Wow guys, i wish I'd joined this forum far sooner!

Thanks to Rich 0 Brien & Pilou Those ideas worked magnificently.

I didn't use Gilles' method, but thanks very much too for showing another way

Edit:

Finished item

Hi there, I am very much a beginner at 3D modelling, i only started last year because i got a 3D Printer.

My dad has asked me to try and make a type of shield for he grass strimmer, a cone shaped item to stop grass from clogging his strimmer.

This is his drawing of what he would like;

I have tried to make one myself, I've attached it to this post for your amusement.
Unfortunately, when i slice to model to be printed, it just makes a weird mess really.
And getting the measurements correct is very difficult to me, because obviously, in the drawing there is no wall thickness, i think 0.8mm would be fine, but i have no idea how i would do that in Sketchup. Also, for some reason i can't explain, the model is at an angle, i have no idea how i managed to do that.

So if anybody could help me out, either by making a model for me or giving me some guidance on doing what i need, i would be very grateful.

Cone Shield.skp