Create Centerline Datum From Drafted Faces

[abrewmaster]

I am trying to figure out a way to call out a center plane as a datum to a molded part that has only drafted faces to reference.

I understand that it is possible to call out a center plane as a datum using a width dimension as shown in the attachment but I am trying to figure out if this is still the case if the surfaces are drafted.

The main issue I see with this is that then you are dimensioning from edge to edge, not surface to surface. Or is this not an issue if you still call out a perpendicularity to datum A (in the attachment).

 

[AndrewTT]

You are most likely going to have to use the concept of datum targets.

 

[3DDave]

Just because the surfaces are intentionally sloped is not a reason to avoid using the width as a datum. There is nothing that prevents individual surfaces that are not intentionally sloped from ending up that way, presuming there is some tolerance that allows the slope.

The problem to solve is to define what it is about the relationship between the faces and the features. Since draft is usually taken as originating from an ideal edge then that same ideal edge can serve as the reference for establishing a datum, which is, ultimately an idealized plane simulated with tooling that might only contact a single point on a realized part.

So, if the holes are to appear in relation to a main face, then ordinary datum references are probably OK**. If they are to be related to the narrowed part of the molded item, then the datum references can be restricted using areas, lines, or points on the drafted surface as datum targets. Essentially, pointing out to the inspector what should influence his setting up for measurements.

** I have never understood why people probe parts on CMMS rather than setting up precision angles as the datums, probing those and then putting the part against the precision angles to set the part in the datum reference frame. This avoids the need to hunt for high spots and allows rapid inspection of a series of identical (supposed to be) parts without repeating the datum step. This doesn't substitute for all datums, but using tooling as datum simulators can make measurements like this more straightforward.