Composite Profile Tolerancing Clarification

optomist1

A Sea of Statistics
Trusted Information Resource
#1
Hi To All,
I need a clarification or rational regarding the attached figures/drawings;

The first figure 24-6, shows that the 20 basic x 12 basic true profile opening can vary in location more than its orientation or size...I get that. It also states that the upper segment controls the location relative to Datums A & B and orientation relative to Datum B.

The second figure 24-7, control 1 upper segment states "controls the orientation relative to Datums A & B and location of the opening relative to Datums B & C."

Given that the DRFs are similarly constructed, why does the upper segment control location of the opening relative to Datums A & B in Figure 24-6; yet in Figure 24-7, the upper segment controls the location of the opening relative to Datums B & C? In fact for controls 2 & 3 in Figure 24-7 orientation of the opening is again controlled by Datums B & C. Why not Datum A & B?

What am I missing?

Thank you for your assistance..

Marty
 

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tonefordays

#2
Optomist1,


In figure 24-6
Datum -A- establishes the Y axis
Datum -B- establishes the X axis (centerline)
Size variance allowed for the window is ±.6mm all around from the basic dimensions, this will also establish location.
The composite callout to Datum -A- requires the window be oriented parallel to -A- with .4mm (±.2mm) from 24mm. Without the composite tolerance, orientation allowance would be 1.2mm (±.6mm) from -A-

Figure 24-7
Datum -A- establishes the Z axis (orientation)
Datum -B- establishes the Y axis
Datum -C- establishes the X axis
Size variance allowed for the window is ±.6mm all around from the basic dimensions, this will also establish location.

The composite callout to -A-B- requires the window be oriented perpendicular to -A- and parallel to -B- within .4mm (±.2mm) to both datums. Without the composite tolerance, orientation allowance would be 1.2mm (±.6mm) to datums -A-B-c-

Figure 24-6 is using one side and centerline to establish location and one datum to establish orientation, Figure 24-7 is using 2 sides to establish location and one plane and one side to establish orientation. The primary feature control frame will control size, and location. The composite feature control will tighten up the orientation requirement.

Hope the explanation isn't too confusing.
 

Paul F. Jackson

Quite Involved in Discussions
#3
In all composites the upper segment controls both translations and rotations relative to the datum features specified while the lower segment is limited to rotational control only to the specified datum features.

The lower segment of a position composite retains both translational and rotational control among the pattern members but only rotational (orientation) control to the specified datum features.

The lower segment of a profile composite constrains size/form/and orientation of the specified profile but only rotational control (orientation) not translational (location) to the specified datum features.

Furthermore all composite callouts release their pattern members from simultaneous requirements with other identically referenced callouts on the drawing.

So the size of the composite controlled profile feature in figure 24-6 is limited by the lower segent profile tolerance 0.4 or +/- 0.2 from the basic feature profile.

Paul
 

optomist1

A Sea of Statistics
Trusted Information Resource
#4
Good afternoon Paul and Tonefordays,

Thank you both for your thorough responses....you both are obviously very experienced in GD&T so bear with me a GDTer in training.

From the responses it appears as though my inability to understand why in Figure 24-6 Datum A a side/plane and Datum B a centerplane are used to control the location of the slot whereas in Figure 24-7 Datum B and Datum C (instead of Datum A & B) both sides, control the location of the opening...is due to how the feature or slot is oriented relative to each Datum feature.....

For instance in Figure 24-7 the slot opening is oriented perpendicular Datum A a plane, therefore one cannot expect to use it to locate the feature. And in Figure 24-6 Datum A and Datum B are parallel to the sides of the slot. Establishing "location requiring two Datums at 90 to each other.

I understand all else, the Composite Heirarchy, segment rules, refinement, upper controlling location, the lower never controlling location, etc. It is the difference in Location control vs Datums that is not clear.

This is gnarly stuff....hopefully I'm on the right track. Thank you for your patience..In the mean time I 'm reviewing my Datum/Datum Feature Type instruction material.

Regards,
Marty
 
T

tonefordays

#5
The designation of datums is usually going to be dependent on form, fit, and function of the part. When a part is being used in service, it may be more important that the window line up with a mating part, or perhaps it's clearance for a mechanism, or maybe it's used to align something. These are just a couple of simple examples of what different datums can dictate. The examples are just showing different ways that similar features can be called out.
 

Paul F. Jackson

Quite Involved in Discussions
#6
In figure 24-7 Alex is highlighting how orientation degrees-of-freedom are constrained/released by changing the references in the lower composite segment.

1) slot profile 0.4 size/form, perpendicular-to-[A], parallel-to-
2) slot profile 0.4 size/form, perpendicular-to-[A]
3) slot profile 0.4 size/form
4) seperate segments same as 3) when orientations are uncontrolled in the lower.

In Figure 24-6 Alex is just highlighting the difference in control of the upper and lower segments.

middle box... actual profile located within 1.2 wide zone at basics relative to [A|B].
bottom box... actual profile size/form constrained by 0.4 wide zone oriented to [A].

Two requirements must be met simultaneously the 0.4 wide zone is permitted to translate for location within the 1.2 wide zone while remaining parallel to [A].

Paul
 
Last edited:

Paul F. Jackson

Quite Involved in Discussions
#7
yet in Figure 24-7, the upper segment controls the location of the opening relative to Datums B & C? In fact for controls 2 & 3 in Figure 24-7 orientation of the opening is again controlled by Datums B & C. Why not Datum A & B?

What am I missing?
Because in the upper segment the profile only has a 90 degree implied perpendicular orientation to the primary [A]... The basic locations are relative to and [C].
Paul
 
Last edited:

optomist1

A Sea of Statistics
Trusted Information Resource
#8
Good Day Paul and Tonefordays,

Thank you both for the detailed responses.....a lot to digest, from your perspectives is the area of GDT one of the more gnarly or difficult to fathom? I'm trying to gage (no pun) my knowledge and learning pace....

Regards,
Marty
 

optomist1

A Sea of Statistics
Trusted Information Resource
#9
Good Day Paul and Tonefordays,

I kind of had an Edison moment......by realizing or remembering just what "Orientation" entails Perpendicularity/Parallelism/Angularity...dahhhh!!

Marty
 
T

tonefordays

#10
Glad to hear you're understanding it better. It is alot of info. to digest. Definitely not an overnight training course.

Figure 1-8, on page 1-9 in the Advanced Concepts of GD&T book shows the basic application of the symbols in a pretty meaningful way. I'm assuming this is the book you're using based on the attachments in your first post.
 
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