Creating Two Additional Splines With Equal Distance From a Mid-Spline

[squarer_pett]

In the attached file, you’ll see three spline curves. The middle spline (the "mid-spline") serves as my reference. I am trying to create an inner and outer spline that remain equidistant from the mid-spline throughout the entire sketch while maintaining the three x/y intercept points shown in the file.

However, as you can see, the inner and outer splines are not consistently spaced from the mid-spline.

My current idea is to create multiple perpendicular lines at various points along the mid-spline and apply a coincident constraint to the inner and outer splines. However, I’m concerned that unless I add perpendicular lines everywhere, there may still be points where the spacing is uneven.

Does anyone have any suggestions on how to achieve equal spacing more effectively?

Thanks!

[g-andresen]

Hi,

try it this way

günther

[johnsonshiue]

Hi! Have you tried using Offset command in Sketch -> Modify? Another way to do is to extrude the mid spline as a surface and offset the surface inward and outward. Then project the offset surfaces edges to another sketch.

Many thanks!

[bwalker145]

It's more than close enough for design/manufacturing purposes, but it's interesting that even with the symmetric offset, there's still a measurable difference in the spacing. Curious if that's an artifact of the offset or the tangency functions.

[TheCADWhisperer]

@squarer_pett 

Would you be willing to start over from scratch?

[squarer_pett]

Thanks for the detailed replies.

I should have been more specific in my original post. The distance is not fixed. On the y-axis, you will see the distance between the two outer splines is 5mm, whereas on the x-axis, the distance is 9mm. There is an expansion of the distance between the inner/outer splines from 5mm to 9mm. 

It seems the offset tool only allows a fixed distance.

[etfrench]

You can do this with control point splines:

Notes:

Left side shows splines tangent to a circle while the right side is just to a straight line.

The spline endpoints may need to be made coincident to the circle before making them tangent.

[squarer_pett]

Hello,

Thank you. Two follow up questions.

1) Does this method you are using having a name I can Google and read more about? What search terms would be relevant to your proposed solution?

2) Using three spline points doesn't fit my template (Sketch 1 in my new attachment). I need an additional point. Adding this additional point, I am stuck and am not able to recreate what you did (see Sketch 2). Any thoughts?

Thank you again for your assistance!

[etfrench]

The circle and the horizontal line were just to show two different solutions to the problem.  Use one or the other, not both.

Starting with the middle spline is probably easiest, but since the end and middle coordinates are known for all three splines, it isn't necessary.

All of the spline endpoints must be constrained using the Horizontal/Vertical constraint tool.

Construction lines can be used to control positions of the end points.

[johnsonshiue]

Hi! To get the precise "offset", Ruled Surface may help this case.

Many thanks!

[squarer_pett]

Hello,

Thanks for continuing to help me with this.

I've spent all morning trying to figure out how to constrain the control points, and I'm still struggling. Specifically, I'm not understanding how to constrain the control points so that the spline itself does what I want. It is possible I don't fully understand how the control points work, even after reading multiple tutorials.

As a summary of what I'm trying to do:

1) I have three splines, and inner, mid, and outer spline.

2) For each spline, I have three defined coordinates, the two x-axis points, and the y-axis (maximum) point.

3) The distance between the inner and outer spline is 5 mm at the y-axis (maximum) and 9 mm at both x-axis points.

4) Between the x-axis and y-axis points, I want the distance between the inner/outer spline and the mid spline to be equal.

For example, at some point the inner/outer spline will have a distance, for example, of 7.5 mm. At this point, I want the distance between the inner/mid spline to be 7.5/2=3.75 and the distance between the outer/mid spline to be 3.75. 

Any other thoughts on how to accomplish what I'm trying to do?

Thank you again

Edit: I don't think this is possible, but what if I added two constructions lines on both sides of the y-axis (red lines in the attached image below). Then I would constrain each line so that they are perpendicular to the inner and outer splines, and constrain the mid spline to be the midpoint. At this line, the inner and outer splines would be equidistant to the mid spline. Note that the mid spline would be constrained such that it can not move. So the inner/outer splines would move.

A) I'm not sure if this would make the inner/outer splines equidistant the whole way around.

B) For the life of me, I cannot figure out how to apply a perpendicular constraint to a spline and a line.

[TheCADWhisperer]

@squarer_pett 

Can you File>Export your *.f3d file to your local drive and then Attach it here to a Reply?

[squarer_pett]

Here you go!

[etfrench]

Use a combination of dimensions and constraints:

Once you have the splines fully constrained, editing the dimensions can further refine their shapes.

You can verify the shapes using circles:

Adjust the dimensions of the spline control points.  Use a curvature comb to verify the splines' curves are fair.

@TheCADWhisperer can probably walk you through creating this much better than I.