mkcski
Mechanical
- Feb 18, 2016
- 589
I have a somewhat unique application for GDT that I can find no examples of in the Standard or in any of the reference materials I have access to. So I’m requesting some assistance with some symbology and its interpretation. See attached partial sheet 1 and sheet 2 drawings
We have a component with a circular pattern of (20) “features”. A section through one “feature” is shown in Detail C – upper left on sheet 1. They that are BASICally located to datums in the FCF at the top of the drawing; Datum plane A - 3 motions (tol. zone perpendicularity to primary datum), Datum axis B - 2 motions (center of BASIC circle), and Datum axis C - 1 motion (rotation–clocking with other features). At each of the (20) Detail C “features” there are (2) 3.625 diameter coaxial holes.
As I see it, there are two patterns operating here: a circular pattern of (20) upper “features” with each of the (20) “features” being a coaxial pattern of (2) holes. The position tolerance for both the 20X circular pattern and between the (2) coaxial holes is the same value because: 1) an oscillating shaft must simultaneously passes through the two bushings in Detail C and a single bushing in the mating part, and 2) when the components are assembled the (20) shafts do not interface with each other. Material condition modifiers have been analyzed and are RFS/RMB - (Rule #2).
Y14.5-2009 only illustrates examples of a single location for position of a coaxial pattern - figures 7-43, 7-44, so we have “concocted” the dimensioning schema in Detail C to functionally dimension the part.
The intended interpretation of the GDT in detail C: The FCF for the upper holes controls the position of the entire 20X circular pattern of upper holes to Datums A, B, and C. Because the oscillating shafts that pass through the bushings do not interface with each other, there is an independent relationship between the (20) individual pairs coaxial holes. Given these parameters, and no examples to refer to, we specified the upper hole as Datum D - 20X INDIVIDUALLY and positioned the lower hole to Datum A to maintain the required functional perpendicularity, and to Datum D to maintain coaxiality with the upper hole. The tolerance zone for the lower hole would be oriented perpendicular to Datum A and be aligned with the RMB related actual mating envelope established from the upper hole.
Composite position was considered. But the fit-up analysis determined the position tolerance for the circular pattern and the coaxial holes is the same, so the PLTZF and FRTZF tolerances are the same value – not a smaller refinement – a requirement for composite position.
Questions for Forum members:
1) Does your interpretation match the intended?
2) Is there a “better” approach? Could we eliminate Datum D, the FCF and size dimension for the lower hole by applying the by adding 2X to the upper hole diameter so that a single tolerance zone projects “down” from the upper holes.
3) Would a continuous feature “CF” concept apply? The 3.38 “neck down” between the holes causes me concerns with this.
We have a component with a circular pattern of (20) “features”. A section through one “feature” is shown in Detail C – upper left on sheet 1. They that are BASICally located to datums in the FCF at the top of the drawing; Datum plane A - 3 motions (tol. zone perpendicularity to primary datum), Datum axis B - 2 motions (center of BASIC circle), and Datum axis C - 1 motion (rotation–clocking with other features). At each of the (20) Detail C “features” there are (2) 3.625 diameter coaxial holes.
As I see it, there are two patterns operating here: a circular pattern of (20) upper “features” with each of the (20) “features” being a coaxial pattern of (2) holes. The position tolerance for both the 20X circular pattern and between the (2) coaxial holes is the same value because: 1) an oscillating shaft must simultaneously passes through the two bushings in Detail C and a single bushing in the mating part, and 2) when the components are assembled the (20) shafts do not interface with each other. Material condition modifiers have been analyzed and are RFS/RMB - (Rule #2).
Y14.5-2009 only illustrates examples of a single location for position of a coaxial pattern - figures 7-43, 7-44, so we have “concocted” the dimensioning schema in Detail C to functionally dimension the part.
The intended interpretation of the GDT in detail C: The FCF for the upper holes controls the position of the entire 20X circular pattern of upper holes to Datums A, B, and C. Because the oscillating shafts that pass through the bushings do not interface with each other, there is an independent relationship between the (20) individual pairs coaxial holes. Given these parameters, and no examples to refer to, we specified the upper hole as Datum D - 20X INDIVIDUALLY and positioned the lower hole to Datum A to maintain the required functional perpendicularity, and to Datum D to maintain coaxiality with the upper hole. The tolerance zone for the lower hole would be oriented perpendicular to Datum A and be aligned with the RMB related actual mating envelope established from the upper hole.
Composite position was considered. But the fit-up analysis determined the position tolerance for the circular pattern and the coaxial holes is the same, so the PLTZF and FRTZF tolerances are the same value – not a smaller refinement – a requirement for composite position.
Questions for Forum members:
1) Does your interpretation match the intended?
2) Is there a “better” approach? Could we eliminate Datum D, the FCF and size dimension for the lower hole by applying the by adding 2X to the upper hole diameter so that a single tolerance zone projects “down” from the upper holes.
3) Would a continuous feature “CF” concept apply? The 3.38 “neck down” between the holes causes me concerns with this.