End of Slot as Datums? 7

[SDETERS]

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.

 

[KENAT]

Functionally, what MechNorth says more closely reflects reality.

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.

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[MechNorth]

That's what I understood as well, fsincox. Peter, your datuming scheme doesn't represent that functionality. Thoughts?

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

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TecEase, Inc.

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[SDETERS]

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?

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.

 

[axym]

Yes, fsincox and Jim are right, the datum features in Peter's drawing don't match the actual DOF constraint and need to be changed.

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

 

[SDETERS]

You have got me thinking about another way. Please take a look at the drawing. Once again it is simple I am looking at the datum structure. Yes there is dimensions and profiles missing. Thoughts please?

 

[KENAT]

No, I wouldn't do that.

I think MechNorth and others are proposing:

http://files.engineering.com/downlo...-4d6f-bcc7-a053361f623d&file=ANGLED-DATUM.tif

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.

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[SDETERS]

Ok sorry it was just a thought that came to my mind as I was going through the posts. Thanks

 

[axym]

sdeters,

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

 

[KENAT]

The cover is located by the side walls of the casting.

If this is true, and at some point we have to take the OP's word for it ;-) then I think using the widths adequately constrains it. If the obround bosses are 'over long' then the rounded ends might come into play but the wouldnt' there be a fit concern? I suppose if they are relatively narrow compared to the slot then your specific concern becomes more significant.

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.

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[fsincox]

axym,
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.

 

[axym]

KENAT,

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.

http://www.axymetrix.ca

 

[MechNorth]

Tks Kenat; excellent graphic of exactly what I was thinking.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

Jim,

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.

http://www.axymetrix.ca

 

[MechNorth]

Evan, first, keep your life jackets nearby ... how are you enjoying the weather in Vancouver this week? Next, didn't notice the gap, sorry. Interesting question on how to reference them; a hyphenated datum reference (/X/Y-Z/) would likely be better than two individual references (/X/Y/Z/), though it would still be legal. The only way that I can see value in using the radial ends & therefore the net boundary would be if the the boundary is specified at MMC so that there is mega-slop so that the radial ends may actually have a chance of coming into play. Typically with something like this, I'd design it with clearance at the ends of the slot and maybe even a light-press fit on the widths of the bosses; the inclined boss would do a great job of constraining left/right motion and fully constrain the vertical motion in conjunction with the horizontal boss. Because of the flexible nature of the material, I don't think that Y will do an adequate job on vertical constraint by itself.

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

http://www.profileservices.ca

TecEase, Inc.

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[NormCrawford]

Hey all,
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.

http://www.GDandT.com

 

[MechNorth]

Tks Norm. Didn't see you in Salt Lake. Dayton next Spring?

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

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TecEase, Inc.

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[NormCrawford]

Jim - most likely in Dayton since I live very close! But never know, I could be in China too.

Norm Crawford
GDTP-S
Applied Geometrics, Inc.

http://www.GDandT.com

 

[KENAT]

Going on the comment of "The cover is located by the side walls of the casting. " is why I'd go with the width as the datum, using logic like MechNorth. As to how to reference them, it depends what you have to relate to them, if it's just the non critical outside profile then it doesn't make much difference does it?

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[axym]

Interesting comments KENAT.

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.

http://www.axymetrix.ca

 

[MechNorth]

Not advocating a redesign for the OP, Evan, just proposing "what ifs" as they would simplify(?) the situation. I'm not comfortable using a partial contact of the radial end of the boss to constrain motion as a first-contact will in reality be at a couple of points, and full contact won't be made until the rubber is compressed. The long surfaces of the boss will (probably) make greater contact with the channel and therefore are less likely to deform significantly vs the end radii in actually constraining motion.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com