# need help machining eccentric sheave



## Ironman2011 (Feb 7, 2011)

I have partly machined the eccentric sheave for the Stuart Twin Launch steam engine and I am stumped as how to proceed. Enclosed a couple of pics. I machined the first part with an offset 4 jaw chuck. Thanks for any info.


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## Bill Gruby (Feb 7, 2011)

From what I can see you have only machined the smaller diameter. If so, chuck again on the small diameter using the same 4 jawed chuck. Offset it to cut the back essentric first. Once that one is done you will offset on the same small diameter to cut the other eccentric. I have not made a part like this before so I am going by what comes to mind here to make the task as simple as possible. I am sure there are others that will guide you also. Hope I have helped in a small way.

 "Bill Gruby"


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## jpeter (Feb 7, 2011)

Keep in mind I've never made a thing like that but if I was to I'd use the same technique I use for crankshafts. I'd center drill a piece of stock with centers for the mains and two more in each end for the eccentrics, that would be 3 centers in each end. I'd mount it in the lathe between centers and have at it. I'd turn the eccentrics first cuz when I turned the mains I'd lose my eccentric centers. Its a challenging thing to make. Have fun.


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## Ironman2011 (Feb 7, 2011)

Thanks for the input, I guess what confused me is the 20 degrees and the 9/64th throw. So do I offset it by 9/64ths? How do I offset it by 20 degrees ? Hope I explained it well enough.


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## Bill Gruby (Feb 7, 2011)

Offset by the Throw. 

 "Bill Gruby"


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## Dan Rowe (Feb 7, 2011)

Ironman2011  said:
			
		

> Thanks for the input, I guess what confused me is the 20 degrees and the 9/64th throw. So do I offset it by 9/64ths? How do I offset it by 20 degrees ? Hope I explained it well enough.



Well I have machined a few of these for Shay locomotives. To make the valve gear work properly both numbers are important. The center point of each eccentric is on the 20 degree line offset by 9/64" 

The center points can be layed out on the far eccentric and the boss chucked in the 4 jaw chuck simply adjust the chuck untill it runs true with the center mark for the eccentric that is being turned.

Dan


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## Ken I (Feb 7, 2011)

Chuck the bar in a 4 jaw - turn the smaller diameter and hole.

Using one pair of jaws and a dial gauge offset the (smaller) distance for the 20° offset (Throw x sin 20°) then using the other pair of jaws offset the larger distance (Throw x Cos 20°) - do one set of large diameters then using the same pair of jaws repeat by going twice the amount in the opposite direction for the other large diameter.

ie you are offsetting by the Opposite (first) and then the Adjacent - with the throw being the Hypotenuse.

You can double check this second offset as your total indicator reading will be twice the throw = 0.2812"

For 9/64 (assuming this is the radial throw) Sin 20° =0.0481"
9/64 Cos 20° = 0.13214

Hope this helps.

Ken


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