Another Angled Hole Positional Tolerance Question 4

[R1chJC]

Hi All,

I have a question regarding the use of positional tolerance on angled holes.

Is the attached legit?

Thanks,
Rich.

 

[3DDave]

I would not dimension that way. Using A as primary I would dimension to the place the axes intersect that feature and show the angle with respect to that surface. In the sketch it is implied that the angled surface plays some part in locating those axes, but that isn't a surface that is referenced.

 

[R1chJC]

If I want those holes normal to the surface (which I do) then isn't right to say that surface plays a part in positioning them?

 

[3DDave]

If you want the angled surface to be the orientation control it needs to be the primary reference.

 

[R1chJC]

Ok I gotcha, that makes sense.

So how about the attached. Green or Red Basic Dimension setup though? Both go back to Datum's so I'm struggling to decide.

Thanks for the help by the way.

 

[drawoh]

R1chJC,

Yes, it is legitimate, just not necessarily the best dimensioning scheme. Fixturing it to hit your Ø0.1mm positional tolerance will be a challenge. As the designer, I would be looking for a way to open that up.

I am not going to propose a dimensioning scheme because I do not know how your part works. If perpendicularity to the angled face matters, you need to specify a datum, and call it up on the block. You can add a second line to your feature control frame with the additional datum.

--
JHG

 

[3DDave]

Since "X" is at an angle to , then depending on how far away from [D] that measurement is made the part can always be correct or almost always be wrong. The dimension should be to the intersection of [D] and .

 

[chez311]

I would second 3DDave's conclusion - your x-dimension is dangling and is not fixed so it will change depending on where along B it is taken. It should be dimensioned from the origin created by the intersection of your datums.

See attached for an example, the figure is closer to the A|B|C DRF in your original post/attachment but can obviously be extrapolated to your modified D|B|C DRF.

 

[pylfrm]

So how about the attached. Green or Red Basic Dimension setup though? Both go back to Datum's so I'm struggling to decide.

You can't have a linear dimension between two lines that aren't parallel, so your red scheme isn't going to work. Your green scheme is valid, although perhaps somewhat unusual.


pylfrm

 

[3DDave]

I think the green version does not work because the hole location depends on the intersection between the axis and an uncontrolled, unrelated surface.

 

[pylfrm]

3DDave,

The surface is not unrelated anymore. It's the primary datum feature.


pylfrm

 

[3DDave]

pylfrm,

You're right - I think I disliked the way that [D] is potentially backdriven in orientation from using the angle dimension on the holes and misinterpreted that dislike.

A dimension parallel to [D] and to the intersection of with [D] avoids the ambiguity.

 

[axym]

Hi All,

Keep in mind that the arrangement of the basic dimensions does not affect the tolerance specification in any way. The only thing that matters is the datum feature referencing.

If the drawing referenced the CAD model and dimensions X, Y and Z were omitted, then the tolerance specification would still be the same. Each hole axis has a theoretically exact orientation and location relationship to the datum reference frame, and it doesn't matter how those theoretically exact relationships are dimensioned (or at all). The basic distance and angle dimensions would not be measured on the actual part - they are only used to lay out the position zones.



Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

Evan,

That's a great point and is also an unrelated 'what if' because this is not a CAD model where a programmer has taken some other dimensional input and predigested it for the user.

In this case, the holes are dimensioned to be both nominally perpendicular to [D] and at an angle of Y degrees to . This has the effect of saying the nominal angle between and [D} is determined by the dimension that locates the holes. Dimensioning to the intersection of and {D] eliminates that back-driving for the person who has to do the layout.

 

[R1chJC]

Hi All,

Thanks for the help. Just when I think i'm getting to grips with GD&T..

Picking up on the points made, how about the attached scheme?

 

[CheckerHater]

Or maybe something like this:

https://files.engineering.com/getfi...26-0b6d249fb1b1&file=Angle_of_the_Dangle..jpg

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[SeasonLee]

A sketch is worth a thousand words, pls ref to the Before and After figure.

Season

 

[greenimi]

In the latest post from SeasonLee with the "Before" and "After" sketches shown, may I ask you why the "After" method is the preferred one?
Does anyone can offer the relevant verbiage from the book? What would be the justification in the textbook?

I am trying to learn something here.

 

[SeasonLee]

@greenimi
Its a snapshot from Paul J. Drake "Dimensioning and Tolerancing Handbook" page 5-117/118.

Season

 

[drawoh]

SeasonLee,

Your "after" drawing is very much more inspectable, but can you bend the sheet metal accurately enough for the Ø0.010" positional tolerance? One of the benefits of good drafting is that it highlights bad design.

--
JHG