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Rectangular Ordinate Dimensioned Drawing 6

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sspatriots

Aerospace
Feb 11, 2011
10
Hi,

The company I work for does a lot of machining for other companies as well as our own. I came across a drawing last week where there was a front view, right side view and to the right of that the view from the backside. In the front view there was a bore that was dimensioned as 0, 0. The bore was about 2/3 the way down from the top in that view, that is, it was not in the middle of the part vertically in that view. Then in the adjacent view looking in from the right side there were 4 tapped holes (one at each corner). The horizontal (x direction) 0 baseline in this view was shifted up to be at the center of the pattern for the 4 holes. I had always learned that the horizontal 0 baseline should carry across to all adjacent views and not move up or down if they are indeed standard orthographically projected views. Am I off base here in my thinking? If not, can someone point me to a reference that either explains my thinking or refutes it?

Also, that third adjacent view to the right of the right side view actually had the 0 baseline moved back up where it aligned with the front view.


Thanks,

Steve
 
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sspatriots,

Ordinate dimensioning should, as opposed to "shall", be done from a common datum. It sounds like poor drafting.

Do you have a question?

--
JHG
 
JHG,

Actually, I had two questions in my post. Looks like you agree with my thoughts, (first question). For me to convince anyone here that it is wrong, I was hoping to find it spelled out in an industry standard or military standard somewhere. Would you know of any such standard or is it simple as Drafting 101 and everyone should know it?



Thanks,

Steve
 
sspatriots,
Is there any chance that we could see the print? At the moment I am leaning towards drawoh's opinion that there is an error on the drawing, but seeing the print would help.
 
Take a look at ASME Y14.5M-1994 paragraph 1.9 not sure it's quite explicit enough for what you want though.

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484
 
Found it! Questions denoted by question marks at the end.
OP said:
Am I off base here in my thinking? If not, can someone point me to a reference that either explains my thinking or refutes it?
 
Likely bad practice, BUT ... Is it a GD&T drawing? If it is a Y14.5 drawing and the dimensions all can be chained back to the datums using basic dimensions, then it's technically ok if still bad drafting practice.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
That is why I asked for print, Jim.
I smell that initial description did not provide all important details.
 
pmarc,

I don't think I can provide a sample, unless I draw something similar, or water down what I have to make it unrecognizeable. How would I put a sample on this forum. I've not done anything like that on this forum before.


Regards,

Steve
 
Use "…or upload your file to ENGINEERING.com" link which is at the bottom of the page.
I think PDF, PNG, JPEG, DOC, PPT formats of files are preferred by most of us here.
 
sspatriots,

Actually, dimensioning from more than one datum makes perfect sense to me. There are circumstances where you might want to do it. I can interpret it as per ASME Y14.5M.

It might not mean what the drafter thinks it does. GD&T is a language, not a procedure.

A good exercise with a marginal drawing is to take everything seriously, and ask yourself how you would inspect the part. What sort of part would you accept?

--
JHG
 
Hi,

I've uploaded a sample .png file to "Engineering.com" called "Ordinate Dimensioning Sample". Have a look. I did have to remove a lot of lines and dimensions for proprietary reasons.


Thanks,

Steve
 
You have to paste the link in the post, otherwise we won't see it.
 
Looks like someone wanted to tie the pattern of 4 holes with large hole visible in center view and not with the smaller hole visible in the left view. There are way clearer ways to express it (unless one wants to stay away from GD&T, no matter what).
 
There was very little GD&T used on this drawing. The orientation of the views was correct. I did move/remove some features slightly so as not to share too much detail on the forum for the example. However, I think you get the idea. The datum in the middle view was clearly moved up to center for the hole pattern. If it were left aligned with the other two views, the part could be measured every bit as accurate as it can with the way it was done. I was hoping to find something documented in a standard that states the 0 datum running between the views is not supposed to shift.
 
Here is the general problem with the lack of GD&T.

Unless certain features are clearly identified as datums, and dimensions on your example are shown as basic, there are no special rules in the standard about your baselines being associated with one single point thru the entire drawing.

Dimensioning you use is known as Rectangular Coordinate Dimensioning Without Dimension Lines, and it is treated like any other dimension, only “without dimension lines”

You mention that there is “very little” GD&T used on the drawing. Is any of it associated with the features you use in your example?
 
I agree with CH.

The drawing is poor, but not because of the origin moving up and down depending on the view. It is rather because there is no clear identification as to which features are datum features, which features are related to which datum features and that the dimensions locating 4 holes are not basic.

Your idea of having the origin in the same place is based on assumption that each and every feature in center view is located from the hole in left view (or maybe from the triangle in right view, who knows?). But what if the intent is to locate the pattern of 4 holes from the biggest hole? Would it really be reasonable to use the same origin as in left view for this purpose?
 
CheckerHater,

I asked about this a while ago.

thread1103-261904

The attachment is not there any more. It was a rectangular plate with some holes in it. All dimensions are plus minus dimensions from one edge, and there were no datums or GD&T. There was no satisfactory answer to my question.

If the second edge is marginally within the angular tolerances, do I measure from the edge, or to I (arbitrarily) select the primary edge for orientation, and measure from the furthest point on the second edge?

--
JHG
 
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