Advice on Enlarging Encoder Aluminum Boss

[Al_The_Man]

I have a quantity of rotary glass encoder discs that have a turned aluminum centre boss that I have to enlarge from 12mm (.4724) to 0.500. They are fairly delicate to handle.
I checked out reamers and the .500 straight reamers start at .482, so I was wondering if going with an adjustable up to the .482 and then using a .500 straight type to finish. Is this an accepted way to go or does anyone see another way?
Al

[Ken_Shea]

Al just thinking out loud here, are they such that you could make a fixture to hold them and then chuck that fixture in the lathe, then bore?

[Al_The_Man]

Ken, I was thinking of doing something like that, its just that the wall of the boss is not very thick and there is not much to attach to, I guess if I could get a collet to hold it. Keep in mind my machining skills are not up there yet.
Al

[HuFlungDung]

Al,

I like Ken's suggestion, too, because reamers can often give you a surprise about the size you end up with.

Perhaps you could get a large 5C blank collet (like with a 2" or 3" diameter face) and bore it to hold your disks by their perimeter. Because of the large contact area, chucking pressure does not need to be extreme. The back side of the disk gets supported by the counterbore in the collet, too. You could sandwich some protective thin plastic around and behind the disk before inserting it into the collet. Think Stretch and Seal.

[ToyMaker]

Al:
CNCZone member "Swede" has a photo journal on his 5bears site that does exactly what you describe.
Start here http://www.5bears.com/cnc.htm for his whole cnc mill adventure.
Or here http://www.5bears.com/cnc22.htm for how he did his encoder discs.

robotic regards,

Tom
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[Al_The_Man]

Thanks Guys, I think I have enough to proceed with a try.
Al