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03-14-2014
12:43 PM

03-14-2014
12:43 PM

Can you make a closed and tangent-to-itself datum curve in Creo 2.0?

Hi everyone, I've been trying to create a closed spline that will represent the edge of a round cavity I measured.

So I'm wondering if I'm missing something with regards to creating closed datum curves made out of splines.

I'm using Creo 2.0 M080, and here's what I did:

I've created datum points using "Offset Coordinate System" method. I made the last point in the group same as its first point. Now I can create a datum curve through these points, but I cannot make it tangent to itself at the start/end point. I know I can do it if I specify that it should be tangent to another entity at that point, but it seems impossible to create the datum curve that is "naturally tangent", if that's the correct mathematical terminology.

I tried another way to generate the spline and I can get what I want in a more tedious way by:

1) creating a sketch that uses the above datup points as references; 2) creating a sketched spline through these points; upon specifying the start point, the curve becomes closed and is automatically tangent everywhere.

Obviously, this is not a satisfactory method - for example it is not feasible if the number of data points is large.

I get a feeling I should be looking at a completely different tool to do this properly. any ideas?

19 REPLIES 19

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03-14-2014
01:19 PM

03-14-2014
01:19 PM

Re: Can you make a closed and tangent-to-itself datum curve in Creo 2.0?

What is the curve going to be used for? If it's planar, put a plane through it, create a section, then create curve from section?

Or possibly project a curve.

I mean not 100% sure what you are trying to do, but reading your description, it sounds tedious. hehe.

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03-14-2014
03:18 PM

03-14-2014
03:18 PM

Re: Can you make a closed and tangent-to-itself datum curve in Creo 2.0?

If it is truly round, use the datum curve by equation.

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03-14-2014
03:59 PM

03-14-2014
03:59 PM

Re: Can you make a closed and tangent-to-itself datum curve in Creo 2.0?

Hi Matt,

I'm actually trying to create the model of a real part I physically measured - so I don't have anything to section.

Hopefully this picture will explain the issue:

I created the datum points by the "offset coordinate system" method, which allows me to easily import the coordinates as a .PTS text file.

Then the blue curve is what I get if I use the "datum curve through points".

The red curve is what I get if I sketch a spline on a separate sketch through the individual points (its last point coinciding with its start point).

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03-14-2014
04:22 PM

03-14-2014
04:22 PM

Re: Can you make a closed and tangent-to-itself datum curve in Creo 2.0?

Oh ok. So which one is correct? I am guessing the blue one is what you want?

Also, why would you start with datum points? Based on the picture, it looks like you can start with a circle (cylinder). Or is there some not-so-obvious shape there?

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03-14-2014
06:23 PM

03-14-2014
06:23 PM

Re: Can you make a closed and tangent-to-itself datum curve in Creo 2.0?

well, the datum points are what I measured with the CMM.

and it's the red curve that's correct - my actual part is a disk with an elliptical hole cut out of it.

I'm trying to reverse-engineer that shape.

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03-14-2014
07:38 PM

03-14-2014
07:38 PM

Re: Can you make a closed and tangent-to-itself datum curve in Creo 2.0?

Paul, you can add 2 points where you make the curve segment straight on both ends (choose the line option for that segment). This will make the ends of the curve tangent as well.

The 3 points on top are linear.

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03-14-2014
07:50 PM

03-14-2014
07:50 PM

Re: Can you make a closed and tangent-to-itself datum curve in Creo 2.0?

I do see what you mean about a sketch not behaving once you close the spline.

The way around it is to divide the spline somewhere along the spline and letting that be tangent. You can then close the curve. In other words, you need to do this with 2 splines in a sketch.

I recommend the datum curve through points, however. This is a more stable, or reliable shape. Dividing the spline in a sketch will create a new degree of freedom that needs a definition.

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03-14-2014
08:03 PM

03-14-2014
08:03 PM

Re: Can you make a closed and tangent-to-itself datum curve in Creo 2.0?

The little glitch will be much smaller when a decent number of points is chosen. They should be much closer together where the curvature is higher to better follow the surface.

I thought there was a way to have the spline ends tangent to a construction line. If your shape is symmetrical, that construction line is perpendicular to the plane of symmetry. Maybe the deal was to sketch line segments on either side of the plane of symmetry, establish tangency, and then convert them to construction entities.

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03-14-2014
08:36 PM

03-14-2014
08:36 PM

Re: Can you make a closed and tangent-to-itself datum curve in Creo 2.0?

You'd think that's true, huh

I tried that but if it is a single spline in a sketch, it jumps to closing the spline even if you have other geometry there. The Datum Curve version doesn't do that because it is not -closed-... it just crosses itself.

And yes, there is a lot of guessing going on in actually duplicating an accurate "cam" in this manner.

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