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Unread 17-01-2017, 13:55
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Re: Machining precision required to make custom gearboxes

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Originally Posted by JesseK View Post
We do have a reamer, but have had mixed results when using it on oblong holes. Using a micrometer on a sample after under-sizing still shows a position margin of error of around +/- 0.001 in a dimension.
This makes sense. A reamer will not change the center position of a hole, it will follow the existing hole. If a roughed hole is not on center position wise you'd need to use a boring head or plunge with a correct size end mill to fix that. Neither of those solutions will help you if your machine is positioning incorrectly to the center location though.
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Unread 17-01-2017, 13:56
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Re: Machining precision required to make custom gearboxes

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Originally Posted by FrankJ View Post
If you don't have a CNC or a manual mill, you can do this manually. With the typical FRC gears and a simple gear box. you can lay out and drill holes using a machinist scale, scribe, prick punch, a drill press, and a little OCCD. Clamp & drill the side plates together. This takes a lot of care, but is very doable.
I agree it is doable. On my team we made a stainless steel template for the toughbox mini. Which allows us to clamp it to the material drill pilot holes out and slap a gearbox on anything in minutes. So i guess our process is this.

1. If you are going to be repeating patterns get your hands on some stainless and make a decent template either by drafting the dimensions onto the metal or by tracing the pattern off of something else and finding the centers. only drill it out to 1/8" for pilot holes

2. Clamp the template to the piece you want a gearbox on and drill out your pilot holes.

3. Bring your holes into size for the smaller holes (<=1/4") we just use a drill bit but step your way up to keep it straight. for the 7/8" and 1-1/8" holes just buy a regular hole saw. the normal hole saw will be over size so you have to put it in a drill and run it on a grinder or belt sander for just a second or two. this will bring the size of the saw under size. pull the 1/4" drill bit out of it and just shove a 1/4" rod in there this will help to keep the saw straight. Bring your pilot hole to 1/4" then used the smooth shaft on the hole saw to bring it to size. for the holes less than 7/8" and greater than 1/4" I highly recommend every team owns a rotabroach kit specifically this one https://www.mcmaster.com/#4093a81/=15ybt9g

4. The bearing holes are undersize so open them up with a deburring tool. another highly recommended if not necessary to pass inspection tool https://www.mcmaster.com/#4253a16/=15ybuy8
Just take your time with it, try to keep the cut perpendicular to the surface of the material and keep checking the bearing until you can press it in snuggly

It isn't fancy but it works. I think it is hugely advantageous to be able to integrate a gearbox into just about any surface. Then you can start doing weird stuff like perimeter mount gearboxes to save space in your bot. There are a lot of COTS gearboxes but it is awfully nice to feel like you can crank out exactly what you need in an hour or two.

this is a test set up on a piece of scrap;

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