The next step in my DC motor mobile robot was the 3D printing of the top dome part. The purpose of this part is to contain some sensors so that the mobile robot can obtain information about the environment in front of the chassis. By making this a rotating dome it enables the use of single dimension sensors such as ultrasonic, infrared or even laser rangefinder sensors, as well as having the potential to include one or more cameras, as well as appropriate displays to provide information to the user or just provide a friendly 'face'.
As with all the other 3D printed parts I wanted this to be an interference fit, if at all possible, so that it can be assembled and dis-assembled as needed. I had already decided that I wanted to make this top part a dome or some sort of circular shape rather than the box like structures used for the rest of the mobile robot chassis. I also decided to use 180 degree micro servo motors as I have used them before in similar situations and they have been very easy and reliable to use. There is no room in the battery compartment below the dome to place the micro servo motor there so it would need to be placed into the dome itself. This would place a minimum on the size of the dome. Fortunately, a dome of radius 40 mm seems to be sufficient to hold the micro servo motor as well as leaving space for other sensors. I wanted to have a flat face to the front of the dome as this makes it easier to place sensors, The first 3D printed part therefore needs to be the base of the dome and will have diameter of 80 mm with a cut-out to hold the micro servo motor. Again, I wanted the micro servo motor to be an interference fit rather than having to fix it in with screws or glue. This is shown in the video below and seems to work quite well. With such a wide base piece which is in contact with the top of the battery compartment there is almost no wobble and as the two surfaces are (relatively) smooth there is almost no sticking.
TinkerCAD which is the software I use to make my 3D prints, is not that good as curves so designing the dome itself was a bit of a challenge. Fortunately one of the basic parts is a solid dome so I made a solid dome 83 mm in diameter and then put another dome of 80 mm diameter inside it which I converted to a hole part. This resulted in a thin shell dome with an internal diameter of 80 mm, the same size as the base, with an external diameter of 83 mm. This makes the dome wall thickness of 1.5 mm, which is enough to create a sufficiently rigid shape without using too much plastic or taking too long to print. The print time was still 2 1/2 hours. The base part of the dome, see above, did not print out all that well as it lifted quite a lot in one corner and some of the surfaces were very rough. I decided I had to do something about this before printing the dome so decided to re-calibrate the printhead height above the printer base place. This seemed to have changed 0.6 mm and when this new value was used the print quality was virtually perfect, see the video below. This also resulted in the raft and support material being very easy to remove, it practically fell off.
The dome itself then fits perfectly onto the base, see video below. Although it is the correct interference fit, as designed, because there is so little contact between the dome shell and the base just along the perimeter of the base plate, it does not really stay together. I will either have to stick it, glue, or find some other method of more permanently connecting these two parts together.
All that remains now is to create the front face plate, which is flat, and the mobile robot chassis will be complete. The I will insert an Arduino Nano controller somewhere, add a few sensors, programme it and away it will go.
Dubbie
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