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crank it!
- Thread starter joegamma1
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joegamma1 link=topic=2979.msg20905#msg20905 date=1312549039 said:Excellent! I think that is along the lines if what Tony was explaining as well.
Wonder how long it would have taken me to come up with such great ideas had I not asked.
Thanks for sharing guys, helps A LOT
Joe
Hi Joe,
I didn't invent the technique.
In fact, it's probably as old as the lathe itself.
We - ALL of us - stand upon the shoulders of giants.
If we are serious about the art of machining, it's our duty to pass the hard won knowledge on to the newer members of the tribe.
It's the appropriate way to honor the nameless men who DID invent all these things we take for granted.
Good luck with your crank. Let us know what you learn.
PaulS
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Well done, Joe! Now you can try a two cylinder....or three....or seven. 
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There's a lot to be said for proof-of-concept projects. Now when you want to tackle an engine or pump, you'll already have a workable method proven out. Now to think about counterweights to balance it.....
Also, I couldn't see really well, but generous fillets in the corners will add tremendously to the strength of the shaft.
One think I would be concerned about when running between centers on a crank turn is the tailstock pressure. If you use too much pressure, a bend could show up between the arms. I'd be curious to measure the gap while in the machine, and in a free state. That's one reason I prefer to machine them using a chuck on a cathead as I described. You still have to be careful with tailstock pressure, but you can get by with less that way than between centers.
Did you leave stock on the end journals for finishing? In nearly all cases, removing metal in a non-symmetrical way will cause the part to bow or twist. If you finished the ends, chances are that they would no longer run true after uneven material removal in the throw.
Also, I couldn't see really well, but generous fillets in the corners will add tremendously to the strength of the shaft.
One think I would be concerned about when running between centers on a crank turn is the tailstock pressure. If you use too much pressure, a bend could show up between the arms. I'd be curious to measure the gap while in the machine, and in a free state. That's one reason I prefer to machine them using a chuck on a cathead as I described. You still have to be careful with tailstock pressure, but you can get by with less that way than between centers.
Did you leave stock on the end journals for finishing? In nearly all cases, removing metal in a non-symmetrical way will cause the part to bow or twist. If you finished the ends, chances are that they would no longer run true after uneven material removal in the throw.
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Maybe 15yrs ago for my sons science project it will be a one cyl steam engine. I do not remember the
stroke probably around 1/4". Out of 1" brass I figured the stroke I wanted, then center drilled each
end for my stroke. Then I turned a brass center and a taper and stuck it in the head stock (set in about
1/2 way of 4 jaws. Now I have two dead centers with the rod throw 1/4" off center. Then tighten the
4 jaw chuck to that. I took most material out with a cut off tool then finished with a small bull nose
cut. I forgot to mention I first center drilled the stock, so I could turn the main jounals which is last. I
had no dimensions for this crank I just made it because I made the block to fit the crank. I dont know
where this thing went but it did work. all done with a SB9A no special tooling and dont offset the tail
stock, and a 4 jaw chuck is a must or you will have to use a dog. Too bad I didnt take pics. Practice on
some soft junk stock. sam
stroke probably around 1/4". Out of 1" brass I figured the stroke I wanted, then center drilled each
end for my stroke. Then I turned a brass center and a taper and stuck it in the head stock (set in about
1/2 way of 4 jaws. Now I have two dead centers with the rod throw 1/4" off center. Then tighten the
4 jaw chuck to that. I took most material out with a cut off tool then finished with a small bull nose
cut. I forgot to mention I first center drilled the stock, so I could turn the main jounals which is last. I
had no dimensions for this crank I just made it because I made the block to fit the crank. I dont know
where this thing went but it did work. all done with a SB9A no special tooling and dont offset the tail
stock, and a 4 jaw chuck is a must or you will have to use a dog. Too bad I didnt take pics. Practice on
some soft junk stock. sam
I machined a crankshaft for a small gas motor from a piece of rectangular bar stock by drilling center holes in the ends for the center of the crank and for the center of the rod journal on the mill. Then I cut the majority of the offset for the throw out with the band saw. I used a faceplate, a dog and a center on the headstock and machined the rod journal, then I shifted centers and machined the main journals. You may need to put a jack screw between the crank throws to keep from bending the rod journal. Turning that deep is always sort of scary. Good luck.
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