Saturday, March 30, 2013

Drawing an archimedes screw in Autocad

Recently, a man at my work wanted me to draw the junction of a double screw conveyor. It was a long screw conveyor, and had a hanger bearing in the middle that was causing some trouble.  This was the junction.

In the "olden days" I used to use a 2D lisp routine to draw a screw conveyor.  This was my first request since those days to actually draw a screw.  Usually we just draw the outside casing and this is all you really need, unless you are a manufacturer of screw conveyors.

No worries, I thought, I'll just draw a helix, and sweep a rectangle along the helix. Yes, it is that simple. If you have release 13 that is!  At work I am on release 12 and spent 2 hours trying to persuade Autocad to this.  In the end I googled it.  Humf! It appears no easy "how to do it" is there. The nearest I got was someone drawing an acme screw thread.

After about 2 hours I discovered you could do it, but you had to input in a value for twist of about 30 degrees.

On another subject, my website, seems in for the chop. At one stage I had advertising on it and thought it might pay it's way.  This was not to be. The people at BlueHost did give me an extra year for free though.

Anyway, here is my method for release 13:

1. Draw a shaft - usually these are from 80NB pipe, ie about 89mm outside diameter, say 3000mm high.

2. Draw helix. In my example, I used a base and top radius of 123.75mm, 10 turns and a height of 3000mm.

3. Draw a rectangle. I drew one 158.5mm x 10mm so that my outside diameter would end up around 406mm.

4. Issue the sweep command, pick the rectangle and then pick the helix: Job done!