Help with circular slots, anchoring to edge of a resizable object

[Anonymous]

The thing I am trying to make is a 3D printed coil former for a metal detector - basically a donut shape with slots in both inner and outer edges to keep the wire in the right place. Around this former will be wound wire. The basic shape is elliptical. Since I would like to be able to experiment with different configurations I am trying to make everything parametric. These parameters are the x and y dimensions of the ellipse (and they could be equal, in which case a circle), the outside diameter of the wire being used and the thickness of the coil former. Thickness probably will not vary much. Wire outside diameter changes the required depth of the slots, there are six passes of wire stacked in each slot. Changing x and y will allow me to try smaller or larger coils, or various configurations of ellipse.

In the first sketch the outside of the ellipse is given by x and y values, then extruded by thickness to form an elliptical plate. The inside of the donut is achieved by sketching another ellipse with dimensions (x - span) and (y - span), concentric, where span is calculated as a function of double the slot length + a gap between the slots. That is extruded by -thickness to remove the donut centre. No problems so far - this all sketches and adjusts nicely as parameters are adjusted.

Around the inside and outside of the donut there must be slots to keep the wire coil in place. These are radial wrt the centre of the donut and there are 37 on both inside and outside. These are the problem. 

If I position one slot for inside and another for the outside edge, finish the sketch, extrude them to remove material, I have a donut with two slots in it. So far so good. If I use the circular function to make 37 of those pairs then I have a perfect coil former - but only when the donut is round! As soon as the x or y values are changed the donut moves but the slots stay where they are. I have spent hours trying to adjust constraints so that the slots are coincident with the donut borders and colinear with construction lines at the correct angles for the slot orientations but it is just a mess of conflicts and incredibly time consuming. Another thing I thought of is to define positions parametrically using the ellipse formula but that seems just as clumsy.

So what's the smart way to do this? The slots should be anchored around the donut and remain attached to inner or outer edge, radial to the centre, no matter what x and y are, ellipse or circle.

(Sorry if any of my terms aren't exactly right, I'm new to this. Is a donut a toroid when it is a flat plate?)

[Anonymous]

To further illustrate - here's an image of a coil former someone made with a saw and what looks like PCB substrate.

[TrippyLighting]

Please post a screencast. It'seasier to digest than your unbulleted paragraph 😉

[Anonymous]

Thanks for looking. I did not know about screencast, but it looks pretty useful - I started with an empty canvas and did just the minimum required to show the problem.

You can see the issue after the third sketch. When changing coilX or coilY the slots do not move as the parameters are changed, just hang in space - the idea is they remain on the inner and outer edges of the ring and I can make any size ring by just changing coilX and coilY.

[TrippyLighting]

This looks like buggy behavior of the sketch solver. I've tried a slightly different approach without using user parameters, but the sketch goes haywire.

[Anonymous]

I don't know what to think about this. If Fusion 360 is really so broken I probably have to look for some other package.

[TrippyLighting]

What makes you think that Fusion 360 is "so broken" ?

Discovering a bug is nothing unusual.

[Anonymous]

OK that's harsh. I'm impressed with what I have been able to learn and do in just a day, but this is a showstopper for my purposes. 

[TrippyLighting]

Perhaps try to find a workaround ? What you'll learn in that process is invaluable.

Also, the first thing you should hve done is create a component and activate it 😉

[Anonymous]

I've just been prompted to accept an answer but I don't think there is an answer yet. I did spend another two hours trying to find the workaround but no good - the strategy of using radial construction lines with slot anchor points tied to both the lines and the ellipse curves seemed to be working, but after adding seven slots everything went haywire on resizing - construction lines lost their anchors, slots went all over the place.

At this point I know to avoid Fusion 360 where this kind of operation might be important. I did use it for a small PCB housing with no trouble, so won't stop using it completely -  If I notice a software update will see if ellipse resizing with attached constraints begins to work.

[TrippyLighting]

Looks like we may have to invite one of the good souls from the Fusion 360 team @jeff_strater to look into this and log it as a bug if it is one.

The ability to invite other users directly to participate in a thread is one of the realy cool features o this Forum that i've not seen on any othe other forums.

[jeff_strater]

I think what you are discovering here is a few (maybe more than a few) bugs related to sketch ellipses.  I tried to copy your workflow, and I think I may have gotten even worse results than you did.  To be honest, ellipses are not as well exercised as other sketch geometry.  In fact, in testing this, I found a whole passel of bugs.  We will take a look at them, and start fixing them.  However, this does not help you...

I did find another way to do this, though.  Below is a screencast.  The basic idea is to split this into multiple sketches - one for the basic elliptical shape, and one for the slots.  At least in my limited testing, this seems to work relatively OK.   Give it a try and see if you can make it work for you.

Jeff Strater (Fusion development)

[Anonymous]

Thanks for your reply - watching taught me a new way to add a dimension when the line is not horizontal or vertical but the slot problem is the same. After creating around seven slots in the right position (checking for correct resize after each one) the sketch just goes plain strange. Construction lines with one end at origin randomly move, or slots that previously did follow the shape of the ellipse suddenly appear in the wrong place.

I'm going to keep trying different ways to do this and will update if I find a way. It might even be that OpenSCAD is the better tool for this job since it is a very repetitive geometric construction. For now I'll stick with the circular coil former and keep on trying to work out how to do the universal elliptical one.