How to pre load screws

[Jonathan]

C5 / C7 is just the grade of the screw which specifies the lead error. Lower the number the better, so C0 is the best. The common C7 grade screws people get are 0.05mm per 300mm. You don't just add the error for longer screws though, so it's not going to be 0.1mm over a 600mm screw for instance - it will be less. Getting higher grade screws doesn't immediately help with eliminating backlash since that depends on the nut. Double or preloaded (by using larger balls which has limitations) nuts are available for the higher grade screws, but they cost an arm and a least one leg...Zapp Automation Ltd - Stepper motors & Stepper Motor Drivers - Servo motors and Servo Motor Drivers - Ballscrews & linear motion products. : R1605T3-FDID-P1 Ballnut [R1605T3-FDID-P1] - £120.00

Instead of that, as you've read, you can get two cheap ballnuts and preload them against each other to eliminate the backlash. Bearing in mind the RM1605 (or whatever size you choose) ballscrew from China is much cheaper and the ballnuts are about $20 each this works out a lot cheaper even with the added cost of springs and the mount. I just got three RM1605 ballnuts for £44.70 including postage.

Preloading in principle works by applying a constant force between the two nuts trying to push them apart. One nut is fixed to the machine as normal, but the other is only rotationally contrained so that it is free to move axially. The easiest way to do this is using a spring, but normal coil springs are bulky so we use Belleville washers, otherwise known as disc springs. So long as this force is not exceeded the backlash is eliminated. Therefore to choose the amount of preload you need to consider the maximum force the axis will experience (the sum of peak forces due to cutting, friction and acceleration) and set the preload a little above this to make sure. Since setting too high a preload will shorten the life of the ballscrew significantly, aim not to exceed about 10-20% of the ballscrew static load rating.

The only worthwhile pictures I have of doing this are in my micro lathe build log.

Having said that, this all depends on what you're expecting from the machine and the machine itself. If the lathe itself is wobbly and not particularly accurate then pre-loading the nuts isn't going to sort that. Equally if you're only turning quite basic profiles then backlash can be compensated for in the same manner as you would when using a manual machine.