points to define a datum
points to define a datum
(OP)
I know that we need two point to define a line, so two points can be used to define a datum that is a line. I also know that we need three point to define a surface, therefore, three points can be used to define a surface that is a datum. But how about cylinders or spheres? Are three points still enough?





RE: points to define a datum
RE: points to define a datum
If I understand your problem correctly, the surface could be used s a secondary or tertiary datum feature along with the clyindriical surface.
RE: points to define a datum
RE: points to define a datum
RE: points to define a datum
RE: points to define a datum
RE: points to define a datum
I guess I don't quite understand the question. The cylinder would be defined by two points to establish the axis and length, and one point to establish the radius.
RE: points to define a datum
RE: points to define a datum
I still don't know enough about your situation to be of much help, but will await any further clarification.
RE: points to define a datum
What you seem to have stated does not seem to clarify. Are you wanting to make an element of the cylinder a datum feature? You have referred to a surface of the cylinder, or are you referring to the ends of the clyinder?
RE: points to define a datum
ewh, I agree with you on datum feature. What you quoted from ASME is definition of datum feature and not datum. But this difference is not what my question is about. I am trying to define a datum feature, not a datum, that is a curved (cylindrical i.e part of a cylinder) through points. Just as you can define a planar datum feature by three points on the part. How many points do I need?
RE: points to define a datum
I apologize; you are correct in your definitions. I blame my focus on drawings for my narrow mindedness.
As far as defining a cylinder, I would think that you need at least six pts. Three in a plane perpendicular to the axis to define your arc, in two places.
RE: points to define a datum
Three points completely define a circle, so four points completly define an infinite cylinder.
RE: points to define a datum
I think ewh has a good handle on your problem. You might not have to define precisely the 6 points, but to establish the location of either the ID or OD of the part at 2 specific loctions. 2 diameters either MMC or RFS at those locations should get the job done.
RE: points to define a datum
What about the second example on this page?
http://www.tec-ease.com/tips/september-99.htm
As far as a datum of a cylinder, I think the same rule holds true as for using true position on a radius--it has to be greater than 180° to constrain/capture the cylindrical surface and make the three points of contact. But then, the resulting datum is not an axis, it is two mutually perpendicular planes (the intersection of which creates an axis, but I think only total runout uses the axis).
RE: points to define a datum
Thanks for the example. But this example shows a the surface of cylinder used as a datum feature to stablish it's axis as datum. I quote from the example:
"Placement of the new datum feature symbol (triangle) can be critical. In the first three views below the datum feature symbol is associated with the size dimension of a feature of size. They indicate that a datum axis should be established using the feature indicated."
My problem is that I have a cylindrical surface (part of a cylinder) and I want to use points to define it as datum feature, much like how three points on the part can be used to define a planar datum feature. I don't completely understand your answer. Is that an answer to the question I just stated? Thanks
RE: points to define a datum
One answer might be we define three point on the surface to define the circle. Now that we have the circle, it's center is established, the two other point, again on the surface of the cylinder. define the direction. But then this define an axis and a plane all we need is another point for clocking to have a cylindrical datum frame! So do we need six points
I think I am confused a little and I have to go. I hope we can continue this discussion later.
RE: points to define a datum
I am in agreement about the benefits of this type of discussion, and greatly value this site for that reason.
You seem to be making good progress with this problem. I look forward to seeing how you finally resolve it.
RE: points to define a datum
Actually I was referring to the last example as the second, as the first three all illustrate the same thing.
The last example: "the datum may be interpreted as a line lying in a plane tangent to the feature indicated." In other words it is defining a tangent linear element of the cylinder wall as the datum.
Again, unless you have a cylindrical section that has MORE than 180° you can't constrain it. You can't define three actual points on the curve and have all three of them contact. If the diameter of the cylinder is even ±.001 off the diameter defined by the three points, one of them won't contact. I believe that cylinders greater than 180° are fixtured by contacting any three linear elements around the cylinder to fully capture it, then that defines the actual axis of the imperfect physical part. Less than 180° has no opposing elements to "nail down" the diameter. I don't think you can define a datum on such a partial cylindrical feature.
I don't think, but I'm not positive.
Wgchere
RE: points to define a datum
If you will look at Fig. 5-43 in the ASME Y14.4 std. You will see that the dia does not necessarily have to be for the full extent of the circumference. I believe this would relate to your situation in not having full circumferential surface. Therefore, I will still conclude that you can locate 2 diameters at a specified distance from one end of the cylinder and use the axis between them as the datum. Relatively simple.
RE: points to define a datum
Jim Sykes, P.Eng, GDTP-S
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