Z Axis Ball Screw Question

[rcazwillis]

I am putting together the Z-Axis ball screw for my HF 12 sp mill. Based on some of the great designs on this forum, I am going to turn the ball nut to raise & lower the z-axis. I have the design basically figured out. I am missing a few things. Maybe someone can help.

When mouting the ball screw between the top & bottom plate, a bearing of some sort is required. Will a ball bearing thrust washer surfice? I am concerned this provides no lateral support. Should I use a tappered bearing in this area to insure the assembly stays centered? If so, how do I mount up the bearing? What specific bearing will work for this?

Thanks in advance.

[2muchstuff]

When turning the ball nut the ball screw stays stationary. You will need to clamp the screw in place top and bottom. On the rotating nut assembly, the nut gets clamped in the pulley with thrust washers on either side that run up against a yoke assembly mounted to the head casting. The nut stays stationary and rotates while the screw stays stationary but goes up and down.

[rcazwillis]

I have all this designed and it is going to work. What type of bearing is used between the ball nut and the yolk assembly?

As I type, I realize that I need a tappered bearing because the belt will want to pull the ball nut towards the motor. Correct?

Now I need to figure out how to mount the ball nut to the inside of the tappered bearing. How has this been solved on other machines?

[Runner4404spd]

you are correct, you need a tapered roller bearing.

[2muchstuff]

I drew this very thing out once before when I was going to use a ballscrew for my x-axis but I can't seem to find it to post it. I will try my best to describe it the best I can. The ball nut is mounted into a tube. That tube is mounted, pressed into a bored out timing pulley such that the ballnut is centered in the pulley. The tube has been machined to receive an angular contact bearing to be pressed on on either side of the pulley. There is a shoulder on the tube so the bearing can be pressed on only so far. Have I lost you yet. The yoke will have a hole machined on either side to accept the bearing. One hole will have a shoulder on it so the bearing can be pressed in and will bottom out in the hole. The other hole will have no shoulder but will be threaded at the end to accept a threaded "cap" with a hole in the center to pass the ballscrew. This bearing fits loosely, allowed to slide in it's hole. There is a wave washer between the bearing and cap. As you tighten the cap it puts pressure on the bearing taking the play out of the assembly.

After all of these parts and pieces and machining wouldn't it be easier to have a stepper drive the ballscrew directly.

[rcazwillis]

I found two post with some good pictures. I think I have it figured out now. One question, should the spindle that the bearings and pulley are mounted to be steel or will aluminum work?

[2muchstuff]

It can be made out of either steel or aluminum. I was going to make mine out of aluminum to keep the weight down and lessen the rotational inertia.

[rcazwillis]

I was planning on AL because my mini-lathe can handle it easier.

I was thinking about some wheel bearings that I replaced on my '72 Chevy truck. I think the outside one might be about the right size for this application. Might have to check that out.

Thanks for the info, I will post some pics when I get something worth showing.