Plastic is cheaper, lighter, more corrosion proof, easier to cut and shape and drill, has a very low thermal conductivity, and a higher strength to weight ratio than metals. So why isn't your robot made of plastic already?
It is probably because you do not know where you can get large cheap quantities of the stuff, nor have you actually realized the advantages of it.
HDPE, or High Density PolyEthylene
Where to Use HDPE
Click the above image to enlarge an example of mounting robot parts to your HDPE robot base. This was actually my very second robot so it should make a good example for beginners. It is a differential drive style robot, implementing two side DC motors and a castor wheel in back. What you are viewing is actually the bottom of the robot.
As you can see it is very artistically shaped, showing how easy it is to cut with basic tools. I used steel brackets to mount my wheels for high stiffness, using screws between the HDPE and the steel. At the rear of the robot is the castor, which just required a few screw holes drilled into the HDPE to attach it. In the front, if you look carefully, is a small camera. I attached it with a bent piece of aluminum, drilling two holes into the HDPE for screws. Lastly, you will notice this big black rectangular thing in the center. This is my NiCad battery. I placed it on the bottom of my robot to keep the center of mass low to the ground. I used zip ties to attach the battery. But if I was to go back and redo it, I would have used velcro. Velcro actually sticks really well to HDPE if done properly. Just take sandpaper and scratch up the surface a little, blow the dust away, and stick it on. This roughing is for increasing the surface area for the sticky glue to bond to - a method good for all other glues and epoxies too.
Designing Your HDPE
Here I used AutoCAD, but you could just as easily have used Paint or Solidworks.
Whatever you have available. I highly recommend 3D CADing if you have the option. Then
just print out the image to scale, cut out the paper parts, and trace them onto your HDPE
(or aluminum or whatever other material) using a dry erase marker.
Dry erase is great as you can easily wipe it off of HDPE when you are done. Permanent markers on
HDPE is . . . well . . . permanent.
HDPE Construction Techniques
The bottom layer is a thin sheet of flexible aluminum, and the top layer is a thin sheet of HDPE. Joining them together are four threaded spacers. Servos were mounted with screws to the aluminum plate, 2 NIMH batteries were velcroed inside between the plates, and a hole was drilled through to allow the servo and battery wires to come out to the top. My microcontroller was then attached to the top using screws in the corners. I later mounted many other electronics and sensors to the top of the omni-wheel robot base.
Tools For HDPE
Lastly, Disadvantages in Using HDPE
- Do not use it when you expect a lot of wear applied to it. HDPE is a soft material and can easily be worn away with abrasive contact. However I have seen HDPE used quite well for chain tensioners because of it's smoothness. Chain tensioners are just a big block of material that pushes against a moving chain to make the chain tight around the sprockets.
- Do not tap HDPE. This is when you put a screw thread into a material. HDPE is very weak when it is made thin, so the tapped threads will end up being lose and will fail after only a few uses. However what is great about HDPE is that is a great self tapping material. Instead of tapping, just screw a screw into a tight hole and the screw will stay in really tight and firm. I use this method very often as it is very reliable.
- Do not use HDPE if you need highly rigid strength. HDPE, unless the dimensions are chosen carefully, will bend and flex. This is likely to happen if you use long thin pieces to hold heavy weights at the end. It is possible to mathematically calculate how it will bend so you can design around the problem, but I will not go into it as it is somewhat complex theory. Below is a robot arm I made with such calculations. It uses long thin horizontal beams of HDPE, but I used a combination of 4 of them and located them strategically so the bending was almost zero. This particular robot arm lifted decks of playing cards, but could easily lift objects much heavier.
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