Need help with finding center of pinions.

[Coldwave]

I have a Sherline 5000 MANUAL mill, that I need to know how to find the center of an unbored pinion gear to bore a .198" hole. I thought you would find the edge first using an edge finder, but being that the pinion is round it would seem useless since after finding the edge I would still have to fiddle with the X and Y axis hand wheels to get the cutting bit to hit the exact center of the round pinion. So how do I go about doing this? I know the math way to find the center of circles obviously, but trying to adjust the dials on a mill in the real world is a whole different story.

One other question I have is how do I properly secure this so that it is perfectly level to insure a straight hole?

[christinandavid]

You don't need to be perfectly on the circle centreline to find the centre. Just position the gear so that you can use an edgefinder/DTI to find centre in 1st axis (X), then move to the centre position in X and repeat the procedure to find centreline in 2nd axis (Y). Double-check the 1st axis. Done.

The most accurate way to ensure the gear is sitting perpendicular to the spindle is to machine a fixture plate 'in situ' that will accept the gear and locate the appropriate face/diameter. Clamp over the part/fixture mating faces (ie don't clamp over 'fresh air') to eliminate clamp-induced distortion.

DP

[christinandavid]

If you have the necessary tooling, a small DTI (Verdict T8 or similar) on a clocking-arm (bent piece of bar) is an easier and quicker way to find the centre. Saves the working-out if you don't have a digital read-out.

DP

[JCVineyard]

a coaxial indcater would solve all your problems. on keeping it flat, if you have a machined surface on the gear. If you have alot of these items to build, make a fixture you can mill flat in the machine. then everything should be perfectly level with the spindle.

[christinandavid]

Assuming spindle is square to table

DP

[JCVineyard]

If the head of your machine isn't tramed in, most likely all you will make on the machine is decoy anchors.

[Perfect Circle]

Do you have a combination square set? If so use the "center head"
"crow's foot" or "center finder" whatever you want to call it.
Scribe 2 lines at approx. 90 deg. from each other and poof you have center of the circle. Then just take a center punch and punch a spot put it in the mill with a center drill spotting drill and line up the work to the tool in the spindle.
Its the fastest way but not the most accurate.
Or you can use the dowel method where you put 2 dowels of the same dia. down in the teeth of the gear and indicate that. Its the most accurate way.

Hope this helps.
Good Luck~!

[Coldwave]

I have been using the edge finder method, but now I would like to know how to use a Coaxial indicator to do the same. Can someone on here show a video or step by step pictures with explanation for using this tool to find the center of a circle?

[ErnieD]

Probably the most accurate way to find the center of a pinion would be to take 3 gage pins of the same diameter, place them approximately 120 degrees apart in 3 tooth space's and then tram around the OD of the pins. This will get you the closest to the center line of the pitch circle.

Ernie

[Perfect Circle]

Here ya go.

[nomedia="http://www.youtube.com/watch?v=ZfioLDhBNBQ"]YouTube - Tramming a mill with a coaxial indicator[/nomedia]

Good Luck~!