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How to CNC machine 3D scanned data

3D scanning and reconstructing all 27 hand bones

The human hand is a complicated mechanism, containing no less than 27 skeletal bones and many muscles and tendons. Dr. Richard Siderits of Robert Wood Johnson University Hospital in New Brunswick (NJ, USA) endeavoured a project to generate accurate 3D CAD models for all those 27 bones. Aim of the project is to make these datasets Open Source information, in order to use them in different research projects. For instance for robotic hands, or when replacing destroyed bones for trauma patients.

The NextEngine scanner
CAD picture of one bone
The 3D scanner (NextEngine), and the result in Truespace.

Current medical research uses CT scan information for creating bone models, however CT scanning is a complicated and expensive procedure, while the accuracy is limited. Siderits found that 3D laser scanning of bone models was much faster, cheaper and also more accurate. He used the lowcost NextEngine Desktop 3D scanner to obtain 3D datasets. One bone could be scanned in just 90 seconds.

3D Scan data typically needs to be processed after the scan, in order to remove scanning errors (spikes), to close gaps at non-scanned areas, etc. Some modelling is needed too as in many cases the CAD model should not be an exact replica of the original. For instance it might be needed to add attachment points for tendons and muscles. For this modelling Dr. Siderits used the TrueSpace CAD software by Caligari Corp (no longer existing).

Toolpaths in DeskProto
The milling machine in action
Toolpaths in DeskProto, and the CNC milling machine with rotation axis.

Complicated medical models often are created using additive RP processes. Dr Siderits however decided to use subtractiver RP, because of the cost, the accuracy, the surface quality and the freedom to choose any material: even biocompatible materials (for implants) can be machined.
Also here a lowcost setup was chosen: the small MDX-15 machine by Roland of Japan is about as lowcost as you can get. For this project rotation axis machining was needed (rotating the workpiece during machining, on a secalled 4th axis), and so the MDX-15 was equipped with an optional rotation axis device created by Fourth Axis in Australia.
The toolpaths were of course created using DeskProto.

Unfinished result
Finished result
Fresh from the milling machine, and a bone with an attachment point added.

Applications for this technology are many, and need to be explored. Imagine an artificial hand, where the bones are connected by "muscle wires": a wire that shortens when an electrical current is transmitted. Or imagine a patient with a crushed finger: the bones in his/her other hand can be scanned, mirrored and machined to create replacement parts to be implanted.

27 hand bone models on a table
The 27 bones of the human hand.

Dr Siderits will be extending his research by miling models in different materials. For that aim a more powerful CNC machine will be used, made by MaxNC in the USA.