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

[aardvarkdw]

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

 

[ewh]

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. - [small]Robert Hunter[/small]

 

[TheTick]

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

 

[ewh]

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. - [small]Robert Hunter[/small]

 

[TheTick]

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

 

[ewh]

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

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[aardvarkdw]

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

 

[aardvarkdw]

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

David

 

[ewh]

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. - [small]Robert Hunter[/small]

 

[PaulJackson]

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

 

[aardvarkdw]

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

 

[drawoh]

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

 

[aardvarkdw]

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

 

[TheTick]

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.

 

[ewh]

True 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. - [small]Robert Hunter[/small]

 

[PaulJackson]

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

 

[fcsuper]

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

 

[KENAT]

thread174-218986 Already had me thinking about this.

Linked is a sketch showing what I was thinking and I think it matches the above posts.

Dimensioning to the CL still feels wrong to me but I think it makes sense.

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

 

[aardvarkdw]

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

 

[ringman]

Kenat,

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