Today's episode of Making It Up As I Go Along is the Z axis on my Sieg X2D. When I initially did my CNC conversion, I used the "make it work, and upgrade later" method. Originally, I modified the mill's fine Z feed system to be motor driven. Inside the original fine feed assembly, it uses a worm gear to turn the sprocket to move the quill up and down on the rack.
All I can say about it is that it worked. It didn't work well though. It was slow. It had almost .060" of backlash and gravity was the main backlash reduction system, the weight of the head kept it tensioned in one direction. There was also a ton of lead error in the rack, meaning if I told the mill to move .100", depending on where in the travel it was sometimes it would move .095", sometimes .103", it was just all over the place with no consistency. TBH, it would probably work fine for most people's machining projects, but it was a pain to work with when trying to actually hold a tolerance.
So, just like the X and Y, a ball screw was the solution. There are a lot of ways people have added Z screws to their X2s. The easiest is a side mounted screw with the ball nut attached to the head. Since I've already had problems with the Z axis binding, I didn't want to side mount it because lifting from one side would only make it worse. So I decided on a center mounted screw. In order to keep the screw out of the chip zone, the only practical way to do it is to fix the screw to the head, and drive the ball nut with the motor. There are a couple of designs using this method out there, but I didn't really like any of them.
So I came up with my own. I, of course, didn't take any pictures of the finished part, so you get a CAD model instead. It's very thick so there shouldn't be any flex in it like some other designs have.
Actually mounting it to the mill was another sticky point for me. The easiest way would be to have brackets on the sides that bolt into the column, but I am always hesitant to modify original parts when I make things up like this in case I change my mind half way through. So what I did instead was use some eye hooks cut down to fit inside the column, then a pair of bolts goes through the back of the column, and the hooks get tensioned against them which pulls the Z mount tight against the top of the column. It is not the strongest mounting system, but for the amount of load the rest of the mill is capable of handling, it's more than enough.
Making the ball nut the part that spins was a bit of a challenge too, much more complex than turning the screw would be. Not only do you need to turn the ball nut, you need a way to take up any axial thrust in the system. I ended up with a stack of parts that has an annular bearing on the bottom to handle the side load and the axial load pushing down, and a standard flat thrust bearing on top(for reference the bearings are an NTN SF0725 35mm x 56.4mm x 14mm annular bearing, and a 35mm x 52mm x 2mm thrust bearing). I designed the mount to use shims under the top cap to set the bearing preload. Here's how the ball nut/bearing stack looks:
With all that sorted out, I could finish the assembly and test everything out. The screw is fixed to the head using the bolt holes for the gas spring that the mill used to use to lift the head. All in all, this ball screw conversion was a fair amount of work but definitely worth it. I've got about $160 in ball screw conversion parts, and my backlash, which started at .006"/.008" X&Y, and .060" in Z, is now down to .001" in all axis(and even that I could get rid of by ordering slightly oversized balls for my ball nuts if I really wanted).
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