View Full Version : Turning a Crankshaft

12-24-2005, 10:59 AM
I have had a chance to spend some time on the John Deere model, and decided to machine the crankshaft casting.

The castings with this kit are of very good quality, and machine well. If I have any complaint, it is that they are too good, and were made very close to final dimensions, leaving little material for machining. This can be a problem if there is any shifting of the mold halves.

The rough casting was cleaned up by grinding and filing the warts, pimples and parting lines. The ends were ground flat and center was scribed and drilled on the lathe.

I use a center button in the chuck, and clean it up to 60* when setting up a critical job;

I use a nut and bolt between the cheeks of the crank throws to take the thrust of the center and keep the crank from springing. On this set up, I machined the faces of the throws and rough machined the crank journals. While doing this, I also fine tuned the tailstock to remove any taper. Once set, I do not move the tailstock until all machining is complete.

Using a tool with a radius, I finish machined the journals to final dimension.

12-24-2005, 11:04 AM
Cool, just what I need.

Ok, some dumb noob questions for ya.

How in world do you set that lathe up to turn the rod journal?

I don't get it, I can't see how it can be done since the rod bearing surface isn't concentric to the rest of the shaft that has to be mounted in the lathe centers.

Edumacate me please!

12-24-2005, 11:10 AM
Moving on to machining the throw, I milled the keyways on the journals for the flywheels.

I then made two cheeks to accomodate the centers when turning the throw. These were clamped to the crank and centers were layed out and drilled.

The crank was set up back on the new centers and the inside of the cheeks finish machined with a boring bar;

With a setup like this, care must be taken as there is a lot if iron flying around in several different orbits. Personal safety and interference with fixed tooling requires that you take a close look at all possible interference.

The throw was turned with a cutoff tool with a radius on the tip. This crank has a 1-1/2" throw, so reaching that far in results in a less then rigid set up. I used very light infeeds and passes. I removed and resharpened the tool for the final passes.

12-24-2005, 11:12 AM
ahhh HA!

12-24-2005, 11:56 AM
good job, that last photo is during the nail biting stage, when there is no choice but using the flimsiest of set ups

Dan Craig
12-24-2005, 12:50 PM
this is very informative. great info here. thanks for posting!

12-24-2005, 01:04 PM
Very nice work on the crank. I'm always afraid of the tool catching and having a catistrophic event when turning the journel.

Is the crank C.I.? looks like it might be from the chips.

Please keep us posted with your prigress on the JD. It's a very nice and interesting model.


12-24-2005, 01:16 PM
It's all in the setup, it seems. Someone should collect all these cool setup ideas for a "tips" work.

I find that old (1930's and older) metalworking books are a wealth of such ideas.

Nice Work,


12-24-2005, 02:16 PM
JC, it looks like in the last pictures when you are turning the rod journal that a bolt between the sides of the rod journal out to the end where the centers are would increase the rigidity of the crank. Do you find that it isn't necessary on this like when you turned the mains? Just wondering because it looks like the crank would still want to spring in that last setup.

12-24-2005, 03:02 PM
Thanks for the kind words.

Yes, it is cast iron.

As far as further support when turning the crank throw, some castings will include the cheeks with support bars cast in. These are cut away after machining the throw.

The usual procedure is to machine the throw first, then the mains. Since this casting had so little material to play with, I reversed the sequence. There was probably some flex, but it is a very rigid crank, the mains and throw are 5/8" diameter, and I felt that I could get away without the extra reinforcement. If you are of a different persuasion, it would not be too difficult to add.

12-24-2005, 04:13 PM
Thanks for the info JC. I wasn't sure how big the diameters were on that crank. Looks real good too by the way. I am building Rudy's Pioneer engine and I had to tack weld two gussets between the counterweights to be able to turn the mains because any tailstock pressure at all was flexing the whole crank. The rod journal was only 1/4in. though.

12-24-2005, 06:33 PM
Thanks for a very good and clear explanation of a workflow for this operations

Very inspiring

Mogens Kilde

12-24-2005, 06:56 PM
Thanks JC. I have never machined a crank journal but that is how I imagined it being done. I really like the center button in the chuck, why didn't I think of that?

12-25-2005, 10:16 AM
Nice job,did the prints call for a radius in the journal shoulders?

12-25-2005, 06:42 PM
I am working at a slight disadvantage, as the drawings are for a slightly different version of this engine and many of the dimensions are different. The drawing does not call for a radius at the shoulders, but I prefer to leave a nice fillet on the crank journals to prevent stress risers.

The center button is not my idea. It is handy, but it is a good idea to true it before a fussy job.

12-26-2005, 02:21 PM
Nice job on the crank. I have seen some turning cheeks with jack screws to provide support along the center line when turning the rod journal. It takes the place of the cast-in support bars. Can't wait to see the finished engine.


[This message has been edited by PolskiFran (edited 12-26-2005).]