BCN3D MOVEO Robot Arm

This arm is and open source project by the generous people at BCN3D. You can find their page on it here. All of the .stl and Solidworks files are available on Github along with assembly manuals and the bill of materials. Bear in mind that it is a Spanish company and most of the file names are in Spanish.  

 
 
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The Point:

The MoveO arm design isa five axis arm using stepper motors with the gear reduction on most joints done via toothed belts and sprockets. Being more concerned with gaining more experience with non-artistic usages of my 3-D printer and stepper motors than spending the time designing and developing an arm myself I decided to use BCN3D's existing design. Once completed I intend to start modifying their design my longer term goal of developing designs that allow an arm such as this to assemble other machines from 3-D printed parts. Such as a small lathe to improve the run-out on 3-D printed pulleys and wheels. This will likely require several interchangeable manipulator ends for actions such as bolting joints or heating and pressing threaded inserts. Though one must keep in mind that many of the difficulties of "self assembling" machines made from FDM thermoplastic prints are solved if one assumes that in an environment such as space a metal powder printer would be used and techniques such as laser welding could be used for fusing. 

Hardware/Datasheets

 

A good page describing stepper motor decay Here.

From "User Manual BCN3D Moveo.pdf"

From "User Manual BCN3D Moveo.pdf"


The two second axis servos in the CAD model are NEMA 23 54mm long stepper motors, where as the servos described in the BOM are NEMA 23 112mm. Looking at their promotional videos for the arm the longer servos were used. Likely they found that the 54mm holding torque of 1.5Nm insufficient. This is unfortunate as the 112mm servos, shockingly, cost twice as much as the 54mm. These two along with the 3rd axis NEMA 17 servo with 5:1 gear reduction make up the majority of the cost of this arm.  

 
fig 4.

fig 4.

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Note on 3-D printed shaft:

In an attempt to make lower the cost of the arm and test out the limitations of my 3-D printer, I attempted printing out the 8mm shafts for the 2nd, 3rd, 4th, and 5th axis. Both Taulman bridge nylon and polycarbonate were tried. The nylon ending up being much easier to insert through the bearings and part bores due to its flexibility and lower surface hardness. The polycarbonate was so rigid that the imperfections caused by printing would catch against the edges of the bore on the other 3-D printed parts.

The shafts were printed vertically as shown in fig. 4 on the left. Horizontally printing the shafts was tried but they layering would cause the cross section to be far from round. Vertically printing the shafts causes them to be much weaker to axial flexing due to the layers delaminating but in this design the distance between loads on the shafts are small enough its almost entirely shear forces. Thus far the nylon appears plenty strong for the application.  The ridges cause the shaft diameter to increase so some trial and error was needed to adjust the model dimensions. For a shaft with an 8mm diameter the model needed to be 7.7mm to fit into the bearings.

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