Symmetrical Tolerancing 2

[TomFin]

Hello World,
The part shown in the link is a hinge block. It is to be sent out for fabrication. I’d like to dimension the pin hole in such a way that variance occurring to the width of the bar stock will not cause the pin hole to be off center due to the fact the pin hole is dimensioned from one of the two vertical edges. This block is used multiple times in a weldment and both sides are used as positioning surfaces which is why I’d like to convey the importance of the pin hole lying mid-plane rather than just dimensioning off an edge.
Our drawing checker promoted a symmetry callout referencing the mid plane, but symmetry is only appropriate for non-cylindrical features and can be expensive to inspect (Geometrics III). The only dimensioning scheme I’ve come up with is positional tolerance relative to a datum, but then again the hole would still be dimensioned off 1 edge.
Then again it is bar stock which shouldn't have much variation. What do you think?


Failure is a prerequisite of successful design

 

[ringman]

I believe the locus for the hole should be a rectangle rather than diametric. If that is the case there is no need for the 1.000 to be a basic dimension. Nor is there a need for datum feature C.

Has it been determined that Y14.5 is applied to the drawing in question?

 

[aardvarkdw]

Correct me if I am wrong but I don't think you actually need the 1.282 dimension.
You have already specified the width of the part and that controls the size of the radius (hence the "R" callout) and if the hole is concentric to the radius (which I infer from there not being a second centermark) then an additional height dimension is redundant.

David

 

[powerhound]

...nor would a feature control frame be legal to use, nor would there be any need to use GD&T at all or reference anything like the ASME standard. That would effectively be converting the whole thing to coordinate dimensioning which in itself has so many disadvantages that that was the reason that GD&T was developed in the first place.
The person that posted the OP is the one doing the drawing so I would imagine he is trying to conform to the ASME standard if he's asking us about it.

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[powerhound]

The above is in reference to ringmans post, not aardvarkdw's

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[TomFin]

Powerhound, you're right. I am trying to veer away from coordinate dimensioning and implemement GD&T for the sake of assembly.

Failure is a prerequisite of successful design

 

[KENAT]

Ringman, why square? The location is to the radius as well as width so I think circular makes sense. The Ops main concern was on centering the hole wrt the width. What you’re proposing would lose this, in fact you don’t even address it.

That said a square zone could be achieved by dropping the diameter symbol from the FCF while still having the benefit of centering the hole wrt the width.

The OP & I traded quotes from 14.5 so I think it’s reasonable to assume it applies. However, just to be sure TomFin do you invoke ASME Y14.5M-1994?

aardvarkdw, did you not see the other thread on coaxiality?;-) You cannot infer coaxiality/concentricity just because the radius is shown coaxial to the hole. That is why the height is given. Now if you want to use a geometric control of the radius to relate it to the hole you could lose the height but this seems over the top for the application.

However, this is based on my best guess of function as I don’t’ have all the information.


KENAT, probably the least qualified checker you'll ever meet...

 

[TomFin]

Yes I am invoke ASME Y14.5M-1994 as I referred to it as the bible in a previous post.

Failure is a prerequisite of successful design

 

[ringman]

Well, if the hole is to be located symmetrically wrt the width, the result would be a zone with the width allowed extending the length of the part. The 1.00 dimension with the tolerances provided would likewise result in a zone bounded by 2 planes the width allowed. The resultant zone for the location of the hole then would be a rectangular, if not square zone rather than diametric. Diametric zones work well for round fastener holes when they are a part of a patterm of holes.

 

[powerhound]

Actually diametric tolerance zones work well in all cases for which they are applicable. If you can think of a single instance where a square tolerance zone works better than a cylindrical one, please explain it to me.

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[KENAT]

Ringman,

I suppose you could just use positional on the width and use coordinate for the length but this doesn't seem like good practice. Also I think a circular zone is probably appropriate in this case.

Powerhound

I had a case recently where square was better than round. Basically a case of intersecting holes. A screw came down one and had to 'clamp' a round shaft in the other as I recal. I could actually use a larger 'square' zone than I could 'round' zone. Make sense?

KENAT, probably the least qualified checker you'll ever meet...

 

[ringman]

IF you would liook at the fig 5.21 in the Std., and at the pattern of holes in the lower left portion of the part. The composiite tolerancing has a diametric zone specified for both the pattern locating tolerance zone and the Feature locating tolerance zone. I believe if you will explore the possibilities on an enlarged drawing, you will find that the rectangular zone would provide not only a '(bonus)' tolerance but would also allow a more precise method of determining the min. edge distance for the hole pattern.

This would approximate what was used years back when the pattern was allowed to be located by plus and minus dimensions. My personal opinion is that something was lost in the updating of the std.

 

[KENAT]

ringman, is this in response to Powerhound 'cause you've lost me as it applies to the OP?

Because of the impact of 'wall thickness' in the portion of the hole sat in the radius, I think based on the information to hand that a diametric zone is appropriate.

If the function dictates otherwise then the designer should consider this but from what information he gives I think diametric makes sense.

KENAT, probably the least qualified checker you'll ever meet...

 

[powerhound]

I'm starting a new thread on square tolerance zones.

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[ringman]

Powerhound,

Good idea! It will work for rivet holes too.

 

[ringman]

Going back and rereading the OP, the piece is used in a weldment. On that basis, it is unlikely that interchangeability is a consideration in any manner. Hence it wsould seem overkill to get carried away with much if any positional tolerancing. Comments?

 

[TomFin]

Ringman,

You appear well-versed in the standard and thankyou for chiming in. But in response to

"the piece is used in a weldment. On that basis, it is unlikely that interchangeability is a consideration in any manner. Hence it wsould seem overkill to get carried away with much if any positional tolerancing. Comments?"

Yes this block and it's mating block are used in separate weldments call them A&B,that must align for the hinge pin. As a matter of fact interchangeability is a consideration, for the block is ambidextrous. With our mfg production rack containing ~30 weldments each of type A & B, an assembler with a random pick of an A and of a B must be able to tap the pin through both mating blocks without too big of a hammer Unless I've missed the boat on what you define interchangeability as, then I believe it isn't overkill to use positional tolerancing for this case.

Failure is a prerequisite of successful design

 

[ringman]

Tom Fin,

Again, if I am reading this correctly, the matter of alignment or interchangeability would be handled at the next assembly level of the drawing. Not on the single hole of the detail part.

 

[MadMango]

I would find out how these parts are fixtured in the weldment to determine what the datums need to be. Does it hold the part in space by the sides of the part, or does a pin slip in the hole for alignment? Is there slop in the fixture, or is everything tight? I'm starting to see where Ringman is coming from, there is no need to increase the cost of this part by applying unnecessary GD&T requirements if the fixture does it's job correctly.

Having said that, it does sound more and more like the width of the part and the location of the "hinge pin" hole is critical in your part. I'm thinking perhaps a parallelism control between the indexing faces and tight tolerances on the hole position should be applied (Basic dim on the width and Positional location of the hole). The diameter of your hole may also require some additional care to allow for misalignment, distortion during welding, pin insertion, etc. How far apart is the second block?

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[ringman]

Mad Mango,

Thanks. I believe that this thread illustrates the necessity for having more than just the detail part to make a determination as to the best scheme for dimensioning.
There is an example in the 1988 Standard which I believe supports that.

 

[KENAT]

Agree one can't be definite without having all the information. I thought I'd made this pretty clear by stating that I'd made assumptions.

That said, I really don't see what the problem is using positional for the hole. It seems appropriate. The value of the tolerances/hole size need to be carefully considered but fundamentally using a position control would seem reasonable.

This is what I was trying to show the OP. The main point was finding a way to center the hole wrt the width. I believe the scheme I proposed does this.

KENAT, probably the least qualified checker you'll ever meet...