GD&T Positional tolerancing 5

[rwild]

Can you have a positional tolerance feature control frame on a hole without referance to any datums? If this is a correct way of dimensioning, then what is it controling? Perpindicularity, hole to hole, something else?

 

[ewh]

A positional control without any datum to be controlled relative to serves no purpose that I am aware of.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[dingy2]

No!

One could have a perpendicularity of a hole to a datum surface but that is it.

Dave D.

http://www.qmsi.ca

 

[SeasonLee]

For a hole pattern, a position tolerance without any datum reference, it means to control the spacing within the pattern.

rwild : Would you mind to post a sketch, it will easy to catch the designer's intent.

SeasonLee

 

[PaulJackson]

Yes!

Coaxial pattern of features.

See Fig 7-59 pg 153 of ASME Y14.5-2009

 

[rwild]

Hello SeasonLee

Attached is the sketch that you requested.

 

[SteveMartin]

I cannot think of a situation where the FCF in the provided sketch has a definite meaning. It needs datums.

 

[SeasonLee]

Obviously, its a hole pattern, composite position callout should be used for this case, and the "position tolerance without any datum" should be the lower segment (FRTZF) of the composite position callout.

SeasonLee

 

[KENAT]

What about title block default position tolerances as some have proposed?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[pmarc]

There are two cases in Y14.5 standard which allow using position control without any datum reference:
1. Lower segment of composite position tolerance - then it controls spacing between the holes within the pattern.
2. Coaxiality control for cylindrical features shown in line on a drawing.

However you must be careful, becasue if you follow ISO standards, they allow such callouts for position of pattern of elements. The location and orientation of the pattern depends on the considered actual features of the workpiece.

 

[restedup]

Is it possible they are using the Model Data Set for the definition?

 

[321GO]

To me the (this) drawing also needs datums. Without them it becomes a guesing game.

SeasonLee -
to my knowledge a (PCD) hole pattern still needs at least one datum (on a concentrical bore).

This way the origin of the PCD is constraint and as such one has a reference to measure from.

http://www.tec-ease.com/tips/november-98.htm

 

[SeasonLee]

321GO

If one datum reference on the lower segment (FRTZF), it means to control the spacing within the pattern and the perpendicularity of the pattern relative to the datum refered to.

If no datum reference on the lower segment (FRTZF), it means to control the spacing within the pattern only.

You will get more detailed information from fig 19-6 of the book by Alex Krulikowski "Advanced Concept of GD&T"

SeasonLee

 

[rwild]

Is it a common practice to have just the lower segment of a composite feature control frame?

 

[pmarc]

rwild,

It is not a common practice simply because Y14.5 std. does not allow using single-segment position tolerance without any datum reference in pattern of holes applications.

 

[Belanger]

The only time datums can be omitted on a position tolerance is if you care only about hole-to-hole relationship. As mentioned above, there is the example of "coaxial holes," but I suppose you could also do it for a pattern of holes laid out in a flat plate -- as long as perpendicularity is not important to you.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[fsincox]

Season,
I know this has been discused before, do you agree with John's last statement? It was certainly OK in earlier versions of the standard. (MIL-8 & 1966). I believe dingy had a srong opininon on allowing it too. I have been telling people (based on the discussions in this forum), it is still allowed though not tecnically what they want in many cases, agree/thoughts?? The fact is it exists the question is is it still allowed?
Frank

 

[SeasonLee]

fsincox

John mentioned "hole-to-hole relationship", I think this is exactly same as what I said "to control the spacing within the pattern" if no datum referenced on the lower segment (FRTZF).

I don't have the standard of MIL-8 & 1966, sorry for I can't give you a reply regarding it.

SeasonLee

 

[dakeb]

Positional tolerances without a datum is allowed. Imagine a group of spheres in infinite space. The spheres are not dimensioned to any other feature except the other spheres in the group. They may have a positional tolerance, so that any sphere in the group automatically becomes a datum for the position of any other sphere in the group. No datum is declared.

In real life the position of the group may not matter, but the relative postion of the holes to each other does (e.g. to align with a mating part). The holes may be dimensioned from an edge to a single hole in the group. The dimension to the group of holes does not need to be controlled by the positional tolerance (unboxed dims). The hole positions are still only controlled by the positional tolerance relative to each other as if they were in infinite space.

If you constrain the position of the holes unnecessarily to a datum you add cost to the manufacture of the part, and unneccessary rejections.

Dave

 

[pmarc]

I can imagine a case when datum reference in a single segment positional FCF for pattern of features is not needed at all - as Dave just said, we can consider pattern of 4 spheres where, for the functional reasons, the spacing within the pattern is more important than the location of the pattern itself. And that's fine for me, because spheres are very specific features which do not have any orientation defined. So the mating part with 4 spherical hollows in it will fit as long as a spacing within the pattern is sufficient.

But if we consider mating of two parts like: a flat plate with a pattern of 4 cylindrical holes and similar plate with 4 pins, and we do not use any datums for position of these patterns, we will not be controlling perpendicularity of holes/pins in relation to mating surfaces at all. So there might be a case when pattern of holes is tilted in one direction and the pattern of pins in opposite direction, and then the alignment is not possible.