Quote Originally Posted by Fozz84 View Post
Hi I'm looking for a suitable ball-screw to fit a Nema 34 8mm shaft for a simple direct drive.

also what size lead screw is commonly used in a 8x4 gantry router.

thanks for your time
Ok, No disrespect meant when I say this but you clearly don't know what you are doing otherwise you wouldn't be asking these questions. So I'm going to make a suggestion that you STOP buying and do some research.

Building an 8x4 requires a completely different approach to a 4x4 or smaller, the components you use and the motors sizes, voltages, etc are all different. What works well on a 4x4 will be rubbish on an 8x4 or larger machine. Like the Motor you are asking about, I'll take a guess and say with 99% certainty you have bought the wrong motor or drive, probably both.!
Yes on a 8x4 or a 4x4 you would use a Nema 34 motor but that's about where the similarities end, the Spec, and voltages you will run them at will be very different.

Likewise, the system you use to drive the gantry needs to be chosen very carefully. Like RouterCnc mentions Rack n pinion is often used because using ball screws is more involved but it can be done provided you chose the correct size and pitch. 25mm is the minimum diameter you would use and 10mm is minimum pitch for an 8 x 4 machine. But this depends on other factors like motors and the feeds you are looking to achieve.

The problem with ball screws at this length is they whip at high feeds so you need to control this in some way. The best way is to choose a pitch higher than you need and then apply a ratio using timing belts. This allows you to lower the screw speed and still get the feeds you require.

So like I say building a large machine is very different to a smaller machine and sizing motors, drives, voltage, screws, etc becomes very important and it's a fine line between success and failure.!! . . . . This why so many people fail or are very disappointed at the result when they build a large machine.