Creating an S-curve using a given tangent length an given arc radius

[cynthia.paish]

Hi folks,

I'm using an engineer's instructions to create a length of track. The track has to jog over about 3230mm [10'-7 1/4"] to make room for rail equipment using an S-curve. The radius of the 2 curves must be 184099.2mm [604'-0"] and the tangent line connecting the 2 curves must be 30480mm [100'-0"] long. I'm having trouble figuring out how to do this other than trial and error. Any help is appreciated.

[SEANT61]

There is probably a mathematical formula for this, but I cheated and used Excel Solver.  Here is the result:

[j.palmeL29YX · Accepted solution]

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@cynthia.paish wrote:

I'm having trouble figuring out how to do this other than trial and error. 

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Let AutoCAD's Parametric do the job: >>video<< 

If my reply solves your problem, click the "accept as solution" button. This can help others find solutions faster

[SEANT61]

Nice. 

Parametrics never were my initial goto choice.  You're showing me the error of that mindset.  

[j.palmeL29YX]

Off topic:

@SEANT61 

Here's the opposite: for years before parametric was implemented in ACAD, I fervently hoped that this functionality would finally be added. (I have seen that before in Mechanical Desktop).
A simple example, why:
In the >>video<<   you see a very simple geometry - two arcs connected by two tangential lines. It's not a hard job to draw that (even if some steps are necessary):
- draw two circles in the wanted distance with the wanted radius
- draw the two lines using the TAN osnap
- trim the circles at the lines.

But what if you want/need to change one or some of the dimensions, e.g. the distance of the centerpoints? It is not enough to simply stretch the lines. A lot of additional repair will be necessary. Or you want to change one or both of the radii. The same amount of effort. If you try it you see, it boils down to the fact that the entire geometry must be completely redrawn.
But now we have the Parameric. Very simple to change the dimensions if the geometry was defined with parametric constraints and dimensions!
(Unfortunately this doesn't [yet?] work in 3D. However, hope dies last ... )

SCNR 

cu

[SEANT61]

Indeed.  The power of constraint based associations do simplify this task such that multiple parameters can be explored at a whim.   Interestingly, much of the behind the scenes work by AutoCAD's parametrics and Excel Solver are the same.  It's crazy, though, not to go all ACAD when working with graphical data.

I typically work in a 3d with AutoCAD, so don't often use those excellent 2d features - need to hone my skills therein.  I'd probably benefit from one of Autodesk's other, more 3d oriented, offerings.  Perhaps somewhere down the road.

[leeminardi]

Having lived through the development of parametric modeling I thought I would offer the following. Constraint-based modeling first appeared in the early 1980’s in a product named Designview by Premise a company founded by Jon Hirschtick who later founded Solidworks. Hirschick sold Solidworks and then founded Onshape, a web-bases constraint solid modeler. The concepts of parametric modeling (a.k.a. variational geometry) were the basis for several CAD systems including PTC’s ProEngineer (later named Creo), Autodesk’s Inventor, SDRC’s I-DEAS, as well as other products.  These CAD systems typically have the user create 2D sketches with constraints similar to AutoCAD’s parametric features. One or more 2D sketches are then used, along with features (e.g., holes, ribs, extrusions) to build 3D parts.  Then an assembly modeler is used with a collection of 3D constraints to create a 3D assembly.  Constraints typically fall into two categories, dimensional and geometric and may be applied implicitly or explicitly. A significant challenge for the user is to determine how all the constraints interact to yield fully constrained geometry. I have found that Inventor is pretty good compared to its competitors in helping the user identify constraint conflicts.  Like Excel’s Goal Seek and Solver, these systems use numerical methods to converge (hopefully) to a solution.

A side note.  When constraint-based modeling first appeared it put pressure on Autodesk to upgrade AutoCAD to provide some sort of “associative dimensioning”.  Constraint-based modelers enabled the user to change the value of a dimension line and the geometry would change accordingly. AutoCAD had no such feature and implementing true parametric capabilities was too difficult with the existing database. So instead Autodesk improved the linear dimension feature to use the reference point of the extension lines to determine the dimension line value.  This enabled a user to use, for example, a stretch window to modify geometry and its corresponding dimension lines.  The AutoCAD parametric features were added some time later

[cynthia.paish]

Thank you. It's been at least 6 years since I've used constraints, and that was just during my time in school. I've never used it at work yet. That works great.

[cynthia.paish]

Thank you.