Spring with a very general pitch

ladislav_schejbal

A spring with a completely general variable pitch.

Unfortunately, I couldn't make it out of round wire.

I wanted to use the Loft function, but it doesn't work with the Bézier curve.

I wanted to use the edge of a square profile as the leading curve.

NeilCooke

Hi, not sure what question you are asking? Is this a custom feature? If so, take a look at the wave spring example. 

ladislav_schejbal

No, it's not a function.

I wanted a spring whose windings thicken at the beginning and end until the coils lie close together.

I've got it in square wire, but I don't have round yet. 

I tried to create a spring with a very general variable pitch defined on the auxiliary plane, using a Bézier curve. I used the Wrap function to project the curves onto the cylinder.

So far, I have only managed a spring with a square cross section.

I then wanted to use the Loft function. And use the edge of that cross-section as a guide curve. Anything less, in this case, you probably can't use a curve that doesn't lie in a plane.

I guess this is it:

 I'm going to use a normal spiral composed of several parts with different gradients.

Square section
Spring | Part Studio 1 (onshape.com)

NeilCooke

Have you tried using Sweep? Also, what is the end goal? Just a modelling exercise? In general, you wouldn’t bother modelling a standard purchased part to this level of detail since it doesn’t add any value. 

ladislav_schejbal

Yes, that's what I came across when modelling.

I do orienteering maps, yes, I'm very fond of the Bézier curve.

 The truth is: "generally you wouldn't bother modelling a standard purchased part to this level of detail as it adds no value". 

I am a beginner in 3d modelling, I made do with 2d drawings before I fell into 3d printing.

I've tried Sweep, but it's not quite the same. 

 Hi, Hi, I probably shouldn't, as a one year user, get into dealing with such details.

 I just wanted to discover a solution, but unfortunately I have very little experience here.

The Onshape program has been very useful for me, although getting started in 3D was quite difficult.

Just for info:

Spring | Part Studio 1 (onshape.com)

NeilCooke

If the sweep is not touching the path it can go wonky. But, since you are using Wrap, you could try just creating a surface and then use Thicken.

mahir

Here's an example of a variable pitch spring using sweep and my Parametric Curve FS.

https://cad.onshape.com/documents/57acdfaae4b005c413ed9b6f/w/3fd585a46d3af1b3ba413c53/e/9f334d884b522c64159c0322

Evan_Reese

@mahir
I'd love to see a video explaining your workflow or the equations you're using.

Evan_Reese

here's how I'd approach it since I'm not sure how to use Mahir's awesome feature.
https://cad.onshape.com/documents/509fbb825e08a1856bbe5dde/w/8b721817ffbe23752bd59d97/e/fe41ec3c9dca768c5eb95bc9

mahir

Evan_Reese said:

@mahir
I'd love to see a video explaining your workflow or the equations you're using.

Evan, the workflow is pretty straightforward. 

1) Figure out how I want a curve to behave in 3 dimensions and which coordinate system would work best. In this case it’s cylindrical (R, theta, Z). 
2) Craft the pieces of each function for the various behaviors. 
3) Combine into 3 functions to make the final curve
4) Sweep


For this model:

R = 20mm
Constant

Th = #t*10*360*degree
10 circles one after the other corresponding to the number of coils

Z = 1.19*50mm*(atan(8*(#t-.5)))/(180deg)
Inverse tangent yields an S shaped curve with a slope/derivative that looks like a bell curve. So, Z ends up moving slowly, then fast, then slowly again. That’s how you get the bunching up at the beginning and end of the curve. 

ladislav_schejbal

Spring Bézier curve - Wrap -Thicken - Sweep

So it was done, 

thanks in no small part to your help. 

Even though we are already in a rather not very practical area, there will probably be no use for it. I only solved this for the presentation of the jig, for cutting the thread with a tap on the lathe.

Here's an example of a general use of a bezier curve that doesn't lie in one plane, but in a general area.

SpringBézier curve - Wrap -Thicken - Sweep | Part Studio 1 (onshape.com)

BrettKuenkel

@mahir

Could you explain the Z function a little more in depth? (what each number in the equation means, or where did it come from)

I want to use your feature script to make a bunch of closed and ground springs and I was able to play with the numbers to get the helix close to what it is supposed to be but it is not exact. I want to take the guesswork out of making these helices and understand the function to find out what numbers are correct.

mahir

b_kuenkel said:

@mahir

Could you explain the Z function a little more in depth? (what each number in the equation means, or where did it come from)

Sure, no problem. I hope this helps

Here's the Z function in question:
Z = 1.19*50mm*(atan(8*(#t-.5)))/(180deg)

I agree. It's not very intuitive at first glance. It took me a second to remember why I used this particular form.

Replacing the numbers with generic variables:
Z = D*E*(atan(A*(#t-B)))/C + F

A: This number is somewhat subjective. Multiplying the input to the Arctan function scales/stretches the shape and dictates what portion of the function you want to use, or in other words how much the curve/spring pitch is compressed at the beginning and end. It also dictates where this change in pitch occurs. Increasing this number stretches the spring. Decreasing it compresses it. 8 just seemed like a reasonable compromise


B: Arctan(t) naturally starts negative and has an inflection point at t=0. I want this inflection point to correspond to the middle of the spring. Since #t goes from 0 to 1, the midway point corresponds to #t=.5. Replacing "t" with "#t-.5" shifts the inflection point so that it occurs at #t=.5. A similar effect can be obtained by instead changing #t to go from -.5 to .5, but then the other functions may need to be updated as well. You also may

C: This is a normalization factor. Since Arctan has natural asymptotic limits of +/- 180deg, dividing by 180deg makes those limits +/- 1 instead.

D: This is also a normalization factor, but it's dependant on C. In this case, setting C=8 yields a maximum value of approximately .42 (after normalization) instead of .5. And .5/.42=1.19. C and D could be combined, but it seems easier to separate them out so the overall length (E) can be changed independently. D changes inversely with C.


E: This is how long you want the spring to be end to end. In this case it was 50mm.


F: This parameter is optional and was not used in the original Z function. But if you would like Z=0 to correspond to the start or stop of the spring instead of its center, set F=+/- E/2.