This is a very bad idea in practise.
If I understand you correctly you are cutting a helical groove in a bar with
a saw. If so you are in danger of bending or breaking the blade on both the
forward and backstroke of the saw, especially if you don't stop the axial
and rotational feed of the bar during the stroke of the saw. At best the saw
blade will deflect during the cut, making the groove an irregular shape. On
the other hand, if you stop the axial and rotational feed of the bar during
the saw stroke, you will not get a perfectly circular groove.
I think you can model the theoretical groove section with a helical 'vee'
section sweep:- \_/ . The width of the top of the groove should equal the
horizontal distance between where the blade enters the circumference of the
bar and where the blade leaves the circumference of the bar, with the saw at
the depth of the cut. This will depend on the speed of the axial and
rotational feed. The dimension of the bottom of the groove will be
calculated the same way, but at the circumference of the bottom of the
If the saw is set at a certain angle to the axis of the bar, the helical
section could roughly be rectangular.
I must say unless you have a relatively strong saw blade that will not
deflect under the sideways forces during the cut, and the groove quality
need not be good, you should not attempt this method.