Simultaneous requirement 5

[DesignBiz]

In a previous thread regarding “profile and a datum”.

http://www.eng-tips.com/viewthread.cfm?qid=241508

A dwg was posted in support of the datums callouts. However as I looked at the dwgs from this post more closely I saw at the bottom a dwg with a different dimensioning scheme.

http://www.tec-ease.com/tips/dec-07.htm

The link for the drawings claims the lower dwg is the same effect as the upper drawing dimension scheme, based on the simultaneous requirement. (pg 92 para 5.3.6.1 and 5.2.6.2 for the ASME Y14. 5M standard)

In the 1st (upper) dwg, a large hole in the center of a square part with a pattern of 4 holes located around a center hole is dimensioned to datums A (back surf of part); B (center hole); and C (height of part). Simple enough, however the bottom drawing shows the same part with the same callouts minus the B and C datums and claims this 2nd dimensioning scheme is the same as the 1st dimensioning scheme based on the “simultaneous requirement” rule.

I disagree. When datums B and C are taken away there is no datum to orient and locate the features to each other. The 2nd drawing only controls mutual perpendicularity to datum A, and the dimensional relationship to the pattern of 4 holes to each other. There is no longer any horizontal or vertical relationship to the features of the lower dwg as in the upper dwg.
I don’t believe this is a valid interpretation of the standards “simultaneous requirement” rule comparing these 2 drawings.

The lower drawing indeed shows a “simultaneous requirement” for all of it's feature callouts, however it is not comparable to the 1st or upper drawing.

Agree? Or tell me how am I incorrect.







DesignBiz

 

[ewh]

'94 also states in 4.3.2 "The datum feature symbol identifies physical features and shall not be applied to center lines, center planes, or axes except as defined in paras. 4.6.6 and 4.6.7."
4.6.6 refers to equalizing datums and 4.6.7 refers to those established from complex or irregular surfaces.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[ringster]

DesignBiz,

To illustrate my concerns over any attemts to label the centerline of a part as a datum feature, let's look at the Fig. 5-18 in the 94 standard. Imagine a datum identifier B attached to the vertical CL and C attached to the horizontal. I know of and dealt with a similar arrangement on a aircraft part years ago. It made no sense then nor would it now. The drawing at that time had been in existance for some time and NO ONE previously apparently had the capability to see the flaws in the application. (in that case there were actully more holes in the plate that were on the same cl)

I can verify the MO thing.

 

[DesignBiz]

Ringster,

I am in complete agreement with you.

I only was trying to make a point regarding the 2nd drawing where the center hole, profile of the plate, and the pattern of the plate share the same centerlines without any reference to an explicit datum in the simultaneous requirement example. This to me makes things very unclear as the former datum symnbol practice was also confusing.

Like I said before, I will trust you on the MO thing.

DesignBiz