I've noticed a lot of posts in threads all over these forums voicing questions and concerns about synchronising two drive screws on a single machine axis. Thought I'd post this just to hear reactions.
There's a relatively simple mechanical way to synchronise screws. The pic below illustrates the basic idea.
Obviously the large circles are sprockets, the small ones are idlers which can be adjusted up and down.
By adjusting them you change the total belt/chain length between the drive and driven spocket on that side of the run, and loosen or tighten the position of the other one to keep belt/chain tension. This effectively moves the tooth position of the slave side of the setup. You only need a total adjustment of one tooth if you use common sense in assembly and setup.
An elegant application would be to mount your drive motor dead center at your table end and use two of these arrangements to drive both leadscrews. The advantage is you would eliminate any need for flex couplings. Just mount your drive sprockets on the motor shaft and support the loose end of it with a bearing. It would also cut down on belt/chain whip problems by keeping the free runs short.
If you want a zero/low backlash setup using allthread, use two screws (stacked) on each side of the table and install one of these arrangements on the opposite end from the drive motor. Adjust the sync of this extra arrangement to take up the backlash by opposing the two nuts against each other.
Could be used with belts or bicycle/motorcycle chain, makes no diff. Of course the idea wouldn't be worth spit in tight quarters, but for some of these big routers and plasma tables it would be a natural since the drives tend to ride outboard and there's plenty of room available.
It could be a bit pricey for some, might be a huge economy for others, depending on how much accuracy you design into your application and what sort of screws you use.
Tiger