Turning a smallish crankshaft.

[Munty Scruntfundle]

Hi folks.

I have a small project coming up where I need to machine a simple 3 journal crankshaft. I'm fine with the measuring and marking out, I need to come up with the final dimensions yet, but I'm thinking we're going to be somewhere near a 35mm cam od, 10 or 11mm journal od and a total length of around 160.

As for machining I think I'll be ok cutting the first outer journal in an offset 4 jaw then swapping ends and cutting the second outer journal in a 5c collet chuck. I'll be leaving the cams pretty chunky so I should be able to chuck one of those for the inner journal.

The problem I'm having is deciding how to remove the material around the inner journal. I have cut off tools, a couple of round style insert holders and a few small boring bars that I think I'll have to use turned around. The inner journal could be anywhere from 20 to 40mm in length with a cut depth of around 25, not a lot of space to play with.

Just wondering if anyone has any advice they could pass on for the inner journal, best practice, bad practice but works, any tools that may make the job easier.

Many thanks.

 

[Spectric]

You don't say what this is going to be for which may help or if it is a steel or wooden item. A picture would give us a better vision of this.

 

[Munty Scruntfundle]

You don't say what this is going to be for which may help or if it is a steel or wooden item. A picture would give us a better vision of this.

Oh yeah, it'll be steel of some kind, possibly a bit of 4140 which I have here, or EN28, possibly boring old mild steel if that's all I have to size. There aren't any pictures, I'm trying to work out all the complexity of the build before starting. It's just a simple 3 journal crank that will be driven at one end by a belt and pulley.

As I say it's the middle journal I'm not sure how to cut. I don't have, nor want to build (if I can help it) a crank jig.

 

[Spectric]

Look at how many two stroke motorcycle crankshafts were built, they were an assembly rather than a single component where steel disc had the offset crankpin and the central shaft supporting the assembly. In your case six disc each one supporting a crankpin, two interconnecting pins and two end shafts and with the 120° offsets. Your crank is working in reverse to this in that you are powering the crank and not pistons powering the crank but it is an easy way to meet your needs.

 

[ChaiLatte]

The inner journal could be anywhere from 20 to 40mm in length with a cut depth of around 25, not a lot of space to play with.

A left hand, a right hand and a neutral tool might assist for bulk material removal. For example, see the SCLC-L, SSDC-N and SCLC-R types from ArcEuro (ARC Indexable Turning Tools - Arc Euro Trade). Buy the deepest (tallest) ones that fit your tool holder and then reduce the width until they are suitable for the gap. The difficulty with the Arc ones for your application is that the larger shank sizes are only available with 09 inserts so the opportunities for width reduction are limited. I think if you look around, maybe Glanze ones are available in 12mm or 16mm shank but with 06 inserts, so you can modify them to be thin but deep. For final sizing a 2mm or 3mm parting blade (again, as deep a section as you can find).

 

[Pilgrim]

If you are visualizing problems with the center journal how are you planning on turning the crank pins?
access for the cutting tool is twice as difficult.

 

[sawtooth-9]

Oh yeah, it'll be steel of some kind, possibly a bit of 4140 which I have here, or EN28, possibly boring old mild steel if that's all I have to size. There aren't any pictures, I'm trying to work out all the complexity of the build before starting. It's just a simple 3 journal crank that will be driven at one end by a belt and pulley.

As I say it's the middle journal I'm not sure how to cut. I don't have, nor want to build (if I can help it) a crank jig.

A sketch of what you want to build would help.
When I turned my first crank, I avoided making a jig - as it was only a "one off"
That was a mistake - it's worth the small amount of time to make a jig.

 

[Fergie 307]

Another issue that might be relevant is that you would normally have a small fillet where the pins meet the webs. This may also have a bearing on your choice of tooling.