参考答案：

已知d₁=28mm、α=20°、m=2.5mm、z₁=2

d_a1=d₁+2m=28mm+2×2.5mm

=28mm+5mm=33mm

d_f1=d1-2.4m=28mm-2.4×2.5mm

=28mm-6mm=22mm

tgγ=P_z/πd₁=z₁πm/πd₁=z₁m/d₁

=2×2.5mm/28mm=0.17857

γ=arctg0.17857=10.12459°

=10°07’29"

s_x=πm/2=3.1415×2.5mm/2

=3.927mm

=3.866mm

蜗杆的齿顶圆直径d_a1是33mm，齿根圆直径d_f1是22mm，导程角γ是10°07’29"，轴向齿厚s_x是3.927mm，法向齿厚是3.866mm。