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Datum as plane tangent to two cylindrical surfaces 2

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tw_0407

tw_0407

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Mar 19, 2023
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I have a part with two partial cylindrical surfaces and I want to make a datum that is the plane tangent to the two surfaces, as the mating part has a flat surface that is seated up against them.

Part is similar to this example I mocked up:
datum1_ojheja.png


And I want the datum to be tangent to both surfaces, like so:
datum2_lendmk.png


Can I just sketch a line between them on a top-down view like in the above picture, and attach a datum to that?
 
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Use two datum target lines, indicating each at the tangent of each arc. Then, target A1 and A2 will be understood as the plane that you are asking about.
If you're not familiar with datum targets, I can post a picture from the ASME standard.
 
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Graphically, Y14.5-2018 accepts a line/circle at the peaks of features for indicting a datum feature.

Examples:

Figure 7-40 Irregular and Regular Features of Size as Datum Features

Figure 7-42 Possible Datum Feature and True Geometric Counterparts From Three Pins Used as an Irregular Feature of
Size

 
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The datum plane is intended to be tangent to the curved surfaces at two line elements (well, nominally) at the peaks. This means only those local elements will be used to establish the datum and not whole surfaces. The standardized way to communicate this is to use datum targets - specifically in this case, datum target lines.

Then, I agree with Garland. And, I would additionally suggest drawing a phantom line between the two datum targets lines (which would be shown as cross-type points on the true view) to clarify that the targets are formed by a single planar datum feature simulator tangent to the two curved surfaces.
 
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The standardized way is shown in the standard in the figures I noted.

Datum target lines will have a definite location. The tangents will not, but the tangents will represent the action of the mating part which the target lines will not.
 
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Hi, Burunduk:

Do you know how to create this phantom line shown in Figure 7-52 (see attached)? I use Solidworks 2023, and I don't see a function or a command to create the line. I ask this because this is supposed to have gaps between the features and ends of the line. Do you just draw the line tangent to the features with whatever length you need?

Best regards,

Alex
 
 https://files.engineering.com/getfile.aspx?folder=63f297c2-5836-4a49-aefe-1659c80323ae&file=Figure_7-52.PNG
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Yes, you are correct. It is a thin solid line, similar to extension lines in dimensions. Now, do I have to control the gaps manually?

Please take a look at Figure 7-42 attached. How do I add these 3 arcs?

Best regards,

Alex
 
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I use to add drafting aids as model curves and put them on a layer "DRAFTING_AIDS" so they could be hidden on the part/assemblies and shown in the drawing. Just create them the same as any other curve. That way they are tied directly to the model and cannot be left behind if the views move.

Since the datum in Figure 7-42 is a surface I would likely create a surface in the model in order to attach the datum feature symbol. I had no need to use it, so I am not sure it would be a DRAFTING_AID, but the visibility needs to be managed separately from the curve.
 
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3D said:
The standardized way is shown in the standard in the figures I noted.
Click to expand...
In the figures you noted the datum feature symbols are associated with dimension lines related to irregular features of size. In OP's case there is no feature of size so it is not the standardized way to treat his case.

3D said:
Datum target lines will have a definite location. The tangents will not, but the tangents will represent the action of the mating part which the target lines will not.
Click to expand...
They don't have to be basically spaced if this is what you mean. The basic relationship would be them lying on the same plane.

 
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So, the datum target lines are on a datum plane in contact with the features? Just want to be clear why you want to add another, less defined layer to the description of the datum feature.

The standard way shown is to identify mutually tangent items.
 
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Alex,

In 7-52 it is an extension line from the planar feature(s), as Garland noted. When you place the datum feature symbol on one of the lines representing the surfaces and move it away until it is offset, the extension line will appear (unfortunately without a gap in most CAD software). Then some software will allow you to prolong that extension line manually (and you would until it reaches the other feature).

For Figure 7-42 you would probably need to draw a circle as a construction geometry (or three concentric and "co-radial" arcs if you insist on the gaps and not too pressed for time) or use the method 3DDave proposes.
 
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3D said:
So, the datum target lines are on a datum plane in contact with the features?
Click to expand...

They would be interpreted to be simulated by a single planar datum feature simulator (or theoretically speaking, derived from a single planar true geometric counterpart).
 
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Selective quote?

"why (do) you want to add another, less defined layer to the description of the datum feature."

is the completion of the implied question.

You cannot establish the datum target lines without the datum plane; once defined, the datum target lines are not required.

I feel this is another argument you have made to save face. Here, have it.
 
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3D, you got it backwards.
The datum targets are used to establish a datum and not the other way around.
They are designated elements of the part surface(s) that are intended to get in contact with the inspection fixture that simulates the datum (or the selected portions of the scan that are involved in the digital simulation of the datum).

What exactly is "less defined"? If you want to implement a datum target line on a planar datum feature you use a cylindrical simulator intended to contact the planar surface. Similarly, for a curved datum feature you can use a planar simulator to make contact along a line.
 
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Datum targets are used to establish datums. However, that isn't required in this case.

The regurgitation of the standard isn't explaining why you state that using a datum simulator to locate the tangencies so the tangencies can be used as simulators for datum target lines and those datum target lines used to locate the part on the datum simulator to establish a datum (theoretical) makes sense.

There is no need to have "designated elements of the part surface(s)" for this application.
 
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