[83]

Looking more closely, the slider screw has a pitch of approximately 0.545"/rev and a diameter of 0.25", for a helix angle of 34°. This means that less than half of the slider force is actually used to turn the screw, undermining any force gain. That is, the jaw force is less than is applied to the slider due to the screw's inefficiency as over half of the applied force is lost pushing against the screw's supports.

Is that accurate that over half the force is lost? As the helix angle gets lower wouldn't more force be transferred to the screw instead of the supports? I'm no mechanical engineer but my gut says 45 degrees would be the point where forces are balanced between screw and support and as the helix angle decreased (to 34 degrees for example) more than half the force would be applied to the screw.

[snovv_crash]

Consider the friction coefficient between the two parts. If it slid perfectly, ie. both were made of greased teflon, then at 30 degrees half would be in the correct direction - sin30° = 0.5

However adding friction makes it much worse. Unlubricated steel on steel is really bad.