Half dimensions
Half dimensions
(OP)
Friday’s question:
If datum feature B were the left side of the shown plate (or the right side for that matter) AND 14 baisc dimension is added between the datum feature B and the closest hole, THEN do we need 30 baisc or not?
In other words, as shown with datum B, being the centerplane of the part, 30 basic is not needed. Agreed with that.
What about in my modified scenario? (again, datum feature B moved to the side and 14 basic added). Do I need 30 basic or is considered implied? (30 basic is the dimension from the center holes)
Thank you very much for your help
If datum feature B were the left side of the shown plate (or the right side for that matter) AND 14 baisc dimension is added between the datum feature B and the closest hole, THEN do we need 30 baisc or not?
In other words, as shown with datum B, being the centerplane of the part, 30 basic is not needed. Agreed with that.
What about in my modified scenario? (again, datum feature B moved to the side and 14 basic added). Do I need 30 basic or is considered implied? (30 basic is the dimension from the center holes)
Thank you very much for your help
RE: Half dimensions
Thank you again
RE: Half dimensions
RE: Half dimensions
So, do I need 30 basic (is it mandatory) or not?
RE: Half dimensions
In other words, IF I leave it out, is the design intent clear?
RE: Half dimensions
If you have 60 basic, you don't need 30 basic. You could have 30 basic on each side.
As far as I know, either system is clear and convenient for the machine shop. Back in they day, they wanted everything taken from a reference corner, but that was before CNC. Any machinists out there are welcome to chip in and disagree with me!
--
JHG
RE: Half dimensions
Not exactly.
Once you move your datum to the side edge, your part is not symmetrical anymore. So mid-plane is out of the game.
Now you need at least SOME dimension to tie your feature(s) to the datum.
RE: Half dimensions
Yes, so I have added 14 basic.
But IN ADDITION of 14 BASIC, do I need or not 30 basic?
Drawoh said no.
RE: Half dimensions
My own personal preference is to avoid confusion.
Either your part is symmetrical, so go as tec-ease suggests all the way, or you dimension everything from one side, then provide half-dimensions "for clarity"
Definitely adding dimension 30 will not be "wrong" or "illegal" because basic dimensions are theoretical - there is no tolerance accumulation or other nasty thing happening.
RE: Half dimensions
RE: Half dimensions
RE: Half dimensions
So, 30 basic is needed.
RE: Half dimensions
RE: Half dimensions
tbuelna,
Can you explain a little bit more why the positional HOLE tolerance is not correct? I do not understand your statement. What is missing?
RE: Half dimensions
RE: Half dimensions
Add, what? Remove, what?
RE: Half dimensions
RE: Half dimensions
Is the "drawing intentionally incomplete" (first example or the second example for that matter)?
What is missing? Still confused.
RE: Half dimensions
RE: Half dimensions
RE: Half dimensions
Perhaps Tec-Ease needs to include a disclaimer similar to that of Y14.5 - "The figures in this Tip are intended only as illustrations to aid the user in understanding the principles and methods of dimensioning and tolerancing described in the text," which in this example refers to half dimensions.
"Know the rules well, so you can break them effectively."
-Dalai Lama XIV
RE: Half dimensions
C'mon Dave. A trick drawing? Really?
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Half dimensions
RE: Half dimensions
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Half dimensions
People are lazy by nature, and if left to their own they often tend to do a poor job of thoroughly checking their work. If an inspector looks at a drawing and does not see a frame around a dimension he expects to be basic, he will be more likely to check into it further. But if none of the dimensions have a frame showing them to be basic, it is easy for someone looking at the drawing to overlook the fact. The same is true with drawings that use general notes to define the tolerance of two-place or three-place dimensions. When making a drawing with lots of dimensions, it is very easy to forget to check that the number of decimal places or tolerance used for each dimension is correct. And the cost of scrapping parts due to an incorrect dimension/tolerance is exponentially greater than what it would have cost to do a more thorough effort on the drawing to begin with.
Glad to see SSG Acosta has successfully transitioned to a career as a GD&T specialist who seems to know his stuff!
RE: Half dimensions
Regardless of whether you like it or not, this method of identifying basic dimensions (with no rectangles) is clearly legitimated by the Y14.5 standard - see fig. 7-1(c). See Y14.8 for more examples.