compound datum, simultaneous requirement

[bxbzq]

This sketch is incomplete intentionally. The values of dimensions and tolerances are arbitrary.
In first sheet, even datum C and D are of equal importance to locate or orient features on the part like the hole, I still have to call out datum C first then locate datum D to datum C.
If the datum feature C and D were of same size, 143.1, I think I can simply attach datum C to the 2X 143.1 slots, just like the two 95.1 slots, datum feature B.
Just because the slot sizes are different, the simultaneous req't does not work. This is not very logic to me. Or is the approach in second sheet better?

 

[powerhound]

If you position everything to Datum A only, then your simultaneous requirements will work for this pattern. You will have to do the same with the missing dimensions but the outside profile should controlled with profile of a surface.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[fsincox]

There are some differences here between ASME & ISO so be careful to know what you want.

 

[powerhound]

Who is your comment directed at Frank? The print he submitted invokes ASME as does the tip I provided. I'm confused.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[bxbzq]

Powerhound,
This is a nice tip. Thanks. I'm just afraid there are fewer people understand the simultaneous requriement here.

fsincox,
What would be the iso approach?

 

[fsincox]

powerhound,
Sorry, it was intended as a just general reminder, not applicable to anyone who does not reference ISO.

 

[powerhound]

bxbzq,

The simultaneous requirements rule is one of those concepts that is mostly understood by those who have taken a deliberate approach to learning GD&T. Datum shift is the same way. You are correct when you say that few people understand it but that doesn't mean it isn't a rule or that it isn't documented in a standard.
I am not a fan of making prints based on what I think a supplier may or may not know. If I have invoked ASME on a print and the supplier quotes the part, he is telling me he will build the part per the print, which includes per the standard. Rest assured that if the people there don't understand simultaneous requirements, there are dozens of other rules and principles that they don't understand. Does that mean you need to exclude everything they don't understand? Probably not.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[pmarc]

bxbzq,
You can simply do the following:

http://files.engineering.com/getfil...0-8165-15274876b309&file=comp_datum_pmarc.pdf

If someone asks you why positional FCF is attached to an empty dimension associated with only one of the slots and not both, use fig. 7-51 from Y14.5-2009 as an explanation.

Of course I am not saying it is the only, the best and the most user-friendly solution - just offering an option.

BTW: Perpendicularity callout for datum features B should be changed to Position (otherwise locational relationship between two widths 95.1 is not controlled at all) and datum feature symbol B should be attached to that positional FCF.

 

[bxbzq]

powerhound,
Thanks for the comments.

pmarc,
Thanks for the offering and pointing out the perpendicularity callout lacking of locational relationship.
But I don't get "datum feature symbol B should be attached to that positional FCF." To me this is same as the way shown in my sketch in terms of datum feature callout. Both mean the datum feature B is the centerplane of the two slots(being 2 slots because of 2X note). Can you elaborate?

 

[fsincox]

I would suspect pmarc's concern is because he wants it to be clear the datum is being derived from the (2) features, also.

 

[pmarc]

That is right, Frank. Attaching datum feature symbol to dimension on the left might suggest that datum B is derived from left width only (despite '2X' callout preceeding dimension value). At least this is my impression.

I see at least one more option since the print is according to Y14.5-2009. <CF> modifier could be used tying locationally two 95.1 widths together. In this case '2X' prefix would not be needed, perpendicularity callout could stay unchanged and the datum feature symbol could also stay as it is now.