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True Position Callout on Old Drawing

True Position Callout on Old Drawing

True Position Callout on Old Drawing

(OP)
Hello, could someone please have a look at the true position callout on the left-hand view in the attached image and let me know if this makes any sense at all? I'm thinking not, as it's placement doesn't seem to indicate that it is associated with any feature of size. I'm thinking that profile of a surface is the correct callout for this surface.



I've found multiple other issues as well... The runout tolerance is total overkill on this part as that chamfer is only there to clear a radius in a molded plastic part it mates with. The surface I first mentioned really only needs to be perpendicular to datum feature C and have a relatively tight tolerance on the positional dimension to ensure interchangeability.

RE: True Position Callout on Old Drawing

(OP)
Datum feature A is supposed to be the shank of the thread. That's a good point though, I didn't notice that... It should be positioned to indicate the FoS is the datum feature.

It is an ASME-inch part, hence my confusion about the position application.

RE: True Position Callout on Old Drawing

(OP)
Ugh - yeah, Datum A is located based off of the position of the pitch diameter, datum B. That doesn't make much sense does it? The thread should be a secondary datum, or the shank should not be a datum feature at all perhaps. I suppose for the purposes of inspection it doesn't matter that the shank is measured and its position validated based off of the pitch diameter though correct?

A needs to be in line with the FoS dimension.

B should probably be positioned in relation to A, not the other way around.

C looks OK.

Profile of a Surface should probably be used for the surface currently being controlled with position. Or perhaps, because the orientation of the flat on the head diameter doesn't matter (180 deg. vs 222 deg.), we could control it's form with an appropriate size dimension and use Rule #1 to control the form.

Runout should be removed.

RE: True Position Callout on Old Drawing

In ASME, replace position with profile (basic .272 stays)

Re:"B should probably be positioned in relation to A, not the other way around."

Ask yourself, which is driving which is driven?

I would say it is correct as is, depending by the functionality of this part/design intent.
Was designed to center it/the part (in the assembly) on B or was designed to be center on A?


RE: True Position Callout on Old Drawing

While the intent might be clearer with profile rather than position, I don't think it makes any difference. The Datum A should be inline with the shank diameter as already stated. You do need the runout on the chamfer (maybe not that tight, need to know mating component(s)) but without runout there could be big chamfer on one side and no chamfer on the other, causing interference in the radius of the mating part.

I have no problem with datum B or the position of A to B.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: True Position Callout on Old Drawing

Anyone remember why splitting the datum symbol from the FoS dimension and using a toilet plunger symbol was argued for as a better idea (ISO compatibility is not sufficient)? It would have avoided the ad hoc approach taken to signifying the thread was a datum and eliminated having a datum apparently apply to the tangent of a cylinder when it makes more sense for it to be the FoS.

And how much tolerance is there for the toilet plunger to be misaligned with the FoS dimension for it to no longer apply? Is .0001 misalignment enough or does it depend on eyeballing that relationship?

Any clues on why the spherical radius is reference and there's no tolerance for its location?

RE: True Position Callout on Old Drawing

When indicating the datum feature symbol on a cylindrical part (such as the shank above), it really doesn't matter if it's in line with the dimension arrow or not. You guys are thinking of a planar part -- then it would be a huge difference: surface vs. FOS.

I'm not staking my comment on ASME's letter-of-the-law, but what other interpretation could there be with a cylindrical feature? Could datum A be misinterpreted as, what... a tangent line across the top?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: True Position Callout on Old Drawing

How better to indicate that it is the tangent than to make it unrelated to the dimension - just like the interpretation for a plate?

RE: True Position Callout on Old Drawing

How better? To use a datum target, if that line is what was desired.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: True Position Callout on Old Drawing

@ 3DDave:

This situation was discussed long and hard some time ago: http://www.eng-tips.com/viewthread.cfm?qid=334165

I myself stopped arguing about it ever since.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: True Position Callout on Old Drawing

(OP)
In any case, it's not meant to be a tangent line (Datum A), it's meant to be the shank diameter, and so should conform to the standard for consistency sake. So I will make that change. And I think that it merely needs to be visually in line with the FoS dim, or otherwise, clearly offset.

dgallup: As far as runout is concerned... Doesn't the FoS dim (45°±1°) control the chamfer enough to ensure that the chamfer is consistent throughout the entire diameter (flat excluded)?

dgallup/greenimi: I thought that position can only be applied to the centerplane or axis of a feature of size (the standard says that you need two opposing elements of a feature to control it with position)? So, I don't believe you can legally apply true position to a surface... I would think that flatness or profile would be the only way to control that feature. In actuality, I was thinking that flatness may be better than profile as we're not really concerned with orientation of the feature radially, but because we are concerned with its perpendicularity to the axis of the part, profile is the way to go I believe.

3DDave: No idea about the spherical radius... Perhaps they start with a purchased part and so it's basically reference information.

RE: True Position Callout on Old Drawing

CH: Thanks for the link. I'll still fall back to the position that placing the datum symbol with the dimension eliminate ambiguity and that adopting the ISO toilet plunger was a huge mistake.

ModulusCT: Even if it is made from a purchased part, there needs to be some limit for the location of the feature relative to all the other features that are being created.

Belanger: Adding a datum target line would add significant geometry to the drawing; adding this interpretation does not. It requires adding a symbol to the left hand view and requires adding a top view to show that this isn't a target point. 2009 - Figure 4-45 should have applied the datum symbol to an indicator for the cone angle to eliminate ambiguity. For certain if it had been applied to either the large or small circle in the side view it would not be assumed to apply to the cone even though those are part of the conical surface.

<soapbox>
Harmonization by parroting ISO makes Y14.5 only superficially similar, in spite of fundamental differences. There was no published explanation given for parroting this element of ISO. The lack of published explanation leads me to speculate as to the motives for doing so, and I see no benefit to the users of the standard, not only because it generates ambiguity but it also takes more space on drawings, as can be seen even in this example. There should have been a really good reason to adopt a symbol from an incompatible standard.
</soapbox>

RE: True Position Callout on Old Drawing

Dave, you should notice that I never advocated for the terribly confusing use of the offset datum feature symbol in the OP's case. All I meant was that it's more of a stretch to think of it as a tangent line than it is to simply think of it as an axial datum.

In the case of Fig. 4-45, I think the escape hatch in that interpretation is that datum A's reference in the feature control frame clearly spells out the degrees of freedom, so in that example there is no chance of misinterpretation. I guess I agree that it's still not the best way to do it, though.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: True Position Callout on Old Drawing

I can often guess when a homonym is used based on other clues in a sentence, but it still is either incorrect or ambiguous.

At a first glance I would assume, absent of any other clue, it applies to the tangent plane, just like it does to one face of a width. If they wanted to apply it to the diameter, there are multiple locations that would express that exactly. It should not be required to look at any FCF to determine which feature a datum symbol refers to.

Since that's not the most likely case, and the datum location is not one that is depicted for a cylindrical feature, I would kick it back for correction rather than accepting it. It takes 2 seconds to make it match similar examples to eliminate any question of interpretation.

As an example of an ambiguous typo, I offer '2009 figure 3-3. Is [C] a plane through one face or is it in the center of the width of the tab?

RE: True Position Callout on Old Drawing

Not sure why you dislike Fig. 3-3, datum C. It's clearly a plane formed from the top surface of the tab. It's not in line with the dimension line, which would be indicating a feature-of-size datum. No typo that I can find.

Is your beef something like "how close must the datum symbol be to the dimension line for us to call it a FOS datum?"

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: True Position Callout on Old Drawing

That's not a dimension line. There is no value associated with it. It's a typo.

RE: True Position Callout on Old Drawing

(OP)
Recently, I was in Indianapolis for Solid Edge University. I took a class on 2D/3D GD&T given by the gentleman who created all of the figures in Y14.5 since the 60's... I have his email. I may email him about this, ask why the plunger was adopted and if I get an answer I will post that answer here.

RE: True Position Callout on Old Drawing

Quote:

That's not a dimension line. There is no value associated with it. It's a typo.
You really are a stickler! I guess all of these figures have typos too: 1-11, 1-12, 1-13, 1-49, 1-52, 1-53, 1-54.
(And that's just in Section 1 of the standard...)

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: True Position Callout on Old Drawing

Quote (ModulusCT)


dgallup: As far as runout is concerned... Doesn't the FoS dim (45°±1°) control the chamfer enough to ensure that the chamfer is consistent throughout the entire diameter (flat excluded)?

That is only the angle of the chamfer, not the size of the chamfer. The size of the chamfer is controlled by the diameter .7600. But that diameter could be way off center, hence the need for the runout control.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: True Position Callout on Old Drawing

JP - if you are going to criticize, nail it - The figure is an attempt to show the application from paragraph 3.3 indicating the surface is intended when the datum is clearly separated from the dimension.

In the 1994 version of the standard that paragraph, 3.3.2. (a), was short and to the point, as was the figure. In the 2009 version they wanted to add more examples to the similar paragraph and increased the number of surface type depictions, but still messed it up and failed to change the pluralization of the caption.

Originally, the diagram had two datum symbols, one on a surface and one on an extension line. Now it is on multiple surfaces and one on an extension line, but still is captioned "Datum Feature Symbols on a Feature Surface and an Extension Line"

Still a typo, just a different one than I called it out for, and still exceptional in that most of the diagrams that show a relationship between datum symbols and dimensions show a value for the dimension.

The rules for dimensions indicate that for unbroken horizontal dimensions the values should be above the dimension line. There is no rule describing unbroken vertical dimensions, so what to do with Fig. 1-49?

15 years between releases and yet rushed to publication.

RE: True Position Callout on Old Drawing

(OP)
Perhaps they'll fix it in the 2018 revision.

RE: True Position Callout on Old Drawing

3DDave,
It is really interesting to read a debate splitting hairs on a caption of a single figure from the standard (BTW, I agree it is a typo), yet at the same time see a "toilet plunger symbol" being called the "datum symbol" instead of the "datum feature symbol" wink

As for the original question, the answer has been already given - profile of a surface should be a choice. Position would be legal per ISO, but is illegal per ASME. Flatness tolerance does not have power over controlling location of the flat.

From other issues that have not been mentioned yet (if I see correctly), the position callout for the shank (datum feature A) does not have the diameter symbol in front of the tolerance value.

RE: True Position Callout on Old Drawing

While we're at it, there is a surface texture symbol applied to an extension line that is associated with the .125 MIN dimension, but not an actual surface. Presumably it's meant to apply to a portion of the shank, but there are better ways to indicate this.

Also, pitch diameter is the default for a threaded datum feature, so the datum feature symbol for B could simply be attached to the horizontal portion of the leader line for the thread specification.

pylfrm

RE: True Position Callout on Old Drawing

pmarc - you got me. I'll be more careful when I publish a standard intended for national and international use.

pylfrm - I agree on the surface texture symbol.

In looking at the drawing it appears to have been done using Pro/E or Creo. They have a terrible Feature control frame creation dialog that 'remembers' the last geometric characteristic symbol and it divides selection of material size condition modifiers from selection of the datum reference, leaving it easy to create a FCF with the wrong symbols.

Also, the reference to datum C is meaningless even if the head flat is converted to use a profile of a surface symbol.

That underhead flat diameter is apparently intended to be very precise as is the fillet radius. Not many calls for 4 place dimension values for chamfer base diameters.

Is there anything that says that increased decimal places must indicate decreased tolerance values for title block tolerances? I would find it interesting to see how long it would take to notice if the 4 place tolerance was +/-.1250 and how many would argue that I can't do that.

RE: True Position Callout on Old Drawing

3DDave,

Is the reference to datum feature C really meaningless? ASME Y14.5-2009 Fig. 4-21 illustration (c) would imply that it is not.

As for title block tolerances, the largest I've come across for 4-place was +/-.0050 in. I wouldn't complain too much about +/-.1250 though. I'm not aware of any standards that say anything on the subject.

pylfrm

RE: True Position Callout on Old Drawing

Ohh ugg. Yes - I'll move from meaningless to 'makes it worse.' Having a design where control of orientation can shift from the primary datum to the secondary depending on on feature size is pretty poor.

I wonder why they included the pathological '2009 example. It's an example that clearly shows how to tolerance a part so one can't be certain which feature is intended to orient the part.

It wasn't present in the '1994 version. It looks like someone looked at the '1994 example 4.18(d) and thought there was a similar problem and didn't realize that the virtual condition of [A] needs to be calculated in the [B] primary frame of reference; that A@MMC is not necessarily the diameter for the virtual condition of [A] needed to create the B|A(M) datum reference frame. If the virtual diameter of A in B|A(M) is correctly calculated, B is always the primary orientation surface for (d).

I think the '2009 example the case is not as clear as it is presented. Absent dimensions for the features required to do the trigonometry to determine the angle of the head and the angle of motion allowed in the hole the condition can't be verified, so (c) is not a done deal; even at LMC for the feature used for A the underside of the item may not lie flat as depicted.

RE: True Position Callout on Old Drawing

All good points here.
But, I'm curious, how is this installed? I see only one flat.

Chris, CSWP
SolidWorks '16
ctophers home
SolidWorks Legion

RE: True Position Callout on Old Drawing

I would assume (with all the incumbent possible errors) that the bolt is inserted so that the flat mates with a corresponding feature that prevents rotation and a nut is tightened on the other end.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: True Position Callout on Old Drawing

3DDave,
You mentioned fig. 4-24 in the other thread about the drawing with vague general notes. I agree with your observations about fig. 4-21(c), but does it mean that in your opinion datum features scheme/sequence shown in fig. 4-24 falls into "that makes it worse" category too? Would you recommend changing datum feature order in fig. 4-24 to avoid "having a design where control of orientation can shift from the primary datum to the secondary depending on on feature size"?

RE: True Position Callout on Old Drawing

pmarc,

I only dislike the 4-21(c) because it depicts that LMC datum feature B will be certain to control the orientation of the part, when that is not always the case. 4-24 doesn't do that, so it seems OK for the purpose it is used for.

As an example of another pathological case*:

Picture a shaft with a loose fitting thin washer and parallel to the shaft is a pin. The washer has a hole that fits closely on the pin and both the pin and shaft are set into a planar surface.

One could identify the bore of the washer as datum feature A and one face of the washer datum feature B.

Set the location tolerance of the hole at 0(M) | A | B, where the MMC size of the hole was the same as the MMC size of the pin at 0 position error. It is possible that the washer will end up flat against the planar surface, but it isn't necessary.

If the fit of the washer is loose enough, an acceptable solution might be the pin hole is a 30 degrees from perpendicular to the face of the washer in order to fit and the washer makes only point contact with the planar surface, meeting all requirements.

Some would argue that B should control the orientation for this example, but that's not what the FCF calls for. Any solution that allows the washer to fit over the shaft and contact at least one point on B is compliant as long as the pin fits the hole, even if mechanically is seems nonsensical. If the other solution is desired, one needs to select a different FCF.

*Result of an actual discussion. The person suggesting A|B was holding that since the shaft assembly step was first before contacting B, the datum representing the shaft had to be first as well and it was a loose fit anyway.

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