Turning a crankshaft with temporary added-on centers

[Mainer]

Has there been any past discussion on turning a crankshaft that requires temporarily attaching auxiliary centers? I did a search and didn't find anything, but I could easily have missed it.

I'm working on Westbury's "Unicorn" and a cast crankshaft comes as part of the kit...with no supplied centers for turning the journal. I know the general theory of making a couple of blocks with center holes at the proper offset and attaching them to the ends of the main crankshaft...but I've never done it and would like to get a description from somebody who has actually done it.

(After I try it and ruin the crankshaft casting I'll make a built-up crankshaft, but I thought I may as well try using the casting first. ;D)

 

[te_gui]

Kozo Hirokoa's book "Building the Shay" locomotive does a great job describing the process. I don't know where my copy is right now or I could send you the pertinant pages. Lemme look around or maybe someone else will speak up.

Brian

 

[Maryak]

Mainer,

First you turn the main journals between centres.

2nd you mark and centre drill the big ends on the outside of the throw.

3rd you make a lockable clamp which fits the main journal and has a centre drill on each side giving the offset.

4th a piece of BMS with a 60 deg cone on each end clamped between your temporary clamp centre and the ones on the outside of the big end. Line these up parallel with the main journal shaft - make sure they are at right angles and tight and machine the big end. TAKE LIGHT CUTS.

Hope this helps ??? ???

Best Regards
Bob

 

[RonGinger]

Here is a 'what can go wrong' photo

I think this was a Stuart Turner #4 crank. I thought I was doing it right, I had the blocks up as close to the throw as my centers would permit. I was taking nice gentle cut, with a long overhang square end tool.

I wound up making a built up crank.

Good luck!

 

[kf2qd]

I have only used that method for grinding cranks - would only try that for turning if I could hold one of the blocks in a four jaw chuck, as that would add lots of rigidity to the setup.

 

[Hilmar]

Mainer, go to "Engines From Castings" -"A Stuart Turner Triple Expansion Engine « 1 2 3 4 », page3 and look.
Hilmar

 

[Mainer]

Thanks folks. I will ponder the situation a while and see what develops.

I looked at the crank casting a while yesterday afternoon and the idea of a built-up crank is seeming better all the time...but I may yet give it a try.

 

[gbritnell]

Mainer, go to the following link. It has a step by step of the machining for my Holt crankshaft. Yes you can build a fabricated crank but that doesn't teach you how to make one from a forging or from billet. Personally there is a great deal of satisfaction in machining a crank "the old fashioned way".
gbritnell
http://bbs.homeshopmachinist.net/showthread.php?t=26132&highlight=holt+crankshaft

 

[Mainer]

I've made a crankshaft from the solid before -- that's certainly an option. In this case the crankshaft has balance weights, so to do it completely from a solid blank I'd have to start with a 2" dia. bar and turn it down to 3/8" dia. for the journals, which seems a bit much. I could use attached weights...but then it's starting to be a built-up job, so why not just build the whole thing.

Thanks for the link -- good job!

 

[T70MkIII]

Thanks for posting the link to your crankshaft, gbritnell.

Very informative.

 

[Mainer]

Well, I decided to do a built-up crank instead of dealing with the casting. It worked pretty well (see photo), though in the long run I'm not sure it was any easier.

I assembled the parts with Loctite 609, using a Delrin spacer to get the distance between the webs correct. After the Loctite had set up I pulled out the spacer and drilled and pinned the joints, again using 609 on the pins.