Biagio, My router lathe is one of the newer tork craft ones, it doesnt have a place to mount templates, except I can put a screw in to turn cylinders(that will run on one of the rails). I have downloaded the manuals for the original craftsman router lathe, and I completely understand regarding the pantograph effect. Because the following pin is saw twice the distance, the high spots needs to be twice the distance you want it to be. In addition, you cannot mount an existing item (table leg, baseball bat, etc) and you have to create a template for everything.
Now - think outside the box. Think see-saw. if you extend the router carriage to the other side and put the pin/follower at the exact distance your router bit is from the hinge, and your pin will ride at the bottom of the Template or existing item
it doesn't even need to be straight, as long as it is fixed. top left is what the first thinking is, but if the router gets in the way, you can do it like the picture below. on the right is where and how - it doesn't have to be a plate. it just needs to be sturdy
Does the picture make sense?
Hentie, I have an old Vermont American model that I have not used in about 30 years, after I got a lathe. I used to do twists and open spirals on it. From what I remember, one could use a template, but the template follower is at the front of the router carriage.
I don’t see how it could be otherwise. If the follower were in the midline, where would you place the template? The workpiece is in the midline.
If you are worried that the workpiece diameter will be smaller than that of the template, because of the “pantograph” effect of having the follower at the front, you can compensate by adjusting the bit height, or by increasing the template diameter, but the calculation involved in doing so, is beyond me. Trial and error, in my ignorance.
I do remember having some dissatisfaction with the bit not being perpendicular to the workpiece throughout its travel, because the carriage pivots on the rear rail, and the tip of the bit therefore travels through an arc. This was only a major problem when cutting open spirals - the spirals landed up having a “rhomboid” cross-section. For surface decoration (e.g. diamonds cut with a twist and reverse twist) it was not that noticeable.
I had had a vague notion of making a parallelogram-based carriage to try and keep the bit travel perpendicular to the workpiece axis, but the model I have is very basic, with a fixed ratio of turn to travel, so probably not worth it.