# Crankshaft turning fixture



## Lakc (Oct 2, 2011)

I have a pice of 6" round x 3.5" mystery steel (probably 1018), and need a fixture to turn a crankshaft that I cannot turn between centers. I drew this up, but am looking for some feedback or other ideas before I start, as its a heck of a lot of metal to remove.

In use, the stock is clamped in the V notch of the sled, which is on dovetails, and adjustable with a 3/8 screw running the width of the tool. Not shown are the screw and the gib for the dovetail locking, never designed a gib before, anything to watch out for?

The backside has a bolt pattern and recess for a standard 4" minilathe mandrel, as well as 1.75-8 threads for my dividing head.

I doubt the idea is new or unique, so any similarity to anyones favorite tool or setup is probably because I read about it somewhere before. Let me hear your comments, tell me if I am missing anything, before I dull a bunch of tools with all this cutting.


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## bearcar1 (Oct 2, 2011)

Hi Jeff, that is a great looking set of drawings you have done there but I am lost. (believe me it doesn't take much)  I do not understand the point really ??? It seems that the vee block clamp riding on dovetails is a clever idea and all but aren't you still only clamping the C'shaft to the 'back' side of the thru hole? If that is the case, would not a simple, off-set, reamed hole through the backplate that is the size of the shaft with a radial locking screw do the same thing? Something in the back of my mind tells me I am not seeing the entire picture here. I'd like to hear/see more of what you have in mind as I am certain it will be pretty slick. :bow: 

BC1
Jim


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## MachineTom (Oct 2, 2011)

Jeff, nice idea, but a couple of concerns. It appears that the work would be clamped between the vee block and a SETscrew which would be used to set the offset for the throws. In my opinion what would not provide enough rigidity to prevent chatter, as well as enough friction to prevent the work from slipping under machining forces. Remember the stroke of the crank will add to the working diameter of the piece. RE: a crank with a main and crankpin D of .250 and a stroke of .5 would be the same as turning a 1" D piece of stock, held by a .250 spud with a single setscrew, and to add difficulty the first few cuts will be interupted. Not the job of a setcrew IMO.


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## MachineTom (Oct 2, 2011)

Having been around crankshaft grinders the design is to have a slide plate mounted on the spindle, attached to the slide is a lathe chuck, first set for throw distance, which can be indexed to the number of throws, as well as adjusted to get the first throw centered. 

So a slide plate on the spindle, a backplate the matches a chuck and has index holes drilled to match at 90, 120, 180, 240, 270, 0 should cover all common cranks.


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## Lakc (Oct 3, 2011)

bearcar1  said:
			
		

> If that is the case, would not a simple, off-set, reamed hole through the backplate that is the size of the shaft with a radial locking screw do the same thing?


Yes, it would be the same thing, however, it would be dedicating a very large chunk of steel to only one single use. 


			
				MachineTom  said:
			
		

> Jeff, nice idea, but a couple of concerns. It appears that the work would be clamped between the vee block and a SETscrew which would be used to set the offset for the throws. In my opinion what would not provide enough rigidity to prevent chatter, as well as enough friction to prevent the work from slipping under machining forces.


I was intending on a gib lock, but wasnt happy with a standard polished gib squeeze. Thanks to that comment, I gave it some additional thought and think I found a way to "push" the dovetails together ala a toolpost lock. You are quite correct the screw wouldn't hold, it was only for convenience, and is actually removable if the full throat is needed. 


			
				MachineTom  said:
			
		

> Having been around crankshaft grinders the design is to have a slide plate mounted on the spindle, attached to the slide is a lathe chuck, first set for throw distance, which can be indexed to the number of throws, as well as adjusted to get the first throw centered.
> 
> So a slide plate on the spindle, a backplate the matches a chuck and has index holes drilled to match at 90, 120, 180, 240, 270, 0 should cover all common cranks.



Which brings to mind a good point, should I go through all this trouble, or just build a crankshaft grinder? At some point, its gonna be time to shoot the enginner and enter production.


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## bearcar1 (Oct 3, 2011)

Hi Jeff, yes, I see now what you have in mind. Sorry, I can not help in any way but if you do decide to build a crank grinder for model sizes, I would be quite interested. Best of luck

Cheers

BC1
Jim


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## Lakc (Oct 3, 2011)

It has been done before, I am sure.


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## bob ward (Oct 4, 2011)

I've always thought this jig by rklopp is pretty neat.

http://www.homemodelenginemachinist.com/index.php?topic=7427.msg78002#msg78002


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## Jasonb (Oct 15, 2011)

This simple set up any help

http://picasaweb.google.com/Tug423/RW29DieselEngine#5453059957719035042

There are a couple more shots in teh parent album

http://picasaweb.google.com/Tug423/RW29DieselEngine#

The build of these engines is being covered in Model Engineer at the moment

J


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## Lakc (Oct 15, 2011)

Thanks Bob, and J. 

I had intended on a clamping arrangement similar to the one J uses. The more I dug around in reference to Tom's comments, the more I realized I was creating half of an imperfect crankshaft grinding machine. This project was at the balooning stage, where the more work I put into it I began to realize how imperfect the outcome would be. The work involved wouldnt justify the increase in capacity or ease of manufacture. It was a valuable exercise, however, and taught me a lot. Thanks all!

I went back to the old drudgery of kerchunk kerchunk for this last crankshaft. The next project will likely start off with a real crankshaft grinder. Ill have some pics up of the last crankshaft soon in my boxxer thread.


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