Symmetrical Tolerancing
Symmetrical Tolerancing
(OP)
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?
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





RE: Symmetrical Tolerancing
Use positional tolerance on the hole, can't find an example with a hole in the standard but 5.60 is similar idea.
I'm not sure you properly define your end radius/overal length either but my brains aching, we just had lay-offs so someone check what I'm saying.
KENAT, probably the least qualified checker you'll ever meet...
RE: Symmetrical Tolerancing
From Fig 5.60 refers to a center plane and a datum plane of the actual mating envelope. If I were to insert a diameter cylinder into the feature control frame to represent a cylindrical axis, then perhaps the same logic would apply, but to cylindrical tolerance zones. Is this sound logic?
Failure is a prerequisite of successful design
RE: Symmetrical Tolerancing
It assumes A is a surface the hole is perpendicular to.
I think per 14.5 para 1.8.4 it's R not FULL R.
Anyone want to pick my sketch apart
KENAT, probably the least qualified checker you'll ever meet...
RE: Symmetrical Tolerancing
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RE: Symmetrical Tolerancing
Chris
SolidWorks 07 4.0/PDMWorks 07
AutoCAD 06
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RE: Symmetrical Tolerancing
Is it too early? Or am I right?
V
Mechanical Engineer
"When I am working on a problem, I do not think of beauty, but when I've finished, if the solution is not beautiful, I know it is wrong."
- R. Buckminster Fuller
RE: Symmetrical Tolerancing
RE: Symmetrical Tolerancing
RE: Symmetrical Tolerancing
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Symmetrical Tolerancing
Doesn't the width .563 (datum B) need to be basic as well? Seems if both sides are critical, the width must be controlled as well.
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RE: Symmetrical Tolerancing
Back on topic...
Refined sketch.
In mine I'd think 1.000 should be basic as the it locates the hole which is controlled by position. I'm sure I made it basic originally but then it must have got deleted or something and I didn't put it back (hey I said I was the least qualified checker you'll ever meet).
I dropped the R.281 as I think giving the overall length and width then "R" is probably better. Otherwise you have to look at locating the R on the hole which seems overkill.
Who said anything about R being basic?
I didn't initially want to assume the bottom edge was a datum because I didn't know all the details but it probably should be.
Anyway, I've made more assumptions, corrected an error and take a look.
KENAT, probably the least qualified checker you'll ever meet...
RE: Symmetrical Tolerancing
Maybe I'm going a bit overboard.
V
Mechanical Engineer
"When I am working on a problem, I do not think of beauty, but when I've finished, if the solution is not beautiful, I know it is wrong."
- R. Buckminster Fuller
RE: Symmetrical Tolerancing
The designer would have to decide but even I'm thinking it's probably overkill for the application and that yes, you are going overboard
KENAT, probably the least qualified checker you'll ever meet...
RE: Symmetrical Tolerancing
I could probably specify the width of the bar stock using a basic dimension, but according to the drawing checker bible I'd have to call out both surfaces comprising the thickness as datums. I'm trying to keep this part as inexpensive as possible. Although now that I brood over this point I'm starting to worry about the consequences of thickness variance.
Kenat,
Congrats on Tip-master of the week! Hope you were able to finalize your roughness call out dilemma. I think you hit the nail or in this case pin on the head with your drawing. By the way is it necessary to specify the front face as a Datum? I'm assuming 'Yes' if I want the hole to be perpendicular to the face.Then again wouldn't we want to reface the oppositte face, the face it sits on as the drill approaches the part.
Failure is a prerequisite of successful design
RE: Symmetrical Tolerancing
You'd have to pick which surface is A based on function.
Someone else may be able to provide better guidance on this.
KENAT, probably the least qualified checker you'll ever meet...
RE: Symmetrical Tolerancing
Could you just reference the hole center in subsequent assemblies and then the width isn't important?
Otherwise, I'd draw the hole off center and put a centerline on the bar and the hole. Then, just dimension the distance from the bar centerline to the hole centerline as 0.000 +/- 0.XXX. Of course, visually the drawing would not show it centered, but the dimension should control, not the visual representation.
On a side note, is the radius on the end of the bar even required? Wouldn't the bar width and the fact that it is shown as 180 degrees indicate that the radius is 1/2 of the bar width without need to dimension the radius? It could still be put in as a reference dimension of course.
-- MechEng2005
RE: Symmetrical Tolerancing
I'm not sure of the modeling procedures at the OP's place of business, but your suggestion would be breaking a primary rule at places I have worked. The part should be modeled at nominal, and that offset hole would never pass checking.
Then again, if solid models aren't used, the rules mqy be different.
"R" or "FULL R" should still be called out for the radius. The fact that it is 1/2 of the width only permits the omission of the actual size.
RE: Symmetrical Tolerancing
Your suggestions probably wouldn't be 'best practice' for CAD as ewh points out and also to some extent from a drawing point of view.
Using the hole as essentially the assembly datum for alignment may have merit, depends on other unknown factors.
On your side note:
In my sketch the dimension of the radius is not needed however "R" to indicate full radius is necessary per ASME Y14.5M-1994. Simply showing 180 degrees isn't adequate, would most people spot the difference between 175 degrees and 180? For certain applications though this could make a difference.
The OP original sketch didn't have an overal length so did need the R.281 although I was wrong in my earlier post and it is fully dimensioned though possibly not in the best wasy.
"It could still be put in as a reference dimension of course." Excessive use of reference dimensions is generally considered bad practice and discouraged by the standard, can't recall the reference.
KENAT, probably the least qualified checker you'll ever meet...
RE: Symmetrical Tolerancing
"... No more dimensions than those necessary for complete definition shall be given. The use of reference dimensions on a drawing should be minimized."
RE: Symmetrical Tolerancing
Has it been determined that Y14.5 is applied to the drawing in question?
RE: Symmetrical Tolerancing
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
RE: Symmetrical Tolerancing
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.
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
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RE: Symmetrical Tolerancing
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Symmetrical Tolerancing
Failure is a prerequisite of successful design
RE: Symmetrical Tolerancing
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?
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...
RE: Symmetrical Tolerancing
Failure is a prerequisite of successful design
RE: Symmetrical Tolerancing
RE: Symmetrical Tolerancing
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Symmetrical Tolerancing
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...
RE: Symmetrical Tolerancing
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.
RE: Symmetrical Tolerancing
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...
RE: Symmetrical Tolerancing
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Symmetrical Tolerancing
Good idea! It will work for rivet holes too.
RE: Symmetrical Tolerancing
RE: Symmetrical Tolerancing
You appear well-versed in the standard and thankyou for chiming in. But in response to
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
RE: Symmetrical Tolerancing
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.
RE: Symmetrical Tolerancing
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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RE: Symmetrical Tolerancing
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.
RE: Symmetrical Tolerancing
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...
RE: Symmetrical Tolerancing
That was the original intent of the post and I will implement that scheme.
MadMango,
The second block has .060" separation. I've checked the welding fixture and as I expected the fixture does it's job best when the hole positioned as close to the center as possible.
Once again, thank you all for your admonition.
Tom
Failure is a prerequisite of successful design
RE: Symmetrical Tolerancing
Can you provide a sketch showing the weldment configuration for this part? That would go a long ways in the solution.
RE: Symmetrical Tolerancing
If the fixture used the holes for locating the part, that should take care of the alignment problem. Is that not a possibility?
RE: Symmetrical Tolerancing
I believe this justifies using positional on the hole. I've previously explained why I suggested diametric tol zone.
KENAT, probably the least qualified checker you'll ever meet...
RE: Symmetrical Tolerancing
The fixture uses either the left side or the right side of the hinge block, depending on which of the 6 locations it is welded to. The fixture does not use the hinge pin holes for fixing the hinge block into position.
It uses: 1. The bottom Face, 2. The LH or RH face, and 3. The Front Face. The fixture does have a feature to align the block via the hinge pin hole as secondary alignment assurance.
I agree that if the hinge pin hole was used as a primary alignment feature this increase overall ease of assembly alignment. The only issue I see if the pin hole is used as primary alignment is that off center pin holes, or thickness variance will cause gaps between the mating surfaces of the hinge block and the tube it is welded to. Waning the gap at the fixture would only delay alignment issues at assembly.
I think the root cause solution would be to control the symmetry of the pin hole as well as the width of the block and employ the pin hole as a primary alignment feature at welding.
Failure is a prerequisite of successful design
RE: Symmetrical Tolerancing
What is the function of the hidden lines in your original sketch. Are they somehow associated with the alignment.
RE: Symmetrical Tolerancing
That was just a pependicular hole for a spring pin to keep the hinge pin from sliding out the back end. So, no it is not associated with the alignment.
Tom
Failure is a prerequisite of successful design
RE: Symmetrical Tolerancing
Ed
RE: Symmetrical Tolerancing
meaning the mid plane is datum B of what the actual size of nom .563 feature is machined. Hence .316 dia is located about datum B midplane within postional tol specified. Of note datums A and C include.
ray Doyle
Ray Doyle
SolidWorks 99-07
RE: Symmetrical Tolerancing
Also, I don't believe CL is usually used any more.
KENAT, probably the least qualified checker you'll ever meet...
RE: Symmetrical Tolerancing
Using "CL" in that matter is like wearing a belt and suspenders. The line font defines that it is a centerline. The only time I specify "CL" is when identifying an axis or other specific feature (i.e. CL DATUM X).
Otherwise, makes sense to me!