[AR] Re: Electroforming Experiment
- From: Peter Fairbrother <peter@xxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sat, 13 Jul 2019 17:00:54 +0100
On 13/07/2019 15:16, Ed LeBouthillier wrote:
On Sat, 2019-07-13 at 10:06 +0200, Uwe Klein wrote:
How are you going about creating buried pathways?
For combustion chambers of this scale, the cooling channels will be
quite small. It's still experimental right now. The current plan is to
use a rotating fourth axis (actually replace the Y axis on the 3D
printer with a rotating axis) holding the chamber under a plotting
head. Using this, the chamber can be coated with a non-conducting
material (resist ink) where you don't want additional material plated
(I'll try permananent markers and heated glue gun plastic). After the
resist is applied, the chamber will go back into the tank for
additional plating.
It is hard to get good channel walls that way. If you use a thin layer
the copper will overflow and thin, and eventually close, the channels.
With thick fillers, to fill the channels completely then be removed, you
will need to use a high-throw copper to get into the spaces and corners.
Can be done, but it's a different bath, and changing baths has difficulties.
Perhaps the best way is to built up the thickness to wall+channel
height, then cut the channels in the copper.
Once the cooling channels are plated, then the closeout material can be
added (if needed).
Closeouts are needed. Pretty much full-stop.
The pressure in the channels is high anyway, and transient pressures in
the channels can be very large, especially at startup and shutdown.
Closeouts should be the same electroformed copper.
Fill the channels with eg wax, brush with graphite powder (which should
stick to the wax, not to the copper!), reverse electroform until you can
see the copper is being eaten away evenly, then forward Ho!
After building up the closeout layer you can then coat the chamber with
nickel for strength if needed. But do not skip the copper closeout
layer, the copper-nickel interface is not strong, and differential
expansion (both thermal and pressure) is acting on it - you need the
interface to be as large in surface area as possible.
This process approach is considered over merely machining out channels
because machining likely requires 5 axes to do properly. The equipment
for 5 axis micromachining is (likely) more prohibitive to obtain or
develop.
Use a high-power high-speed spindle instead of a pen? If it is high
powered and fast enough cutting forces will be light so you don't need a
highly rigid base or high power servos.
Use a (semi-) ball-end mill, the channels will not be perfect but you
save an axis.
Best of luck, it all sounds very interesting.
Peter Fairbrother
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