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I'm inclined to agree with spider007. Not because a model or drawings of a tooth brush may be hand to find, but because of the exercise. In the real world, you will be called upon to reverse engineer a part or an entire design. This procedure would be a great exercise! It would also add to the value of your thesis.
Tools you may need:
Assemble a good set of calipers, micrometer and radius gages. Then have about 3 tooth brushes that are identical because you will be destroying one or more of them. Cut one up in cross sections every 5-10mm and measure each section. Use the cross-sections as sketches in your model.
Cut one section through the middle of one or more of the brushes if they are different. To make it interesting, you could buy some of the new battery operated sonic tooth brushes and model the entire assembly!
If you have access to the Quality Control department of local design and manufacturing facility, you may be able to convince them to let you use some of their equipment. I band saw with a good new blade would do for cutting and if they have a CMM (Coordinate Measuring Machine) and shadow graph to help with the measuring, that would be great!
Sounds like fun and brings back memories
But I am glad it's you and not me
Like the others have said, get a caliper and or a ruler, or if you don't have either of those GUESS! I am constantly creating rough models of OEM parts that we use in our assemblies and most of the time I am stuck with a set of calipers a ruler and a mark one eyeball rather than complete dimensions. If you don't need to match a part exactly, get some rough dimensions and then fake it. As long as the form factor is the same and everything else is close no one will know the difference. Best of luck.