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DIY 3D Printing Prosthetic Hand - Make it Real Challenge

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3D Printing Prosthetic Hand - Make it Real Challenge Scand, designed by Scott Allen, and is a piece of functional body art for consumers with single upper-body amputations.  Scand is designed to enhance the user's quality of life

Technical Specifications: 

Materials

Block of Cystacal D dental stone

Powdered Cystacal D dental stone

Rotary sander with hose outlet

Milkshake mixer or equivalent food style mixer

Vibration machine

Mixing cup/pot/container

A new mixing stick - scrap wood/spoon etc.

Scissors

3D scanner such as a z-corp 800 hand held scanner.

Step 1: To create the CAD model for the 3D printer we will need to use a hand-held 3D scanner.  The scanner does not scan actual body parts as well as it does models, objects and casts.  For this reason, as well as not having to always have the client/person's hand available to re-scan (should you need to), it makes more sense to scan a stone cast.   

Step 2:  Using the mold to create the hand cast

1. Grind your block of Cystacal D dental stone using the rotary sander, mix with the out hose with water to create a milky liquid

2. Mix this liquid with the powdered Cystacal D dental stone using the food mixer

3. Place your mold onto the vibrating platform (machine) and pour your thick, gloopy mixture into the cavity.

4. Turn on the vibrating platform and let the vibrations cause the mixture to enter all the tiny gaps inside.

5. Turn off the vibrating platform and leave to set.

6. Cut out the cast from the mold.

7. Admire your model and all its amazing detail including finger prints!

Step 3:  3D scanning

There are two ways in which you can scan the cast, you may stick the reflective positioning dots onto the model directly or onto a board beneath, meaning you can reuse the relatively expensive dots to scan many objects, although you will need to conduct both a top and bottom scan.

1. Plug the scanner into your laptop

2. Calibrate the scanner

3. Stick the dots to your model or board and begin to scan

4. Make sure that when scanning that your optimum proximity bar remains green such as in the photo (to the left of the laptop's screen).

5. Use the real-time model on screen to watch your progress and find any missed patches. Slow sweeps work best; you may of course recover previously scanned areas, providing you have not moved your model.

Step 4:   CAD Modeling

1. Took the solid '.prt file' and created several new work planes from which to sketch onto (a scanned model will not instantly have logically located work planes as it was not built within the CAD package).

2. Next I created a simple rectangular sketch and cut the wrist nice and flat with an extrude (remove material selected), the wrist was previously a bit lumpy from the liquid mixture poured into the mold.

3. Then I removed the thumb and used that as a separate part file named "thumb".

4. Then I shelled the hand to a wall thickness of 2millimeters.

5. Then I created a circular cut in the wrist for inserting the internal mechanism (to come).

6. Then I created three screw holes to affix this mechanism, 120 degrees from one another, which I strengthened with thicker wall thicknesses around the holes.

7. To build the watch rim I then I sketched a circle on a projected plane above the top of the hand aka 'back of the hand'. The sketch was then projected onto the surface of the hand itself.

8. A smaller circle was then sketched on a projected plane and the two circles were merged or 'blended' using influences curves around this blend.

9. I filled this blend to create the solid rim walls (you cannot print surfaces using 3d printers, surfaces have no thickness!)

10. I created a simple circular extrude to remove the inner material, but not all the way through as I needed to retain a base.

11. Then I cut a smaller hole in the base allowing the clock (or other insert able object) to be popped out from the inside of the hand.

12. To clock itself is from habitat, I reverse engineered this clock to check its fit within the hand, throughout the modelling process.

13. To create the rubber sleeve I copied the geometry of the organic wrist profile to build a new part which I then blended to a circle profile on a projected plane.  I thickened these surfaces to give a wall thickness and I created a protrusion to fit into a recess within the wrist ensuring a smooth transition from the hand shell to the rubber sleeve component.

14. The thumb was just left solid for strength; however it could have been shelled to reduce cost.

15. Cut a slither from the index finger and use the cut part to form a new part (rubbers insert)

16. All parts were used to build an assembly file and check the overall model appearance.

Now you will want to save your individual part files as '.stl' files.

This is different depending on your modelling package, but generally speaking try and input the minimum step heights etc. that your system will default to as this will give you the best surface finish, but may increase your file sizes.

 

Send them to a 3D printer or company who can offer this service. I printed the hand shell in the most rigid of the rubberized materials (DM_9795), as I wanted to avoid brittle materials which would shatter on bashes or knocks.  I printed the thumb and sleeve in a more flexible material (DM_9770).  If you have to pay for support material then you may which to consider which orientation you print the model in.

 

Once it has been printed you will need to carefully remove the support material with a jet wash or by hand using water only.  Caution; smaller or delicate parts are likely to break under high pressured water.  Let the model completely dry out for 48 hours (it will sweat), then finish with fine grade wet and dry paper, make sure you remove all support material, sometimes you can feel it better with your fingers than you can see it with your eyes!

Step 5: 3D printing

Send files to a 3D printer or company who can offer this service. The hand shell is the most rigid of the rubberized materials.  The thumb and sleeve are printed in a more flexible material (DM_9770).  If you have to pay for support material then you may which to consider which orientation you print the model in.

 

Once the file is printed you will need to carefully remove the support material with a jet wash or by hand washing with water only.  Caution, smaller or delicate parts are likely to break under high pressured water.  Let the model completely dry out for 48 hours (it will sweat), then finish with fine grade wet and dry paper, make sure you remove all support material, sometimes you can feel it better with your fingers than you can see it with your eyes!

Step 6:  Internal Mechanism

To make this mechanism the author modified an existing prosthetic hook, made by Hosmer USA.  You need to:

1. Disassemble and store internal ball bearings safely, then saw off the hooks themselves

2. Saw off the cable arm too, but not completely, just leave a little metal to weld onto.

3. Take some aluminum rod (mine was 8x120mm) and bend it into the desired curve then weld this to what was the cable lever arm. Now file and wet & dry.

4. Drill a hole into the new welded arm to form a loop hole for the cable to be crimped around.

5. Take your cabling and some brass tubing, now crimp them very tightly.

6. Drill the screw holes and another hole to create the channel for the cabling to fit through, keeping all cabling internal. I printed a drilling template from my CAD to do this.

6. Drill a hole in the thumb to slot the levering arm onto.

Step 7:  Painting

1. Smoothen your parts using wet and dry.

2. Blast off any dust with compressed air.

3. Sit the hand on a jig and spray whilst rotating it.

4. Allow coat to dry, then sand back again, particularly in between the fingers.

5. Apply another coat and repeat the process until you're satisfied.

Step 8:  Assembly

Ensure that you fit the mechanism from the inside of the sleeve, screw the mechanism in and then mark out the slot for the cable to slot through.  Then drop in the ball bearings and screw all together, pretty much as you disassembled it.  Finally screw it into the fixing holes that were created on CAD. It would be much better to incorporate brass fixings than screw directly into the material as it doesn't bite well.  The holes were filled with glue and rethread as this was more sturdy, however the source brass fixings of the correct size.

Then glue the thin slither of printed rubber into the index finger cavity to create your enhanced grip.  I used gorilla glue as it’s very strong!

Step 9: Final Piece

Available

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as of: 
05/19/2015
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DIY 3D Printing Prosthetic Hand - Make it Real Challenge