Implied Centerlines/Symmetry

[KENAT]

This topic has come up before most recently in thread1103-239885, but on a quick look in ASME Y14.5M-1994 & a search of this site I can't find the text/section in the standard that says explicitly when using datum feature center planes you get implied symmetry/centerlines.

It's shown in figure 5-4 amongst others but I'd like to refer to the actuall text that says when you show a pattern or similar centered to "datum center planes" you don't have to add an extra dimension centering it.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[KENAT]

Drawoh, your new sketcyh is clear to me;-) The common center are the "datum center planes". Your additional holes are still related to these.

In your 1st sketch, you had a different datum structure so to me in that case the centering/symmetry wasn't as obvious.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[DesignBiz]

DrawOh,
I am not sure where you are coming from. If you are thinking that because you pointed at a hole diameter; that the particular hole that the leader is pointing to becomes the Datum; this is not true. As on shown on your drawing , there is a Datum feature symbol that is attached to a FCF pointing to a single hole in a pattern of 6. This indicates that the pattern IS the datum, NOT a single hole. This is why I made the jpg showing the DRF at the center of the hole pattern. It is not correct to claim a single hole in the pattern of 6 is the datum as shown on the drawing you posted. (paragraph 4.5.8 pg 68 and fig 4-22 pg 69). You will notice that in figure 4-8 on page 58 that Datum B is established by a separate leader pointing to a single hole of 2 holes, which allows 1 hole in a pattern to be the datum. Not at all like your posting.

If that is the DRF (similar to fig 4-22) that you suggested for centering the part around the pattern of holes, then like MintJulep's response to that drawing in which he didnt see a "left/right" control; I agree with him. He is correct, there is no centering in that callout.

With your latest posted figure, you do show to place the hole pattern to the center plane(s) of the overall part profile. Your continued reference to the part being symmetric as shown on pg 84 fig 5-4 is simply NOT true.
You are centering the hole pattern in a "symmetrical relationship" with that callout. This is not symmetry according to the standard. There is a difference in the two!!! (See 5.13 and 5.14 page 149)


DesignBiz

"Quality is in the details"

 

[ringster]

Designbiz,

I rather take issue with the statement that an axis 'establishes' 2 mutually perpendicular planes. My old school of geometry says that an infinite number of planes may be passed thru a line. It is only when we provide a datum for the orientation of 2 of these planes which are perpendicular, that they are established, and not by the line alone.

 

[KENAT]

ringster, take a look at figure 4-22 & sectin 4.5.8 of ASME Y14.5M.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[DesignBiz]

Ringster, On guard!!!

I agree with Kenat, take a look at those figures..

until tomorrow

DesignBiz

"Quality is in the details"

 

[drawoh]

DesignBiz,

Okay, on my original sketch, I should have attached datum[ ]B directly to the first hole. This would have expressed my intent. My nominal geometry was symmetric, not my intended datums.

My second sketch was not a hack or re-intepration of my first one. In response to KENAT's original question, I hacked the absolutely symmetric and unambiguous figure[ ]5-4 by deliberately adding an asymmetric feature. I do not think I would do this on a real drawing. I am trying to make the point that the format and clarity of the drawing suffers before you start not conforming to the standard.

JHG

 

[DesignBiz]

DrawOh,

Understood

DesignBiz