Question about dimensioning with center-plane datums

[RooneyJC]

Hello,

I'm detailing some parts to send out for fabrication. A lot of the parts are shaped like plates and have symmetric features. Because of this, I tend to set up my datum reference frame using the longest surface as my A datum and my B and C datums as center-planes. I was told at my first job that if you call out a center-plane datum and put in a center-line to show this plane, you can dimension features with the symmetry implied. Follow the link for an example:

http://imgur.com/GBRELWa

Have I been doing this correctly or do I need to include extra dimensioning to fully define the features?

Thanks,
Justin

 

[KENAT]

I'd be OK with your datum scheme as you show it and do similar, there are examples in ASME Y14.5M-1994 that show similar though it's not explicitly stated that you don't need the additional 'centering' dimension as I recall it.

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[randy64]

Don't you need dimensions locating the holes from the datums (B and C)?

 

[KENAT]

If you work to ASME Y14.5M-1994 take a look at figure 5-4. In this example they do not dimension from the datums but symmetrically across them as per the OP's example.

Now someone may invoke 1.1.4 and claim the main point of that figure isn't about the dimensioning scheme... but it's good enough for me.

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[CheckerHater]

There was also a tip on Tec-Ease about not needing "half" dimensions. Unfortunately now it's all premium content.

 

[RooneyJC]

Thanks for the responses. I would like to leave these "half" dimensions out as they can clutter the drawing.

Here's a Tec-Ease video explaining it:

https://www.youtube.com/watch?v=3hCAfVgAMCA

Also, do you guys also know if center-plane datums are burdensome to machinists? I feel like machinists would always prefer edge plane datums as they are much easier to zero their machines to.

 

[CheckerHater]

I feel like machinists would always prefer edge plane datums as they are much easier to zero their machines to.

For some reason I always remember the movie “National Treasure” where FBI agent played by Harvey Keitel keeps repeating the phrase: “Somebody has to go to jail”.

Somebody will always be burdened by something.

You dimension part from the edge, machinist takes one edge as reference, QC flips the part over and rejects it because holes are too far from the edge. Then you apply unnecessarily tight tolerance to maintain the edges, and the part cost goes up.
This is why we say dimensions have to be functional.
You want your part to be symmetrical? Somebody has to take care of that.

 

[KENAT]

Like CH says, primary intent of the drawing is to capture functional requirements & detail what you'll accept - not how to make it.

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[RooneyJC]

Right, functionality of the part should take the largest precedence for my application. Thanks again everyone for the help.

 

[jassco]

Hi,

I see nowhere in ASME Y14.5-2009 that explicitly indicates symmetry implied with center plane (line). ASMY Y14.5 does mention symmetry symbol that can be used for this purpose.

Best regards,

Alex

 

[Tunalover]

RooneyJC,
I hate to kick a dead horse here but this is an important concept. KENAT and CH speak wisely. The drawing is to generally show function first and foremost. How the part is made is normally not a consideration in how the part should be dimensioned. The dimensioning should be functional dimensioning. If that agrees with your drawing I won't know unless I understood your application.

Tunalover

 

[powerhound]

jassco,

Look at section 1.4(k). That fundamental rule states what's being discussed here.

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