Engagement Ring - CNC Router, Graphite Moulds, Bronze and Gold Casting

This section gives a snap shot of the design, CNC routing, casting and finishing of my fiance's engagement ring.

The first initial concept.

CAM tool profiles for CNC routing the 50x50x20mm graphite blocks. 

First set of completed moulds.

(Note: brass pins melted later on when I discovered that the mould needed quite a lot of pre-heat to get the metal to flow all the way around).

First cast with solder, showed that air pockets could form and prevent metal flow.

Design iteration 2:

The first attempt at matching the engagement ring to the existing wedding band

Mould set No.2 

The design incorporated a large sprue and riser section with the filling point entering at the bottom of the mould.  The two bolts used to hold it together used a conical seat for mould locating.

Mould held together with steel bolts.

First test cast using pure Tin.

Cast showing a successful melt.

An attempt with pure copper was undertaken.  Two problems were found. 1. Pure copper doesn't cast well, the metal doesn't flow well.  2. Borax seems to stick to the graphite and cause all sorts of issues.

The next cast I mixed up a Bronze alloy using 11% Tin and 89% Copper by weight.  This flowed a lot better.

First decent bronze cast.  The alloy was quite strong, hard and brittle.  Which with later research I found that copper based alloys tend to behave in an opposite way to steels.  When hardening copper based alloys, you let then cool slowly.  Where as annealing them is done by quenching them from a dull red down to room temperature.

This one cleaned up okay.

Polished even better.  Though it was found that trying to set a gem into this geometry was going to be difficult.  The design needed to be evolved.

Design iteration No.3

A six pronged type design was considered, where the CNC router could be used to do the accurate work.

Mould No.3

This mould used more flat sides where tapered tools could be used instead of 3D profiling with ball mills which greatly reduced the cycle time.

Moulds 1, 2 & 3

Bronze casting in mould No.3

Casting trimmed and in the rock tumbler for a clean up.

Held in a 3D printed fixture and CNC routing the prongs.

First attempt at the routed prongs.

3 stage process of manufacturing the ring.

Design Iteration No.4

A final tweak to geometry and mould draft angle to aid release.

Moulds No.4 and a bronze casting.

As cast bronze

As machined bronze in CNC routed graphite fixture block.  Note the graphite block was machined to obtain higher accuracy than the earlier 3D printed fixture block design.

Hand trimmed bronze version.

Australian Sapphire and 18k Yellow Gold selected.

Gold in the furnace ready for the first pour.

And it did not go so well....

All the preparation work of dialling in the bronze casting process, controlling pre-heat etc didn't seem to apply to the gold and I had to come at it with a revised casting strategy.

The mould was pre-heated to a glowing red hot, yet it still wasn't enough for complete flow.

I thought that perhaps the thermal conductivity of the graphite block was allowing heat to be leeched away via the contact surface with the heat proof mat.  So a lump of steel was pre-heated to a glowing red heat to act as a thermal mass / buffer for the mould to be placed on top of.  This was still not enough to give a good casting.

To obtain a successful casting, I had to place the whole graphite mould with the gold in it.  Into the furnace and allow the gold to melt in the mould itself.  This worked, but was risky if the mould spilt or split and mix the gold in with the dross and scrap metal in the bottom of the furnace. 

In addition to the thermal conductivity of the mould being and issue, the graphite crucible was also a problem.  As I lifted it out of the furnace the heat was able to leech out of the gold very quickly and reduce its fluidity before I could get it into the mould.

The end result was absolutely worth it though and we are both extremely happy with it.