Re: [CAD_CAM_EDM_DRO] Modeling a twisted blade on autocad?? PLease help!

> I was wondering if somebody teach me how to design a twisted blade
> such as a fan blade. I know how to make a straight blade with
> polylines but I cant twist it and make the contours i want.

Nic is right about NURBS being easier. It is interesting to note that ten+
years ago wireframe was the 'easier' way... Also, NURBS or solids
capability doesn't mean 'twist' capability, and you'd still need to draw
your basic 'wireframe' cross section(s) to start off.

As far as learning how to do 3D objects in wireframe, I'd say it comes more
by doing than reading texts. Nowadays it's useful for building up multiple
surface intesection fillets and contours that solid modellers won't hack.

I've added a description of a way I'd go about doing a prop type blade
below. Hopefully the description isn't too obtuse!

- Keith


Basic Description

Don't know about twisting existing models in ACAD, but the following works
for building in the twist.

This sequence would be to construct a wire frame (for use as NURBS/toolpath
boundaries), using cross sections. I wouldn't use polylines, just plain old
lines & arcs.

Additional note: Beware when using splines for your contour. Splines can be
messy if you're not careful. They often have tiny elements that are easy to
miss when selecting, blanking, or deleting; and can have hard to match
entity count and proportion when surfacing.

Start in a 2d plan view with your root shape. Worry about hub and hub fillet
later.

Draw your tip shape using corresponding entity end points and
the same amount of entities for each part of the cross-section . (this keeps
surfaces clean and behaving in an expected fashion when it comes time to
surface.)

Draw the in-between cross-sections as needed. More sections will give you
more control of the final surface.

From there you can rotate the sections to the degree twist you need. Use
different layers and colors to keep the cross-sections separate.

Next move/copy the sections out into your 3rd dimension axis, and/or rotate
them as needed in the 3rd dimension. You can then use the cross-sections as
boundary curves to create surfaces.

I'm not sure what AutoCAD version you've got, but you may need to define the
longitudinal curves so you have four edges for the surface patch. (edgesurf
in v13 is this way...). Easiest way to do that would be using three-point
arc between sections, then breaking the arc at the intersecting point. Might
need to do that anyway if the cross section surface tool flattens things or
brings the surfaces in at mis-matching angles.

Mickey Mouse ear blades and impeller blades you can use the same method,
but it is much more involved. It'd be easier if you could slice and dice
solids/surfaces, extrude shapes along 3d paths, etc. (using programs such as
Nic mentioned - Mechanical Desktop, for example. Rhino, Vector w/ Nurbs,
OneCNC, Pro/Engineer/Desktop, Inventor, SurfCAM, MasterCAM, Think3,
SolidEdge, SolidWorks, IronCAD, etc)

Keep in mind entry level CAM programs will use the wireframe only, so a
cleanly constructed wireframe will also give better toolpaths.