Question on splines and mirrors and constraints

[jmillerXL6J2]

I'm starting with a circle. 

This object will be 3D printed, and then metal cast. The resulting part will be machined.

I need to create tabs on the part to give the CNC operator somewhere to clamp it. It will look something like this:

I have created a parameter ("MachineAllowance") that indicates how much additional diameter to add to the circle to give extra material for subsequent machining.

I am using a spline to create the first half of the tab, then mirroring it to create the full tab.

When I create the spline, I set it to where one end is on the circumference of the circle, and then adjust it so it is correctly constrained (tangential) to the circle.

When I execute the mirror, the mirrored side is not constrained. It is also ever so slightly off the circumference of the circle.

When I change the MachineAllowance parameter, things get really wonky. The mirrored side of the tab is now completely off, and even the mirror line I created is no longer on the axis.

Ultimately, I need to get this spline stuff sorted so I can then to a circular pattern of these tabs. This is all prototype phase work, so I need to be able to adjust the MachineAllowance parameter to get things dialed in. As it stands, I'm more or less forced to rebuild this by hand each time.

Surely I am missing something simple. 

Any guidance would be most appreciated!

Cheers!

Jeff

PS: I'm not sure if my screencast is coming through. Here is the link: https://autode.sk/3Dl7NQT

[davebYYPCU]

Your sketching is not accurate enough for changing parameters, black sketch articles will behave as expected, your top spline point was not vertical above the origin, but the mirror line was.

ditch the spline, use lines and later model fillet, 

Might help...

[jmillerXL6J2]

I think you're half correct.

I want back and re-verified, and the placement precision of the various elements couldn't be more exact.

I've just concluded that parametric modeling and splines are a bad fit.

I went back and re-did the sketch, using rectangles. I added the fillets where the straight lines intersect in the sketch. I had to add the fillets where the straight lines intersect the circles after I created the body.

Not exactly what I wanted, but it works.

Cheers!

[TrippyLighting · Accepted solution]

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@jmillerXL6J2 wrote:

I've just concluded that parametric modeling and splines are a bad fit.

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That is not generally true, but you have encountered a "hidden" caveat with fit point splines in Fusion 360.

Mirroring a fit point spline in Fusion 360 is really not a good idea, while mirroring a control point spline works just fine.

Mirroring and patterning in a sketch generally does not mirror constraints.

When mirroring a spline initially, the mirrored spline is and exact copy of the original. However, once you start editing one of these splines minor differences will appear.

It is always better to create half the 3D geometry and mirror that.

Again, a control point spline does not have that behavior.

In general I agree that for this design lines and a/or arcs are the better choice.