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CNC machine custom braces and prostheses

NY Rehab creates a lower leg brace using DeskProto

Manufacturing prosthetics and orthotics is a fine example of creating bespoke products: each product is designed to fit only one unique person. This is a perfect application for 3D technologies like 3D scanning, 3D printing and CNC machining. DeskProto user New York Rehabilitative Services (NY Rehab) in Valley Stream, NY (USA) has embraced the use of these new technologies to manufacture orthotic braces and prosthetic devices.

This Gallery project shows the design and manufacturing of a brace for the victim of a stroke. One of the results of a stroke can be a 'drop foot': the patient cannot hold the foot horizontal while walking. The brace is meant to limit the angle the patients foot can bend downwards. Such brace is called an "Ankle Foot Orthosis" (AFO). As it needs to exactly fit the lower part of the leg a custom shape is needed.

The patient being scanned
3D scanning a complete leg (image by Techmed).

The first step that is needed in order to create a bespoke brace is 3D scanning the client's lower leg. Based on the resulting 3D geometry a perfectly fitting brace can be created.

NY Rehab uses a 3D scanner made by Techmed: the Structure Sensor. This scanner is in fact an add-on to be mounted on an iPad, which turns this tablet into a powerful 3D scanner. The sensor uses 'structured light", which means that it projects line patterns on the object and then scans these lines. No laser involved, so perfectly safe for the eye. The complete leg can be scanned by walking around the client. On the iPad the app 3DsizeME is running, which has been especially created for this application.

Screenshot of the Vorum Canfit software
Designing the brace in Vorum Canfit.

After this scan the result is exported as STL file, which then is imported into the Canfit CAD software by Vorum. This program is especially created to design all types of custom prosthetic and orthotic devices.
In Canfit the geometry can be modified to correct the patient’s issue. More modifications are made: some extra material around the ankle to insure comfort, and a ridge of extra material to indicate the trim lines to be used when cutting out the brace later on.

Screenshot of the Autodesk MeshMixer software
Adding supports in Autodesk MeshMixer.

The next step in the design process is the Meshmixer software by Autodesk. NY Rehab uses this program to prepare the geometry for CNC machining. On both sides supports are added, if needed extra material (visible in dark pink) is added to prevent the cutter from travelling too deep into the block, and a check is made if the model will indeed completely fit within the block, also in case the block is not rectangular. Extra rotations may be needed to make it fit.

DeskProto screenshot
Rotary toolpaths in DeskProto.

The resulting STL file then is loaded into DeskProto. One special roughing operation is used, creating roughing layers above the first toolpath only: clearly visible in the screenshot above. After that the complete part is carved with parallel toolpaths along A, at a path distance of 0.1 inch. The part keeps rotating in the same direction (conventional), using A-values (far) above 360 degrees.
You can see that the addition shown in Meshmixer (pink) still is present, however as the brace does not cover that part of the foot this is not a problem.

CNC foam carver in action
The special CNC carver, doing the roughing pass.

The CNC milling machine that is used by NY Rehab is dedicated to this application (custom made by ThommCarr Company). As you can see a rotary axis is present, however no Y axis. So the machine can only perform (X,Z,A) movements, which is perfect for this type of shape.
In DeskProto the strategy was set to "Along A-axis reversed" (so from the right to the left), as the motor of the rotary axis is on the left side.

The CNC carver just after finishing
Just after finishing the model.

The carver uses an extremely long cutter, which can create the complete model in one pass, without using roughing layers (except for the first toolpath, as mentioned above). To keep such long cutter stable the spindle speed needs to stay below 6000 rpm. Of course this is only possible for light materials like this green PUR foam: a "CAD carver block" made by Friddles (foam density No 3, which stands for 3 pounds per cubic foot).

The maximum feedrate of the machine is very high: 16 inches/sec. Total machining time for this model was about 44 minutes.

The foam model as it came from the machine
The foam model after sanding
The foam model: as it comes from the machine and after sanding.

The foam model that comes from the machine is mounted on a rod (after drilling a hole first) and the rod is clamped in a vise on the workbench. As you can see the finish is not yet perfect as the large toolpath distance leaves slight lines. Manual sanding is used to smoothen it, as well as to finish the bottom of the foot (that surface was not yet complete because of the support block).

After sanding, the green foam part is a copy of the leg, with some extra material round the ankle bones and a ridge along the trimming line as just mentioned.

Adding Tamarack joint dummies
Adding molding dummies for the Tamarack joints.

The rod is then replaced by a perforated pipe that is connected to a vacuum pump, allowing to suction air out of the foam. While drawing vacuum a thin layer of hot foam is draped over the foam and vacuum sealed: the white layer in the above image. The foam is the first part of the actual brace: needed for a comfortable fit.

The actual brace will be built of two separate parts, that are connected with joints. These joints are standard parts made by Tamarack Habilitation Technologies. The green parts in the image are "Molding Dummies", to create cavities during the vacuum-forming process in the next step. The actual joints will be inserted into these cavities later.

Thermoforming a sheet of plastic
The sheet now formed around the model
Thermoforming steps...

The brace's main body is also created using vacuum forming (also called thermoforming): a hot sheet of plastic is draped over the foam and vacuum sealed. Manual assistance (with special heat-resistant gloves) is needed to insure suction. Excess material is cut away in order to limit the plastic from drooping down and thinning.
The plastic is a polypropylene copolymer. To explain the different colors in the photos: when heated this plastic becomes transparent, when it cools down it becomes white.

Trimming the brace parts with an angle-grinder
Trimming the brace parts with a band saw
Cutting the two actual parts for this brace, along the trim lines.

When the plastic has cooled down the two parts for this brace can be cut out. First step is removing the front side, after cutting it loose with an angle grinder. After removing the foam core the two parts then can be separated, using a band saw. These are quite rough cuts, to be perfected later.

Grinding the edges of the brace
Sanding to smoothen the rough edges.

As the angle grinder leaves very rough edges, sanding is needed to make these smooth: the finishing touch. Both the white foam layer and the hard plastic layer are sanded at the same time. The photo above clearly shows the cavities for the joints.

The two custom made parts for this brace
Adding the joints
Assembling the two parts to form a complete brace.

Final production step is the assembly. The tamarack joints are mounted. These allow the brace to flex alongside the anatomical ankle joint. At the heel a 'stopper' is present, needed for a comfortable resistance when going past 90 degrees. The straps are needed to keep the brace comfortably in place. Both the joints and the straps are mounted using a type of rivet, as shown in the image below.

The resulting brace
The resulting brace, completely finished.

The finished brace, with joints and straps.

The client testing the brace
The client testing the brace
The client is testing the brace, which also involves again learning how to walk.

The ultimate goal of this brace is of course to make the client mobile. The pictures above have been taken while the client was testing his new AFO brace. It exactly fits his lower leg, so in case adjustments are needed these will be very small.
It will be obvious that having to wear an AFO is no one's dream, however as it allows the client to walk again without his foot dragging over the ground the brace means a significant improvement in mobility. And the fact that the brace is custom made to fit exactly this one leg means an optimum wearing comfort.