# Turning a one piece crankshaft



## CMS

With some time off this upcoming holiday, I would like to try my hand at turning a one piece crank. I'd like to try one from shaft stock as well as flat stock. So what kind of tips, tricks, and pointers can you pro's out there lend my way?


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## cfellows

Is this going to be a single cylinder crank or multiple cylinder?

Chuck


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## CMS

A single cylinder to start with and work my way to a twin. I've got two projects going on and the single's five piece crank is off a bit, so all the better reason to start with it. My twin project, the crank turned out nicely to be in nine pieces. So when I master the single, I'll shot for the double.


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## Diy89

The only way i know, i read in an old Army manual. 
Using bar stock, scribe a centerline on both ends. Spot drill the center, and the offset (crank throw) on both ends. Using centers and a drive-dog, set up on the offset and turn the stock for your rod bearings. Then move to the center and cut the mains. Here is where cut your web thickness too.
I'll try and find the link to that manual, it explains it better. Mind you i have not had a bunch of success. Also you have to start with a bar big enough to cover your crank throw. I think this is where i go wrong, having a "want it done right now" and cranking the handles for deep cuts. :

Hope its ok to post, here is the link. Page 62/63 it starts. Also shows the simular technique as Blackvette's link.

http://www.metalwebnews.com/machine-tools/ch7.pdf


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## black85vette

Go check this thread:

http://www.homemodelenginemachinist.com/index.php?topic=5060.135

Go to page 9 and start there. Nice step by step by Maryak on turning a crank.


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## black85vette

Then go over to here:

http://www.homemodelenginemachinist.com/index.php?topic=5234.30

Go to page 3 and Brian does a nice job of explaining how it did it following Cfellow's instructions.

Between those two posts you should get an idea of how its done. I am sure there are other really good posts but I just happen to remember those two from when I did my first crank.


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## CMS

Thanks for the links.


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## rklopp

Here is a description of the fixture and procedure I used for making two-throw cranks for six modified Upshur twins I am building. It is a marked departure from the well-known technique of making offset center holes in each end of the stock. The only times center support was used was for turning the main journals and shafts, and the inner end was always held and driven using a collet, not a headstock center and dog. I rarely ever turn between two centers.

1. Using conventional chucking techniques, I turned up eight slugs (two extras) like the proverbial Monty Python dinosaur - skinny at one end, fat in the middle, and skinny at the other end. The skinny ends were left oversized for finish turning to main journal and shaft sizes once the throws were complete. One skinny end was temporarily threaded for a nut to pull it into the fixture. The root diameter of the thread was bigger than the finish diameter of the shaft.

2. I cut a notch in the corner of each slug in the mill. The notch is a close fit to a dowel pin in the face of the fixture, and is for transferring the driving torque and allowing repeatable removal and replacement of the cranks-to-be.

3. I built a fixture based on a cheap Asian D1-3 chuck backplate to hold each slug for turning the throws. A nut in a counterbore in the back pulls the slug hard against the face of the fixture, making a very stiff setup. Stiffness is critical for making the turning fast and easy. The fixture can be offset left and right for each throw, plus it has a center position for facing and boring the fixture to accept the slugs. Dowels between the back of the aluminum cross-bar and iron back-plate give good locational precision and accuracy for the offsets. Obviously, I have to have decent control of the nascent shaft ODs that fit in the fixture.






4. The throw furthest from the fixture was turned first, relying on the main body of the slug to provide stiffness. 











Then, the inner throw was turned.






These are the cranks before profiling the webs or finishing of the main journals and shafts. I made two extras.






5. The cranks were transferred back to the mill for profiling of the webs.

6. The cranks were transferred back to the lathe for finishing the main journals and shafts to size, turning the tapered seats for the flywheels (one of my many deviations from Upshur's plans), and left-hand-threading of the stubs for the flywheel nuts. The turning had to be gentle, because cutting the throws ruined the stiffness. Sorry, I don't have pictures of the finished cranks yet.

I used 1144 (LaSalle Stress-Proof) to make the cranks. The fixture materials consist of cast iron for the back plate, 6061 aluminum for the cross bar, and a steel cylinder shrunk-fit into the aluminum.


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## tel

Now _that's_ an impressive bit of work!


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## Diy89

Very nice cranks!


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## Maryak

rklopp,

Great job on the cranks. :bow: :bow:

Best Regards
Bob


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## bearcar1

Say RK, that is a very interesting fixture. Could you put up some drawings of it and some more pictures with explanations for us old slower learners. It would be a big help.

Thanks

BC1
Jim


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## rklopp

OK, more photos, but no drawings. I believe I did some CAD, but it's not readily accessible from this computer.

1. The fixture "exploded." You can see the pair of dowels and three pairs of dowel holes that provide left, right, and no offset positions.






2. One rough crank I saved, held as if ready to insert into the fixture. In actual use, I used a long extension socket wrench to reach through the headstock to attach and remove the nut. That way, I did not have to undo the fixture from the lathe spindle nose when swapping cranks.






3. Counterbore on the back of the fixture for the crank retaining nut.






4. Rough and finished (but for some deburring) cranks. You can see the dowel notch in the rough crank, which was machined away when forming the webs on the finished crank.






5. Finished crank, except for some deburring. This one happens to have a right-hand thread. I did this one first, then recognized that a left-hand thread would be self-tightening in my application, so made the remaining six with left-hand threads. I single-pointed the threads and made the nuts myself, using a left-hand tap I bought. The threads are 1/4-32 for better scale looks.


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## vlmarshall

Neat photos! I wish they were bigger.


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## rklopp

Vernon  said:
			
		

> Neat photos! I wish they were bigger.



Bigger? Holy moly, they're plenty big in Firefox on my Mac laptop. What details are you trying to see? Any bigger, and you'll start seeing burrs and tool marks, and think my work is crappier than it is. 

Anyway, if anyone wants a particular picture enlarged, let me know, and I will try to accommodate.


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## bearcar1

Pretty slick setup RK, I get the big picture now, thank you. I'm not too keen on the one end threaded routine but I need to digest ore of this jigs potential. A very nice setup.

BC1
Jim


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## rklopp

bearcar1  said:
			
		

> I'm not too keen on the one end threaded routine but I need to digest ore of this jigs potential.
> 
> BC1
> Jim



You're right, you have to like cutting temporary threads and then turning them into swarf. Fortunately, I'm mental and really like single-point threading, and I'm pretty fast at it. It's a lot easier with the lathes I have compared to something like a 7x12, AA109, or Sherline. I could have tried turning the final main shaft, journal diameters, and threads when I machined the original slugs, and made the fixture to fit the final sizes accordingly, but I was afraid the steel would move around on me as I cut the throws and webs. The 1144 material is supposed to be stress relieved to minimize residual stresses, but it's not perfect.

I am fortunate enough to have a Colchester Chipmaster 10 x 20, which I used to make the cranks. For mills, I have a Deckel FP2NC and an Aciera F-2. I've since added another lathe: a Monarch 10EE, which means I am badly spoiled. You should judge the quality of my work in light of the equipment I have. You folks who make masterpieces on things like Sherlines and worn out Atlases should be very proud.


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## bob ward

rklopp, that is a well thought out jig. I like the way the work is solidly mounted and that the crank throws are accurately indexed off the jig. You eliminate the inaccuracies that can creep in with scribing and punching.

I guess also that with extra holes in the back plate, or different back plates, the idea could be extended for use with 120° and 90° throws.


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## CMS

Rklopp, that's one sweet holding jig you've got there. And as posted before, ANY further info that you may be able to share would be great. Think some of my engine projects are going to be put on hold because of other projects that I need to build so that I can learn other processes.


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## doc-zeus

I'd like to add my positive comments, along with the others here! 

What would it take to convince you to make drawings available, or at least pictures of the individual parts? 

Also, your method of calculating how far you set the device over for a given engine stroke?

Since I'm not the sharpest tool in the drawer, I'm not grasping fully how this is constructed!


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## ariz

what a neat fixture you have done, compliments :bow: and thanks for sharing it



			
				rklopp  said:
			
		

> I am fortunate enough to have a Colchester Chipmaster 10 x 20, which I used to make the cranks. For mills, I have a Deckel FP2NC and an Aciera F-2. I've since added another lathe: a Monarch 10EE, which means I am badly spoiled. You should judge the quality of my work in light of the equipment I have. You folks who make masterpieces on things like Sherlines and worn out Atlases should be very proud.



compliments for your honesty too, not many people do remember the difference in machinery that come into account when you compare similar works


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## Paul_C

Not sure if this will work. http://s207.photobucket.com/home/Paul_C_XJ/index I left the ends with the centers until finish machining. The journals were 5/8 and with the thickness of the webs I didn't need a filler in the connecting rod fillet. I did the rough machining of the connecting rod journal in a Bridgeport manually turning it with a dividing head.


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