This is my very first instructable and I hope you will enjoy it.

Please any mistakes caused by my bad english. :-)

Some of you will ask me the question, why not just buy a ready to go gear from the shop or ebay for the usual $19,95. You don't need to ask me that, as I have produced these gears as POC for myself to know if it works this way and if I am able to it. Maybe some tim will come and I will need the procedure to make some gears quick and will not be able to buy any replacement parts. Also remember, these parts are much stronger than that you can get in your store.

Step 1: 1. What Do We Need for the Mould Mixture

For the first step we will need any silicone fill, but not the one you use for the bath works.

For the first step I have used "Creato RTV/NV" available almost everywhere, but quite expensive. It is about 30€/kg.

It is simple to use. Just take a smaller sample form, put it on the scale and set tara. Keep a piece of paper between the scale and the form as the Creato components and not easy to be taken off any surface.

Once done, fill the RTV (the white silicon rubber) into the form filling it at about 80%. The wight you can read at your scale is now 96-98% of your mixture. The missing 2-4% will need to be filled by the hardener (NV).

Once you're done having the 100% mould mixture, mix it for about 10 Minutes to be sure having a constantly deployed the hardener.

Step 2: 2. Preparing the Gears to Be Copied

ok. The silicon mixture is ready and now the gears, which are not damaged need to be cleaned ( I have used a usual industrial cleaner). Afterwards a simple electronics contact cleaner "Kontakt 60" can bee used to prevent any resting of silicon an plastic gears. I know there are those very special chemicals, but the $3,99 spray works like a charm. Use it only as much as you can see the gears being wet. than put them with the bigger site to the bottom. That's it!

Now the tricky part. Spray the cleaner on the surface of your mould.

For the smaller gear I've used the "Ferrero Rocher Chocolates - 16Pcs" Box.

For the bigger one a 50 DVD spindle is the best choice, but wasting more silicon! It is up to you.

The tricky part of this step is to keep the silicon at even surface, preventing uneven moulds. This will cause difficulties on creation or resin forms later on.

Step 3: 3. Silicon - Cosmetics After 24-36h

After 3 days I took a look at the forms. It looks like it is better to keep it in a warm place. MY work bench was too cold and I moved them to another room. The result was a finished form after 12h. You can just touch the surface after 24h and it should feel like,... a piece of hard silicon.

Remove the forming gear (i was using the contact 60) and once it is out, remove the silicon form out of the plastic form.

Once done, turn the form upside-down and prepare the surface cutting any faulty fillings. This Step is very important. However, it only works this way as the biggest part has to be at the top. You will want to be able to pull it out afterwards.

Ok, here is my favorite: Use the contact-60 to prepare the surface for filling with resin. ;-)

Step 4: 4. Preparing the Epoxy Resin and Filling the Form

Somewhere, I really do not remember where, but it was mentioned in any blog to use metal powder or smaller parts to make larger resin fillings stronger and quick force resistant.

I did take some of those Metal parts from my work bench and mixed it with the resin.

Oh, yes the resin is the "L" with 2:1 given mix proportion requirement. It needs about 5h to begin to get hard.

You will ask me, why not to take the 3 or 5 minute version. It is very simple to answer those questions. : It would get too hot and break by itself, or get burned or get unstable. This is the right component to create a stable gear.

Once you have a form filled with 2pcs of resin and 1pcs of hardener, put the metal powder into it and begin to mix it together. You don't need to hurry. 5 hours is a long time. I have spent 5 Minutes on mixing all together until the resin did not look like two different fluids being mixed, but like one constant fluid with metal dirt.

After filling about 80% of the form you have to start to put some attention to the surface and look if it is still even.Fill more and more of the resin and look at the leveling. When everything is ok, than put more resin than needed and it will be a little bit less once dried. It should not be more than 1%, but maybe you will be missing it like I did. Try to prevent it as it is simpler to polish the surface, until You have the exact height, than trying to put more resin after 2-3 days and expect the "sandwiched" resin will last like it was a one piece construction .

Step 5: 5. Pulling Out the Gears, Cleaning, Polishing

My favorite KONTAKT-60 has been used to pull out the gears out of silicon preventing any damage in the white mass. The cleaning has been done with the usual industrial cleaner. The polishing was easy. I used all my existing wood tools for that purpose.

Step 6: 6. the Result

the very final and funny step was to compare the original gearbox and the refurbished one. Of course the original one was very quite due to soft plastic. I suppose, the this is the reason for building such devices will need to be tough. My rebuild would last a lot but maybe when getting broken, it would just destroy both new gears directly.

We will see. The test with 18V on both gear boxes in parallel was ok for a 10 Minutes run without any resistance on the wheels. For now it is very stable built and holding the gear by myself did not stop tu make the turns and went forward forcing the turns to slip through my hand easily.

Please let me know if also you have been able to rebuild any gears using this method.