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# 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

(OP)
I see that my question is misleading, to use a cylinder as a datum feature results in a datum that is a line, but supose we have a surface on the part which is part of the cylinder and it makes a lot of sense, from assembly stand point, to use this surface as a feature. How do we do that?

### RE: points to define a datum

ALi,

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

(OP)
No, We have a surface which is Cylindrical (part of the cylinder) and we want to use this surface as a (pimary) datum.

### RE: points to define a datum

The surface of the cylinder would be the datum feature. The centerline would NOT be a datum, but the axis of the datum feature.

### RE: points to define a datum

A three point datum is a PLANE defined by the points.

### RE: points to define a datum

(OP)
ewh, I am not sure if I understand what you said. I agree that the cylindrical surface (Part of the cylinder) is the datum feature. That will probably make the axis of cylinder as the datum (so in would be a datum). But my question is how many points do I need to define this cylindrical surface? think of it as datum target and how we use three point to define a plane, what happens if we have a cylindrical surface rather than a plane?

### RE: points to define a datum

You cannot define a datum as a centerline.  The feature is the datum, in this case the cylinder.  The centerline is the AXIS of the 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

(OP)
ewh, I think the correct vocabulary is that the cylinder is the datum feature and it's axis is the datum. The datum feature is different than datum. I cannot use three point to define a surface as you mentioned. Because the datum feature, that is the cylindrical surface would not SEAT on this three points as planes do on three point or line on two points. My personal thought is that I need four points to define this cylindrical datum feature. I need to think about it a little bit more to be able to explain it right, mean while please let me know what you think. Thanks

### RE: points to define a datum

Per AMSE Y14.5-1994 "The datum feature symbol identifies physical features and shall not be applied to center lines, center planes. or axes..." except in the situation of equalizing datums or those of a complex or irregular surface.  I realize that it is semantics, but the center line is not a datum (at least it can not be defined as such on a drawing).
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

Ali,

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

(OP)
Ringman, I do not mean end of the cylinder, I mean a curved surface witch is cylindrical in shape but it is not a complete cylinder (circle) just part of it.

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

Ali,
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

How many points do I need to define a cylindrical surface?

Three points completely define a circle, so four points completly define an infinite cylinder.

### RE: points to define a datum

ali,

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

Ali,

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

(OP)
wgchere,
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

(OP)
ewh. There is no need for apology. I very much enjoy this kind of discussions and benefit from it in educating my self. I think we need at least four points. 3 points to define a circle and 1 point to hold the part axialy. I think if we have 4 special points we can seat a cylinder on it so that it touches all four point and stays in space (fully constrained). A special case can explain this a little better. Suppose we have three points in a plane (every three point make a plane) called  point A, B, and C. Now if we connect A to B and B to C and draw a line perpendicular to AB that passes through the middle of AB and another line perpendicular to BC that passes through the center of BC, the intersection of these two latter line will be the center of a circle that passes through A, B and C. So holds a plane of cylinder Now another point not on this same plane but on the surface of the cylinder can hold and constrain the cylinder. Stated differently, 3 points not only define a plane but can be used to define a circle in that plane. No another point out side that plane is needed to stablish the axis (from that point perpendicular to the plane). The problem is that only in this special case, where 4 points is enough, we are using the concept of a line perpendicular to a plane. If we add another point so that points 4 and 5 define an axis then our probem is solved. So 5 points are needed. But then how we define those 5 points on a cylindrical surface.

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

Ali,
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

Ali,

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

ali,

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

Typically, you would use a collet (a multi-jaw chuck or other such device) to close down around the cylindrical datum feature.  The centre axis of the collet simulates the datum axis of the cylindrical feature.  If you don't want to use this method, you can take two groups of three points (prefer 7 & 11 pts to minimize lobing effects) to generate two circles, resulting in one common datum axis.  Not a great way to do it, but useful if a taper is present.  Personally, I'm always concerned about how well the generated axis represents the real one, but that's a decision for you.  This is a fairly advanced topic as far inspection is concerned, so be careful using the two circles to generate the datum.  Hope it helps.

Jim Sykes, P.Eng, GDTP-S
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