Lets Make a Crankshaft

[rdean]

My last engine build was a 90 degree V Twin engine and after some run time I noticed the flywheel was not running as true as it had. The crankshaft was coming apart so a new one was in order. I had made the crank from a 6 piece design glued up pressed together with steel pins. I had used this design before but not with this large a flywheel. I decided to make a 1 piece crankshaft for the added strength and I made some other updates while I had it apart.


All is working fine now the the engine is becoming quite reliable but that brought me to this subject. A friend of mine, upon seeing the new crank asked me to explain to him just how I had made it and show the setups for machining. I thought maybe someone else would also like to see the steps that I use so here it is.

This Crankshaft specs
1 1/2" throw
Two cylinder
One piece steel
And since it would be visible and not inside a crankcase it must have with counter weights and look good.

The chunk of steel must be at least 2.25 in diameter and I wanted it 7" long so I made it 7 1/2" long.
The shaft I had was somewhat larger than I needed so I cut it off and reduced the diameter.
That is a hefty chunk there.

I center drilled the shaft in the lathe and moved it to the milling machine where I milled a groove the entire length about 0.020 deep and 1/2" wide.

This groove was to be my index for the next operation which was to center drill the offsets for the two throws.
The shaft was secured in the vise vertically and using the original center hole I was able to move over 0.750 each direction and drill the new holes. The shaft was then turned over and the the same thing was done to the other end. The index groove insured that all the holes were in the correct place.
Please excuse the photo as this was the only one I had taken to show that process and it was out of focus.

Back in the vise and horizontal I started cutting away bits that I didn't need. The more material that can be removed now will make the job easier later. Here again the index slot kept everything in alignment.


I wanted a 0.625 wide by 1" section in the center of the crank and the rod throws to be 0.560 wide (That was the size of the end mill I had) and the webbing to be 0.250 thick.


This shows milling off the corners after indexing again.


Flip the shaft around and do the same thing on the other end and remove some more material in the center section.
The shaft is now ready for the lathe.


More to come later.

Thanks for looking
Ray

 

[Chris Hamel]

How did you locate the rod journals and other features. Did you use the dros?

 

[rdean]

Yes all measurements were taken using the DROs and I also took a cut at the outside edge of the outer webs. This gave me easy to see stopping point for later operations.

Not visible in this photo is a morse taper 5 dead center in the spindle bore. It is long enough to extend past the 3 jaw chuck so no reason to remove the chuck. Turning will now be between centers aligning each end with the corresponding center hole. I am using a 3/16" parting tool mounted up side down in a home made holder. The lathe is run in reverse at slow speed and the chips just fall down out of the way. I really like this style of parting.


This shows the tool a little better.


After the rod journal is to size the piece is flipped over to do the second journal. Always do the cutting to the journal that is closest to the drive end so the twisting forces will be reduced. That is why the shaft is flipped over.

We now have both journals completed and are done with turning between centers for now.


One end in the 3 jaw chuck and the other in a live center. This is the time that some people recommend to attach blocks or plugs to the rod journal gaps to stiffen up the shaft but I will be very conservative in my cuts and see how it goes.

This shows how I have reduced one end of the shaft so it will fit inside my 3 jaw chuck.


The advantage to this is the chuck jaws can now grip on the center web which eliminates any twisting force on the inside throw. In the past I made a similar crankshaft with a 1" throw and I was able to grip the shaft at the far web and so eliminated all twisting forces.


So now just a matter of reducing the shaft ends down to a little over 5/8" diameter on both ends.


A 3 jaw chuck will not be true enough for these final operations so back to turning between centers. First the center land is cut down to 1" diameter and the webs cleaned up.


Next the shaft end is turned to 0.625 and then a land for the bearing and then the final shaft size of 0.500 and flipped over for the other end.


Al the lathe work done and running very true and straight.


The crank shaft is ready to be used just like it is but I want to make some counter weight cuts first so back in the mill.
The webs are cut off at an appropriate pleasing angle to the center line of the crank. And a second cut was taken at a different angle to smooth the corner.
You may ask how do you make all the cuts the same on both sides and both ends and how do you figure the angles. Well with the shaft loose in the vise I placed a 1/2" square short block on top of each of the vise jaws. I held a straight edge on top of the blocks and rotated the crankshaft so that the crank throw touched the bottom of the straight end and tightened the vise. For the second angle I used 1/4" blocks instead of 1/2" and worked from the center of the crankshaft to the outside. Not very precise or scientific but they look good.
Sorry I didn't get a picture of those setups.


After some cleanup and bearing install here is my crankshaft.




Thanks for looking
Ray

 

[Chris Hamel]

Great job. Now all you have to do is build an engine around it.

 

[rdean]

Great job. Now all you have to do is build an engine around it.

Exactly what the problem is.
I am open to suggestions.

Ray

 

[rwm]

At least you know how many cylinders.
Robert

 

[Pointon_Machine]

...and you know it's 180 degrees between throws.

 

[Charles scozzari]

Hi, Really great work. Where are you in New jersey? Sorry now I see, Lawrenceville.