Larry,
Do you mean backlash or back drive? Backlash is the play between the nut and lead screw. Do you know the rating on your THK ball screw? Some are pre-loaded and contain less "backlash" than others. As with any system, both parts play a role in quality. Both the nut and the lead screw can play a role in backlash.
Back drive is the tendency of the Z-axis plunge arm to coast back down when supporting a vertical load (the router). The tendency to back drive in the vertical will depend on several things, weight, pitch of lead screw, holding torque of stepper/servo, efficiency of lead screw/ ball screw, and the friction of the linear bearings holding that assembly.
When you hear of back drive, they are usually referring to when the machine is off and the motor and is not powered. There shouldn’t be any back-drive while the motor holding the Z-axis lead screw is on, or the motor is undersized.
This is still a problem though because you do not want to lose you position when your machine is off.
Ball screws do have more of a tendency to back drive than ACME screw because of their efficiency (less friction) and usually having a high pitch. Even if an ACME screw and a ball screw have the same pitch (lead), the ACME will be less likely to back drive. ACME screw efficiency percent may be 20-60 while ball screws usually run from 50-80%. So for a ball screw there is less friction to keep it from turning due to the linear force.
For ball screws with pitch less than 5 turns per inch, there will be a tendency to back drive with the weight of a normal router spindle and accessories, assuming that the linear bearings offer little friction. This is also the recommendation of Nook industries.
Since you have a 5 (TPI) ball screw this is something to consider. You can use springs pistons etc. to make the assembly neutral weight or use a brake system. On one of my first designed I had this problem and made a brake that clamped the linear bearings. I just had to remember to lock it before cutting power and then cut power on and then unclasp before starting. After doing that I had little problems.
On my other machine I used ACME screw with active suppression wedge nuts like the ones offer from dumpster.com. The lead screw was ½-10 2 start (5 TPI), and I had no problems with back drive.
As a note, another solution is to build a simple contact plate. Apply an electrical charge (ground wire) to the spindle, and have small metal plate with the pos wire. Then you can create a simple macro to run to that plate and touch down with the tool. The circuit is complete when they touch (like a switch) and the Z-axis is always true. That way, even if you change the bit it will always have the same Z-height. With the driver software like Mach 3, this is very easy to do as it can detect switches.
Sorry if I didn’t explain that very well but I hope this helps.
Regards
Brian
WWW.CNCRouterSource.com