# Turning eccentrics



## Mcgyver (Aug 1, 2008)

Steam engines need them, and if with Stevenson reversing gear you need doubles. Here's what a double eccentric looks like







This particular engine, a stuart triple, needs one in the middle of engine as well as the two ends. As it would be impossible to install in one piece, its made it two halves...a little trickier to make. I included it here because it ends up giving a great cross section of the eccentric. Both of these are about the size of a quarter, 7/8" OD.






One of the tricky parts is that the part must be turned on three separate axis. Its important that each eccentric is the correct distance from the crank section AND that the angular separation of the two eccentrics is reasonably accurate. 

Hilmar referred to an approach published by Tubal Cain. His required levels and protractors to set things up the lathe which always struck me as a bit awkward. There may not be a more revered ME author than Tubal Cain, however I think he mustn't have had a mill at his disposal - if you do here is an alternate method.

In a nut shell, I made an arbor who's endcap accurately fits the bore of the eccentric and has some very small holes spotted exactly on the centres of the crank and the two eccentrics. This way each axis can be positioned in the four jaw.

first, face on both ends and carefully bore the eccentrics to fit the crank

turn the arbor as per pic - be precise. also turn a bolt that also has a section that accurately fits inside the eccentric.

Load the arbor in a vise at the mill, use a V block and accurate square - make sure things are vertical. Indicate the portion of the arbor that goes inside the eccentric.

Doing some math from the plans, figure out the X/Y coordinates of each of the three centres. make a small spot drill mark or something a wiggler can pick up.

the arbor is done











In use, eyeball the alignment of the eccentric and firmly clamp on the arbor with the bolt. Using a wiggler and indicator in the four jaw, in turn, do the lathe work each of the three axis. DO NOT unbolt the arbot until the eccentric is done. Turn at one axis completely, then re indicate to the next and so on.

in the pic there are five spots shown - this is because on some engines the angular spacing the eccentrics is different for different cylinders - only three are used for each eccentric


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## zeusrekning (Aug 1, 2008)

Great write up mcgyver. I'll keep up with this post.


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## Hilmar (Aug 4, 2008)

Hello Mcgyver,
I made this gadget to prick punch in the late.










And also have a Degree Wheel on the back of my lath.





   Her is how I work on my eccentric,
In this case I will need three pieces, so I cut material for that size.
Put in four jaw chuck, and face the end and center drill. 
Line up the prick punch in the center mark
Move the punch over for the distance with the compound. 
Turn the lathe shaft over by the given degrees (let say from 0° + 15°) and mark with the punch. 
Don't touch the compound 
Turn the lath shaft back to the 180° mark and now move  15° and mark again with the prick punch. 
Job done
Now center the offset marks and you proceed with the job.


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## Mcgyver (Aug 4, 2008)

hey that's cool, so you index off the dial on the spindle and offset via crossfeed. have you got to turning them yet?


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## Hilmar (Aug 5, 2008)

The one You see is work in progress.

That should read Move the punch over for the "Throw Distance with the compound."
Hilmar


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## BobWarfield (Aug 9, 2008)

Very cool setup!

Now I'm curious: how would the Cnc world tackle this? We have several pros here. Is there some other approach? Closest alternate I've seen ivolved making a pot chuck, but it seems this app would require multiple chucks. I guess you could circular interpolate on a mill, but this is a lathe job.

Hmmmm


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## wareagle (Nov 9, 2008)

On my Crusader project, I had to turn the rear of the crankshaft which was nothing more than an eccentric. While looking through Maryak's thread on his little diesel, I saw his method of turning his crankshaft and more or less ripped off the idea from his thread and used it for my crank. It worked like a charm. For those that haven't seen the two threads mentioned above, the thread by Maryak is *here *, and the thread for my Crusader is * here*.

Without further delay, the fixture is very quick and easy to make. Attention to detail will affect the accuracy of the part to be made as with any fixture. For the purposes of the crankshaft, I took a piece of 1.500" aluminum bar stock and faced the end off, center drilled and turned the nose down to get a good round surface for getting a reading for the TIR once back into the lathe. The fixture was "keyed" to the chuck jaws and taken to the milling machine (for info on the process of "keying to the three jaw, * this thread *will explain it).

On the milling machine, the fixture was placed in the vise and squared to be vertical. The center was then located and the table was moved in the amount of the offset needed. A hole was drilled through and then reamed to the diameter of the crank's main journal. Opposite of the center line of the fixture, a hole was drilled through for stress relief. The fixture was then remounted at the edge of the vise and then cut through to the relief hole to enable it to firmly clamp the crank. The fixture and crank are seen below.






Once the fixture was made, the crank and fixture were put together and the unit was placed in the lathe. The TIR was checked with an indicator to make sure things went back to where they needed to be. Below is a picture of the process.





_Note: this picture is for demonstration; the part was placed against the fixture to rigidity._

Once the fixture and crank were clamped down, the machining commenced. With this set up, heavy cuts aren't a good idea as things will move in the fixture. Light cuts were taken and the part was successfully finished. This technique can easily be adapted for other shapes and diameters. Just use common sense and patience when machining a part with this method.


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## Brass_Machine (Nov 9, 2008)

This looks a lot easier than what I was planing on doing. I have turned up a face plate that I was going to build a sliding vice on... I got the idea from a few pictures a few other users here had made. I think I am going to try this one instead.

Eric


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## mklotz (Nov 10, 2008)

I'm curious. Why not just use the 4jaw?


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## wareagle (Nov 10, 2008)

Marv, in my case I wanted to protect the polished main journal on the crank shaft. That was the origional intention for going this route. It worked well so I thought I would share. For those that don't have a three jaw this could be a method that comes in handy. It can also save the changing of the chuck. This isn't the best method for all cases, just another way of skinning the cat.


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