Monday, November 20, 2017

Z 2 CNC: Dark Money; Black-Scholes

Production Log for "Black-Scholes", Coin1 of Dark Money, 
a Limited Edition Series of sculptures in Ebony, Graphite and Brass.

 2.85 in. diameter by 0.65 in. thick
Mexican Royal Ebony & 260 Brass
Stained with Liquid Graphite 
Sculpted in ZBrush4R8
Milled on a PocketNC v1
CAM Software Fusion360
Aluminum Mounting Plate by Randy Kopf
17hrs milling time
Smallest bit, Harvey#989725 / 0.025 in.

Each "Coin" in the Dark Money Series will have the same 
material / stock structure.
0.5in Royal Ebony
0.04in Brass Plate
0.25in Royal Ebony

 Wood being wood, it is less accurate (in terms of being square, flat and unwarped) than metal.
With multiple layers laminated together, and wooden Sacrifice Stock (pine) underneath,
there is a potential for error of 0.01in.
(Actually, the potential for error is a lot greater than that if you don't
pay serious attention to clamping everything when you glue, or screw mount)

0.01in doesn't sound like much unless your primary target is a brass plate 0.04in thick.
So I need to adjust every cut, per block, using MDI G-Code, altering my Z axis 
movements to make sure I just kiss the brass with a flat end mill. 
Then I know I will have enough remaining depth in the brass for the engraving
and I wont cut all the way through. 
All the other cuts conform to this alteration.

My first attempts at this were done with wood veneer rather than metal,
as I didn't want to risk the end mills on my learning curve. Various types of wood 
were tested as well. Below is Black Oak with Maple veneer.

Cutting too deep not only takes me through the veneer, but it brings the other 
geometry deeper with it, so those cavities are also cut into the veneer. 
(see below bottle)

 Once I know the exact depth of the brass plate, to 0.001 accuracy, all the other
cuts fall into place. There are 13 separate paths generated in Fusion360
that I use to cut "Black Scholes". This path below is called Adaptive Clearing,
designed to remove large amounts of material as fast as possible, 
so that the detail cuts are removing as little material as possible.

Bits Used;
 A 0.25in dia Flat End Mill is used to clear the stock of excess material.
A Tapered Ball End Mill with a 0.032in dia tip is used for Finishing the wood.
A Helix Tapered Ball End Mill with a 0.025in dia tip is used to engrave the metal.

This is a Harvey#989715 @ 0.015in. I broke two of these in my original attempts 
to achieve the ultimate in detail, as that is the smallest bit I can find on planet Terra.

The total milling time is 17hrs, but its the last detailed engraving cut 
on the brass that takes 8 hrs.

Here you can also see why I am mounting my Ebony to the pine Sacrifice Stock.
It allows me to cut around the Ebony, cutting into the soft pine, 
without hitting the aluminum mounting plate.

The final step is to apply
Liquid Graphite to the finished wood.
Hand rubbing brings out detail.

Some are too fast, some are too thick, some are too brittle.
I found that a thick Cyanoacrylate type works best, 
otherwise, these two materials wont play nice together. 

It's loud. Particles and broken bits can fly.
Exotic wood dust can cause allergic reactions.