The taper to be measured is in blue color.
Let the yellow and red tubes contact with the taper to get A dimension (distance between two faces).
tanα = ((D2-D1)/(A+L2-L1))2
α: haft taper angle
D2 and L2: inner diameter and length of the yellow tube
D1 and L1: inner diameter and length of the red tube
If L2 = L1 then
tanα = ((D2-D1)/A)2
In case of go-no and go control the red area on the scale of the red tube should be used. It is determined according to the tolerance of taper angle α.
This measurement is faster but less precise than other known methods (using sine bar)