# RailRevolve oddity

Can anyone enlighten me as to why I am getting the result I am with RailRevolve in the attached file? It doesn’t make much sense to me…

Thanks, --Mitch

RailRevolve.3dm (53.9 KB)

Hi MItch - you mean the askewedness (like my new word?) of the result? @lowell can explain, but he’s out at the moment. There is a ‘good’ reason, I’ve heard it before, but I do not recall what the deal is with that.

-Pascal

Hi Pascal,

I can’t imagine what the “good” reason might be… The rail is a simple polyline - and essentially a segmented part of a circle centered on the axis to boot - the profile is also “on-axis” so to speak, so I can see no reason why it should be skewed. Also, even if you rotate it back into place on the profile end, the shape is still incorrect IMO.

Cheers, --Mitch

May be something with the relation of the rails local tangent vector to the profile curve. If you place the profile in the middle of your rail or at a corner, the rail will be “tangent” to the profile and everything looks a I would expect it. Move the profile away from those positions and you get weird results.

At the middle of your rail, the tangency is easy to see. At the rail corners, there is no defined tangent but the average of both connecting rail segments is tangent to the revolution circle. You don’t have a tangent rail at the ends. That may cause the weird behavior.

I can explain what happens to cause it, which is different than giving you a good reason it should be that way.

A closest point on the rail to the profile curve comes out a little way along the rail.
Then everything gets rotated to start with that point on the profile closest to the start of the rail.

I can’t say I’ve ever understood the nuances of RailRevolve very well.

Just spouting off something, it might work better more of the time when the profile is planar to use the intersection of the profile’s plane with the rail, if there is one, instead of the closest point. Not sure what else that might mess up.

offset the blue rail to the inner quadrant of the profile circle (green)… then it works fine

 oh… i guess i see what lowell is saying… or, if you move the blue rail vertically a tad, it will revolve ok… with the rail being in the exact middle of the circle, it messes up since the closest point on the rail to the curve is everywhere…
or, keep the rail in place but change the profile to something other than a circle

Hello There.
I don`t know if this is what you need but here are an example of your file:
RailRevolve 2.3dm (241.9 KB)

There are plenty of workarounds, the file was just posted because I feel that this particular case should be considered as a bug. --Mitch

Ok… my miss understood then.