How to define a spline?

john_eisenlohr500

I tried to do what what it recommends here but have had no luck defining my spline sketch. https://forum.onshape.com/discussion/13288/fully-defining-a-spline

Any advice to help me figure it out?  This is my sketch - https://cad.onshape.com/documents/d41e72db0d4e393bc9baa02b/w/80e78cbbec7ceb2388eef2fe/e/a59407838a90f4cfb39a47b2

MichaelPascoe

Hi @john_eisenlohr500, it looks like your off to a great start. Normally, I would recommend just a few more dimensions then your set. I'm not sure if your document is showing the same results as mine, but I ran into a bug which would not let me constrain the last two spline points even though they have degrees of freedom. I have seen this bug a few other times with other geometries. If this happens, contact support with the question button at the top right so they will fix it in the future, then try a new sketch with a fresh start.

If you are wanting to define the spline via spline length, you will need to set up dynamic constraints between the spline points for it to work well. Or, you can leave the entire sketch undimensioned to get the right shape, then only apply the spline length; this way the sketch will grow or shrink and keep proportions to fit the spline length. Or, fully dimension it with the shape you want, offset the sketch face via Offset surface so you have a surface. Then use the Transform scale tool to scale the spline length to the size you need.

Creating a new sketch with a defined spline:





Sketch with bug:

S1mon

It seems like the issue is that the spline handle at the leading edge is not constrained vertical. I got rid of the curve length dimension (10.448), and I fixed all the internal spline points just to quickly check, and once I set the leading edge to vertical, the sketch went black. Unless there's some really good reason to control the curve length (flat material stock size?), it doesn't make sense to use that for an airfoil.

MichaelPascoe

Hi @john_eisenlohr500, it looks like your off to a great start. Normally, I would recommend just a few more dimensions then your set. I'm not sure if your document is showing the same results as mine, but I ran into a bug which would not let me constrain the last two spline points even though they have degrees of freedom. I have seen this bug a few other times with other geometries. If this happens, contact support with the question button at the top right so they will fix it in the future, then try a new sketch with a fresh start.

If you are wanting to define the spline via spline length, you will need to set up dynamic constraints between the spline points for it to work well. Or, you can leave the entire sketch undimensioned to get the right shape, then only apply the spline length; this way the sketch will grow or shrink and keep proportions to fit the spline length. Or, fully dimension it with the shape you want, offset the sketch face via Offset surface so you have a surface. Then use the Transform scale tool to scale the spline length to the size you need.

Creating a new sketch with a defined spline:





Sketch with bug:

S1mon

It seems like the issue is that the spline handle at the leading edge is not constrained vertical. I got rid of the curve length dimension (10.448), and I fixed all the internal spline points just to quickly check, and once I set the leading edge to vertical, the sketch went black. Unless there's some really good reason to control the curve length (flat material stock size?), it doesn't make sense to use that for an airfoil.