How do I draw an epitrochoid (Wankel rotary engine)

onshaper

Hi there,

How can I go about drawing an epitrochiod in Onshape?

http://en.wikipedia.org/wiki/Epitrochoid

3dcad

Freehand?

andrew_troup

@Onshaper
For a four-to-one ratio as per the wikipedia example:

Maybe you could model a single generatrix circle at 12o'clock on the base circle, with 3 equal length lines equally spaced radiating from its centre (draw one, then sketch pattern - Yay!)
Then sketch pattern these four elements, as a single group, twelve times round the base circle.
Draw a twelve point closed spline, snapping the control points to successive endpoints, incrementing anticlockwise around the generatrix as you travel anticlockwise around the figure.

billy2

A spline would only approximate your curve. Maybe this is good enough.

jakeramsley

I would probably make two circles, one for the inner radius and one for the outer.  I'd then make a construction line from the center of the outer circle to the edge of the inner circle, place a mid-point on it.  Circular pattern this mid-point around N times about the center of the outer circle.  I would then circular pattern those patterned points about the inner circle N * inner_radius/outer_radius times.  This should give a full set (and more) of points to attach a closed spline to.

This is all just an approximation though.

andrew_troup

Indeed, and I should perhaps have made that clear .... it probably goes without saying the approximation could be refined by increasing the number of control points

onshaper

@JakeRamsley
You guys probably already have this on the radar, but being able to do equation driven curves (polar & rectangular) would be nice.

maxdotreynolds

onshaper said:

@JakeRamsley
You guys probably already have this on the radar, but being able to do equation driven curves (polar & rectangular) would be nice.

definitely it's cartesian btw..

mahir

This post is pretty old, but check out FeatureScript Parametric Curve. It does exactly this. Kind of klugey, but it gets the job done.