David,
I'm an occasional user of RoboCAD and from a user's point of view, there is
little difference between drawing long hand and using CAD. Probably you
have to think a little more with CAD, but that is mainly due to the ease
with which something can be accurately specified wrongly. Measure twice,
cut once has a cultural equivalent even in the world of CAD & CAM.
Probably your greatest problem will be that most model engineering designs
are badly dimensioned (some missing, others duplicated with errors). For
example when cutting out frames long-hand, you might well round certain
corners until they either looked nice, or you were bored with all that
filing. With CAD, you must apply that sort of discretion in advance of
seeing the parts. Many smaller holes will be labelled 'to mark through from
the other piece', but if you drawing both pieces, it make sense to ensure
the holes are drilled in both. You may find a little redesign is
inevitable, particularly on platework.

That said, it is worth checking if your package can draw curves properly and
spot open contours and overlaid lines.

Drawing curves may seem like an essential aspect, but many CAD systems
either never record the curve or loose it when exporting to other formats.
You end up with a thruppenny bit curve, approximated by short straight
lines. Steer clear of such packages if you can, but I've heard some
versions of AutoCAD are amongst the worst offenders.

If you have open contours, the cutting path is not a complete loop. With a
closed contour, you can accomodate the thickness of the cutter (or laser,
water jet etc) with an offset, having drawn the component at 'finished'
size. With an open contour, there is no inside/outside, so the cutter can
only go straight down the middle. This could result in parts coming out the
wrong size. Of course, if you know the size of the cutter diameter and have
allowed for this offset, then the above would not apply, but if you choose a
cutter size that does not exist, the whole drawing will need to be redone.

Overlaid lines can result in the same area being cut twice. This can result
in a poorer finish where the cutter has scuffed the edge of the work.

If in doubt, it could be worth giving a sample file to your engineering firm
to see if they could cut it, before commiting much time & effort to drawing
the real thing.

I hope at least some of this makes sense. It sounds like a good topic for
discussion, so please point out where I'm wrong, or where something needs
clarifying.

Simon.

----- Original Message -----
>
> Do we have anybody activly working on any CAD projects? If so what
> are the pitfalls and how about a putting a file on tis group
>
> David
>