LBS hand v2 (design + CAD + CNC machined)

In our previous blog, we made a prototype of a mechanical finger using the concept of 4 bar linkages. In this post, we are gonna write about the design considerations for LBS hand v2.

LBSv2 on Google Sketch

Our aim is to:

1. form an actual robot hand
2. make space for the servos (Decided to use HTX900 since they are available from Ray)
3. create space for the light sensors on the finger tips
4. have the entire thing made out of plastic with the exception of the screws and bolts
5. Reduce the weight 
6. Make it smaller 

Lets elaborate each point.

1. Form an actual robot hand

LBS v1 was only a prototype to test if the mechanism fits our purpose. We did not consider where we will mount the servos. It was also very chunky. The second version, LBS v2 will take all these into consideration.

2. Make space for the servos

The servos will have to be placed directly behind the finger. We decided to use the HTX 900 since the department has ordered extras. The dimensions of HTX900 were easily found (shown below).

http://www.hobbyking.com/hobbyking/store/__662__HXT900_9g_1_6kg_12sec_Micro_Servo.html

3. Create space for the light sensors on the finger tips

The finger tips should have some space to fit the sensors. One IR sensor and One LED.

http://www.modeltrainsoftware.com/led-sizes-explained.html

4. have the entire thing made out of plastic with the exception of the screws and bolts

The main reason why it seems attractive to build a robot hand ourselves is due to the facilities available in the department. We decided to use the CNC router again. Why not the 3D printer you ask? We explored the potential of the 3D printer (previous blogs) however there are many limitations. Two reasons why we didn't' want to use 3D printer anymore:

• Each piece has to be individually printed with CARE, which can be time consuming and frustrating. We have had many opportunities to observe how the 3D printer can accidentally screw up the design. It is just not reliable. For example, the ABS filament is melted in the extruder and comes out of a nozzle but when it requires 'cleaning' (removing excess plastic material), it simply uses the edge of another plastic to 'bump' off the excess plastic. This mechanism was not reliable and was THE reason why so many of our 3D printing attempts failed.
• Once it was printed, difficult to modify it manually. We tried drilling a hole into previous 3D printed parts and it started melting! 

Therefore we decided to use the CNC router machine to cut out our pieces. Since all the required pieces are FLAT, our choice of CNC router was clearly the most obvious choice.

The fingers will be made from plastic BUT the palm will be aluminium. The aluminium will add strength to the entire hand.

5. Reduce the weight
One of the biggest problems that we foresee is over estimating how much weight the mentor arm can carry. We shall try to reduce the weight by making the hand as small as possible!

6. Make it smaller
To mimic a human hand, LBS v2 must be smaller. We will be using Shen's hand as a guide.

Shen's Hand!

After considering all these factors, we spent a few days to design the hand on Google sketch!

A proper sketch on paper!

Parts for the fingers (Plastic)

The palm (aluminium)

Assembling the pieces on Google SketchUp

The next step is to convert the SketchUp file to a STL format for the CNC router.

CNC router cutting out the pieces. Plastic sheet is 2mm thick

Hi Five!

Assembling in the work shop

Comparing the Flexy hand with the LBS v2 hand

These are just some snap shots taken over a period of mid July - end of June 2014. Took us a long time and we encountered some problems.

1. Some parts were too big/small and wouldn't fit properly.
2. Had to order some new 2M 16mm plastic screws

 These problems will be further explained in the next post.