End of Slot as Datums?
End of Slot as Datums?
(OP)
I have a part that has two protuding slot features on it. The part is made of Rubber. The two protruded slots are what locates this part in the assembly. I want to call out a profile of these slots but I also want to call the end radius as y and z datum Please See PDF. What is the proper way of doing this? Will the R.16 be basic or have a tolerance? Should I say three surfaces on the profile instead of all around? Suggestions. Wow two questions from me in same day. Sorry.





RE: End of Slot as Datums?
Certainly if you want to use the surface profile all around you either need to make the entire slot controlled by it and make the rads basic or, explicitly identify which surfaces it applies to as per Fig 6-13 from ASME Y14.5M-1994.
Can you explian really what you mean by using the end radii as datums? I think your secondary and tertiary datum between them want to fix it's translation (top to bottom and side to side as shown in the print) and its orientation (rotation).
Given the way one of the slots is angled to the other and to the overal part, I wonder if you'd be better using the horizontal slot feature to get both your secondary & tertiary datum and then relate the other 'slot' to it.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: End of Slot as Datums?
RE: End of Slot as Datums?
To get the rotational orientation you add another datum (4.4.3). My Concernt is how your Z is interpreted in this context.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: End of Slot as Datums?
RE: End of Slot as Datums?
Kenat I looked at the veiw you mentioned. All the datums are in the same plane.
RE: End of Slot as Datums?
Some things are "legal" but very questionable.
I would use the width of the "Y" slot as the horizontal datum and the length or an edge as a vertical datum.
-OR- use the straight edges as datums and constrain the slots as a pattern, perhaps with a composite tolerance.
The slot ends are not going to be easy to pick up with any accuracy, even with a CMM.
Given the generous tolerance you have on the profile, I would wager there are many better ways to dimension and inspect this part and get a good result, even if the scheme is not 100% aligned with your design intent.
RE: End of Slot as Datums?
However, this is assuming the fit to the next part is very much constrained by the location of the slot.
Given that you say it's rubber, and the fairly loose tols, I wonder if this is a case as Tick says where you don't have to stick really close to theoretical function in order to get a workable solution.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: End of Slot as Datums?
Also, I ASME has a method that better controls slots, though it is a bit more work to set up, and profile allows you to short cut your datum scheme (if you want the centers of those radii to be your datums).
For style, attached the Z and the Y to the bent portion of the radius leader. This is a bit cleaner than globbing it directly onto a radius.
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: End of Slot as Datums?
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: End of Slot as Datums?
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: End of Slot as Datums?
I will review these suggestions and update the drawing. Maybe GD&T with datums do not need to be used. I am going to review this with the eng and post the results when we get something that can agree upon. Thanks again.
RE: End of Slot as Datums?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: End of Slot as Datums?
RE: End of Slot as Datums?
If both slots are controlled by the same control frame (w/ two arrows), then you can control their relative position to one another with just the "X" datum (maybe add a second datum for orientation). Then a second frame to control the position of the pattern on the plate.
Can composite tolerancing be applied to profile?
RE: End of Slot as Datums?
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: End of Slot as Datums?
i would caution you that the horizontal slot is actually a TAPERED slot as shown. even changing the r .16 to basic leaves you with a tapered slot.
that said, the y14.5-2009 standard will allows the use of a 'linear extrusion', i.e. the horizontal slot, as both secondary and tertiary datums. this allows you to locate the angled slot with basic dims, and gives you rotational control of the entire part (after locating the slot to one of the edges of the part, of course.)
teddykaye
RE: End of Slot as Datums?
Peterstock.
Thanks for that feedback. I was thinking that same thing and I am moving torwards that way of setting the datums up.
I have taken that type of info from Kenat you and thetick and assumed that it meant what you have posted in your file. Thanks for all the help
RE: End of Slot as Datums?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: End of Slot as Datums?
Before recommending a datum structure, I would need to know exactly how the part's degrees of freedom are constrained when it interfaces with the mating part. Specifically,
1. What is the geometry of the features that the slots mate with? From your descriptions, it sounds like the mating part has two pins, with basic spacings of 5.444 in the X direction and .398 in the Y direction. Is this correct?
2. What is the size tolerance of the mating features? Or are they adjustable in size to achieve a snug fit with the slots?
Once these questions are answered, then the datum features (or possibly datum targets) can be specified appropriately. Tolerances can then be applied to the various slot surfaces and to the features that need to clear the channel.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: End of Slot as Datums?
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: End of Slot as Datums?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: End of Slot as Datums?
This part covers a channel. Think of a long c shaped channel. This part just covers this channel using the two slot shapes to keep the part aligned in the channel. So the width of channel in the mating part is sand cast and has sand cast tolerances. The tolerance on the channel is +/-.030. This part is acting as a cover and we need to have the slots act as the guide during assembly.
RE: End of Slot as Datums?
Okay, that helps a bit. But more details are needed on how the two slots align and guide the part. Exactly what are the slots in contact with? Are there some sort of pins or bosses on the channel?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: End of Slot as Datums?
RE: End of Slot as Datums?
Okay, now I see it. The latest diagram cleared things up a lot. I had been picturing something very different. I got hung up on the term "slot", which I normally associate with some sort of cutout and not a solid feature.
I'll think about it during lunch and post something later.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: End of Slot as Datums?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: End of Slot as Datums?
RE: End of Slot as Datums?
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: End of Slot as Datums?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: End of Slot as Datums?
Looks like the part locates on the face and the two widths. Why there is that angle to the maiting psrt I don't know but if it's the way it is so be it. I like where you are going MechNorth.
RE: End of Slot as Datums?
RE: End of Slot as Datums?
RE: End of Slot as Datums?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: End of Slot as Datums?
So the first functional requirement is that the two bosses must both fit into the channel without interference.
The second functional requirement is that the outer edges of the cover must maintain some relationship to the channel. This needs to be clarified - perhaps sdeters can give some guidance on the requirements for the cover's orientation and location relative to the channel.
In any case, we know that the bosses are the datum features. That is, contact on the boss surfaces constrains the cover's remaining degrees of freedom (one rotation and two translations). Exactly which of the boss surfaces participate in the DOF constraint, and how, will determine how the datum features need to be defined and referenced.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: End of Slot as Datums?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: End of Slot as Datums?
RE: End of Slot as Datums?
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: End of Slot as Datums?
Fit and function requirements for the assembly! Now we're getting somewhere.
You mentioned that the tolerance on the channel is +/- .030. Is this tolerance on the width of the channel, or the profile of each side of the channel? Also, what are the nominal inner width and outer width of the channel, and the nominal outer width of the cover?
Jim,
The profile accuracy of my sketch is what a mouse and two cups of coffee will allow. The thing is, the parts might literally look like that and still function. The as-cast channel surface probably does.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: End of Slot as Datums?
RE: End of Slot as Datums?
However, I had to go back to the standard to remind myself how the 3 perpendicular reference frames are developed, fig 4-4 & section 4.4.1.1 cover it.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: End of Slot as Datums?
Evan ... just like my old shop!!! Actually, probably worse when someone missed a nicotine break and was loaded up on caffeine!
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: End of Slot as Datums?
The tolerance is on each surface of the channel and in the drawing it is dimensioned by a surface profile .050 back to XYZ. The width of the Channel inside surface to surface is .380. The width of the channel outside is .730. On the cover to start with I was going to make it line to line with the outer width of the channel with the slot bosses centered in the channel. Then let the part slide on the X datum until the cover slot bosses hits channel.
Thanks for all the feedback again the info that has been provided has been great and very usefull.
RE: End of Slot as Datums?
The attached sketch is a depiction of what would happen on my actual part if this datum scheme was used. The black blocks are the two vise-like datum feature simulators, which contract onto datum features Y and Z. The datums are the centerplane of simulator Z and the center point of simulator Y. The red geometry represents profile boundaries for the bosses and the outer edge. These boundaries have the proper basic relationship to the datums. Note how the slight skew of the right-hand boss skews the simulators, and thus the datums, and thus the tolerance zones, along with it. Because the boss is so short compared to the length of the part, the projection effect magnifies the apparent error. This wouldn't happen in the real assembly, because the right-hand boss doesn't constrain the rotational DOF on its own. If the parts were inspected with this datum scheme, a lot of functional parts would probably fail.
Both bosses play an equal role in constraining the degrees of freedom, so the datum referencing needs to reflect that. We need some way of referencing the surfaces of both bosses as a combined datum feature, without giving one boss precedence over the other.
Unfortunately, Y14.5M-1994 does not provide very good tools to do this on "irregular" features like obrounds. We may have to cobble something together that gets close to the right result.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: End of Slot as Datums?
RE: End of Slot as Datums?
I think MechNorth and others are proposing:
ht
Based on my extrapolation of figure 4-4 & section4.4.1.1 of ASME Y14.5M-1994.
This most accurately reflects function as I understand it. However, if the cover truely is rubber and hence compliant, part of me wonders if it's worth the effort. Though that's probably 'cause I'm a bit out of practice.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: End of Slot as Datums?
RE: End of Slot as Datums?
I agree with KENAT that your latest version is not a good direction to go in. The way I like to think of it is that we don't really choose the functional datum features, we have to discover them. The geometry and the assembly method determines how the part's degrees of freedom will be constrained. Our task is to find a way to describe the constraint using the tools in Y14.5. We sometimes find that the Y14.5 tools are not able to completely describe the functional situation, or require overly obscure and confusing callouts in order to do so. It is often more workable to sacrifice some functional "correctness" and go with a more palatable approximation.
This is what we'll probably find with the cover bosses. Those obrounds are nasty, because they're irregular shapes that masquerade as simple features of size. We try to create simple datum features on them and it doesn't work, because they don't function as features of size.
That said, let's try to discover the functional datum features and find some way to reference them.
The arrangement in the ANGLED-DATUM.tif diagram is getting closer, but I still don't think it's quite right. Imagine the cover being pushed and turned on the channel, and picture where the contact will be. I would argue that the "outer" round ends of the bosses constrain the DOF's just as much or more than the flats, particularly translation in the left/right direction. So those outer round ends need to be included in the datum feature(s). The inner round ends of the bosses won't contact the channel, so those should not be included.
We need a datum scheme that would result in the datum feature simulators shown in the attached diagram. The two simulators are fixed in location relative to each other. Their inner surfaces represent the worst-case outer boundary of the combined boss pattern. This boundary would be derived from a Surface Profile tolerance on the boss pattern. I believe that the way my actual part sits on this simulator set is in line with how an actual cover would sit on a worst-case (i.e. smallest) channel. Neither boss is aligned exactly with its simulator, and there is some clearance.
Now how can we get this using Y14.5 datum referencing techniques? Any ideas?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: End of Slot as Datums?
If this is true, and at some point we have to take the OP's word for it
I think tolerancing it so that the ends of the bosses don't touch the ends of the channel and are not too sloppey on width then simplifies the problem to one that my sketch of Mech North's idea adequately addresses.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: End of Slot as Datums?
That is it, "The way I like to think of it is that we don't really choose the functional datum features, we have to discover them."
That is one of the things I am trying to say in my philosophy thread, the parts/assembly works the way it works, we need enabling tools to help us describe it and not arbitrary prohibitions that stop us from doing the job. We as designers can determine some of it if we in fact have a clean sheet. Other factors come in and in the end we just want to document the actual condition.
RE: End of Slot as Datums?
I went the other way with the round ends and the widths. I guess I didn't take the "located by the side walls" thing that literally. I thought that the two outer rounded ends would constrain the DOF's very well on their own, and the widths just get in the way. The bosses are only obrounded because the engineer "likes the look of them" and sdeters would have preferred to make the bosses cylindrical. This would be a much more efficient datum feature design - the extended length of the bosses just provides more material to control. The datum referencing would be much simpler as well - something along the lines of sdeters's original drawing would work fine.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: End of Slot as Datums?
Evan, how very Zen of you! I have to remember that one; mind if I use it in classes? From a mechanical design perspective, unless the widths of the bosses and slots are modified at LMC, you would never get the radial ends to make contact with the ends fo the slots without fighting against the angle and therefore distorting the rubber part. The configuration as provided would find its own natural origin during assembly, and it would be independent of the ends of the slots/bosses; in Kenat's graphic, the origin of meaurement is at the intersection of the two datum planes. If the gasket was to be press-fit between the ends as suggested in Evan's graphic, and buckling or compression of the material was ok, then I'd use datum target areas. Could use targets K1 & K2, referencing only A/K in the DRF, or use K1,M1, and reference A/K-M in the DRF; same effect.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: End of Slot as Datums?
I'm not sure what part of my graphic suggested a press fit, but that definitely wasn't the intent. Are you referring to Boss_datums_3.pdf, with the two U-shaped simulators? If you look closely, the bosses aren't actually touching the simulators (perhaps I should have exaggerated the gaps a bit more). The idea was that the simulator geometry is derived from the bosses' outer boundaries, so that the bosses would always have at least zero clearance with the simulators.
KENAT,
In your ANGLED-DATUM.tif diagram, what was the intention for how datum features Y and Z would be referenced in a feature control frame? Specifically, in what sequence and at what material condition? Perhaps Jim can shed some light on this as well. I'm still having trouble seeing the advantage of using the two widths as datum features. It doesn't seem like they would control the left/right translation as efficiently as the round ends would.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: End of Slot as Datums?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: End of Slot as Datums?
Sorry I have been out of the Eng-tips forum for so long. I even kept a bunch of notices trying to get back. Anyway, I be trying to be more active.
As for this thread, already long enough, I have to go with Jim on this through all the speed reading.
I know perhaps you want more out of me, but I have a lot of catching up to do and fundamentally what I read is that Jim, not surprising, has a handle on this one.
Norm
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: End of Slot as Datums?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: End of Slot as Datums?
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: End of Slot as Datums?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: End of Slot as Datums?
It still surprises me how different my instincts often are from those of you in the design field. How you reference the datums makes a lot of difference, especially to the inspector! And to the material review board tasked with dispositioning nonconforming parts, when this "non critical" outside profile somehow failed inspection even though the parts work fine. And to the quality manager, when the assemblies have less than the required .050" of overlap even though the detail parts passed inspection.
The functional datum features are what they are. You can specify something else if you choose, but that doesn't change how the DOF's will get constrained in the assembly. All it changes is how the inspector will be obligated to constrain the DOF's during inspection.
Even after all of this discussion, we never did give DSETERS a good answer for how to specify a good datum scheme for the functional situation as described. Jim, you said that the only way the rounded ends would come into play is if everything is at MMC and there's mega-slop. Well, that's what the situation actually is! These bosses interface with an as-cast channel with .050 profile tolerances on the surfaces. There is going to be slop! But you suggested changing the design to take the rounded ends out of play and perhaps put a press fit on the widths. Are we redesigning the part now? If we are, why not just make the bosses round? I'm not a design engineer, but I can't see how the widths can constrain the left/right translation more efficiently than the round ends would given the part geometry. If there was a .005" gap between the boss widths and the channel, the cover could move left or right by about .015". Let me know if I'm completely out to lunch here.
I guess I'm more used to working within the confines of a given situation as opposed to changing it. This mindset probably comes from years in the CMM room, trying to reverse-engineer the real functional datum features of a part in order to make sense of nonsensical inspection results.
Sorry about the bitterness and ranting. I just remember that whenever there was a disagreement over part conformance or inspection results, the three most common causes were:
1. Datum Reference Frames
2. Datum Reference Frames
3. Datum Reference Frames
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: End of Slot as Datums?
You are right that design & inspection often have different (& often conflicting) understandings and instincts. It all comes down to the design intent, and unfortunately I often see that designers pay only casual attention to the actual functionality vs the pereived functionality. I was a victim of this until I was lead through an extended "5-Why" session to get to the root functionality of components in a system. Quite an exercise and experience, but not a popular design philosophy.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: End of Slot as Datums?
If the outside profile is truly relatively non critical then I'd hope it would have a tolerance that reflects this and that is well within process capability and so chances of one being far enough off to be rejected is correspondingly small. Or something like that.
If there is some other interface or functional concern that the OP hasn't shared with us (or I missed) then I'd look more closely.
I sometimes wonder if, especially on non critical parts, we spend too much time trying to find the 100% correct, all things considered solution, when a 99+% solution could be found for a lot less time and effort.
I think most people I work with don't go far enough, but I sometimes wonder if on here we don't get a bit carried away occasionally.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: End of Slot as Datums?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: End of Slot as Datums?
I spent a half hour today arguing with a guy (the chief representative of quality and head of our new GD&T "A" list, we are currently preparing to adopt the 1994 standard) who insisted the standard requires datums to be in alphabetical order ABC, DEF, primary, secondary, and tertiary "the don't have to start with "A" but they have to be in order, that is the way they show it in the book and it's supported by 4.4". I was shocked, I couldn't believe my ears. I finally found the statement in 3.4.3 "datum reference letters need not be in alphabetical order". I said: "if what you say is true, they would always be in order". After reading and thinking he said: "oh, I don't have time to argue about this anymore".
My point is some of us need to have a place were we can discuss issues.
RE: End of Slot as Datums?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: End of Slot as Datums?
This is a good one.
But man it can eat up some time. lol
I would list others but I don't wnat to break any rules or step on toes.
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: End of Slot as Datums?
It might also be that I'm one of those freaks who actually enjoys debating Y14.5 minutiae for it's own sake. Sad but true.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: End of Slot as Datums?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: End of Slot as Datums?
When it is more of a discuss/debate question or just very complex then I tend to just let you smart folks with GD&T letters after your name have at it and just follow along till my brain starts to smoke (usually about ½ way through the OP).
I can't claim to stick rigidly to either of these principles but try to generally follow them.
That's not to say others approaches are wrong, however I know I have got frustrated when my threads have been taken off on a tangent or gone into much greater detail etc. than I desired/needed, so I try to "do as you would be done by".
Now I've gone and taken this off track though, doh!
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?