Introduction: Dextrus V1.1 Robotic Hand

This is an instructable to show how to make version 1.1 of the Dextrus robotic hand from the Open Hand Project. It assumes you have access to a 3D printer and have printed the parts according to the instructions on thingiverse and have purchased all of the parts in the bill of materials included in the attached .zip file.

This project is still underway and it's by no means a completely polished product. It should be a great project for someone that is interested in robotics or prosthetics and want's their very own robotic hand!

Please download the attached .zip file which contains the bill of materials and .stl files (and source files) of all of the 3D printed parts.

This instructable will take you through the assembly. I would rate it as "difficult", but hey, if I can do it, anyone can! If you have all of the parts to hand, assembly should take around 2 hours.

The general process of the build will go as follows:

1. Assemble the finger.
2. Assemble the motor housing.
3. Attach the finger to the motor housing.
4. Assemble the thumb housing.
5. Repeat steps 1-3 for all of the fingers and the thumb.
6. Assemble the hand.

Tools required:
- Small philips head screwdriver.
- Small flat head screwdriver.
- 0.9mm Allen key.
- 2.5mm Allen key.
- A pair of pliers.
- A bicycle chain link remover tool. (amazon)
- A crimping tool (can use pliers instead).
- A drill with 1.5mm, 2mm and 3mm drill bits.
- Vernier calipers (I prefer digital like these). 
- 9V battery.
- M3 bolt.

Step 1: Preparing the Bearings

First off, we'll need to prepare the bearings that go into the joints of the finger. They are friction fitted, and it's a very tight fit.

You will need:

- Bike chain tool (like this one).
- An M3 bolt (6-16mm in length).
- Pliers.
- Vernier calliper (Amazon).
- 3 Standard ball bearings.
- 2 12mm steel dowels.
- 1 14mm steel dowel.

1. Take one of the standard (non-grovved) ball bearings and put a short M3 bolt through it, this acts as a guide to get the bearing positioned in the bike chain tool, and will be removed later.

2. Place the bearing in the bike chain tool and tighten the handle all the way (don't make it tight, just enough to have the bolt inside the bearing.

3. Tighten the bike chain tool from the other end (the bolt), this time you want to make it fairly tight to hold the bearing in place.

4. Unscrew the handle of the bike chain tool all the way and remove the M3 bolt.

5. Using a pair of pliers, get a 12mm steel dowel pin and insert it tapered end first into the bearing. With a little bit of pressure, you should be able to get it to hold in place. Make sure it's aligned as well as possible.

6. Tighten the handle of the bike chain tool so that it pushes the dowel into the bearing, the objective is to get it half way. READ STEP 7 NOW.

7. Measure the dowel to make sure it is exactly half way through. Although it should be 4mm on either side, I've found that due to the tolerances of the bearings and the dowels, 3.85mm works best. You could measure your dowel and bearing first to calculate this for your particular components. If you accidentally overshoot you can unscrew everything, remove the bearing and flip it around.

8. Unscrew everything and jiggle the bike chain tool around until the bearing falls out with the dowel snuggly in place.

9. Repeat steps 1-9 once with another 12mm dowel, and then again with a 14mm dowel.

End of part 1.

Step 2: Assembling a Finger

In this step you will assemble the finger.

You will need

- The dowelled bearings you made in the previous step.
- 4 M2 phillips head bolts.
- 4 M2 nuts.
- The 3D printed parts of an entire finger (should be 6 parts, 2 for each joint).
- Phillips head screwdriver.

1. Take the two halves of the fingertip and place one of the 12mm dowelled bearings into the groove in one of the halves.

2. Screw the halves together with a nut and bolt. The nuts go on the front of the fingers (bottom if the hand is palm down) and the bolts go in the back of the fingers. The nut should fit into it's hexagonal hole although it may need a bit of a push with the screwdriver.

3. Take the back half of the middle joint and place the other 12mm dowelled bearing into the groove. Also place the assembled fingertip into the smaller grooves at the other end.

4. Screw the middle joint together.

5. Take the back half of the remaining joint and place the 14mm dowelled bearing into the groove. Also place the assembled middle joint (with fingertip attached) into the grooves at the other end.

End of Part 2.

Step 3: Assembling the Motor Housing

In this step you'll assemble the motor housing with motor.

You will need:

- Flathead screwdriver.
- DC motor.
- 2 Grooved bearings.
- 14mm dowel pin.
- 2 6mm dowel pins.
- 2 flathead M2 bolts.
- A pair of tensioners.
- A motor housing.

1. Take the two tensioners and drill out the holes at the back with a 3mm drill bit. You may need to move it around a little to loosen them up a bit.

2. Put the grooved bearings onto the 6mm dowel pins.

3. Insert the bearings, with dowel pins, up through the bottom of the tensioners. They should click into place.

4. Insert the 14mm dowel pin through the 3mm holes you've drilled out at the back of the tensioners. This could be a little stiff and pliers may be helpful. These should have a pretty tight fit. NOTE: They do have a correct way around, it's very subtle, but the part that you're putting the dowel through isn't quite central on the tensioner. They should be positioned so that the two shallowest sides are facing each other. If you can't work it out don't worry, if they're the wrong way around it should become more obvious when you try step 5.

5. Click the dowel with tensioners into the back of the motor housing.

6. Insert the motor into the motor housing and put the two M2 bolts through the holes and tighten them.

End of part 3.

Step 4: Attaching the Spool

In this step you'll attach the spool to the motor shaft.

You will need:

- Spool.
- M2 grub screw.
- 0.9mm Allen key.
- 0.6m of steel cable.
- 9v battery.
- Pliers.

1. Using the 9V battery, rotate the motor shaft so that the flat side faces upwards (away from the tensioner bearings).
2. Drill out the small hole in the spool. This is for the grub screw. I use a 1.5mm drill bit and move it around a bit so that the grub screw needs to be screwed into the hole. You might prefer to just use a 2mm drill bit.
3. Using the pliers, insert the M2 nut into the spool and push it all the way down.
4. Screw the grub screw into place using the 0.9mm allen key.
5. Cut a 0.6m length of steel cable (about an arms length).
6. Thread the cable through the centre of the spool. There is a groove that it should sit in on the opposite side to the grub screw. You should thread the cable so the spool is half way along the cable.
7. Threading the loose ends of the cables down past the motor axle, move the spool into position next to the axle.
8. Push the spool onto the axel (this needs a bit of wriggling and flexing but should go on). One end of the spool has a collar, this end goes nearest the motor.
9. Tighten the grub screw using the 0.9mm allen key.

End of part 4.

Step 5: Threading the Tendon

In this step you'll attach the finger to the motor housing and thread the tendon. It's strongly recommended that you read through all of these instructions a couple of times before beginning.

You will need:

- Assembled finger.
- Assembled motor housing.
- Pliers.
- 9V battery.
- 2 Springs

1. Attach the finger to the motor housing by clicking it into place.
2. Thread the tendon nearest the motor first, take this tendon and hold it while you use the 9V battery to rotate the motor so that the tendon wraps around the spool.
3. Keep going until you can see two windings on the spool. The axle should be in a position where the flat side is facing the 3 O'clock position of a clock face when you look at it head on.
4. With the tendon still in this position, take a spring and put it into place on the receiving tensioner (the tensioner that you use is important,it should be the one directly beneath the tendon that you're threading).
5. Thread the tendon around the grooved bearing, it should follow around the bearing so it's not in contact with any of the plastic of the tensioner.
6. Thread the tendon up through the assembly, it should go on the inside of the already threaded bit of tendon and under the plastic strip on the motor housing (look at the pictures for a better idea).
7. Thread the tendon through the back of the finger all the way to the tip.
8. Now for the other tendon. While this is being threaded, try to keep the first one taught so it stays on the spool. Take this tendon and wrap it over the spool, threading it down through the assembly. It only needs one winding as shown in the picture.
9. Insert the second spring and thread the tendon around the grooved bearing.
10. The tendons should now be wound as shown in the image.

End of step 5.

Step 6: Crimping the Tendons

In this step you'll complete the finger mechanism and crimp the tendons into place. It's recommended that you read through all of these instructions before beginning.

You will need:
- 20cm of steel cable.
- Crimping tool.
- 2 Double copper ferrules.
- Pliers.
- A small long thing.

1. Cut a 20cm length of steel cable.
2. Very loosely wrap the cable around the entire motor housing and crimp.
3. Trim the ends of the cable off.
4. Put the cable in place around the motor housing, all the way from the very top to underneath the tensioners.
5. Place your small long thing through the cable and begin to twist it. The purpose of this is to clamp the tensioners all the way into their compressed position while you crimp the tendon. You can use some kind of clamp instead if you have something suitable.
6. Make sure the clamp stays in place. a rubber band might help here.
7. Pull the tendons taught again and make sure they are still wound properly on the spool.
8. Place a double copper ferrule over the two ends of the tendons and move it all the way down towards the fingertip.
9. Crimp the copper ferrule in place while it's as close to the fingertip as possible and the tendons are as taught as possible. It might help to hold the tendons with pliers to keep them taught. The finger should be straight during this process.
10. Cut off the trailing ends of the cable.
11. Remove the clamp and you have one completed finger mechanism!

End of step 6.

Step 7: Making the Rest of the Fingers and Thumb

In this step you'll make the remaining fingers and thumb.

You will need:
- All of the same components that you used to make the first finger.
- The parts to make the thumb.
- You can reuse the clamp you made in the previous step.

1. Repeat steps 1-6 3 times to create the fingers.
2. Repeat steps 1 and 2 for the thumb.

End of step 7

Step 8: Assembling the Thumb Housing

In this step you will assemble the housing for the thumb.

You will need:
- Assembled thumb.
- 14mm Dowel pin.
- Nylon bushing.
- 0.9mm Allen key.
- A pair of tensioners.
- Servo motor.
- Grub screw.
- Pliers.
- Two grooved bearings.
- Two 6mm dowel pins.

1. Insert the nylon bushing into the 3mm hole in the thumb housing. Super glue it into place if you wish.
2. Assemble the tensioners as in step 3.
3. Get the tensioners into position.
4. Thread the dowel pin through the motor housing and tensioners, this will be stiff, and some of the holes may need drilling out a little. It may help to use pliers, and to put the tensioners in one at a time.
6. Click the thumb into place.
7. Push the servo in to place, and using the 0.9mm allen key screw a grub screw through the upper most hole until it goes through the servo fixing. This will hold the servo in place. If that doesn't work, you could try either super glue, or wedging the servo in place with some card either side of it.

End of step 8.

Step 9: Assembling the Palm

In this step you'll assemble the palm of the hand

You will need:
- Palm 1.
- Palm 2.
- A DC motor.
- A 12mm dowel pin.
- 4 flathead M2 nuts.
- The assembled thumb housing.
- Flathead screwdriver.

1. Take palm2 and slot the dowel pin into the hole. Palm2 goes onto palm 1 and remains there permanently, so I would recommend using super glue to fix them together and using M3 bolts to keep them aligned while the glue dries.
2. Screw the DC motor into place with the flathead bolts. It's a good idea to solder wires to this motor first, as it's a tight fit and may be difficult later.
3. Slot the thumb housing into place by slotting the metal dowel into the nylon bushing.
4. Mount the servo horn to the servo. It may be necessary to drill the holes in the servo horn out to 2mm for the M2 bolts. The thumb should be able to move between a grasp grip and a key grip (use common sense here).
5. Bolt the servo horn to the palm with the M2 nuts, these screw straight into the plastic.

End of Step 9.

Step 10: Threading the Thumb Tendon

In this step you'll thread the thumb tendon

You will need:
- 0.6m of steel cable.
- 15cm of steel cable.
- Crimping tool.
- 2 Double copper ferrules.
- The last Spool.
- 2 springs.
- Pliers.
- A phillips head M2 bolt.
- Phillips head screwdriver.
- Thumb cover.

1. Mount the spool to the motor axle as in step 4.
2. As before, thread the tendon nearest the motor first, wrap it around the spool so there are two reels and thread it through the hole in the thumb housing.
3. Insert the springs between the tensioners and the thumb housing.
4. Thread the tendon around the grooved bearing like in the picture.
5. Thread the tendon through the hole in the thumb housing and then through the backs of the thumb joints (as in the picture).
6. Thread the other tendon. This should have one winding on the spool as in the picture.
7. Thread it around the grooved bearing, through the lower hole in the thumb housing and through the fronts of the finger joints
8. You need to clamp the thumb tensioners as in step 6, do the same thing, but this time thread the length of cable underneath the thumb housing.
9. Holding everything as taught as possible, and with the thumb in the fully open position, crimp the ferrule as close as possible to the thumb tip.
10. Place the thumb cover over the thumb housing and screw into place with an M2 bolt.

End of step 10.

Step 11: Assembling the Hand

This will be the final step, where the hand is assembled.

You will need:
- All of the parts made so far.
- Back 1 and back 2.
- 4 45mm hex head bolts.
- 2.5mm Allen key.

1. Place all of the completed fingers into Back 1.
2. Put the palm, back 1 and back 2 together and bolt them into place.
3. You're done! Congratulations.

If you complete this build successfully, please drop me an e-mail at I would love to see pictures of your build and hear feedback on the design.

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