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enlarge gear hole

i need to enalarge a hole in a plastic gear from 3mm to 9mm How can i do it?

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It sounds like your hole needs a good reaming. if the plastic is thin enough,say less than 1/8 " you should be able to do it by hand. You would need the right size drill bit and be able to keep it perpendicular to the gear. Use the hole to keep yourself centered and go about half way through on on side then finish it from the other side.do any touch ups with a very fine file and make sure not ot oblong the hole and you should be good to go.

THAT'S WHAT SHE SAID! (Are you proud of me, CtC?)

As a 14 year old boy, I can understand the urge... 

If you have a stand for you drilling, take a drill at 3mm, without starting the drill, put it thru the hole then you know its center to your stand and fixate it. Then change the drill to 9mm and start drilling thru.

Since precision is the second name of every gear I would suggest the same as Jørgen. Precision drill without a stand can be an mission impossible if you are not an experienced driller.

yes precision is essential in a gear. Unfortunately i don't have a stand so i need to find someone in my city that has one and that let me use it.