This is only guessing. But follow me.
You have lathe spindle rotational speed.
You have these variables on the spinning cutter head.
Number of cutters
Diameter of cutter tip
So if the two are the same speed the cutter will not cut all if the diameter of the cutter tip and the rotational speed of the part in the lathe are equal.
As in if both have the same distance covered in feet per minute or meters per second.
If you change the surface speed of the cutting head to 1/2 that of the lathe you will cut on two places. ( I think ) if it is a quarter then 4 places. I think this applies to a single rotating cutter. If you have two rotating cutters then you are again making two cuts for each rotation of the cutter head.
Looking carefully you will see one of the cut parts is done with one cutter and another of the cut parts is done with 2 cutters.
And if I am correct again, there is room for 4 cutters on the rotating cutter block. So there is a synchronised setup that can be accomplished to cut facets in the round stock.
I may be wrong on the indexing of cuts versus speed fractions. As in 1/2 speed may cut on two sizes of the lathes rotation.
Nice explanation mwmkravchenko, works just like a gear hob where there relationship between work and cutter rotation determine number of teeth cut. I had never thought about that phenomenon being used to cut polygons. The surfaces will not be truly flat but that is of no consequence for a bolt head and may actually be advantages for spanner or socket purchase especially when using a flank drive.
I have seen a great design for a lathe hobbing attachment that was driven off the tailstock end of the leadscrew which enabled the gearbox and a couple of gears to set the work/cutter relationship.
One day .........
I once worked at an aerospace company that supplied a specialised spark plug manufacturer - they supported all sorts of classic/ancient plug types - the various sizes of hexagons needed for these old plugs were generated the same way. The machines in use were six-spindle autos.
Fascinating to watch.
mwmkravchenko (Mar 4, 2021)
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