large size casting parts drawings, ISO vs ASME
large size casting parts drawings, ISO vs ASME
(OP)
I'm working with casting part drawings that are ISO based(ISO 8062). I think the designers use similar to "END ITEM DRAWING METHOD" as explained in para 2.4 of 09' Y14.8, to draft these drawings. What I don't quite understand is there is no datum specified on the casting drawings. None of the casting part drawings has datum specified. These casting parts are typically cylindrical and huge, around 20 tons. Imagine pressure vessel. So I figure there are couple of constraints to apply GD&T for large size casting parts:
1. Features supposed to establish datums are too rough to be datums features. These surfaces will be machined later in machining process, and may be specified as datums on machining drawings.
2. It is not practical to use datum simulators or gauges because of the size.
3. It is not practical to use CMM to inspect the part because of the size. Even for machined parts, I was told inspector uses ruler to measure most of dimensions.
However, we also have ASME based drawings for same parts out of casting. These drawings use GD&T. So I'm not sure if it is the constraints listed (may be more) cause different approaches between ASME and ISO, or I miss something. Anyone works with large size casting parts can share some ideas?
1. Features supposed to establish datums are too rough to be datums features. These surfaces will be machined later in machining process, and may be specified as datums on machining drawings.
2. It is not practical to use datum simulators or gauges because of the size.
3. It is not practical to use CMM to inspect the part because of the size. Even for machined parts, I was told inspector uses ruler to measure most of dimensions.
However, we also have ASME based drawings for same parts out of casting. These drawings use GD&T. So I'm not sure if it is the constraints listed (may be more) cause different approaches between ASME and ISO, or I miss something. Anyone works with large size casting parts can share some ideas?





RE: large size casting parts drawings, ISO vs ASME
As to your specific questions,
1) datum targets are commonly used on castings of all sizes
2) using datum targets, a somewhat simple datum simulator fixture can usually be setup, even for large parts; I've concepted some that were over 25 feet long by 10 feet wide by 6 feet high
3) I've seen very large gantry-CMMs, as well as long-arm CMMs that can be moved around to accommodate large pieces
Most likely the designers of the castings just aren't adequately familiar with GD&T, and / or don't see the value in it for a casting. Alternately, the geometries may be considered "simple" enough that they don't see the value of the effort.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: large size casting parts drawings, ISO vs ASME
Take one as an example. It has OD up to around 4.6m, ID around 4m, more than 3m long. There are a lot of holes, flanges, struts, ribs on it need to be located and oriented. Not a simple part at all.
RE: large size casting parts drawings, ISO vs ASME
Frank
RE: large size casting parts drawings, ISO vs ASME
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: large size casting parts drawings, ISO vs ASME
RE: large size casting parts drawings, ISO vs ASME
Frank
RE: large size casting parts drawings, ISO vs ASME
SDETERS has brought up a good point. I probably should see the machining process and understand how the part is being set up.
RE: large size casting parts drawings, ISO vs ASME
Frank
RE: large size casting parts drawings, ISO vs ASME
Frank
RE: large size casting parts drawings, ISO vs ASME
RE: large size casting parts drawings, ISO vs ASME
Our machining drawings establish datums from cast surfaces (i.e. parting surface), but we use regular (+/- toleranced) dimensions to locate our primary machining datums from the casting, and then geometric tolerances for all machined features based on our primary machined datums.
It hasn't caused an issue with our suppliers (yet). We initially had some tolerancing schemes that involved cast-in pads for datum targets but that was thrown out the window when our machine shop told us they were not needed.
fsincox - we use CT12 without issues from the foundry. Part sizes range from "slender" (~200mm x 200mm x 1700 mm) to "chunky and flat-ish" (250 x 500 x 850). Material used is AA356-T6.
RE: large size casting parts drawings, ISO vs ASME
justkeepgivener,
When you say just plain dimensions, I assume you are working in metric? Meaning you are not dealing with implied decimal place tolerances just the "CT" implied tolerances.
Frank