"Proper" CAD stuff is, as far as I know, expensive, specialist software. I would be amazed that any would be available free (but if any one knows, I will also be very interested!). As an example, I have used CAD for electronic design software that was, if I remember correctly, £50,000 plus £12,000 per year "backup" fees.
However, you may find some that you could try for 30 days. AutoCAD is a much cheaper system (only (!) about £700 I think, but I don't know if they offer a free trial.
As an alternative, you could try using ordinary drawing packages like stuff from Serif. This could be down to £10's of pounds (and early Serif stuff is sometimes found free on computer mags' discs).