Double True Position but same diameter?
Double True Position but same diameter?
(OP)
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
[-A-], [-B-], [-C-]. Why are there two boxes? The note covers 12 holes.
Thanks,
Joe





RE: Double True Position but same diameter?
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.
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Ben Loosli
CAD/CAM System Analyst
Ingersoll-Rand
RE: Double True Position but same diameter?
Check out this web-site for more info:
http://www.tec-ease.com/tips/december-98.htm
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
RE: Double True Position but same diameter?
Interesting link, thanks
Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP2.0 / PDMWorks 05
ctopher's home site
FAQ559-1100
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RE: Double True Position but same diameter?
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.
RE: Double True Position but same diameter?
So for lower segment containing:
- NO DATUM REFERENCE: pattern is free to shift and/or tilt within boundaries of upper segment
- A: pattern can shift and and tilt with respect to datums b & c but tolerance zone must remain perpendicular to datum A.
- A|B: pattern can shift but tolerance zone must remain perpendicular to datum A and pattern must remain oriented to datum B
- 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
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
RE: Double True Position but same diameter?
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?
RE: Double True Position but same diameter?
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
RE: Double True Position but same diameter?