SHORTENED VOLKSWAGEN DRIVE SHAFTS
Originally these drive shafts were made from high tensile steel and heat treated to handle the torsional (twisting) forces being fed through them. The customer was fitting a turbo diesel lump from a Golf into the back of a type 2 vw van similar to pictured below, and needed the original shafts shortening.
Once the drive shafts were cut in half with a grinder, the job of machining them down to size (length) was undertaken. As the steel shaft is High Tensile steel that has been heat treated to give it the correct properties it is quite hard as well, this means we have to slow the speed of the lathe down (approx 540 rpm). Now we can either use coolant when cutting or take smaller cuts with the tool and not use coolant. Either way it is not good advice to start without coolant and then turn it on whilst the tool is hot (during machining) as this does the cutting tip no good at all (fast change in temperature of the tip) and may dull or blunt the tip. The drive shaft halves were machined to length by cutting along the shaft to reduce the diameter as opposed to “facing” off the excess material, when cutting harder material this can sometimes cause the tool to bounce off the hard material and you end up with a radius on the end and not a flat faced off end.
Once machined to length I used a centre drill to pilot dead centre of the shaft in preparation for a drilled hole to accept a machined pin. The idea of this is to make sure that both ends of the shaft to be welded are pressed onto the centre pin in theory this means that each end is “centred” accurately with the other.
Final machining of a chamfer on both ends allows easy access to the “root” (bottom of the joint near the pin) for a quality “TIG” root run.
When Machining is complete, shafts can be pressed onto machined centre pin and then set in vee blocks on a flat surface to ensure alignment; as we do not want any “run out” end to end once the shafts have been welded together.
Notice in the images above I have machined a doubler boss to add strength and material to the joint just as a precaution. The aim of this is to add additional strength to the joint area. I have stated previously that drive shafts are made from High Tensile Steel and then Heat Treated to “Temper” the material to reduce the brittleness and improve the resistance to torsional forces going through the shaft. Once cut and welded I have changed the properties of the material due to heat input from welding, this may well have changed the “tensile” and “torsional” resistance properties of the material; so this is now an unknown. In order to offset this I bulk up the material cross section around the joint as a “belt and braces” fail safe. Notice the heat affected zone (HAZ) areas around the joint (where the blueing of the material occurs). This you can see changes colour the further away from the actual heat input point (weld), showing that heat travels through the material and if looked at closely you would see a range of colours indicating that the heat reduces as it travels through the material away from the weld. This is typically known as thermal conduction. A point to note if welding aluminium you would not see the material change colour due to heat input.
Once a TIG root run has been laid in the bottom of the “vee” a capping run is completed (using a MIG welder) over to fill the joint. This was then sanded off smooth and level with a flap disc (left image) to allow for the strengthening doubler boss to slide back over the original joint. Before welding the boss into place the shaft was “eyed” up for run out simply by rolling over a smooth surface whilst watching for the ends “lifting” and “dipping” Then the boss is slid in place (central over the original joint) the boss was then tacked in place and a nice “hot pass” fillet was laid around both ends of the boss (again using a MIG welder).
A couple of points to note:- welding the root is undertaken in a clockwise direction, heat input causes expansion, cooling will contract the material more than the original expansion. To reduce the amount of times the shaft heats up and cools down, immediately after “root” run a “capping” run is laid in the joint in the opposite direction to offset any contractile forces created by the “root run”. Again the sanding process adds heat to the welded area and hence keeps the area warm. Immediately then the boss is welded in place and the whole job is allowed to cool down. Completing the job like this means that less shrinking and expansion occurs and reduces the effects of the coinciding forces created.
Finally the shafts were then put in a lathe and the welds skimmed down to ensure a nice even weight of weld is left around each end joint, thus hopefully reducing any vibrations due to excess weight of weld causing an imbalance in the shaft.
Once complete the shafts were left to “air cool”, If I had cooled them quickly in water this may have led to cracking or brittleness in the joint. After these shafts will be painted and fitted to the custom engine swap.
I hope this has been a useful blog for some one and if I can help in any way with your future custom modifying needs please contact us at www.flashcustoms.co.uk
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