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Datum Location

Datum Location

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

RE: Datum Location

And just how is the imaginary point being established please?

RE: Datum Location

(OP)
It will be a center point(not centerline)of a closed slot.

RE: Datum Location

PATATMOOG,

   Your datum must be a real feature.  A basic rule of GD&T is that the fabricator and inspector must use your datums for fixturing.  It helps to visualize what this fixture looks like.

   The width and length of a slot can be picked up by a cross shaped pin.  This is necessary if you have a separate datum that prevents your part from rotating.  

   On your drawing, the width and the length of your slot would be two separate datums, i.e. you will need four datums to locate your part.  Both datums need to be specified at MMC.  Whenever I specify a slot, I make the length sloppy. Sloppy features are not good feature-of-size datums.  

   If you need an X/Y locating feature, you should make it round, and slot all the other sloppy features.  This makes the GD&T on your drawings, and your fabricator's and inspector's lives, easier.

   Another possibility is to use your slot to control rotation as well as X and Y.  This will work if your slot has an accurate width and it is long.  Otherwise, this is a bad idea.  Your maximum angle error in radians is your maximum clearance divided by your slot length.   

               JHG

RE: Datum Location

There is no proper way to do it, because it isn't proper to do.

Datums have to be REAL, TANGIBLE features.

V

RE: Datum Location

This will maybe confuse, but it is a critical distinction.  A datum is a theoretical point, line (axis) or plane.  A datum feature is a real, physical feature on the workpiece, which is used to establish the datum (point, axis or plane).  The datum simulator would be the pin or key (in this case) or other representative component that would be used to represent the datum.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Datum Location

gentlemen,  

Are we exercising proper descriptions in our discussion?  If not shouldn't we?

I was taught that datums are theoretical points, planes and lines.  That is rather imaginary is it not?

Datum features on the other hand do exist on the parts.
 

RE: Datum Location

ringster,

   The OP is asking how to add a datum point.  I am interpreting this as how to specify such a datum on a drawing.  My cross-shaped pin will locate the centre of a closed slot, at MMC, with the limitations noted above.

   I suppose my first paragraph is not well written.  How about...

   Your datum must be defined by a real feature.  

               JHG

RE: Datum Location

My guess is that the orig post was to inquire how to identify a datum point.  If that is the case I believe it is covered by the Standard.

RE: Datum Location

Yes it does cover how to create datum points but in all cases they are real points on real surfaces, not points out in space.  

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

RE: Datum Location

Question:  

Can 2 points occupy the same space?  That seems to be the oriiginal question as stated.

Where is Kenat when you really need him?

 

RE: Datum Location

The datum point (which is theoretical), must repeatable; i.e. you must be able to use real datum features (physical features) to establish the point.  An example of this would be a truncated cone, the truncated apex of which is for some reason desired as the datum point.  Again, a datum must always be related back to datum features.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Datum Location

Pat,

After some careful consideration, it appears that you may be where you are looking to be.  That is to say that if you have your 3 mutually perpendicular planes, established by the slot length, width and another surface, these three planes will converge in a point.  Is that not the point you are seeking?     

RE: Datum Location

I agree with staying sticking to the definitions of the standard for discussion of it !

An "imaginary point" would not be based on fact. It lacks factual reality.

A "theoretical point" would be the analysis of a set of facts in their relation to one another.
Relative facts would establish a "theroretical point".

I would stick to the standard's definition of "theoretical datum point" and/or (not to be confused with) a "datum target point".  "Imaginary point" terminology does not lend itself to geometric relationships and seems to confuse rather than to help in discussion.

"Datum points" always seem to be the intersection of 2 perpendicular planes as in the case of a hole's centerline, and of course the intersection of 3 mutually perpendicular planes as in the case of a DRF and it's origin.

Interestingly too is the figure on page 15 of ASME Y14.5m 1994 which demonstrates the way slots are to be dimensioned. Fig (b) depicts the centerline of the slot with a leader of width and length dimensions. In this case I would say there is a "datum point" at the intersection of the slot's centerline represented by a 'single vertical' and a 'single horizontal' line, representing 2
perpendicular datum planes. This "datum point" could be simulated by a gage pin in contact with the datum feature (the physical slot). This is a common way to dimension slots.

DesignBiz

"Quality is in the details"
 

RE: Datum Location

Designbiz,

Not to be argumentative, but, I believe that the intersection of 2 planes, perpendicular or NOT, results in a line, which I believe in theory contains an infinite number of points.  The center of a circle, on a plane, would result in a point I believe. Or the tangency of a circle and a line or plane, likewise.

RE: Datum Location

No one is arguing on whether a point can be created from planes, lines, circles, or otherwise. The question is how to measure to it or from it or how to fixture around it...and how can a single point within a slot possibly be functional anyway?. This is not a valid dimensioning scheme. Re-read what Jim wrote in the post above.

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

RE: Datum Location

Do not the three mutually perpendicular planes in fact establish (determine) a point at their intersection?

RE: Datum Location

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
Taji, Iraq OIF II

RE: Datum Location

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"
 

RE: Datum Location



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.

 

RE: Datum Location

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"
 

RE: Datum Location

(OP)
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.

RE: Datum Location

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)

RE: Datum Location

Pat,

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

RE: Datum Location

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/getfile.aspx?folder=b5324dba-cd4c-4a1c-b836-60a8fce98128&;file=SLOT-DATUM.tif

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Datum Location

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

RE: Datum Location

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  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Datum Location

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: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Datum Location

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  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Datum Location

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: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Datum Location

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?

RE: Datum Location

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

RE: Datum Location

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.

RE: Datum Location

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

RE: Datum Location

Powerhound,

I believe you misread my post.  I stated it might be misconstrued and the CL BETWEEN the holes, not the Cl of the holes.

RE: Datum Location

No I didn't mis-read...the statement about the centerline of the holes was an addition. The centerline between the holes is what I was saying was probably not necessary (so I'm agreeing with you) but I called it the centerline of the part, which makes your point. What I was saying otherwise is that if there was another feature (let's say a hole) on the part and there was a dimension to it from centerline, and the FCF specified the hole WRT to A, B, and C, then it would be measured from the center of the OD of the part, not from the center of the hole pattern. Believe me, I am not from the school of "Centerlines on Everything".

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

RE: Datum Location

The centerlines in the Tec-Ease Nov'08 tip are used to graphically show symmetry; really they don't achieve anything else.  What I've seen too often is that people will assume that the centerline (regardless of where it is on the drawing) represents the datum plane, and that the datum plane is thus in the symmetrical center of the workpiece.  I've tried to illustrate the issue in the following graphic (yes, I know that the workpiece geometry is not the same as we have been discussing in the OP, but I had something similar on my mind for a while, so here it is): http://www.profileservices.ca/files/tidbits/thd1103_237533.pdf

I'm sure that this will raise questions and comments that the misinterpretation is blatantly not correct, however this is what trainers deal with on a regular basis.  I once spent over an hour in one class converting the masses to acceptance that the centerlines are only graphics with no physical meaning ... long hour.  Some still argued that graphically represented centerlines were absolutely critical to convey information ... I asked where the centerline was drawn in the CAD model and on the actual workpiece ... gave them pause for thought.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Datum Location

Jim,

My favorite is a hinged glass assembly drawing that came across my desk.

The previous design was orthogonal...so the datums were the hinge surfaces primary and the pattern slots in the hinges secondary. I didn't think the old drawing was toleranced correctly, but at least I think I could make an educated guess on intent.

In the new design the Hinge Slots were not orthogonal...but, the part seemed to have been dimensioned orthogonally anyway.

Take a look for a good laugh.

Michael

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