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How to locate a part relative to the "center" of another part

How to locate a part relative to the "center" of another part

How to locate a part relative to the "center" of another part

(OP)
I have a requirement from our engineers that a vertical rail, in an assembled machine frame, be located a certain distance from the center or mid-point of a horizontal rail regardless of the horizontal rail's true manufactured length.

How would you dimension/tolerance this per ASME Y14.5m-1994? I may be overlooking something simple but it is Friday and I can't seem to wrap my mind around this today.

Thanks for your help.  

David

RE: How to locate a part relative to the "center" of another part

Off the top of my head (after a huge Friday lunch) I'd add a datum to the length dimension of the horizontal rail in line with the dimension callout.  Control the vertical rail relative to that datum with a true position callout.
I'm sure the others here will let us know if this is a good approach.

Believe it if you need it or leave it if you dare. - Robert Hunter
 

RE: How to locate a part relative to the "center" of another part

ewh's approach seems technically correct but is likely going to confuse the under-schooled.

RE: How to locate a part relative to the "center" of another part

Yes, I realize that... it was the first thing to come to mind, and would effectively control coaxiality through the size tolerance, so I went with it.
What would you suggest, Tick?

Believe it if you need it or leave it if you dare. - Robert Hunter
 

RE: How to locate a part relative to the "center" of another part

I got nothing better.  I suggest schooling for the confused.

RE: How to locate a part relative to the "center" of another part

I meant position, not coaxiality... that food is making me groggy.

Believe it if you need it or leave it if you dare. - Robert Hunter
 

RE: How to locate a part relative to the "center" of another part

(OP)
Ewh,

Would I then place a centerline on the horizontal rail (datum A) and dimension from that? I think it would be because you have defined the centerplane by making that surface datum A.

Thanks Ewh and The Tick for the help. If this doesn't seem kosher to you please let us know.

David

RE: How to locate a part relative to the "center" of another part

(OP)
See section 3.3.2b and figure 3-4c for my reasoning.

David

RE: How to locate a part relative to the "center" of another part

I think that would be correct, using a basic dimension.  You may want to label the centerline as "CL DATUM A" or some such.

Believe it if you need it or leave it if you dare. - Robert Hunter
 

RE: How to locate a part relative to the "center" of another part

Assuming that the horizontal rail is longer than it is cross sectionally wide... I would make the functional surface(s) that establish horizontal - primary, then I would do as ewh suggested and make the size of the overall length (or its functional hard points) secondary.

I am just commenting that the overall length probably will not serve as a stable primary datum feature if I am envisioning your frame correctly.

Paul   

RE: How to locate a part relative to the "center" of another part

(OP)
Paul,

Assume that the rest of the frame has been controlled and that this vertical rail is being added to a fully dimensioned frame but needs to be located from the true centerplane of a horizontal rail. The horizontal rail will become a quaternary(?) datum for the sole purpose of locating the vertical rail.

David

RE: How to locate a part relative to the "center" of another part

aardvarkdw,

   I agree with the suggestions above to use the rail length as a datum, draw a centre line in the middle, and dimension from the centreline.  Presumably, you are following orders.

   How accurate is the length of this rail, +/-.005", +/1"?  Everybody's life will be easier, and your drawings will look more competent if you dimension from one end of the rail.  In the end, that is what someone is going to have to do, anyway.  The least you can do is save them having to measure the length of the rail, divide by two, and add the first number to it.

                            JHG

RE: How to locate a part relative to the "center" of another part

(OP)
JHG,

I agree wholeheartedly with you! It would be easier to do it that way, and I even tried to raise the point with the engineers that you can't measure to a centerplane but they would have none of it. The equipment that will be placed into this frame placed in the "center" of the machine and they wont change their minds. So I get the dubious task of attempting to dimension it the way they think it should be done but also doing it correctly per the standards that I try my darnedest to enforce.

As for, "In the end, that is what someone is going to have to do, anyway". The whole idea is that that is NOT what they are going to do, or if they do their cuts had better have no measurable tolerance. Right now this rail is installed by our people in-house. They measure the rail and divide by two to find center, then use a tool to locate the vertical rail. We are having this outsourced with the rest of the frame.

(longwinded I know, sorry.)

The bottom line is someone else cuts my check, so I have to do what they want and try to limit the collateral damage.

Thanks everyone for the help.    

David

RE: How to locate a part relative to the "center" of another part

Tell your engineers that I said they are putzes.  They are giving us a bad name.  Really, I'd like to tell them myself.

By the rules & numbers, ewh has the solution.  However, none of us would think to inflict such on manufacturing, I'm sure.

Maybe an old-fashioned note, along with an apology-in-advance, disclaimer, and home phone number of said engineers/putzes.

RE: How to locate a part relative to the "center" of another part

thumbsup2True enough, Tick.
Perhaps this is a good case for the judicial use of reference dimensions.

Believe it if you need it or leave it if you dare. - Robert Hunter
 

RE: How to locate a part relative to the "center" of another part

aardvarkdw,

Just for clarification...if you meant by "quaternary" that coordinate system alignments could be assumed from common datum structures specified on the same drawing with X0,Y0,Z0 reference moved via this FCF to the center of the width, that is not permitted under the current standard.

The coordinate system for the measurement of this vertical feature's centrality would need to be fully defined in the FCF controlling it, therefore one would need to declare all of the features primary, secondary, and  tertiary (if necessary) affecting its measurement.

The next ASME standard may provide freedom to unlock translational or rotational degrees-of-freedom to accomplish the intent that "I think" you are suggesting but assuredly the FCF will be more complicated to understand.

Paul
   

RE: How to locate a part relative to the "center" of another part

I also think a note describing what you what is the best solution.  

However, you can ID the center of the rail by placing a datum callouts on its X thickness and Y thickness dimensions, then referring to those on the verticle element.  It forces the recognition of the overalls for the horizontal element.  Though, I doubt anyone is going to be able to implement this without some sort of tooling.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

RE: How to locate a part relative to the "center" of another part

(OP)
Kenat,

Yep, that's a fair example of what I was going for and what I eventually did. I too think that dimensioning to the centerline is "wrong" because there is not a physical feature to measure from, but in this case the design is dependent on these features being located from the center (of mass?) of the part rather than edge because the finished length of the part is less critical than the location of other features.

Thanks everyone.

David

RE: How to locate a part relative to the "center" of another part

Kenat,

I think 2 things are missing on the example.  The depth of the hole and physical ident for the datum surface identificiation.

RE: How to locate a part relative to the "center" of another part

Just pretend it's in ASME Y14.5M-1994 and invoke paragraph 1.1.4.  "...In other instances, figures are incomplete by intent..."

Or if you're that concerned do your own sketch and if you're really feeling that strongly have my original red-flagged.

The sketch show's the items under debate, it's not intended to be a complete drawing.  I don't see that adding either of those things will clarify its input to the OP.
 

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

RE: How to locate a part relative to the "center" of another part

Nice comeback Kenat - he kinda had that coming. I liked your sketch and didn't think it needed that kind of critique, either.
Keep up the good work, 'far from least...'
Kim

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