fixed screw vertical movement

[mccafferty]

I am building a CNC horizontal mortiser to make mortise and tennon joints in wood. Two of the axis are traditional horizontal linear bering arrangement. The third axis moves a horizontal mounted router up and down on linear slides. I have some desire to mount the screw in a fixed fashion to the router carriage base and have the nut turn. For this application, it makes the design a bit more compact and the motion force is being delivered centered on the cutter axis and near the base. I am desiring 6-8" of travel.

Does anyone know a source or design for a zero backlash nut that has the thrust bearings and timing belt shieve all in one assembly? Is this a bad idea?

Also, my Precix CNC router has a brake on the Z axis so that when power is removed, the head doesn't drop. Do many of the shop built machines incorporate some type of brade for this function?

[irving2008]

for that small amount of travel maybe you could consider an alternate non-ballscrew that won't wind down under the weight of the router? Basically a smaller pitch/lead in a ACME or Trapezoidal thread.

[Al_The_Man]

Would you need thrust bearings on the BS if it is stationary?
You may have to make a custom housing for the nut itself that you can mount a timing pulley to.
With this set up the backfeeding force will most likely be increased due to the weight of the servo motor as well as the spindle motor, so a brake may be in order, also possibly the need for counter balance.
Al.

[mccafferty]

Just to make sure I am communicating, I attached a rough sketch of what I am thinking.

[irving2008]

Originally Posted by mccafferty Just to make sure I am communicating, I attached a rough sketch of what I am thinking.

What stops the nut/pulley 'climbing' the screw instead of pushing it down? Surely there needs to be a housing/retainer plate above the nut/pulley with thrust bearings above and below the nut/pulley arrangement

[Al_The_Man]

Originally Posted by irving2008 What stops the nut/pulley 'climbing' the screw instead of pushing it down?

That is the way I see it operating the stepper/actuator climbs the BS and the notaion FIXED part is secured to the frame, IOW the BS is kept from moving at all.
I assume.
Al.

[mccafferty]

Hmmm.... sorry, I'm still not communicating.

The ball nut is fixed to the frame. The router base and square plate around it moves up and down as the stepper turns the ball nut assembly that is fixed to the frame. The screw does not turn, the nut dows. The nut has to be captive in a bearing assembly (not detailed)

[mccafferty]

I think I have found something.... THK DIR rotary ball nut. I have attached docs. It looks like it has angular thrust bearings built in. Probably costs a fortune....

Any comments?

[irving2008]

Originally Posted by mccafferty Hmmm.... sorry, I'm still not communicating. The ball nut is fixed to the frame. The router base and square plate around it moves up and down as the stepper turns the ball nut assembly that is fixed to the frame. The screw does not turn, the nut dows. The nut has to be captive in a bearing assembly (not detailed)

Thats what I thought - I understood what you intended, just not how it was engineered... the nut has to be trapped between the frame and some retaining assembly above it with thrust washers above and below the nut/pulley assembly but I hadn't realised you'd not detailed the retention methods. I suppose the nut could be attached to the frame through a captive bearing assembly, avoiding something above the nut, but that would be much harder to manufacture.

[mccafferty]

In this rendering, the outer bolt circle bolts to the frame. The shieve mounts on the inner bolt circle and as it turns, the screw moves. The thrust bearings are located in the housing. Again, it's probably un-affordable....

[BillTodd]

You mean like this Jeff Davis Z axis design?

[mccafferty]

Exactly!!!! Where can I get one?