Ideas to inspire...
Everything in this project is handmade. As such, all the parts are initially cut larger (by 5mm -10mm) than specified in the drawings and then shaped to size. Precise cutting machines would provide efficiencies in a manufacturing environment.
It is also important to glue all the parts together before rounding off the edges and corners. This ensures that maximum surface area is available for adhesion. Butt joints are used throughout given the width of the material and because they are simple. Each piece will need to be supported during the gluing process and adequate time must be allowed for the glue to dry properly. Drying time may vary depending on temperature and environment.
I undertook testing of the model before gluing the wheels and finishing. The dowels that are to be the axels are a comfortable fit and so support the wheels well for testing purposes. The details of testing are set out below.
After everything is properly dry, sand back the timber to remove all marks and round off all edges and corners. Wipe down in preparation for painting.
The material chosen for making this childâ€™s toy is MDF. Unfortunately, it is not the best material for children's toys and pine or various hardwoods would have made better choices. However, for the purposes of this project, I considered it to be a reasonable choice given its weight advantage.
Given this choice of material and in order to ensure it would be suitable as a childâ€™s toy, it is important to ensure that the finishing compensates for the material choice. In this regard, the only option for finishing is to apply suitable non toxic paint in order to create a hard enamel shell. The first couple of coats will absorb into the material so at least three to four coats will be required. All surfaces will need to be painted ensuring the MDF is completely sealed. A hard shell surface will withstand a child's attention and also enable ease of cleaning.
I decided to use PlastiKote spray paint. This type of paint is used by artists, is suitable for use on wood, is easy to apply and will produce a hard wearing surface. This paint is also used on childrenâ€™s toys. The key is to ensure that the paint is allowed adequate time to dry.
For the colour scheme. this was possibly the simplest part of this project. Everyone knows that red things go fast and the brighter the red, the faster they go. So it makes sense to have red wheels. I read somewhere that blue is a relaxing colour and it seems to me that you would want to be relaxed when going fast. So, blue seemed to be an excellent colour choice for the cab and body. Finally, for the parts that pull away from the model, I obtained inspiration from nature. Looking out at my garden, I noticed it was full of weeds. Weeds are green and you always want to pull those out. Therefore, green appeared to be the ideal colour for the deck and back of the cab which can be removed. (Taking things too seriously could be hazardous to your health.)
The testing described here was undertaken at home on 7 August 2010. The final weight of the vehicle is 1.038kg
To determine the effect of inertia, I chose to test this by seeing how many windings of the rubberband would be required to move the model. I ascertained that between 1 to 2 windings of a single rubberband was all that was required. As such, inertia appears not to be a significant issue in so far as preventing propulsion.
A single rubberband wound up to 15 times will propel the vehicle but falls short of the required distance. It also appears sluggish. This seems consistent with the expectations in my analysis. This testing was undertaken on a flat lino surface and also a carpeted surface with similar results.
A second rubberband was an improvement and the model travelled the required 6 meters on both the lino and the carpet. However, there was still some sluggishness about its performance which may have more to do with some inconsistency with windings.
I made a simple makeshift incline from kitchen trays to test the model. The incline was short of a meter but sufficient for this test. It successfully climbed the incline and had enough momentum to cover further distance. However, the appearance of "sluggishness" was still of concern. A third rubberband resolved this.
The performance identified two other areas that required attention; the tracking of the model and rubbing of wheels against the chassis. The latter was fixed by introducing spacers between the wheel and chassis. In this regard, I fashioned spacers from left over 15mm (ID) pvc piping.The former was more challenging as the wheels will be ultimately glued to their axels. I made guides for aligning the wheels at the time of gluing. Final tests showed improvement although it still tracks slightly to the right.
Although initial tests indicated that the model should perform, its performance on the actual course was problematic. It managed to climb half way up the incline and stopped. This could be due to the following possibilities.
The surface of the ramp is quite smooth and traction is not good. This was the simplest problem to test. I attached slim wide rubberbands to the wheels effectively creating rubber tyres and retested the model on the ramp - problem solved. The model easily travelled up the ramp and completed the distance required.
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