# Assistance in finding exact center of physical gear

1. May 1, 2012

### jordanm

Hey, I am new to this forum hoping that I can get some help with a project. I also hope to be accepted as a new member, hopefully learning from everyone here.

I am sorry to ask a simple question that may be easy. I am learning and experimenting as a hobby, I do not have access to super accurate milling machines, and I understand I will have to have some tolerance. I know I can order a whole new gear from ponoko.com, but I want to work with what I have.

I am twenty years old, playing around in my garage with a welder, and a chop saw, etc... I also have a gear that looks like...

(the gear does not have the holes on the edge or the same number of teeth, it also is not the same size. However, and unfortunately, the gear has the giant gaping hole in the center)

The problem that I am having is that I am finding it pretty difficult to connect another (larger) gear with a chain, and have the two gears perfectly center. The two gears not being centered absolutely perfectly on their shafts causes, well, you know....

I have built a jig (before buying the chop saw), and I got the newly welded shaft to be about .02in off from center, as far as I could accurately measure.

I was just wondering on some of the other methods to attach a shaft to the gear that is perfectly centered horizontally and vertically to the gear. I have searched around the internet and did not find really anything.

Jordan

EDIT: I can do all of the math on paper, drawing the circle with a compass, etc, but it does not come out into the real product.

Last edited: May 1, 2012
2. May 1, 2012

### Danger

Hi, Jordanm;
Welcome to PF.
Your image isn't Mac-compatible, so I can't see it, but I have a pretty good idea of what you mean because you expressed it well in words.
If I understand your problem correctly, the solution is one that has been in use for centuries. You just need to obtain bushings whose outer diameters matches the holes in the gears, and whose inner diameters match the size of the shaft. The accuracy is provided by the manufacturer, so all that you have to do is specify the dimensions.

3. May 1, 2012

### jordanm

Hey,

Thank you!

The picture just shows how the gear does not have the perfect center already milled into it, as in, it was part of a cassette, which is pretty much a circle with teeth.

http://img502.imageshack.us/img502/4136/gearwheel.jpg [Broken]

I did think of something along those lines. In my particular situation, the shaft that I am trying to attach to the gear is already rotating inside bearings. The solution you suggested would mean that the shaft is fixed and does not rotate with the gear, that the bearing rotates.

I have been trying to attach a flat, straight piece of steel to span across the center (diameter) (or a circular sheet metal, which did not work all too well either) and finding the center of a gear, to attach to the shaft.

Thank you,
Jordan

Last edited by a moderator: May 6, 2017
4. May 1, 2012

### Danger

Okay... the second link worked, but I knew what you meant when you mentioned a cassette. They are, though, in fact fairly precise in their manufacture. Those injection moulds cost thousands of dollars, so they don't make them with sloppy tolerances. The "slop' is in the way that they are integrated into the cassette itself, and is for a good reason.
Anyhow... the only other thing that immediately occurs to me is to use a pantograph to trace the outline of the "gear" at full scale and then duplicate it at a much smaller scale to serve as the axle opening. If it were in a strictly visual environment, as I use in Illustrator or Inkscape, I would just duplicate the outer diameter at 10%, and if necessary use the "align" function to centre them on the same axis. Usually, however, a duplicate at any scale is centred equally to the original, so alignment is unnecessary.

Last edited: May 1, 2012