Thanks all for your comments. Hopefully in a couple of days I'll get a chance to dedicate more time to your suggestions.

Keith

I guess looks can be deceiving.
Here's how it goes:

(1) The primary flite has a lead of 1.75" for a distance of 12.25" then transitions in to a 2.5" lead within a 2.875" span.

(2) The primary flite lead remains constant again at 2.5" for a length of 16.25" within what is called a barrier section.

(3) The secondary (thinner flite) begins in the barrier section and it's lead is increasing by 0.150" per turn. It ends at the the end of the barrier section just before it "catches up" with the primary flite.

(4) The primary flite then transistions back to a 1.75" lead for 13.5".

This is pretty normal for extruder screws with barrier sections. This particular screw is small in diameter at only 45mm. Most range between 3.5" to 6" o.d., 8" not uncommon, and up to 15"+. There are many variations on these barrier sections but this probably should give you the basic idea.

Keith

A good youtube presentation and tutorial.
Though ... my main question I guess is creating a secondary flute that as it turns and progresses along the screw shaft it smoothly increases pitch or lead. An example might be it increasing 0.150" with each turn ... but doing it in a manner that has no noticeable stop/start points during the pitch increments.

I am going to play around with all the suggestions I've received.
I was wondering if someone has had to do this incremental pitch several times and
had settled on a way to accomplish it.

Thanks guys for the replies. I am looking into the suggestions.
The models will be used to represent extrusion screws but the models will be redrawn in autocad for actual manufacturing. If I can more or less get a good visualization with most of the dimensioning in Sketchup the next stage can be handled by a design engineer. These models I guess are more or less field drawings but I'd like them to be a good representation. I usually do some in 3d iso and then turn some into parallel projection. I've attached 2 pics of the actual parts ... though not the best quality, and a drawing to some the idea of the incremental flite lead / pitch.

Keith

I'm new to Sketchup ... only using it 3-4 months.
I need to draw parts of a plastics extruder screw.
These screws are somewhat similar to an auger but usually incorporate some more complicated features such mixers, wave roots, etc, etc.
One feature sometimes is a "barrier flited section". A seconday, more narrow, flite with slightly less height will be between the primary flites for possibly 6-10 turns.
This secondary flite also may have a progressive lead. It's lead / pitch smoothly changes with the flite's turns.
My question: Is there a fairly easy way to increase the pitch /lead of something like this ... which is mostly a helix or spiral with a profile?
Plugins like screw and helix, drawhelix, etc, are great but have no input for this type of feature.

Keith