Tec-Ease Blog

Don — Tue, 07 Jun 2011 13:10:56 +0000

Question: Fig, 4-49 of the 2009 standard shows datum target spheres being used to establish a datum axis. Can lathe centers be used as datum targets to establish a datum axis? If so, would the included angle of the centers be included on the drawing?

Hard gaging profile of a surface.

Don — Thu, 26 May 2011 13:42:56 +0000

Question: Hello Don,
When you apply a profile of a surface control to
lets say a slot, are you able to create a functional
hard gage to inspect this callout?

Datum feature symbol placement

Don — Tue, 24 May 2011 12:21:39 +0000

Question: Hi Don,
Because of an ongoing (friendly) disagreement between me and a co-worker regarding datum symbol placement, I decided to turn to your tips. If you reference 3.3.2(f) in the standard it states, “placed above or below and attached to the feature control frame when the feature (or group of features) controlled is the datum axis or the datum center plane.” The tip I looked at said it was added August 1998. My question, since datum b in the second example is a surface (not an axis or center plane) is it allowable to attached the datum feature symbol to the feature control frame? Thanks for your time…
Mike Kay

Don’s 2nd response to Comment 2:

Don — Fri, 14 Jan 2011 17:45:02 +0000

You are correct that you could add the 30 and 10 dimensions and if the 20 had been a toleranced dimension rather than basic, it would be incorrect to add the 10 dimension. See 7.7 Tolerancing For Symmetrical Relationships.
It is paragraph (k) that states the zero basic relationship. There are several figures in this and previous standards that illustrate this. See Figs. 4-29 thru 4-33, 4-39, 4-46, 7-4, and the list goes on. Datum feature B is a width and establishes a center plane.
What I have stated is in accordance with the Standard and is not an assumption or implication. Using your argument, we should put 90° on every right angle, label every tangency, state a note when features are coaxial, and so forth. There are many implied conditions that are explained in the Standard and previous versions of the Standard. Please review the Tips I mentioned previously.

Just as we all had to learn how orthographic projection works to read a drawing, we have to know the rules of the Standard.

Comment 2, Response:

Don — Fri, 14 Jan 2011 17:21:23 +0000

So, for the center lines of the 60 holes, a 30 basic dimension is ok, and a 10 dimension for the slot is ok, because it’s controlled using Profile of the surface….but if the 20 slot was dimensioned using +/- tolerance for size….the 10 dimension is incorrect?

Upon further reading of 2.1.1.4, it doesn’t address the implied center of the block issue…. It talks only of 90 and 0/180 degree relationships of features and center lines. My understanding of GD&T was to eliminate confusion, assumptions, implications etc…. therefore adding the 30 and 10 dimensions to this drawing would be more clear. Thinking about those who read the prints is most important when making drawings.

Is this your understanding from the 2009 standard? If so, how about earlier Y14.5 standards?

Don’s Reply to Comment 2:

Don — Fri, 14 Jan 2011 17:02:06 +0000

You asked if the drawing with GD&T will add cost. The answer depends on who is making the part and if there are any problems with the part once it is manufactured. The drawing without GD&T is open to interpretation. If the supplier makes a near perfect part there probably will not be any problems and some might argue that it will cost less. If there are any problems with the part, since the drawing has multiple interpretations, there will often be issues with scrap, rework, “use as is” decisions and unknown time and money spent resolving issues.
The drawing that has geometric tolerancing on it has one clear meaning to anyone who knows the standard. If the folks making the part know how to read a drawing, there should not be all of the issues there could be with the other part and the cost of the part should be considerably less. If however, the folks making the drawing do not know how to read a drawing–who knows what the cost could be.
I hope this answered your question.

Comment 2:

Don — Fri, 14 Jan 2011 16:58:30 +0000

Thanks for your regular update and sharing GD&T tips with us. After reading below tip of month it will help me to pass on and share this information our engineering centre. There are two different drawing in tip. Could you please tell me if drawing with GD&T will add any cost to the manufacturing, tooling and quality inspection and how much cost it will add?

Don’s reply to Comment 1:

Don — Fri, 14 Jan 2011 16:55:34 +0000

I get this question quite often and should probably create a tip on it. The features are implied centered. There are several examples of this in the Standard. From the 2009 revision it states:

1.4 FUNDAMENTAL RULES

(k) A zero basic dimension applies where axes, center
planes, or surfaces are shown coincident on a drawing,
and geometric tolerances establish the relationship
among the features.

Also, I have done a couple of related Tips in the past. They are:
http://www.tec-ease.com/gdt-tips-view.php?q=122
http://www.tec-ease.com/gdt-tips-view.php?q=234

A lot of folks want to add “half” dimensions. When all of the dimensions are basic, it is OK to add the half dimensions but not necessary.
the
standard shows it both ways. However, if some of the dimensions are size dimensions, adding a half dimensions is totally incorrect.

Comment 1:

Don — Fri, 14 Jan 2011 16:50:12 +0000

I enjoy your tips of the Month, I have a couple questions on this months print…..to me the two hole 60 apart and the 20 slot are not located to an edge or the center line. What do you think?s of the Month, I have a couple questions on this months print…..to me the two hole 60 apart and the 20 slot are not located to an edge or the center line. What do you think?

What you always wanted to know about Y14.5.

Don — Mon, 03 Jan 2011 19:05:47 +0000

As a member of the ASME Y14.5 committee I am not allowed to speak for the committee. However, I am allowed to offer my opinion based on my experience and having attended all but two of the Y14.5 meetings over the past 20+ years. So, if you would like to better understand some of the reasons behind the development of the rules, terms, definitions and symbols found in the ASME Y14.5 standard, join this blog. I expect that many of the questions will generate their own thread.