A helicoid sheet with a generating line at, say, 85 degree to the axis, and a second one with a generating line at, say, {HEAD ~ 75 degree} to {TAIL ~ the axis}, can be attached to the same central cylinder at a small distance from one another, a distance less than the pitch distance 
A helicoid sheet with a generating line at , say , 85 degree to the axis , and a second one with a generating line at , say , { HEAD ~ 75 degree } to { TAIL ~ the axis } , can be attached to the same central cylinder at a small distance from one another , a distance less than the pitch distance
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