Assigning a datum axis through a tapered (countersunk) holes

Not 100 % sure that I understand the problem. But, if I do, there are examples of using datum target points to get C and D and subsequently axis C-D. You could then relate the C-D to whatever feature you desired.

Assign the datum to the conic surface.

editorial comment
A countersink seems like an awfully flaky feature to use as a datum. Difficult to pick up with a gage, lots of variance in a small zone for CMM. Why not use the hole and not the c'sink?

Datum the c'sink hole. If there is not a hole, datum the bigger dia of the C'sink then dim the angle. I would not datum both dia's of the C'sink.

Chris
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Correction: forget I said "countersink." The hole is tapered.


Tunalover

Because of chamfers and/or edge radii, the ends of a hole do not actually exist, so they don't make good datums.

I don't know what 14.5 has to say about it, but the traditional way of measuring the location of a tapered hole or using it as a datum is with a gage ball of specified diameter, the diameter usually being chosen so the ball stops halfway through the cone.



Mike Halloran
Pembroke Pines, FL, USA

TheTick-
From a functional standpoint, hole centers are often good choices for datums especially for the control of coaxial features. It follows then that tapered holes often serve as natural and functionally-significant datums.
I've always held that functionality carries 85% of the weight when choosing datums! Inspectability carries the remaining 15%.





Tunalover

Hole centers work well enough. My concern would be the quality of the c'sink surface. It would need to be a high quality cut to be used as a reliable datum. No chattering and well-centered on its respective hole.

Mike H's gage ball idea would work well enough, though again depending on the quaity of the cut.

ringman-
Where are these examples you mention?


Tunalover

Tunalover,

Try Fig. 4-34 on page 76.

ringman-
Are you referring to Y14.5M-1994?


Tunalover

Yes

Conical features are frequently used in tool holders and on self-aligning tools, and are completely valid as datum features. Put the datum feature callout on the conical surface or on a leader extending away from it. A cone actually produces a datum axis and a datum plane; this gives you three mutually perpendicular planes to work from. The first two planes result from the axis...no surprise there...the datum plane (the third plane) is located at the vertex of the cone. The vertex is a useless point because you can't stabilize anything there, and often doesn't physically exist because the cone is truncated. To get around this automatic generation of the secondary datum plane (the third plane), you may need to clearly specify a limitation/exclusion on the drawing.
Assuming that you want the axis to be the primary datum feature, select the conical surface as the primary datum feature and include a note under the datum callout (or in an addendum or note elsewhere) indicating that the axis is only being used to generate the primary datum (axis); then label another feature as the secondary datum. In the FCF, include the primary datum and secondary datum (and tertiary as needed to clock the part).
It's easier if the cone's axis can be the secondary datum; the primary datum will already override the datum plane that would otherwise have been generated at the vertex.

Another option, rather than using the entire surface of the cone is to use a series of datum targets. A ring (usually 5 points MIN to catch lobing effects) of targets at a location near one end of the cone and a ring (an odd number of points, but more points than at the first ring) of targets near the other end of the cone will be used, with the RESULTANT axis being used as the datum. (Note, the RESULTANT axis DOES NOT correspond to the actual axis of the part).

Another option that I've heard of is to use a toroidal ring to locate the center at one location (essentially line contact) on the cone, and a series of 3 or more (odd number again) points around the circumference at a substantial distance from the toroidal ring. This method actually sets up a compound-datum which, of course, may have nothing to do with the actual axis of the part.

My preference is to specify the whole surface to generate the axis, and meet with inspection to agree on a mutually-acceptable way of establishing the datum in practice (and document it!).

Sorry it's a long answer, but it's a major topic! Hope it helps.



Jim Sykes, P.Eng, GDTP-S
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