Positional Tolerance and Datum Precedence 1

[EnglishMuffin]

Suppose we have a cylinder, with an off-center slot extending right across one end face. Suppose the surface of that end face is defined as Datum A, and that the axis of the cylinder is defined as Datum B by using a datum callout on the diameter. Also, suppose one side of the off-center slot is defined as Datum C, and that its radial location from the centerline of the cylinder is specified with a toleranced dimension of some sort. Now suppose that, using a positional tolerance, it is required to locate a circular set of six holes in the said end face, such that the hole pattern is centered on the axis of the cylinder (Datum B) and is angularly aligned using the full length of the slot (ie Datum C) in some specified manner.
If in the position tolerance frame we use the three datums A, B, C in that order, it would contradict the required 3 point/2 point/1 point rule used to define the datums in the ANSI standard, since C requires two points because of the fact that we desire to use it to define a line for angular orientation. (This does not lead to ambiguity in most of the examples shown in the standard and other texts, because they always show very short slots which in conjunction with Datum B can then provide a localized single point angular locator without ambiguity). If on the other hand we use the sequence A, C, B, as the standard seems to require, would the position tolerance then be referenced relative to a point at the intersection of datum C and a perpendicular lying on Datum A and passing through the cylinder centerline (defined by datum B)? If so, that would of course not meet the original intent. Or is it the case, as I have been told by certain others, that the position tolerance would be implied to be referenced to the B datum with C used to define only the orientation? I realize I am probably displaying my poor understanding of G D & T here. However, any comments ?

 

[ringman]

What is the distance from the center of Dia B to the side of the slot and precisely how is it controlled?

Any way you can provide a sketch?

Again I will restate there is a distinction to be made between orientation and alignment. This may be the point of confusion between your Tech. and the Company Rep.

Personally I think the drawing as you state it has problems.

 

[drawoh]

EnglishMuffin,

If the drawing calls up ACB as the datums, in that order, the diameter only defines one plane. The first plane is datum_A and the second is defined by the slot. If you do not impose datum_C, the housing is free to wobble from side to side on one direction only.

This three points, two points, one point thing is for datums defined by flat surfaces. It has no meaning when you use a diameter as a datum. The diameter is a feature of size, which is why I kept mentioning MMC, above. The slot too can be a feature of size, depending on how your drafter calls it up.

JHG

 

[ringman]

Drawoh,

I believe that Datum feature B, in this case, is related to an axis regardless of the sequencing, and not a plane.

 

[EnglishMuffin]

Now we might be getting somewhere .... which one of you is correct ? What exactly does the standard say ? I think conceptually and logically it ought to work the way drawoh seems to think, but the impression I get is that the standard is actually written the way ringman says. I also think conceptually that drawoh's view that the 3/2/1 thing only makes sense with flat surfaces makes perfect sense - but again - what does the standard actually say ? I have no clue - I don't have one. That is the big issue with Person #2 - the standard. I am neither person #1 nor person #2 - I am person #3 - i.e. an interested bystander. Also, there is no "drafter" involved in the argument - only a drawing provided by person #2, which Person #1 is trying to interpret and measure.

 

[ringman]

If you prefer to think in terms of 3,2,1 points, try this; Datum A, plane, 3 points, Datum B, axis 2 points, (determines a line) and Datum C a single point rather than 2 points for orientation.

Again the example would be helpful in either case.

Sounds like the drawing needs changing, but if I understand you are stuck with it. Been there, done that.

 

[drawoh]

ringman,

ASME Y14.5M-1994 is a practical standard. Theoretical planes and axes are useless as fixturing devices because they do not exist.

If your datums are three orthogonal, flat surfaces, you can design a fixture with three real planes that make contact with your datum surfaces. The three planes shown in the ASME standard are real metal surfaces that you can measure from.

When you call up an outside diameter as datum, your point of measurement is the inside diameter of your fixture. You can visualize planes and axes, but these are imaginary. Reality is a hollow, round piece of metal. Since there is a clearance between the hollow, round piece of metal and your part, you usually need to call up the datum at MMC.

JHG

 

[ringman]

Many drawings require theoretical planes as the basis for dimensioning. Part in discussions involves the use of the axis and the mutually perpendicular planes associated with it in this case.

I agree you will not see the 'theoretical planes and lines' on the gaging or fixturing devices, but they are necessary evil,in the overall scheme.

 

[ewh]

The 3,2,1 datum scheme is indeed for more than parts with flat planes. While they do represent planes, those planes do not have to exist on the part except as datum points.

 

[btrueblood]

Not clear to me you need datum A (face of cylinder) referenced at all, other than to control the depth of the holes. To locate the holes on the endplane of the cylinder, with datums A=endplane, B=cylinder diameter, C=slot face, I'd call out the datum sequence as BCA, or just BC if the holes were "thru".

 

[drawoh]

btrueblood,

This is a fundamental mental block with GD&T as per ASME Y14.5M-1994. You must immobilize the part. Your primary datum should be the best available fixturing feature of the part. This this case, it is probably the flat base of the part.

Your datums are fixturing points, not dimension bases. Of course, it is desirable to dimension from your datums, but not absolutely necessary.

JHG

 

[ringman]

Drawoh,

Was the last statement a typo or just a slip. I believe that datums are the basis for dimensions within the realm of Y14.5. "Best as I recall"

 

[drawoh]

ringman,

My phrasing was not very good there.

It was suggested that the required datums were B and C. The logic of this is that these two datums are sufficient to locate the holes and slots. This is a logical point of view, and it was certainly my first impression when I started applying GD&T specs. to drawings. It is wrong.

The features must be located with respect to the specified datums. There is no absolute need for the dimension lines to emanate from the datums, although it is good practise.

JHG

 

[ewh]

Don't the features still have to be tied in some way back to the datums with basic dimensions? Otherwise, wouldn't you be introducing tolerances other than those in the FCF?

 

[JohnNabors]

EnglishMuffin-

Any way you could post the actual drawing in question?

 

[ctopher]

That was my suggestion also.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)

 

[drawoh]

ewh,

For example, if the holes at the bottom of this thing are specified as a pitch circle, there is no explicit link to the datums. They can be inspected from the datums, which is what matters.

JHG

 

[btrueblood]

Drawoh,

Not sure I follow you. The axis of the holes are aligned with the cylinder axis, as I am envisioning the part, and presumably, like a pipe flange, the holes are intended to be symmetric with the axis of the cylinder. Fixturing on the flat surface does you absolutely no good for inspection, since it may or may not be perpendicular to the axis of the cylinder. Also, if the holes are blind, you'll not be able to see, much less inspect/indicate to the holes if the datum A plane is lying at rest on a granite surface plate.

I think, after re-reading, I follow you: it's not up to the inspector to pick the datums, it's up to him to follow the drawing, and use the order of precedence stated on the print. Agreed. But I will continue to design my flanges referencing the cylinder axis as a primary datum; it's my part, make it my way!

 

[ewh]

As long as there is an implicit link, such as a common center, or there is no FCF controlling the pitch circle. If there is, the pitch circle either shares a center with a datum feature, or should be located by basic dimensions to a datum. True position is established by basic dimensions per Y14.5. I have seen no exceptions to this in the standard, but am interested to know if there are any.

 

[ringman]

drawoh,

In your statement previous about the pitch circle. What is the pitch circle related to? What Standard are you currently using for GD and T interpretation?
Do you have access to either ANSI or ASME Y14.5?

Is there perhaps a company Standard that applies to your drawings?