arrow a 1 in fig4 in such a manner therefore the nip dimension defined between the opposed rolls 10 1 and 12 1 may be altered to thereby alter the compressive pressure exerted on the board preform within that nip i e so as to alter the thickness dimension t 1 to which the board preform bp is compressed by means of the rolls 10 1 and 12 1 each forward end of the arms 16 and thus the forward roll 10 1 is supported dependently by means of a respective threaded support shaft 24 coupled operatively to a worm gear 26 at the terminal end of a drive shaft 28 extending from motor 30 operation of the motor 30 will concurrently rotate each of the drive shafts 28 and the worm gears 26 which rotation will in turn threadably drive the support shafts 24 rectilinearly towards and away from the roll 12 1 arrow a 2 in fig3 depending on the direction of rotational movement in such a manner the forward end of the support arms 16 to which the shafts 24 are attached and hence the roll 10 1 carried thereby may be pivoted about the transverse axis established by the rear axle 20.