Datums on a molded part w/ draft - acceptable practice?
Datums on a molded part w/ draft - acceptable practice?
(OP)
I have an injection molded part that has a blind hole at one end. When this component was being machined I used this hole as the primary datum since it locates/orients the part in the assembly. Now that I'm looking at molding a 1-2 degree draft angle may be required for this hole (i.e. zero draft may not be practical). Can I still use it as a datum now that it's tapered?





RE: Datums on a molded part w/ draft - acceptable practice?
You said that "the hole (cylinder) when it was machined served as the primary datum feature" that would mean that the machined hole constrains four degrees-of-freedom (two rotations orienting the axis) and (two translations locating the axis)... that would leave two degrees-of-freedom for other features to constrain if indeed all six are so constrained (one stopping rotation about the axis) and (the other stopping translation along the axis).
If the molded cylindrical hole with draft (making it a cone) still constrains four degrees-of-freedom as it mates to a geometric counterpart (a coned protrusion) then by all means keep it as the primary datum feature and apply a form tolerance for the conical surface itself to get the integrity of the coordinate system off on the right foot.
I have no idea what your part looks like or how it actually is oriented or located in the assembly... but if a machined hole can be interchanged with a drafted hole and not suffer any critical loss of coordinate system orientation... I would suspect that other features may be doing some of the orientation that the designated primary is supposed to be doing... in which case the feature has been miss labeled as primary.
Take a good look at the assembly and see what feature or set of features control the most degrees of freedom (specifically the most orientation degrees-of-freedom) and that will lead you to the answer to your question.
Paul
RE: Datums on a molded part w/ draft - acceptable practice?
Peter Truitt
Minnesota
RE: Datums on a molded part w/ draft - acceptable practice?
Since we do not have the assembly to understand, Are you saying the hole in it's unfinished state locates the "machined" (your word) part?
Frank
RE: Datums on a molded part w/ draft - acceptable practice?
The blind hole will be slightly undersized to the screw major diameter and the counterbore pilot will be slightly oversized to the screw major diameter. Since the blind and pilot holes are not very deep, both datum features are used to establish the primary datum.
For the machined part, I envisioned a stepped expanding pin could be fabricated (if practical) could be used in inspection to simulate the datum features. Referencing datum features A & B at their maximum material boundary in the FCF may also work. The question is what I do if max draft angle is imparted whether the datum features are simulated RFS or MMB?
Hopefully the above does not confuse matters further.
RE: Datums on a molded part w/ draft - acceptable practice?
RE: Datums on a molded part w/ draft - acceptable practice?
RE: Datums on a molded part w/ draft - acceptable practice?
RE: Datums on a molded part w/ draft - acceptable practice?
Peter Truitt
Minnesota
RE: Datums on a molded part w/ draft - acceptable practice?
Is any of the assembly sandwiched between surface [C] and the surface perpendicular to [Z] with such pressure that it would cause the self tapped threaded piece to attempt orient itself to [C]?
Paul
RE: Datums on a molded part w/ draft - acceptable practice?
Paul: Yes to your first question. The blind hole is undersized to the screw major diameter [A] based on the 80-90% undersizing rule of thumb per research for a Type-23 (aka Type-T)self-cutting fasteners. No to your latter question. Nothing is sandwiched between surface |C| and the step perpendicular to |Z| - think of surface |Z| as a standoff. I'm relying on the pilot to orient the threaded piece. I'm not yet even sure how effectively surface |C| will act as a hard stop to control the depth since the step is so small. I may may need to rely on assembling under a microscope and visual gage when the two features are touching.
RE: Datums on a molded part w/ draft - acceptable practice?
With the info that you have shared so far... I would say that your use of the hole as the primary is "spot on" although perhaps it is not just [A] that is primary... but rather [A-B] as you have so detailed it. The gradually increasing interference fit orients and locates the two details to each other. I wouldn't worry much about the taper in the new molded design for orientation sake... since the mating part is going to cut threads into it. Perhaps there could be a shallow c-bore .095 that encapsulates any thread-cutting debris and gives a square corner as the assembly torque reaches its max.
Paul
RE: Datums on a molded part w/ draft - acceptable practice?
RE: Datums on a molded part w/ draft - acceptable practice?
RE: Datums on a molded part w/ draft - acceptable practice?
RE: Datums on a molded part w/ draft - acceptable practice?
Lets clear up one issue, the OP. Can a tapered feature be used as a datum?
I say yes!
Does it fit here? It looks like it might from your description.
ptruitt,
I think the taper is on the internal feature(s) I had a different picture in mind, too. As they say: "a pivture is worth a thousand words."
Frank
RE: Datums on a molded part w/ draft - acceptable practice?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datums on a molded part w/ draft - acceptable practice?
Thanks, I just wanted to express a clear answer to the original basic question. I realize prdave00 is interested in a more specific question.
Frank