Thank you! This is in my opinion one of the easiest concepts to understand once you are familiar with GD&T, but very difficult to undestand when you start learning or to explain to newbies. It causes a lot of confusion, a lot of questions and sometimes make people think it can be used whenever they feel both datums are "important".
Hi, totally agree. I mentioned the perpendicular common datum thing in the video because I’ve seen it applied before. It’s tough to explain why datums can’t be “equally” important as you mention. The trick with the common datum features is that they only create a single “datum”.
Will there be a significant difference in variability between parts if a single cylindrical mating envelope is created around the 2 cylindrical features vs 2 cylindrical common datum features as shown above? For example, say there exists a cylinder with a surface profile requirement across the entire exterior cylindrical surface. The ends of the cylinder are fairly consistent in terms of meeting nominal dimensions, however, everything in between them is highly variable. Think of randomly hammering the sides of a bunch of empty soda cans; the ends of the cans would largely maintain their shape, but the surfaces between them would vary significantly between cans. Would using a common datum feature (1 datum band at each end of the can) create more consistency between cans versus a standard ASME single collapsing cylinder datum?
I'm trying to figure out the general rules for common datum features. One drawing has a plane as one datum and two studs intersecting a perpendicular plane as another datum. The CMM software will not allow me to make a common datum from these together as an A-B datum. My understanding from the software files is that it is because the cylinders are not perpendicular to the first plane. The standard isn't clear in explaining these rules so where would I find them at?
Hi D.O can you explain in details how a common datum defined? For ex. How to define common axis of two coaxial bearing seats on a shaft with different diameter? Two perfect forms collapsed at the same time with same ratio or what else? Thanks
I think that’s a fair statement. They basically mean the same thing. I don’t like continuous feature for this application personally, it doesn’t make sense to me for datums A and B on the example to be continuous, because they are totally separate in reality. They can be called out as continuous and a single datum per Y14.5 though.
