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Datum Targets

Datum Targets

Datum Targets

I have a large, broad machined face that I want to control. There is one portion of the surface that I am truly concerned with, and I plan to use Datum targets to hold flatness in a specific area. How can I control the flatness of the rest of the surface?

Ex: Datum target area flatness is .002 within a diameter of 16.00", but I want to state the rest of the surface needs to be within a flatness of .010

Please advise

RE: Datum Targets


Datum targets are just a means of specifying a datum.

What you are trying to do is specify a flatness over a limited surface. A quick look at ASME Y14.5-2009 did not show me anything. How about a thick phantom line indicating the area of interest, with your FCF pointing inside it?


RE: Datum Targets

I wouldn't necessarily use datum targets for this, I don't think it's necessarily appropriate.

Identifying a portion of the surface and making it clear it has different requirements as drawoh suggests may be more applicable.

Or... I was once persuaded to relieve a surface outside of the interface area i.e. break it into multiple surfaces. From cost point of view not a real fan of that one but it may have applications.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Datum Targets

drawoh, Kenat,

I believe OP wants to define tighter flatness tolerance of the portion of the larger surface which later will be used as a datum target area for establishing a datum. To me that looks like a smart move, if functionally needed.

But in general I agree with you - probably two flatness callouts are needed;
1) .002 - pointing to the datum target area (which diameter should be defined by basic dia. 16'' dimension);
2) .010 - pointing to the remaining part of the surface.

RE: Datum Targets


This is exactly what I am after. Is this acceptable to do as per the standard? There is nothing in the standard from what I can see that shows this.

RE: Datum Targets

Yes, this is acceptable. You just have to keep in mind/accept that it is rather impossible to have each and every case specifically described in the standard.

I am not sure which version of the standard you are using, but if it is ASME Y14.5-2009 take a look at paragraph 1.1.4 first. Among other things it says: "The absence of a figure illustrating the desired application is neither reason to assume inapplicability, nor basis for drawing rejection."

Then go to figure 1-13 (page 11) and imagine that the rectangular cross-hatched area is your datum target. By changing the MASK FROM PLATE note to a flatness callout (.002), and adding one more flatness callout (.010) to the remaining portion of the surface you will get what you want. Of course don't forget to add basic dimensions defining size and location of the datum target area as well as remember to change line style of datum target boundary from thick chain to phantom.

RE: Datum Targets

The 1994 version was better; it didn't link a process step to the limited area. Instead it said if something applied to a limited length or area, describe it as shown.

I think I would use profile instead for each area and indicate a simultaneous requirement to prevent/control out-of parallel and an unacceptable large step between the two areas.

RE: Datum Targets

Or you could just indicate/state that the .010 flatness tolerance applies to the entire surface if .010 max is acceptable limit of parallelism and offset error between the areas.

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