I read a bit up on the various ideas behind it on the Internet, and it seems as there are a two models normally employed, both tapered along the length of the protruding part of the steel:
One version has a cylindrical shape of the tapered part all the way.
The other version has got a not completely rounded shape of the tapered part. (popular called eccentric)
I guess that the eccentric model can either be of an elliptic shape, or it could just be a circular shape with part of the perimeter moved inwards.
I am going to try to make a set of drawbore pins based on the last idea. I can't really see any advantages of a pure elliptical shape over the flattened circular shape, but there is a lot more work involved in making a tapered elliptic piece of steel compared to the flattened model.
After a bit of testing to try our some ideas I had regarding how to do it, I ended up with this way of getting the wanted result:
First a piece of steel is mounted as usual in the 3 jaw chuck, and the far end is supported by the live center.
I adjust the compound rest to a 1 degree taper, meaning that the including taper will be 2 degrees.
I then take some passes only using the compound rest, to make a tapered section. I stop when the thin end is approximately half the diameter of the steel rod.
I then have to move the main apron to continue the taper. That is because the travel distance of the compound rest is only 2.75". Once I have completed the taper to its final length, I stop.
The next step is to remove the old hole for the live center, so I can make a new one.
The new hole is made eccentric by adding a distance piece under one of the jaws in the chuck. In this case the distance piece is an old washer.
I leave the washer in place and make sure to orient the steel bar in the same way, and again use the chuck and the live center.
The eccentric mounting of the live center and the washer between the steel bar and the jaw now causes the entire piece to be wobbling in the lathe. Or more correctly it is eccentric mounted with a throw equal to the thickness of the washer.
I bring the turning tool into contact with the piece and repeat the process of making a small taper. I removed 0.6 mm (3/128") while making this second taper.
The result is a nice and shallow taper and if the piece is rotated there is a slight difference of the aforementioned 3/128".
As far as I have understood the idea of this is that you insert the flattened part into the drawbored holes, and then you twist the tool to tighten up the joint.
I am going to try to harden the drawbore pins before making some octagonal handles for them.
So far I have made two sets, 4 - 8 mm (5/32" - 5/16") and 5 - 10 mm(25/128" - 25/64")
Eccentric drawbore pins
Turning the taper
Getting ready for making the pin eccentric.
Note the washer between the left jaw and the workpiece.
This should show that there is a flattened ace on the pin.