DonkeyPhysics
New member
- Jul 16, 2009
- 41
I'm looking for the best way to define a plane, and to choose which of a couple options make the most sense for defining a primary datum plane (i.e. Datum Plane A).
Background: I'm fairly new to a job working for an electricity company (in Costa Rica). My department is fairly new, and currently have the task of designing some replacement parts for steam turbines. Most of the folks here appear to be somewhat new to the concept of GD&T. I am NOT an expert, but I have more experience than they do working with ASME y19.5 (about 4 years in the aerospace industry in the US). Thus, I am evaluating one of their drawings for a particularly complicated part.
The part actually an assembly, and is a complete nozzle stage of a steam turbine. In that stage, the construction method is such that they first weld all the blades (64) of the stage into inner and outer bands of a large disc. The technicians essentially do their best to align the blade tips. From that ring of blade tips, they use a CMM to compute a derived, imaginary plane (see
for a conceptual idea). For sake of argument, we'll say that the plane is shown in the image as the "Base" line (just to have an easy picture in your head, though realistically it will be the opposite side). From that plane, the rest of the stage assembly is machined, including attachments to the casing.
The most critical feature, in terms of next-level assembly (and therefore the as-inspected part), is a plane which ultimately forms a seal (metal to metal) with the casing for the turbine (see
for a cut-view). My confusion comes from the fact that this planar surface is also essentially (and necessarily) one of the last features cut in the machining process. So, in terms of next-level assembly, I'm inclined to label this surface as Datum Plane A, as it is the locating feature for the full nozzle stage assembly. However, in terms of manufacturing, since it's cut last, I'm not sure if that makes sense. In terms of manufacturing, the derived plane is primary.
So, my first question is: Is the final planar surface appropriate to choose as a primary datum plane (Datum Plane A) (i.e. utilizing a Next-level assembly reference frame)? Or is the original, derived plane more appropriate for Datum Plane A) (i.e. utilizing an as-manufactured reference frame)?
My second question is, considering that my derived plane is computed from the contact points of multiple blade tips (ref Image 2), is a multi-surface Profile call-out the appropriate way to control this plane? Although the technicians are currently "doing their best" to align the tips, once we determine what is reasonable, we want to be able to define a tolerance zone for that derived plane. So, ultimately, I believe we'll be defining that plane with a FCF calling out Profile of .150 (mm), and a note above the FCF stating "64 surfaces" (possibly with an additional note defining those surfaces). I would assume that if I choose my final, machined plane as my Datum A, then I would define my derived plane with a basic dimension, and add a reference to Datum A in the FCF.
For the record, from my previous training, I believe the correct answer for Question 1 is to use the Next-Level Assembly (i.e. as-inspected) approach, rather than the as-machined approach. However, I'd still like to have the discussion.
Background: I'm fairly new to a job working for an electricity company (in Costa Rica). My department is fairly new, and currently have the task of designing some replacement parts for steam turbines. Most of the folks here appear to be somewhat new to the concept of GD&T. I am NOT an expert, but I have more experience than they do working with ASME y19.5 (about 4 years in the aerospace industry in the US). Thus, I am evaluating one of their drawings for a particularly complicated part.
The part actually an assembly, and is a complete nozzle stage of a steam turbine. In that stage, the construction method is such that they first weld all the blades (64) of the stage into inner and outer bands of a large disc. The technicians essentially do their best to align the blade tips. From that ring of blade tips, they use a CMM to compute a derived, imaginary plane (see
The most critical feature, in terms of next-level assembly (and therefore the as-inspected part), is a plane which ultimately forms a seal (metal to metal) with the casing for the turbine (see
So, my first question is: Is the final planar surface appropriate to choose as a primary datum plane (Datum Plane A) (i.e. utilizing a Next-level assembly reference frame)? Or is the original, derived plane more appropriate for Datum Plane A) (i.e. utilizing an as-manufactured reference frame)?
My second question is, considering that my derived plane is computed from the contact points of multiple blade tips (ref Image 2), is a multi-surface Profile call-out the appropriate way to control this plane? Although the technicians are currently "doing their best" to align the tips, once we determine what is reasonable, we want to be able to define a tolerance zone for that derived plane. So, ultimately, I believe we'll be defining that plane with a FCF calling out Profile of .150 (mm), and a note above the FCF stating "64 surfaces" (possibly with an additional note defining those surfaces). I would assume that if I choose my final, machined plane as my Datum A, then I would define my derived plane with a basic dimension, and add a reference to Datum A in the FCF.
For the record, from my previous training, I believe the correct answer for Question 1 is to use the Next-Level Assembly (i.e. as-inspected) approach, rather than the as-machined approach. However, I'd still like to have the discussion.