Gallery, Projects and General > How do I??

Hard to hold.....

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AdeV:
I've been asked to do a little boring (or maybe a boring little...) job.... See attached pics for info.

Basically, it's a toothed belt pulley, it'll be used to drive a mechanism. I need to bore the centre hole out to 25mm. Unfortunately, the boss is smaller than that & will disappear, so I can't hold it on that. If I do this in the lathe, it'll have to be held on the outer rims. Couple of issues there: They're really thin, and subject to distortion (it's an aluminium pulley too, so REALLY susceptible to distortion), and the spacing looks like I can't get it to sit on the flats on my chuck, at least one rim will always be at some point on the V-shaped grooved "grips".

My thinking is: Get a couple of bits of plastic, screw them together, bore a hole in the centre to fit the OD of the main pulley surface. Then, somehow, hold the whole thing together & turn the OD round. Split them, put them around the pulley, and in theory I can clamp onto the plastic & have it basically concentric (a few thou runout shouldn't hurt, this isn't a precision mechanism); drill & bore.


...Or, I could put it on the milling machine, and do it that way; I can strap clamp it to the rotary table that's on there now, or just straight onto the bed (well, raised up a bit obvs). and drill/boring head it to size...

Unless there's a crafty trick I'm missing on the lathe?  Let me know!

Cheers!
Ade.


PS: OD is approx 3-1/8", or 78mm for those who speak French.

philf:
Ade,

The flanges are some times easy to remove.

Alternatively, if you want to grip it in a 3 jaw, cut 3 small lengths of square bar that will fit between the flanges and clamp on the OD of the pulley.

Phil.

Muzzerboy:
Get 4 short lengths of round stock with a diameter that is greater than the depth of the flanges and a length just shorter than the inner distance between the flanges. Then tape them onto the outside diameter of the toothed body, with their axes parallel to the pulley bore, ideally 90 degrees apart. Then you can grip them in your 4 jaw chuck and dial them in perfectly while holding them securely without damage.

When setting up, use a DTI to minimise the runout of both the bore and the face.

You could use the same technique with 3 of them and use a 3 jaw chuck but for that you'd need a pulley with teeth that are a multiple of 3. Even so, you are better off with a 4 jaw to minimise any runout.

Brass_Machine:

--- Quote from: philf on December 22, 2020, 02:08:22 PM ---...
Alternatively, if you want to grip it in a 3 jaw, cut 3 small lengths of square bar that will fit between the flanges and clamp on the OD of the pulley.

...

--- End quote ---

I second this. Worked for me. 3 (or 4 if using a 4 jaw) pieces of scrap aluminum that fit between the flanges. I also machined a curve on side that matched the radius of pulley (where the teeth are)

Eric

vintageandclassicrepairs:
Hi Ade
You would have it done on the mill in less time than it takes to set up in the lathe !!

John

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