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View Full Version : Turning a crankshaft..Help!



laddy
04-19-2010, 07:33 PM
Hey,
I have to turn a single journey crankshaft for a steam engine. Description is: Think of a "U" with arms extended out from each side of the top of the U and I have to turn the bottom of the U.

I made a fixture to off set the tail stock and figured I would use the 4 jaw chuck to offset the arm but....oops I misconstrued something because that ain't going to work.

I need to spin the bottom of the "U on center so I can reduce its diameter but the arms are out 3.5 inches on each side.

How do I mount it to do it? I have not done this before. I have a 9" South Bend A.

Thanks to all in advance for the help! Fred

KiddZimaHater
04-19-2010, 07:44 PM
Can the bottom of the "U" be mounted between centers since it is only a single crankshaft?
http://img504.imageshack.us/img504/9707/centers.jpg

laddy
04-19-2010, 07:48 PM
I guess it could but that is 3.5 inches in each direction and (as far as I know) impossible to mount a steady to keep the wobble down. If that is the word I will do it. It seemed to me there should be a more controlled way. Thanks for the help! You were fast Too. Fred

winchman
04-19-2010, 07:49 PM
Here's another way:
http://chestofbooks.com/home-improvement/woodworking/Lathe-Operation/Use-Of-Combination-Lathe-Dogs.html

http://chestofbooks.com/home-improvement/woodworking/Lathe-Operation/images/Fig-340-How-to-Use-Combination-Lathe-Dogs-for-Turning-Up.jpg

bob ward
04-19-2010, 08:12 PM
http://www.homemodelenginemachinist.com/index.php?topic=7427.0

Good info at reply #7

Al Messer
04-19-2010, 08:27 PM
http://www.homemodelenginemachinist.com/index.php?topic=7427.0

Good info at reply #7


WHEN I TURNED THE CRANKSHAFT FOR MY STUART 10-V I MADE UP A COUPLE OF BLOCKS FROM SQUARE STOCK, MARKING ONE END AND CENTER DRILLING. AT THE OPPOSITE END, I DRILLED A THRU HOLE AT THE PROPER OFF SET DISTANCE AND AT RIGHT ANGLES TO THIS, DRILLED AND TAPPED A HOLE FOR A SET SCREW TO KEEP THINGS FROM TWISTING. CHECK THE PARALLELISM BY RESTING THE BLOCKS ON THE LATHE BED OR OTHER FLAT SURFACE. TIGHTEN SET SCREWS AND PROCEED TO TURN BETWEEN CENTERS, DON'T GET IN ANY SORT OF A HURRY---TAKE YOUR TIME.

Dr Stan
04-19-2010, 08:30 PM
Use counterweights to help keep it balanced while turning.

KiddZimaHater
04-20-2010, 01:07 AM
Those crankshaft lathe dogs are the BEES KNEES !!!!
I was trying to envision some sort of offset, setup, between centers gizmo.
Looks like an old timer already did the thinking 100 years ago.
Nice.

laddy
04-20-2010, 08:32 PM
Thanks to all for the help. I think I have all my fixtures ready and am looking for time to mount them on the lathe and try it out for swing. I appreciate the help from every one. You guys never fail me! Fred

Mcgyver
04-20-2010, 08:38 PM
laddy, generally turn they crank first. I then make a very well fitting spacer to fit in the U and duct tape around so it stays put. This will hugely increase rigidity why you turn the balance of the shaft. Also, to reach in the crank you need a lot of overhang on the tool - get some bits that are rectangular, even parting tools, so they are more rigid than just small square tools and grind left and right hand versions with a nice curve the where the webs meet the crank is not a sharp corner.

ormachine
04-20-2010, 11:44 PM
Your best bet i think would be to have an automotive machine shop that has a crank grinder do it.
I am sure that wasn't the answer you were looking for, but it is the best way to get it done.
Ron

Ian B
04-21-2010, 02:08 AM
Some good stuff to read here:
http://modelenginenews.org/techniques/crankshafts.html

If you go for the method of fitting blocks to the ends with centre holes in line with the crankpin, the biggest danger is that one of the blocks rotates on the shaft and the whole thing twists - a spot of weld on the outer end may not be a bad idea.

Ian

JCHannum
04-21-2010, 08:32 AM
This is me turning a throw on a crankshaft. I do the shaft first and the throw second. This is using a cutoff tool to do the journal. With a long or limber shaft, spacers between the offset blocks and the cheeks of the throw to prevent flexing are a good idea.

http://i320.photobucket.com/albums/nn351/jchannum/P1000402.jpg

Richard Wilson
04-21-2010, 08:53 AM
This is me turning a throw on a crankshaft. I do the shaft first and the throw second. This is using a cutoff tool to do the journal. With a long or limber shaft, spacers between the offset blocks and the cheeks of the throw to prevent flexing are a good idea.

http://i320.photobucket.com/albums/nn351/jchannum/P1000402.jpg
Thats how I do it too, except that I move the blocks as far down the shaft as I can to improve the stiffness. Also, I tend to rough out the shaft first, then do the throw, and finally finish turn the shaft in case doing the throw has caused any distortion. Packing between blocks and the cheeks when you are doing the throw, and between the cheeks when you are doing the shaft helps as well, but as an earlier contributor suggested, tape the packing in place so it doesnt fly out. A light push fit for the packing is about right, too tight and it can cause distortion itself.

Richard