Position Tolerance on placed objects on a weldment

[jamiethekiller]

Can i use Position to locate an object on a weldment? See attached image. A is the bottom perpendicular surface.

My intent is to have all of my hole patterns and gussets symmetric to weldment centerline. I have a 5x 24 basic dimension doing the spacing of the gussets. I also added a 144" basic dimension that is overall width of the end gussets. Is that 144" dimension even needed? Does the 5x 24" dimension cover intent?

Thanks

 

[3DDave]

Typically a position tolerance is applied to the centerplane or axis of a feature by identifying the width or diameter of the feature. This is indicated by either placing the requirement Feature Control Frame (aka, FCF) in line with the size/width dimension or beneath the size/width dimension. In addition, since the true position of the centerplane or axis is the basis, the dimensions should be to that centerplane/axis and not to one side or a tangency.

I would also recommend against using [B | A | C] as [B ] appears rather small. If I am guessing correctly the far face is used as datum [A]; you probably want the gusset to be perpendicular to [A] and perpendicular to [C] and then located in that orientation to [B ] so the order would be [A|C|B]

The way it is now only [B ] matters as it controls both orientations and the location to the relatively small surface making [A] and [C] redundant.

 

[jamiethekiller]

Typically a position tolerance is applied to the centerplane or axis of a feature by identifying the width or diameter of the feature. This is indicated by either placing the requirement Feature Control Frame (aka, FCF) in line with the size/width dimension or beneath the size/width dimension. In addition, since the true position of the centerplane or axis is the basis, the dimensions should be to that centerplane/axis and not to one side or a tangency.

I would also recommend against using [B | A | C] as [B ] appears rather small. If I am guessing correctly the far face is used as datum [A]; you probably want the gusset to be perpendicular to [A] and perpendicular to [C] and then located in that orientation to [B ] so the order would be [A|C|B]

The way it is now only [B ] matters as it controls both orientations and the location to the relatively small surface making [A] and [C] redundant.

B is attached to the 147.625" dimension, which i wouldnt' consider small(over 12ft long)... B being on a FoS is giving me a centerplane to work from.

There's no datum on the far face. The bottom surface is A(the gusset is perp to that surface) and C is as shown. Its a very long and large weldment with some support gussets and 2 different hole patterns. I chose [B | A | C] because i care more about the symmetry of the gussets and hole patterns than i do the perpendiculary of the gussets and hole patterns. In the end i'm not sure the order of the datums matter on a part like this.

I don't have any dimensions coming from a side or tangency. I'm assuming i don't need a drawing axis denoting a centerplane on the drawing. Since B is on a FoS the centerplane is already defined and doesn't need to be shown? Is that correct?

I take it that i CAN just put a FCF with a leader directly to an object on a weldment and its understood(and legal)?

 

[3DDave]

The far face in the view you have applied the FCF to is [A] or it is to the near face; it's the same sort of surface controlling the same degrees of freedom.

The surfaces for [B ] are small, even if the overall width isn't. If you were to clamp that in a large vice you would have a difficult time getting the part oriented as there's not much control there.

The 5 X [24.00] is to the side of the feature you are putting a position tolerance on.

It doesn't matter if it's a weldment detail or if this is machined from a solid bar - the FCF controls the location of the centerplane of the feature and is applied to either the width of the feature or with opposing arrows from opposite sides of the feature. The face is not allowed to have a position tolerance. The dimension should be to the centerplane OF THE FEATURE. The center of [B ] is assumed as is symmetry.

After the part is clamped on [B ] it can no longer have the orientation further controlled. If it goes in the vice crooked, then that's acceptable and references to [A] and [C} cannot apply; their degrees of freedom are already locked by [B ]

 

[jamiethekiller]

It doesn't matter if it's a weldment detail or if this is machined from a solid bar - the FCF controls the location of the centerplane of the feature and is applied to either the width of the feature or with opposing arrows from opposite sides of the feature. The face is not allowed to have a position tolerance. The dimension should be to the centerplane OF THE FEATURE. The center of [B ] is assumed as is symmetry.

The gusset dimension should be from center to center of gusset for the 5x 24.00 dimension?
Do i need the [144"] dimension or can that drop since the pattern of gussets is implied symmetric to B through the FCF? But the FCF can only be applied to the centerline of the gusset?

After the part is clamped on [B ] it can no longer have the orientation further controlled. If it goes in the vice crooked, then that's acceptable and references to [A] and [C} cannot apply; their degrees of freedom are already locked by [B ]

I think i understand this and it makes sense.

In practice though this isn't going to be clamped in a vise. They're going to touch off one side and then touch off the other side and establish the CL that way. We're not going to have them inspect this. I'm just using GDT as a way to deliver intent. I want the holes and gussets to be symmetric to the part. I have generous clearance on the holes that dimensioning from an edge isn't going to cause a stackup issue(say the part is .02 long and the hole pattern is offset the oposite direction .01" i can still have this part installed symmetric). Is there a better way to deliver that intent otherwise?

 

[3DDave]

The FCF is applied to the width of the feature. The dimension is to locate the center of that feature as that is the true position of the width.

Right now all the gussets would be nominally offset to one side, if position tolerance could apply to the faces, which it doesn't.

The [144] isn't needed and some will complain that it is double dimensioning. Double dimensioning is normally a problem when directly applied tolerances are used - the claim is that features can have conflicting tolerances because of stack-up, but mainly they don't want responsibility for having to meet all the requirements at the same time by controlling features/processing. That does not apply to basic dimensions, but some complain anyway.

They aren't going to just touch off to the sides. They are going to place this on a table on [A] (if that's the top surface it may block access to put in the gussets), square it up to a stop for [C] and then, once is is aligned, they will touch off to each side to find where [B ] is.

 

[mfgenggear]

Can i use Position to locate an object on a weldment? See attached image. A is the bottom perpendicular surface.

My intent is to have all of my hole patterns and gussets symmetric to weldment centerline. I have a 5x 24 basic dimension doing the spacing of the gussets. I also added a 144" basic dimension that is overall width of the end gussets. Is that 144" dimension even needed? Does the 5x 24" dimension cover intent?

Thanks

what is this gusset do?
is the flange to holes important for location?
weld distortion and shrinkage important?
are these just bolt holes for a preload?