Microwelding
 

I need to shorten (cut and shut) a driveshaft (dogbone) and would like to weld it.

Tried sleeving and soldering and it didn't last.

If you have any idea how to weld small bits like that solidly, I need your input. Memories form a distant past revolve around a car battery and a carbon rod electrode, but can't remember how it worked.

Are they aluminium or steel? if they're aluminium then you can solder the two parts together.
Alternatively, dont most manufactures make different length drive shafts(Shorter)??

Aluminium is notoriusly hard to solder because of the thin oxide film on it which forms instantly in contact with air (i.e. instantly after you cleaned it - and it's difficult to clean to begin with).

But they're steel.

And yes, I am aware of the various lengths available but this is in another league altogether. The shortest similar shaft (ECS) is made for Tamyia (or other) minis. This is waaay shorter than that.

I found a place though where they can weld it for me, so tomorrow might be able to post some pictures.

If you can get a well fitting sleeve, have you considered using a high strength epoxy? Just an idea (golf club shafts are epoxied into the head, and they generally hold up very well). The sleeve just needs to be long enough to increase the surface area enough to hold. All the stresses should be in shear, so in theory, this could work.

Depending how important this is and how much you are willing to spend, your best bet would be to use 2 shafts and step cut them then solder/glue to make one. More surface area will hold much better than a surface area equal to its diameter. Crude example pictured below.

http://oi45.tinypic.com/34ish07.jpg

All good ideas. Trying to cut a half like that when the whole length of the shaft is below 10mm is a different challenge. I considered putting slots in both ends and inserting a blade-like piece in both to give it more torsional rigidity and surface area before sleeving and soldering, but again, we're talking 3mm diameter shafts here. How small a slot can one cut at accurate angles and constant width in such a small shaft?

I thought about the epoxy/glue idea, but it is exactly the twisting stress they don't cope with very well. The surface is so small I don't think it would hold for long. Which is perhaps why my soldering didn't last a lot.

The step wouldn't have to be that large. 3-5mm would be plenty. A good acid flux and silver solder would help a bunch too.

Epoxies, and glues in general, are the strongest in shear, and having the two ends of your shaft inserted into a sleeve puts the bond in shear. However, if you do have a small diameter shaft, as you said, the surface area does get small, and it also puts a high moment on the glue bond (you'd want a thicker shaft to move the glue line away form the neutral axis).

Here's another idea. How about threading the ends of the shaft, then threading them into the sleeve, epoxied into place? That would greatly increase the surface area, and provide a mechanical joint as well.

All good ideas, but keep in mind the size of this thing. The "stubs" I am left with are about 3mm long (between the end spheric portion of the driveshaft). Not much to play with really.

I didn't think of threading, that would have worked but I am not sure the driveshaft material is soft enough to cut thread into.

Like I said, tomorrow they'll be welded properly with a jeweller's microwelder which does a fine job of such stuff. I'll post some pictures.