Simon / David,

I'm using CAD for laser cutting in 2.5in Gauge too - but only small wagon
components. I agree with all you've said. I am impressed by the quality of
cut possible with the laser, but the limit to the size seems to be the heat
generated by the laser on small pieces which can burn or even melt
completely.

Having said that, I've just had cut some stanchions for the bolster of a
wagon in 7mm scale (14mm long) and the finish is perfection. There is a
hole for a pin which is less than half a millimetre in diameter, through
steel 1mm thick, and it is half a millimetre from the edge. The reason this
is possible is that the parts are stainless steel, and for cutting stainless
they use an argon shield around the laser which prevents the build up of
heat. Sometimes stainless is not suitable, so I asked my cutting firm
whether they can programme the machine to cut stainless, but insert mild
steel. They laughed and said it doesn't work like that, but can anyone tell
me why?

Incidentally, the firm I use is in Leighton Buzzard and charges £100 per
hour of machine time, plus the cost of the metal. I assume that's about
market rate? Of course, calculating how much time it will take to cut is
just down to experience. The stanchions mentioned about take about 6
seconds each.

One other little point is to avoid square corners if possible because the
laser has to stop, change direction, and start again. This causes a delay
(time = money) and local build-up of heat, so try to put a radius on every
corner you can, even a tiny one.

Mike Williams
www.williamsmodels.co.uk

-----Original Message-----
From: Simon Thomas [mailto:spt001@...]
Sent: 14 October 2004 14:09
To: halfinchlivesteamforum@...
Subject: Re: [Half inch live steam forum] CAD projects


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
>




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