Half dimensions 2

greenimi · Dec 19, 2014

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

greenimi · Dec 19, 2014

Here is the modified sketch---for better understanding on what I am asking?
Thank you again

CheckerHater · Dec 19, 2014

Considering that general rule is "basic dimensions tie features to datums" I don't see anything wrong with your sketch.

greenimi · Dec 19, 2014

Okay,
So, do I need 30 basic (is it mandatory) or not?

greenimi · Dec 19, 2014

Too bad I cannot edit those posts

In other words, IF I leave it out, is the design intent clear?

drawoh · Dec 19, 2014

greenimi,

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

CheckerHater · Dec 19, 2014

greenimi said:
In other words, IF I leave it out, is the design intent clear?

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.

greenimi · Dec 19, 2014

Quote: "Now you need at least SOME dimension to tie your feature(s) to the datum."

Yes, so I have added 14 basic.

But IN ADDITION of 14 BASIC, do I need or not 30 basic?

Drawoh said no.

CheckerHater · Dec 19, 2014

I would say yes.

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.

randy64 · Dec 19, 2014

In your modified version you have to have the 30 dimension to locate the slot as well as the 2 holes in the slot. Without it, there is no way to know where they are located.

3DDave · Dec 19, 2014

If you have a copy of ASME Y14.5M-1994 then you can see on page 84, figure 5-3 and 5-4 the recommendations for dimensioning a set of holes almost exactly like your example.

greenimi · Dec 20, 2014

Thank you 3DDave
So, 30 basic is needed.

tbuelna · Dec 26, 2014

The first example is not correct since there is not proper dimensional definition for the positional hole tolerance shown. The positional hole tolerance must be accompanied by a complete system of basic dimensions based on relevant datum features.

greenimi · Dec 26, 2014

tbuelna said:
The first example is not correct since there is not proper dimensional definition for the positional hole tolerance shown. The positional hole tolerance must be accompanied by a complete system of basic dimensions based on relevant datum features.

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?

tbuelna · Dec 28, 2014

In the first example you linked, the 70, 22 & 15 dimensions of the part are un-toleranced and therefore considered to be basic. So these features could have any location, shape or profile imaginable. And in theory the mating component would not fit properly.

greenimi · Dec 28, 2014

So, again what do YOU think is missing? What is your proposal to make it fully defined? You said the first example is not correct, right? How to fix it, in your opinion?
Add, what? Remove, what?

3DDave · Dec 28, 2014

This is a trick drawing with a general note and a general profile tolerance. It's rarely used because it's more complicated than title block tolerances and is not a useful functional tolerance. The technique is a favorite for 'gd&t' consultants showing off what one could do, and it covers the same territory as "Drawing is Intentionally incomplete" notes.

greenimi · Dec 28, 2014

3DDave and tbuelna,

Is the "drawing intentionally incomplete" (first example or the second example for that matter)?
What is missing? Still confused.

3DDave · Dec 28, 2014

The tolerancing is covered in the first posting as I described. There is no missing information.

tbuelna · Dec 30, 2014

After taking another look at the sketch linked, I would agree with 3DDave that almost every surface or feature is covered with the following exception: The 22 (basic) dimension shown for the plate thickness should be noted as 2X.

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