Homebrew PCBs

Hi Andrew,

Thanks for this, which I found very interesting:

> http://www.instructables.com/id/The-Etchinator-low-cost-spray-etcher/

I doubt if there is an absolute need for the vanes at the bottom of each
rotating tube, which you made from a cut down black plastic toy wheel.

I think that all that would be needed is an end-cap with a hole in the
centre, and for this hole to be somewhat below the surface of the
etchant in the bottom of the tank.  The etchant would flow into the hole
by fluid pressure.  Once in the hole it would be rapidly spun to the
outside, even without vanes.  The vanes would probably help, but I
suspect that only two or four vanes would work nearly as well as 8 or
10, and that the system would still work pretty well without vanes.
Such a pattern of vanes could be made by gluing some baffles into the
bottom of the tube.  They wouldn't need to be particularly numerous or
symmetrical.  Just a single piece of plastic from one side to the other
would work fine, I think.

Likewise, I doubt that the three vertical strips you glued inside the
tube would be necessary, since by the time the etchant has been in the
tube for more than a fraction of a second, it is already spinning with
the tube.  You show the etchant bunched up against each strip, but that
would only be the case if the rotational speed was increasing, which it
is not.  Without these strips, or with much less in the way of strips -
the etchant would be forced against the walls of the tube, with greater
pressure at the bottom due to gravitational force downwards.

Perhaps it would suffice to have a 20 to 25mm PVC tube with an end-cap
with an 8mm diameter hole, and then just loosely stuff the tube with cut
up pieces of plastic pot scourer or similar.  This would allow flow of
etchant along the tube but would quickly cause all the etchant which
enters the hole to spin up to the rotational speed of the tube itself.

I didn't see any lower bearing.  Do you have the end of the rotating
tubes rubbing against the lower plastic framework?  If not, I think that
the whole thing could be self balancing as long as the motor was loosely
mounted, as yours is.  The motor and tube might not be exactly on axis,
but it wouldn't matter.

I wonder if the hole system could be replaced by somewhat flexible
narrow tubes, which exit the PVC tube at various heights.  Each such
flexible tube might flail around somewhat, distributing the drops from
its ends over a larger area of PCB.

With or without the flexible tube approach, I think the problem of
evenly distributing the etchant on the PCB might be solved to some
degree by cycling the speed of rotation.  The exact spray pattern would
depend on rotation speed and this is easily varied from slow to fast
every few seconds via a simple electronic circuit.

Depending on the rotational speed, I imagine you would get more flow
from the lower holes than from the upper ones.  This might be OK,
especially if you staggered the hole spacing to give more holes near the
top, to generally even out the flow pattern.

I had never heard of this rotary spray approach.  I think it is most
intriguing!

I see from your newspaper (The Courier Mail) that you are in Brisbane.
I took a quick look at your page this morning and then went to my
brother's place for lunch.  Lo and behold he had some of the same
plastic containers in his pantry!  The were made by Icon:

  http://www.iconplastics.com.au

near Melbourne.  He recalls he bought them at Bunnings.  I couldn't find
these containers in the Icon catalogue, but I did find they make the
prized plastic crates which are so good for storing LPs and other items.
 These used to be easy to get, but they are quite difficult to obtain
now.  They are on page 7 of the catalogue:

  http://www.iconplastics.com.au/products/materials-handling/

360 x 360 x 270 mm polypropylene "Hobby Box".

I couldn't find an image of the storage box you used, but it looks most
appropriate to the project, because of the relatively clear window on
each side.

 - Robin   http://www.firstpr.com.au/pcb-diy/