tip of the Month for Nov 2008 by Tec-Ease?

As I view it is shows two centerlines which are representative of the datum planes of the length and width of the part. How do you explain or justify this situation?

 

[powerhound]

There's nothing wrong with the graphic on the tec-ease website. I think what Jim was saying is that it is a common error made that the datum identifier is attached to the centerline instead of the feature or somehow someone thinks that there is a centerline that all edges should be worked around, kind of like the September 2007 tip on tec-ease.

http://www.tec-ease.com/tips/sep-07.htm

I see this frequently and it's amazing to me how difficult it is to get an engineer who has never spent an hour on the shop floor to understand this. I had one guy refuse to look at the above mentioned tip. My guess is because he knew it would totally invalidate his case.





Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

Powerhound, that's probably my favourite example of the implications of incorrectly applied GD&T.

KENAT,

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

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

 

[ringster]

Powerhound,

What is the purpose of the CLs in the example? For one thing, it could be construed as the centerline between the holes rather than the CL between the sides. There is no VALUE added, IMHO.

 

[powerhound]

The centerlines on the holes are absolutely necessary per Fundamental rule 1.4 (j). The centerlines on the part itself are probably not necessary in this case but if a feature were dimensioned from the centerline, the FCF would have to call it out WRT the DRF and unless the hole pattern were used as another datum, the meaning would be from the center of the part, NOT the holes.

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II