Machining a crankshaft

[benjames]

I was hoping if someone could elaborate on how a length of metal is held in the lathe when machining a crankshaft.

I understand that I drill the centres to turn down the main centre shaft.

I understand that I drill a centre hole off-centre (5/16" from centre in my case) so that I can hold the piece with the live centre at one end, but I am unsure how the piece is held at the chuck end?

I have included a couple of pictures from a person on HMEM called wesm1957 which was the closest I got to understanding how it works.

He says he held it in a 4 jaw chuck but that would still hold it central wouldn't it? How would I hold something in a 4 jaw chuck to do eccentric turning?

I have attached a picture below which is the only way I can think of doing it. My idea is to hold the piece in a larger piece off centre with a pin holding it in place. Then when it is finished, face off the extra length that holds the pin.

If anyone could shed light on an easier way of doing it I would appreciate it. I have googled and looked at many sites about eccentric turning and they mainly say put a 3 jaw chuck in a 4 jaw chuck.

Any other ideas would be appreciated.

Thanks,

Ben.

 

[Swifty]

Hi Ben, 4 jaw chucks have independent jaws that are moved individually, so you can hold the piece offset. The other option would be to put an offset centre in both ends, however you would have to align them correctly to each other. Then you can put a piece of scrap in your 3 jaw chuck and turn a 60deg taper on it to match the centre drill, with a centre in each end, you will need a carrier, or lathe dog to our US members, which you fasten on to the workpiece at the chuck end and hook onto a jaw. This will turn the workpiece around with the centre.

Google "lathe dog" and you will see what I mean.

Paul.

 

[Philjoe5]

Ben,
A four jaw chuck works fine for making an eccentric. Consider the jaws are at 3, 6, 9, and 12 o'clock. Center the work in the usual way with a dial indicator against the shaft so TIR is at or near zero. Let's say your eccentric is going to have an offset of 0.250" from the center of the main shaft. Keeping the jaws at 6 and 12 o'clock fixed, loosen the 3 o'clock jaw while tightening the one at 9 o'clock. Keep doing that until the difference in DI readings between those 2 positions is 0.500" (one jaw is - 0.250", the other is + 0.250" from your new centerline). You've now established a second centerline coincident with the lathe spindle that's offset from the original centerline of the shaft, which is no longer coincident with the lathe spindle centerline. Drill a new center on the end of the shaft and use a tailstock center in the new hole. You're good to go.

Cheers,
Phil

 

[stevehuckss396]

I'm with swifty when doing mine. I like to drill my centers on both ends and then turn between centers. It's much easier to get setup and get the crank arms indexed properly.

 

[lennardhme]

If you have a boring head for your tailtock, just make a center to fit it & adjust the offset to suit.

 

[sssfox]

If you have a boring head for your tailtock, just make a center to fit it & adjust the offset to suit.

I've heard of a boring head being used to turn a taper, but never an offset turning, like a crankshaft.

How would that work?

 

[Steve J]

OPen up the four jaw for clearence and put a dead center in the taper in the headstock. put some pressure on the crank blank with the tailstock to hold it in place and gently clamp the headstock end with the jaws of the four jaw to drive it. use one center drill for the center line of the crank and another for the offset of the rod journal. not sure what taper your headstock has but i believe standard practice with a morse taper is one size larger than the tailstock.

 

[benjames]

Thanks for your responses guys. The lathe dog idea sounds pretty good and it seems they are pretty cheap aswell. I will invest in one of those.

Thanks again.

Ben.

 

[Lakc]

Here is a pic of a crankshaft, the offset holes are held between two dead centers and the dog drives off a bolt where the chuck usually attaches.

 

[lennardhme]

"I've heard of a boring head being used to turn a taper, but never an offset turning, like a crankshaft."
sssfox,
The boring head is fixed in the tailstock so it adjusts out horizontally.
Make a center to fit into the middle hole instead of the boring bar.
Adjust out the head the same distance as the 4 jaw in the headstock - this should be easy as the adjusting screw should be graduated.

 

[benjames]

Here is a pic of a crankshaft, the offset holes are held between two dead centers and the dog drives off a bolt where the chuck usually attaches.
https://imageshack.com/i/5ms3012759j

Lakc:
It's interesting that you have a faceplate to attach your lathe dog to. The one that I found when I googled was turned by the chuck jaws themselves.

 

[rhankey]

"I've heard of a boring head being used to turn a taper, but never an offset turning, like a crankshaft."
sssfox,
The boring head is fixed in the tailstock so it adjusts out horizontally.
Make a center to fit into the middle hole instead of the boring bar.
Adjust out the head the same distance as the 4 jaw in the headstock - this should be easy as the adjusting screw should be graduated.

Unless the boring head rotates with the work piece, what you are describing would only turn a taper.

 

[deverett]

Originally Posted by lennardhme
"I've heard of a boring head being used to turn a taper, but never an offset turning, like a crankshaft."
sssfox,
The boring head is fixed in the tailstock so it adjusts out horizontally.
Make a center to fit into the middle hole instead of the boring bar.
Adjust out the head the same distance as the 4 jaw in the headstock - this should be easy as the adjusting screw should be graduated.

Unless the boring head rotates with the work piece, what you are describing would only turn a taper.

Using the boring head - with the slide parallel to the bed - and a 60 degree taper in the offset toolhlder is a work around for taper turning so that the tailstock does not have to be offset, which can be a PITA to reset.

Dave
The Emerald Isle

 

[Lakc]

Lakc:
It's interesting that you have a faceplate to attach your lathe dog to. The one that I found when I googled was turned by the chuck jaws themselves.

On my particular lathe, the chuck needs to be removed to fit the 3MT center in the spindle. Kind of typical for import lathes.
Regular faceplates can sometimes have holes for dog "tails", or at least holes to install a bolt to drive any type of dog you have.

 

[TimTaylor]

On my particular lathe, the chuck needs to be removed to fit the 3MT center in the spindle. Kind of typical for import lathes.
Regular faceplates can sometimes have holes for dog "tails", or at least holes to install a bolt to drive any type of dog you have.

Easy way around this is to clamp a short piece of round stock in the chuck and turn a temp center. You then center drill both ends of the stock to be machined with the desired offsets, mount it between centers using the pair of holes for the portion to be machined. A lathe dog is used to drive the stock as shown in the picture.