Can anyone explain to me what is meant by this marking? This model came from a vendor. Is the B datum just the centerline of the circle?
Also, this same hole has a drafted side and mates with another part with an o-ring between them. What would be the best way to dimension this so that the part works as intended. The original 23mm dimension is technically gone since there was a draft added and a round added at the top & bottom.
The only thing that the attachment of the datum feature symbol would affect is the inspection of the component...
The surface has higher importance as I would read it and would drive the inspection method ensuring the surface met the size criteria (most likely driven by mating component functionality). If the datum feature symbol was on the axis then I would consider the theoretical centerline axis more important during the inspection process and use it as the basis for my inspection origin (much harder to do in real life and without sophisticated equipment, i.e. CMM).
This is my experience... but, if this is a customer component and there is any questions, DO over communicate. A phone call can go a long ways! (and save everyone involved much wasted time or scrap dollars)
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I assume there is a datum A. I also assume datum A is a plane (it may not be). Datum A would be formed by taking a perfectly (and I do mean mathematically perfect) flat plane and laying it on the real world part such that it touches three high points at the base of the boss. Then take a perfect cylinder (and again I do mean mathematically perfect) whose axis is perfectly perpendicular to that plane such that the boss is inside the cylinder but it doesn't touch the boss. That is, this cylinder can float around still staying perpendicular to the plane without contacting the boss. Now, shrink that cylinder until it can no longer float around stuck in the perpendicular position touching the boss. This is datum B. Datum B is NOT the axis of the boss UNLESS it is perfectly perpendicular to datum a (not possible in the real world).
Now the technical answer (the truth is more complex).
The interpretation of the datum B symbol depends on how it is used and its order in its feature control frame. If you don't know what a feature control frame is, please read ASME Y14.5M-1994 (not the most current version) page 66, Figure 4-18.
Harry L. Archer Jr. (Code E26HA1) ++++++++++++++++++++++++++++++++ Naval Surface Warfare Center Indian Head Division
Datum B can only be the axis of the smallest cylinder that can fit over the boss, perpendicular to another datum plane. If you create a geometric tolerance, such as true position, there would be basic dimensions to the axis of the boss, not a point on the outer edge of the boss. Besides, which point in the 360° circle would you pick to be the datum? The same goes with the flat datum which is created by three points. You would inspect a dimension from that datum plane, not an arbitrary point on the actulal part's surface, because again, which point on the part's surface would you pick?
Exactly correct. Because the hole may be tapered, or barreled, or even banana shaped, the Axis is the average of this as is the datum. You also have the option of a projected tolerance zone above the datum the hole is located in to further restrict the geometric tolerance of the hole feature. This is somewhat difficult to define/measure with CMM equipment.
Another interesting thing about 'centerline' datum callouts is that you can also have the same condition exist for a slot or key type of feature, where the datum callout flag is put in line with the dimension leader indicating that the midplane of the feature is the datum as opposed to one of the sides. Very handy.
As far as I can tell, it is defined as the axis of a fully inscribed cylinder for a hole or circumscribed cylinder for a boss. It is not defined to be perpendicular to anything else as such. Any tolerance would be described in a perp. tol or a true pos. tol. These tolerances would then describe perfect cylinders perpendicular to the referenced datum, and having a diameter equal to the tolerance described. The feature axis would then have to lie inside of these cylinders.
BTW, it is perfectly OK to have a hole with a true position tol of 0.000 at MMC to the locating datums. I use this for mating jigs and fixtures with locating dowel holes. You can also add a projected tolerance zone (P in a circle) for the expected protrusion of the assembled dowel (or etc...) out of the hole to make sure the jigs are interchangable.