# Sweep 1 rail precision problem

hi all,
why if i draw a 2d arc then i use it to sweep 1 rail and i make it 2d i have not more the arc radius constant but a series of different dimension along all the arc?
thank you,
roy

Check the â€śSimple sweepâ€ť option in the panel which pops up while executing the command. The surface will use the arc point structure and weights and the radius of the inner edge should exactly match the arc.

For most purposes Rhino automatically uses simple splines with all the weights set to 1. However arcs, circles and other conics cannot be exactly represented by such splines so Rhino uses â€śrationalâ€ť splines with weights set to values other than 1 which can exactly represent conics. It appears that Sweep1 uses the simple splines by default even when the rail curve is a rational spline. Simple sweep forces use of the input point structure and weights of the rail curve.

From Rhino Help for Sweep1:

Simple sweep

Simple sweep works in cases where the shape curves intersect the rail at rail edit points. This option generates simpler surfaces in cases when the curves are perfectly set up. The surface inherits the structure of the input curves.

Check the deviation between the rail curve and original surface edge using CrvDeviation and it should be less than the global tolerance. Two curves can locally differ significantly in radius with only a small distance between them.

Hi Roy - I would SimplifyCrv on the rail before sweeping, to get a little cleaner structure on the result (straights separate from tangent corners) but if you want a true arc on the corner, Iâ€™d use a Revolve. Two straight extrusions and a revolve for the corner in this case will get you the simplest and most exact result. The Sweep will be within tolerance but will not have the exact structure of an arc.

-Pascal

hi david, hi pascal,
iâ€™ve did some other test and it seems to me that, as pascal explain, rhino need to convert the arc to a spline so is not possible using sweep 1 rail to mantain the arc structure because rhino approximate the result changing the radial dimension at any curve point.
definitely i need to use another one solution as for example the one pascal suggested.
pascal, will not be possible in the future use the arc instead of a spline?
thank you again,
roy

the method that will give you the most accuracy in this scenario is by using `_Revolve` for the arc section as mentioned earlier by pascal.
something like:

1. revolve the arced section:

2. extrude the straight sections (then Join and cap)

Awesomely excellent tip.

Could this command be improved.
I donâ€™t like gattchas like this.

1 Like

using revolve is great tip, thanks:-)