Modify basic sketch of circular pattern without distortion

[factotuminventum]

Hi,

I clearly seem to miss something important about the circular pattern command. But I can't put my finger on it. So: Help!

In the first attachment you will see a pdf of an equal in out hot cold diffuser as it is used in jet engines to mix hot and cold air before it leaves the burning chamber. Usually these are made out of titanium and so pretty hard to make, but with some recent technological developments it now becomes possible to upgrade the shape toward a more beneficial form seen from the fluid dynamical standpoint. Practically speaking that means that I want to increase the diameter of both in and out channels on the corugated side.

In the second attachment you will find the drawing in which the first sketch shows the circles of the channels I refer to above.

Problem: If I select any of these circle segments and try to change their size, I end up with all kind of distortions and side effects I am NOT looking for. One of the most annoying ones being that the tangent character of the interconnecting lines between the inner and outer circle segment gets lost in the translation. I am already bald, so I can't pull my hair out. How do I preserve element characteristics after circular pattern command?  Can I freeze those or make them an intrinsic characteristic of the part?

If you need more clarification please ask!

Thanks. 

Erik.

[mcgyvr · Accepted solution]

Looks to me like you are missing a bunch of dimensions and constraints... As such I'd fully expect problems when making modifications..

I never leave a sketch until its 100% defined as indicated by a color change of the sketch elements, a thumbtack on the sketch icon and the bottom right of your screen says "fully constrained"

When you force Inventor to make assumptions all bets are off as you have lost control..

[TheCADWhisperer]

Before I spend time on this - are you willing to start over from scratch and follow step-by-step instructions?

[swalton · Accepted solution]

I would tend to pattern a loft feature, not in the sketch.  

My attached attempt works from 10-20 repeats.  Now that I know a way to make it, I would rebuild from scratch so that the  user parameters are in a logical order.   I am not sure I used the critical dimensions to size the tube, ribs or wall thickness.  

The tube sketch is not as stable as I would like.  It lost the sketch coordinate system the first time I changed the number of ribs.  The loft gave a precision error when I first made it.  

[schmidmi · Accepted solution]

What @mcgyvr says is right, it's very difficult (impossible) for us to know design intent, if the underlying sketch isn't fully constrained.  I downloaded the model, and took at stab at doing this (attached).  There are some assumptions about the design, but you can now edit the diameter of the "ridges[?]".  Because of constraints, these do have a limited range of allowed values, but hopefully this gives you an idea of what you'll need to change for your model.

To summarize what I did:

- make all in/out circles equal in diameter
- constrain all the diameter center points to one of the two construction circles (one for inner, one for outer)

- make all connecting lines between in/out circles concentric

[factotuminventum]

---

@mcgyvrwrote:

---

@mcgyvrwrote:

Looks to me like you are missing a bunch of dimensions and constraints... As such I'd fully expect problems when making modifications..

I never leave a sketch until its 100% defined as indicated by a color change of the sketch elements, a thumbtack on the sketch icon and the bottom right of your screen says "fully constrained"

When you force Inventor to make assumptions all bets are off as you have lost control..

Thanks for the input. I've been getting this remark before so I clearly need do improve on that and be more accurate. I'll pay more attention to it.

[factotuminventum]

Hi Schmidmi,

Thank you for your effort and reply. I seem to have an outdated version of Inventor (21.0) and can't open your file. But by the description in your message I do think I understand what you have done. I took all advice at heart and started over making sure all lines where constraint before I moved to the next part of the drawing. I do have a better approximation of the shape I intended to draw, but still am not completely there. I'll finish that attempt later today and post it here when it's done. 

As always, stay curious.

Erik.