Composite Construction Resource Page
For the most part DEW Robots have been constructed out of aluminum and some plastic in the past. This build season I ( Gary Deaver) would like to introduce some basic carbon fiber and fiber glass construction techniques. We need to get some fiber in to DEW bot's diet. Properly used composites can help reduce weight while maintaining structural strength. Two types of composites that could be used are fiberglass pultrutions and carbon fiber laminated boards.
Fiberglass pultrusions are made of long fiberglass threads called roving and polyester resin. The come in many structural shapes like the aluminum that we use on the robot. Pultrusions can be machined with normal wood working tools. The dust from any fiberglass material needs health precautions. Gloves and a dust mask are recommended. Bedford Reinforced Plastics, Inc. is a manufacturer of pultrusions. This link will take you to their home page. Bedford Plastics There are many resources on their site. This page gives a description of how they are made. Pultrusion Process When working and designing with pultrutions, it is important to remember that if a hole is drilled into the piece, the long fibers are cut and a weak point is formed. While this is true for aluminum too, it is much worse for pultrusions. As a general rule try to avoid drilling. Clamping on to the pultrusion is better.
For an example of fabricating with pultrusions look at the following pictures. To make part of a ball grabber, we took a piece of 1.5" box tubing and mitered out a hole for a 1" box tube. Kind of like a mortise and tenon joint. Look at picture 1.
There is a piece of card board in the 1.5" tube below the 1" tube sealing it off. A little epoxy putty fillet around the 1" tube on the outside seals the cavity for the next step shown in picture 2 and 3. That black gooo in picture 2 and 3 is a mixture of epoxy and chopped carbon fiber. The void around the tenon joint is filled with this mixture. A little heat from a hot air gun helps remove some trapped air bubbles and starts the curing process. Yields a surprisingly strong joint.
The next example is for an arm. Remember that above I said it was bad to drill holes in pultrusions? So how do you attach a pultrusion arm or manipulator to the robot frame. This is a very high load joint and with impacts can incur allot of stress. I mention that it is better to clamp, however, clamping may not be good enough for this application. Sometimes you just gotta bolt it down. Let's say we're going to use a 1.5" square tube for the lower arm that will be bolted to a heavy strong double aluminum rail on the robot base. Look at picture 4. For this mount, tube in tube construction works very well. A 1" square tube is bonded into the 1.5" tube. Looking at picture 4, notice the 1" square tube has a piece of plywood super glued on to one end. This tube is inserted into the 1.5" tube, centered and glued. After the glue has set, the next step is to foam the lower 8" of the cavity. In picture 4 notice the small cup with the foam in it. This is 2 part 2LB per cubic foot expanding urethane foam. The foam comes in 2 parts like epoxy. Mix equal parts together and then stir real fast. The liquid starts to foam after 30 seconds. Pour into the cavity. After about fifteen minutes the foam is hard and cured. Any extra foam is dug out of the cavity so that there is a 4" void. This 4" space is filled with the epoxy and chopped carbon fiber like in example 1. After it cures trim it square. The end result can be seen in picture 5. Now there is a reinforced Area to drill and put some bolts through.
For the 2010 season the intention is to have a carbon fiber laminating project to introduce students to the the field of composite construction. A good candidate for this project is the electronics board. A piece of polycarbonate could be used with a support frame. This would be faster and easier. However the potential is to have a lighter and stiffer board by laminating a piece of 1/4" birch plywood with 1 layer of carbon fiber cloth on each side. Making composite parts can be very complex. We will strive to keep it simple. The first step to learning the skills to make the electronics board is to know what epoxy is. Follow the link below to a hand book on epoxies.
Now that you have a little understanding of epoxy and laminating, There are some more documents to study.
- West Systems Epoxy
- Applying cloth Watch the video if you want.
- Soller composites on cloth
- Some examples of different cloth
If you have studied the above links you should have a basic understanding of composite construction. An important part of laminating is to minimizes the amount of epoxy resin. To keep the epoxy content close to the 60 - 40 ratio vacuum bagging is used many times. Instead of trying to describe all the details in this procedure, a video is worth about 2000 words. Please watch this You Tube video.