el "tuercas " lo tengo yo, es el programita que me hizo mi hija
la formula que empleo es esta, donde H corresponde al nº de dientes del engranje del cabezal y L al engranaje final de la lira, las otras ruedas son las que van en la lira (cuatro en total) y el orden en que van colocadas

estas son las relaciones que me salen para el conjunto de ruedas que pongo a continuacion, tienes que escoger las que mas se aproximen al paso que deseas construir y que mejor te encajen en la lira
un saludo, espero que te sirva de algo, ya diras
engranjes para el que esta calculado este paso
20 25 30 45 50 60 60 65 70 75 80
85
H z1 z2 z3 z4 L Pm
40 25.0 20.0 80.0 45.0 65.0 4.102564102564102
40 25.0 20.0 80.0 65.0 45.0 4.102564102564102
40 25.0 45.0 80.0 20.0 65.0 4.102564102564102
40 25.0 45.0 80.0 65.0 20.0 4.102564102564102
40 25.0 65.0 80.0 20.0 45.0 4.102564102564102
40 25.0 65.0 80.0 45.0 20.0 4.102564102564102
40 45.0 20.0 85.0 70.0 80.0 4.098214285714286
40 45.0 20.0 85.0 80.0 70.0 4.098214285714285
40 45.0 70.0 85.0 20.0 80.0 4.098214285714285
40 45.0 70.0 85.0 80.0 20.0 4.098214285714285
40 45.0 80.0 85.0 20.0 70.0 4.098214285714285
40 45.0 80.0 85.0 70.0 20.0 4.098214285714286
40 50.0 30.0 60.0 45.0 65.0 4.102564102564102
40 50.0 30.0 60.0 65.0 45.0 4.102564102564102
40 50.0 30.0 60.0 45.0 65.0 4.102564102564102
40 50.0 30.0 60.0 65.0 45.0 4.102564102564102
40 50.0 30.0 80.0 60.0 65.0 4.102564102564102
40 50.0 30.0 80.0 60.0 65.0 4.102564102564102
40 50.0 30.0 80.0 65.0 60.0 4.102564102564102
40 50.0 30.0 80.0 65.0 60.0 4.102564102564102
40 50.0 45.0 60.0 30.0 65.0 4.102564102564102
40 50.0 45.0 60.0 65.0 30.0 4.102564102564102
40 50.0 45.0 60.0 30.0 65.0 4.102564102564102
40 50.0 45.0 60.0 65.0 30.0 4.102564102564102
40 50.0 60.0 80.0 30.0 65.0 4.102564102564102
40 50.0 60.0 80.0 65.0 30.0 4.102564102564102
40 50.0 60.0 80.0 30.0 65.0 4.102564102564102
40 50.0 60.0 80.0 65.0 30.0 4.102564102564102
40 50.0 65.0 60.0 30.0 45.0 4.102564102564102
40 50.0 65.0 60.0 45.0 30.0 4.102564102564102
40 50.0 65.0 60.0 30.0 45.0 4.102564102564102
40 50.0 65.0 60.0 45.0 30.0 4.102564102564102
40 50.0 65.0 80.0 30.0 60.0 4.102564102564102
40 50.0 65.0 80.0 30.0 60.0 4.102564102564102
40 50.0 65.0 80.0 60.0 30.0 4.102564102564102
40 50.0 65.0 80.0 60.0 30.0 4.102564102564102
40 60.0 30.0 50.0 45.0 65.0 4.102564102564102
40 60.0 30.0 50.0 65.0 45.0 4.102564102564102
40 60.0 45.0 50.0 30.0 65.0 4.102564102564102
40 60.0 45.0 50.0 65.0 30.0 4.102564102564102
40 60.0 65.0 50.0 30.0 45.0 4.102564102564102
40 60.0 65.0 50.0 45.0 30.0 4.102564102564102
40 60.0 30.0 50.0 45.0 65.0 4.102564102564102
40 60.0 30.0 50.0 65.0 45.0 4.102564102564102
40 60.0 45.0 50.0 30.0 65.0 4.102564102564102
40 60.0 45.0 50.0 65.0 30.0 4.102564102564102
40 60.0 65.0 50.0 30.0 45.0 4.102564102564102
40 60.0 65.0 50.0 45.0 30.0 4.102564102564102
40 65.0 45.0 85.0 60.0 60.0 4.092592592592592
40 65.0 45.0 85.0 60.0 60.0 4.092592592592592
40 65.0 60.0 85.0 45.0 60.0 4.092592592592593
40 65.0 60.0 85.0 60.0 45.0 4.092592592592592
40 65.0 60.0 85.0 45.0 60.0 4.092592592592593
40 65.0 60.0 85.0 60.0 45.0 4.092592592592592
40 75.0 45.0 80.0 60.0 65.0 4.102564102564102
40 75.0 45.0 80.0 60.0 65.0 4.102564102564102
40 75.0 45.0 80.0 65.0 60.0 4.102564102564102
40 75.0 45.0 80.0 65.0 60.0 4.102564102564102
40 75.0 60.0 80.0 45.0 65.0 4.102564102564102
40 75.0 60.0 80.0 65.0 45.0 4.102564102564102
40 75.0 60.0 80.0 45.0 65.0 4.102564102564102
40 75.0 60.0 80.0 65.0 45.0 4.102564102564102
40 75.0 65.0 80.0 45.0 60.0 4.102564102564102
40 75.0 65.0 80.0 45.0 60.0 4.102564102564102
40 75.0 65.0 80.0 60.0 45.0 4.102564102564102
40 75.0 65.0 80.0 60.0 45.0 4.102564102564102
40 80.0 20.0 25.0 45.0 65.0 4.102564102564102
40 80.0 20.0 25.0 65.0 45.0 4.102564102564102
40 80.0 30.0 50.0 60.0 65.0 4.102564102564102
40 80.0 30.0 50.0 60.0 65.0 4.102564102564102
40 80.0 30.0 50.0 65.0 60.0 4.102564102564102
40 80.0 30.0 50.0 65.0 60.0 4.102564102564102
40 80.0 45.0 25.0 20.0 65.0 4.102564102564102
40 80.0 45.0 25.0 65.0 20.0 4.102564102564102
40 80.0 45.0 75.0 60.0 65.0 4.102564102564102
40 80.0 45.0 75.0 60.0 65.0 4.102564102564102
40 80.0 45.0 75.0 65.0 60.0 4.102564102564102
40 80.0 45.0 75.0 65.0 60.0 4.102564102564102
40 80.0 60.0 50.0 30.0 65.0 4.102564102564102
40 80.0 60.0 50.0 65.0 30.0 4.102564102564102
40 80.0 60.0 75.0 45.0 65.0 4.102564102564102
40 80.0 60.0 75.0 65.0 45.0 4.102564102564102
40 80.0 60.0 50.0 30.0 65.0 4.102564102564102
40 80.0 60.0 50.0 65.0 30.0 4.102564102564102
40 80.0 60.0 75.0 45.0 65.0 4.102564102564102
40 80.0 60.0 75.0 65.0 45.0 4.102564102564102
40 80.0 65.0 25.0 20.0 45.0 4.102564102564102
40 80.0 65.0 25.0 45.0 20.0 4.102564102564102
40 80.0 65.0 50.0 30.0 60.0 4.102564102564102
40 80.0 65.0 50.0 30.0 60.0 4.102564102564102
40 80.0 65.0 50.0 60.0 30.0 4.102564102564102
40 80.0 65.0 50.0 60.0 30.0 4.102564102564102
40 80.0 65.0 75.0 45.0 60.0 4.102564102564102
40 80.0 65.0 75.0 45.0 60.0 4.102564102564102
40 80.0 65.0 75.0 60.0 45.0 4.102564102564102
40 80.0 65.0 75.0 60.0 45.0 4.102564102564102
40 85.0 20.0 45.0 70.0 80.0 4.098214285714285
40 85.0 20.0 45.0 80.0 70.0 4.098214285714286
40 85.0 45.0 65.0 60.0 60.0 4.092592592592592
40 85.0 45.0 65.0 60.0 60.0 4.092592592592592
40 85.0 60.0 65.0 45.0 60.0 4.092592592592592
40 85.0 60.0 65.0 60.0 45.0 4.092592592592592
40 85.0 60.0 65.0 45.0 60.0 4.092592592592592
40 85.0 60.0 65.0 60.0 45.0 4.092592592592592
40 85.0 70.0 45.0 20.0 80.0 4.098214285714285
40 85.0 70.0 45.0 80.0 20.0 4.098214285714285
40 85.0 80.0 45.0 20.0 70.0 4.098214285714286
40 85.0 80.0 45.0 70.0 20.0 4.098214285714285