Spline precision and performance

[Jef_E]

Hi,

I'm trying to create a spline that should represent the formed shape of a pressure vessel head. I'm making a formula to create the spline but I have some concerns about the performace of the model.

For example if I make the spline average accurate it would take +- 40 points to create the spline, but if I would make the spline highly accurate I would need +100 points.

Would this increase in point affect the performance of my model? Or are these number of points fairly small for Inventor to handle?

[TheCADWhisperer]

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@Jef_E wrote:
....For example if I make the spline average accurate it would take +- 40 points to create the spline, but if I would make the spline highly accurate I would need +100 points...

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Attach file with your average accurate and with your highly accurate splines on two different sketches.

I will wager that I can make a better spline with fewer than 6 points.

[Jef_E]

I did not make the spline yet, I'm thinking of making it this way. hence the performance question

But attached is the shape that I need to create, now I made 2 arcs but I need it as one line and a spline looks the best option.

[TheCADWhisperer]

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@Jef_E wrote:

... but I need it as one line .....

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I am going to assume you mean "one curve".  My next question is, why?  

The most primitive geometry you can use is generally best.

And then I am going to start asking questions about your manufacturing process and the process tolerances.

Go ahead and make your 40 pt and 100 pt splines and then I can graphically demonstrate how more points does not equate to better curve.

[Jef_E]

Why: i'm using this curve as a path to place a nozzle on it. The nozzle is a standard object I can copy, all objects of the nozzles are the same but only a little bit longer dependent on the position on the head. When the nozzle moves towards the edge of the head it first follows the big radius and all things are fine but when the nozzle reaches the knuckle ( small radius ) it should follow the small radius, but Inventor does not allow this, it keeps it track on the large radius and my length is not correct.

You could say just edit the constraint and all is solved, but we want to automate this process. No user interaction required just values.

[TheCADWhisperer]

OK, I seem to recall a similar discussion here from perhaps several years ago.

So a spline it is.

I recommend research into:

the two spline types, Control Vertex and Interpolation.

Handles and Curvature.

Display Curvature. (do this with your 40 pt and 100 pt curves)

...and always keep in mind what is really really possible to measure and control in you fabrication process. (we can't manufacture perfect parts - so you don't need to go out to 8 decimal places)

[Jef_E]

I'm still curious how you make the 6pt accurate spline 🙂

And you are right to know about that parts can't be manufactured perfectly, thats why there are tolerances. That doesn't mean that the drawing can be inaccurate.

Also these heads are cold formed and have a large tolerance on certain values, so they are as imperfect as they can get. But non the less I want my drawing to represent the true shape.

[TheCADWhisperer]

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@Jef_E wrote:

I'm still curious how you make the 6pt accurate spline :).

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Attach your file with your splines.