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

 

[TomFin]

Kenat,

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

 

[ringman]

TomFin,

Can you provide a sketch showing the weldment configuration for this part? That would go a long ways in the solution.

 

[ringman]

TomFin

If the fixture used the holes for locating the part, that should take care of the alignment problem. Is that not a possibility?

 

[KENAT]

TomFin sorry this all seemed to get out of proporation. I think you and I are on the same wavelength and I hope it helped. My last few posts aren't directed at anything you've said/asked.

ASME Y14.5M-1994

2.1.1.1 Positional Tolerance Method. Preferably, tolerances on dimensions that locate features of size are specified by the positional tolerance method described in Section 5. In certain cases, such as locating irregular-shaped feature, the profile Tolerancing method described in section 6 may be used.

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...

 

[TomFin]

Ringman,

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

 

[ringman]

TomFin,

What is the function of the hidden lines in your original sketch. Are they somehow associated with the alignment.

 

[TomFin]

Ringman,
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

 

[highwaystar]

If having the hole in the center is ultra critical, I would either use ground bar stock. That way you can dimension it either way. If it is only semi-critical, you could specify a tolerance range on the bar stock width. If this is a weldment I hope you specify accurate secure fixturing for making the welds otherwise the distortion will kill your alignment. Depending on the rest of the geometry, you might ensure alignment by specifying that a tooling hinge pin be inserted prior to welding. You will still need to fixture it tightly during the weld, but the tooling pin will guaarantee alignment.

Ed

 

[121560]

PLEASE SEE ATTACHMENT. The .563 is a datum feature of size
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

 

[KENAT]

I'm not sure that your .281 rad is properly defined. You don't explicitly define the center point of it. This was part of why I gave overal length and just 'R'.

Also, I don't believe CL is usually used any more.

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

 

[ewh]

I agree with KENAT. Specifying the actual radius size is unnecessary in this situation.
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!