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True Position?

True Position?

True Position?

(OP)
So I'm drafting up some process prints for a run of new tombstones we have in our shop.  Aside from the surfaces being machined we are putting a dowel hole pattern on the faces.  We are looking to control the location of these holes to the bottom and to the center of the tombstone, which I believe will be our primary and secondary datums.  My question is what would be ideal for our third datum?

RE: True Position?

If it is (2) pins I would say you do no need any other. ASME Y14.5M-1994, Fig 4-29.
Frank

RE: True Position?

What you describe most likely will be your secondary and tertiary datums.
Your primary will be face or back.
Hard to tell without seeing some details - how many sides, etc.
 

RE: True Position?

(OP)
Checker
It's a four-sided tombstone ..also I thought I read somewhere that the primary datum should be perpendicular to the feature which is what I think you're saying ...is this true?

RE: True Position?

Is the tombstone rectangular block? Are these holes located on the side faces of the block? A simple sketch of geometry in Paint would help.

Though it's the most common case to have primary datum perpendicular to the hole, it doesn't always have to be what is really needed. For sure it's not a hard rule.

RE: True Position?

Datums are HUGE area of GD&T; and you may need to check some additional sources.
For now, let say primary datum is what is the best to locate your part for machining /  inspection.
In case of simple rectangular plate with holes in it, the "flat" would be your primary datum, then longer side the secondary, and the short - tertiary. Then you locate the holes to sides and orient them being perpendicular to "flat".
As far as I can imagine four-sided tombstone, things are little more complicated. The bottom actually can be your primary.
The 2 center planes derived from "width" and "depth" could be secondary and tertiary.
I will try to produce picture after lunchtime smile
 

RE: True Position?

(OP)
The tombstone is square and the holes are on the faces.  I'm looking to control the position to the bottom plane and the center axis of the tombstone most critically, but need a third datum to keep it from spinning ....maybe a second axis off center?

RE: True Position?

You may not need a tertiary datum. It seems your primary and secondary datum references constrain location of positional tolerance zone in a satisfactory way. Look at attached picture, just imagine there is only one (1) hole not four (4) and forget about two bottom segments of composite position tolerance callout. Tertiary datum feature is not needed in this case.

http://files.engineering.com/getfile.aspx?folder=fa5a55b2-6def-40c1-879d-2e8fd756cec9&file=2_datum_references.JPG

Do you see analogy?

RE: True Position?

Pmarcs, example is a good one for a part not needing the third datum, it is not an absolute requirement. The functional requirement is key for me, which we do not know.
Frank

RE: True Position?

(OP)
Here is a cross section of the tombstone. The bottom of the tombstone and the center of rotation which is the center bore in the bottom are critical.  I want to control the tru position of the dowel hole pattern on the sides to these features.  As it is I have my primary datum as A (the bottom plane)and secondary as B (center axis).  Inspection requires that I have a third datum to stop rotation around datum axis B?  I hope this clarifies and appreciate your assistance.

RE: True Position?

Technically, it is up to you to choose the tertiary datum feature. You are the one who knows which features in the part play the most important functional role and can be considered as canditates for datum feature. From what is shown, this can be one of the holes in the bottom of part, but also width or height of bottom plate or widht or height of the column. It is really hard to tell without knowing how this part functions.

However from theoretical point of view your inspection is wrong. Tertiary datum feature is not needed as long as literally all features here (dowel holes, widths and heights of rectangular elements, holes in the bottom) are located with relation to the same datum reference frame |A|B|. In such case rule of simultaneous requirement is in charge, so everything can indeed rotate around datum axis B, but this is simultaneous rotation, meaning that all those features are in fact tied to each other.   

RE: True Position?

Your tertiary datum could be one of the holes in your base. Possibly dowel hole, or some other feature made with precision.

RE: True Position?

pmarc,
From theoretical point your datums will constrain 5 out of 6 degrees of freedom, so your datum reference frame will be incomplete.
 

RE: True Position?

CH,
That is right, but it does not have to be fully contrained, because the importance of last rotational degree of freedom around datun axis B will be somehow neglected by simultaneous requirement rule which will tie all features together in terms of rotation. The necessary condition is that all holes, widhts and heights are controlled relative to the same datum reference frame |A|B|.

RE: True Position?

Guys,
You cannot have 3-dimensional part desribed using only 2 axis.
Where exactly ASME "likes" it, and where does it say datum reference frame does not have to be constrained?

RE: True Position?

CH,
It is just part of the rules of the ASME game. If they didn't like it they would change it, The ISO doesn't like it. You judge by people's  actions, it has been ASME law since at least 1982. To me it is just like the envelope principle and AME's, these assumpltion of perfection ASME likes to make. Sometimes, like here, It makes the engineer's job easier.
Frank
 

RE: True Position?

Frank,
What makes you think that ISO does not like simultaneous requirement? Do you have any reference to the standard?

It is not that I am defending the concept. I do think it is less intuitive and I mentioned it only to show that there is a possibility of living without tertiary datum feature, however I just wanted to know where did this opinion come from?

RE: True Position?

Simultaneous requirement has nothing to with creating your datum reference frame. It simply means that all the features that reference the same DRF make a pattern. You establish your DRF long before you decide if simultaneous requirement will apply. And without clocking feature your DRF (in this case) will not fly.

RE: True Position?

CH, since all features referencing the same DRF create a pattern, they are all tied together regardless of how many degrees of freedom were constrained by the DRF.

There is this Tec-Ease tip showing idea of the concept. Though I have some remarks to this, it also offers interesting comments about additional considerations that should be taken into account before applying it.

http://www.tec-ease.com/gdt-tips-view.php?q=126

I am pretty sure there was quite hot debate about it way in the past on the forum.     

RE: True Position?

The part in Tec-Ease example can be allowed rotational freedom because it is "spinning" in real life.
How will you inspect "the tombstone" if you allow it to spin?
There is even better illustration of simultaneous requirement. Look at ASME Y14.5-2009 Fig.4-39.
Do you notice that part on the picture has TWO HOLES as secondary datum, while OP's example has only ONE HOLE for secondary datum? The devil is in the detail.
 

RE: True Position?

Pmarc,
As I understand it is not their default condition, it seems it would have been easier for all concerned to just do it the ASME's way, if it made sense to them. I certainly can imagine there is dissention in both camps, though.
Frank
 

RE: True Position?

Sorry, but I don't get you. I do not see a difference between Tec-Ease's example and OP's except the number of datums referenced.

As for 4-39... Well, this is quite a nasty example IMO, exactly because of TWO holes being the secondary datum feature. Please explain me how could the part's geometry look like if the profile callout was specified as a SEP REQT. Where would be the difference between having and not having simultaneous requirement for this particular part?  

RE: True Position?

pmarc,
It's really sad that you don't see the difference.
The idea of using DOF is simple: you constrain part until it stops moving OR the movement of the part has no effect on measurement.
For example: you put flat part on the table - create primary "plane" datum. The part can slide and spin, but you still can measure parallelism because it only requires one datum.
Or the part you make on the lathe: when you rotate it, it still look the same, so you can allow rotational degree of freedom in your DRF.
4-39 is nasty, but legal. Part is going nowhere.
With SEPT REQ for profile part will look the same. It's just, if you decide to make gage to check the part, you can build two separate gages - one for profile and one for 4-hole pattern.
This is the best I can do to present my point of view. Feel free to disagree.
 

RE: True Position?

Pmarc,
Is it your understanding that it is as broad as the ASME understanding?, this says "shown on the same axis", which is a good point for me to remember, but my whole contention is these things are part of the standard, but not even known, or followed, in most shops.
Thanks,
Frank

RE: True Position?

CH,

Pmarc is right.  It is not necessary for the datum feature references to constrain all 6 degrees of freedom, regardless of the part geometry.  Parts can be inspected even if rotational or translational degrees of freedom are left open.

During inspection, any open degrees of freedom can be optimized.  In other words, the part can be arbitrarily rotated/translated within those degrees of freedom.  This isn't always convenient or easily repeatable, so a common practice is to arbitrarily choose additional alignment features provide stable constraint for inspection.

pmarc,

Regarding 4-39, as CH says there could be one gage for the profile and one for the 4-hole pattern if SEP RQTS were specified.  This would change the accuracy requirement for the part, and allow the holes to be shifted one way and the profile to be shifted another way.  The original SIM REQ would not allow this, as the entire part would have to be evaluated on the same gage (in effect, both the profile and the holes would have to be evaluated in the same DRF).

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: True Position?

Evan,
You are right about the datum shift effect. I forgot about that. Thank you.

RE: True Position?

Evan,
I understand you are CMM kind of guy who believes that it's just enough to put part on the table for inspection.
Would you please clarify; when you "arbitrarily choose additional alignment features", do you in fact add missing datum feature(s) possibly not specified on the drawing?
 

RE: True Position?

Frank,
Point for you on whether simultaneous requirement is implied for features shown non-coaxial on ISO prints or not. It seems it's not. I just looked to Henzold's book (figs. 20-29 and 20-30) and he says that in such situations clear indication "simultaneous" or "separate" must be given. Though it is just only his idea of dealing with this, not supported by any standard, I am inclined to buy this explanation. Thank you.

RE: True Position?

CH,

Short answer, yes.

To inspect a feature on the CMM, a fully constrained "part coordinate system" needs to be created by aligning to features on the part.  If the datum features in the FCF do not constrain all 6 DOF's, then additional alignment features are added.  Since these additional features are arbitrary, it is quite possible that they are not specified on the drawing.  These additional alignment features should ideally be convenient to measure and have a good chance of constraining the open DOF's in an optimal way (or at least close).  Often the considered feature itself is chosen as an additional alignment feature.

Many (but not all) CMM softwares are able to use this arbitrarily constrained coordinate system as a starting point, and then "best fit" within the open degrees of freedom from the FCF.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: True Position?

pmarc,
I actually wouldn't mind it either; my main objections come from the fact that, I can never use it in the real world. Just like 0 position at MMC and the envelope all fine concepts if the other guy plays by the same rules but I do not find any shop or inspection people (really the only ones are the ones here) that understand it to be the case. This puts me, as a responsible engineer, in the position of not having it anyway. I would rather start from the same position as they do since I have to anyway.
Frank
 

RE: True Position?

dtmbtz,
you are thinking of the same tombstone I was thinking of! smile
Frank

RE: True Position?

Thank you Evan.
Appreciate straight "yes" as always. There is one thing I still don't understand.
You say "To inspect a feature on the CMM, a fully constrained "part coordinate system" needs to be created by aligning to features on the part."
At the same time you support pmarc's opinion that creating fully constrained set of datums is "wrong".
DOF is mathematically precise way to create comprehensive datum framework. I understand "mathematics" may be a dirty word among some part of 14.5 crowd, but you in your line of work actually use and appreciate DOF, so I feel confused.
At least you made me realize that DOF concept is not widely accepted, even among "the best and the brightest"; Genium manual, for example, ignores it completely.
Well, nobody is perfect sad
 

RE: True Position?

CH,
I do not believe pmarc is saying it is wrong to have a full datum framework just not necessary "under the law" which I am forced to agree is correct.
Frank
 

RE: True Position?

Frank,
pmarc actually used the word "wrong".
Technically it still legal to use plus-minus dimension for location, but I do not say using position is wrong.
ISO "likes" idea of using DOF pretty much. We could discuss it sometime.
BTW, I misspelled your name in other post. I am sorry. sad
 

RE: True Position?

Okay, maybe instead of "wrong" I should have said: "From theoretical point of view your inspection is not 100% correct by requiring tertiary datum, because theoretically there is a way of living without it and having the tombstone's geometry fully defined and inspectable". Would it be better then?

RE: True Position?

The short answer is "yes". smile

RE: True Position?

CH,
I am not really sure what you want to use +/- dims on but I would say it is not well defined once the whole FOS redefinition occured (1994), I am not saying it was perfect before, but, I honestly am not sure what we have now, at least with ISO I know what it is, opposed point measurement.
Frank

RE: True Position?

Frank,
I don't want to use +/- per se. I was using it as example against "it can be done" argument.
Let say I prepared the drawing that can be interpreted 10 different ways. But all 10 will result in good part. Should I consider my drawing "good" because it "can be done", or should I still try to make it more standard-compliant and unambiguous?
After all tomorrow someone may invent the 11-th way.  
 

RE: True Position?

Are you aware of anyplace in the current ASME standard that gives a definition to a non-directly opposed toleranced dimension? Something everyone thinks they know until pressed. I must have missed it.
Frank

RE: True Position?

Directly toleranced or Basic included?

RE: True Position?

No basic, No directly opposed (FOS), just the kind of dimensions on the majority of prints I see everyday.
Frank

RE: True Position?

Frank, the closest example in 2009 would be Fig. 2-5?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: True Position?

The link is showing Tec-Ease's opinion about applicability of directly toleranced dimensions.
http://tec-ease.com/gdt-tips-view.php?q=165
Though no references to the standard given, I like this one.

I also find interesting that chamfers have not been listed there, unlike in Alex Krulikowski's Fundamentals of GD&T. I am not saying they should be, just mentioning the difference.

RE: True Position?

J-P,
Thank you, that example involves the concept of implied datum reference measurement, so to speak, but definitely, thank you for helping me look.
Frank
 

RE: True Position?

Frank,
If you insist on dimensioning non-opposing points, the only thing that comes to mind are dimensions with origin symbol, and, by extension, dimensions without dimension lines.
 

RE: True Position?

So, the counterbore hole at the bottom of the tombstone establishes the datum axis?  Not great because it's so shallow compared with the height of the tombstone, but so be it.  Datum shift also mentioned, but can be eliminated by putting a conical taper instead of cylindrical mating condition.  Looking at the layout of holes and faces on this fixture, there is an evident need for a rotational control.  Otherwise, the holes on the base can be rotated about the datum axis, causing the tombstone faces to be off-square wrt the tool path on the machining center.  Note though, that this doesn't center the dowel holes on the face of the tombstone, but rather wrt the base of the tombstone.


Personally, favor CH's datum layout for centering on the tombstone mating/cutting face.  The cutter is adjusted to that position anyway (center of the width of the vertical face) as its center, and that's what where the programs start from (from my background) rather than the center of the tombstone base interface to the machine bed.  This minimizes the impact of the dowel pattern being shifted to the side.

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

RE: True Position?

Simply, as I see it now there are FOS dimensions and feature locating dimensions, so to speak, the one that are not really considered a FOS. Under the old versions of the standard I always looked at all dimensions as FOS dimensions, I suspect others did, too. That seems to have been one of the things that definitely was intended to be changed with 1994 with the change in the definition of FOS.
Frank
 

RE: True Position?

CH,

I support pmarc's opinion that it is not necessary for the datum features to constrain all 6 degrees of freedom.  There are many examples in Y14.5 in which less than 6 degrees of freedom are constrained (or even none at all).

Degree of freedom (DOF) constraint is a central concept in datum reference frame (DRF) definition.  Another central concept in Y14.5 is that the DRF is defined in the datum feature simulators.  In other words, the coordinate system is defined in the gage elements.  This concept is easy to work with in a hard gaging context, which the Y14.5 standard and most GD&T books tend to focus on.  If certain degrees of freedom are unconstrained (or partly constrained), the part can be shifted around (translated and/or rotated) on the gage to get everything to conform.  Dealing with the datum shift is hands-on and understandable.  It isn't necessary to understand DOF constraint to use a hard gage.

If a hard gage is not available, and CMM or open setup methods are necessary, dealing with the datum shift is very complex.  It's very mathematical, and requires full understanding of the gory details of DOF constraint to do properly.  Instead of moving the part around relative to the coordinate system (which is in the gage), we have to shift the coordinate system around relative the part.  This really clashes with one of the fundamental paradigms of CMM's, in which a fully constrained coordinate system is needed.  The coordinate system must be optimized either by trial and error or by special "soft gaging" algorithms (which most CMM softwares still do not have).

So FCF's with unconstrained DOF's creates challenges for inspection.  This is not to say that it can't be done, however.  I would still resist the temptation to "dumb down" the drawing by specifying extra datum features that add unnessary constraints, just for the sake of making things simpler for inspection.  Your mileage may vary.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: True Position?

Evan,
I does mean more coming from someone who actually has had to do it, or knows it can be done, in my opinion.
Frank

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