Double True Position but same diameter? 4

[JoeBlock]

Working on a part that has two true position feature control frames under one diameter. Both are identical except for the tolerence. The top frames tolerence is 1.25 the bottom frames is 1.00. Both are maximum material to
[-A-], [-B-], [-C-]. Why are there two boxes? The note covers 12 holes.

Thanks,

Joe

 

[looslib]

Composite tolerance.

I think I have this right.
The top controls the holes to each other within the pattern.
The bottom controls the pattern to the reference control frame.

"Wildfires are dangerous, hard to control, and economically catastrophic."
"Fixed in the next release" should replace "Product First" as the PTC slogan.

Ben Loosli
CAD/CAM System Analyst
Ingersoll-Rand

 

[gsagolla]

Actually, its the other way around. The top controls the pattern location (PLTZF - Pattern-Locating Tolerance Zone Framework) and the bottom controls the features within the pattern (FRTZF - Feature-Relating Tolerance Zone Framework)

Check out this web-site for more info:

[URL unfurl="true"]http://www.tec-ease.com/tips/december-98.htm[/url]

Giuseppe Sagolla
Sr. Mechanical Engineer
Immunicon, Corp.

SW2005SP3.0; XP Pro SP1.0
hp xw4100; P4 3.2 GHz, 3GB RAM
NVIDIA Quadro4 980 XGL

 

[ctopher]

gsagolla,
Interesting link, thanks

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP2.0 / PDMWorks 05
ctopher's home site

FAQ559-1100
FAQ559-716

 

[JoeBlock]

Thank you looslib and gsagolla. I read through the link at tech-ease and it's says about the same thing as my
GEO-METRICKS III reference by Lowell Foster. I just don't understand why both frames include A, B, and C. Foster's example includes A, the tech-ease example includes A and B, but both feature frames on the print call out A, B and C the tolerence is the only difference.

 

[gsagolla]

For a composite feature control frame, the lower segment controls the feature-to-feature tolerance and the basic dimensions relating the pattern to the datums do not apply. Therefore the lower segmnet can have no datum reference or may repeat, in the same order of precedence, some or all of the upper segment datum.

So for lower segment containing:
[ul]
[li]NO DATUM REFERENCE: pattern is free to shift and/or tilt within boundaries of upper segment[/li]
[li]A: pattern can shift and and tilt with respect to datums b & c but tolerance zone must remain perpendicular to datum A.[/li]
[li]A|B: pattern can shift but tolerance zone must remain perpendicular to datum A and pattern must remain oriented to datum B[/li]
[li]A|B|C: pattern can shift but tolerance zone must remain perpendicular to datum A and pattern must remain oriented to datums B and C[/li]
[/ul]

The use of the tertiary datum (in this case) really only applies to radial patterns where B is a datum axis (rather than a surface datum). Adding reference to datum feature C (keyway or flat on a shaft) would prevent rotation of the pattern about axis B. This is covered in the ASME standard ASME Y14.5M-1994: Dimensioning and Tolerancing and in Geometric Dimesnsioning & Tolerancing by James Meadows but I don't remember ever seeing it Foster's book.

Hope that helps...

Giuseppe Sagolla
Sr. Mechanical Engineer
Immunicon, Corp.

SW2005SP3.0; XP Pro SP1.0
hp xw4100; P4 3.2 GHz, 3GB RAM
NVIDIA Quadro4 980 XGL

 

[JoeBlock]

Great and easy to understand explanation, but this brings up a few more questions.

What is the proper way to verify the bottom feature control frame?

Does checking each hole to each other mean check all
12 * 11 or all 132 possibilities for conformance?

Or can one hole be made a reference datum to check the others to?

 

[gsagolla]

If the 12 holes are a rectangular pattern and A, B and C are surface datums, then the lower frame shouldn't really have a C specified. In this case, to verify the upper frame, the center axis of the hole must be within 1.25 as established by the basic dimension from the datums. Typically, one hole in the pattern would be dimensioned to datums B & C and all other holes would be dimensioned from this first hole. For the lower frame, you would establish a reference frame with origin at the first hole and axes aligned to B datum. Every other hole in the pattern must then be within 1.00 to this first hole.

The same methodology applies to a radial pattern on a cylindrical part; the C datum simply provides an additional constraint to prevent rotation of the pattern.

Good luck.

Giuseppe Sagolla
Sr. Mechanical Engineer
Immunicon, Corp.

SW2005SP3.0; XP Pro SP1.0
hp xw4100; P4 3.2 GHz, 3GB RAM
NVIDIA Quadro4 980 XGL

 

[JoeBlock]

Yes, I’m in agreement with that measuring scheme. Normally I would let the mating part verify the measurement method when the print has this kind of ambiguity. This product launch has been so hot that every assembly had to go to build, so we couldn’t get any assemblies for measurement methodology confirmation. This is an excellent link, thank you Giuseppe, it will be easier for me to sleep now.