Rotating ballnut?

[cncuser1]

Does anyone know of any proven designs where a Z-axis( for a benchtop mill) is driven by a Rotating Ballnut( incorporatinng drivebelt and appropriate bearings) instead of a rotating ballscrew.

I'd love to see some schematics

[Willbird]

I'm going to do it on a full sized Bport, have not sketeched it out yet. it makes sense to me to do it that way when your machine used bevel gears to originally drive the knee leadscrew like a lot of knee mills do.

[cncuser1]

Originally Posted by Willbird I'm going to do it on a full sized Bport, have not sketeched it out yet. it makes sense to me to do it that way when your machine used bevel gears to originally drive the knee leadscrew like a lot of knee mills do.

I have a tabletop knee mill, I want to mount the stepper motor and ball nut on the base of the machine, and have only bearinngs installed on the knee.

It'd be nice to have some ideas to kick start my design.

[Willbird]

Well the way I see it, the ballscrew would just clamp to the knee, no bearings at all needed on the knee end. The ball nuts need to be held on bearings so they can rotate driven by the stepper or servo motor.

Simple, all you need to do is start working it all out :-).

Bill

[Zumba]

I don't see any benefit of doing it on a benchtop mill. Slapping a stepper on the top of the column to drive the screw is so much easier.

A router with a 10-foot X-axis is a different story.

[cncuser1]

Originally Posted by Zumba I don't see any benefit of doing it on a benchtop mill. Slapping a stepper on the top of the column to drive the screw is so much easier. A router with a 10-foot X-axis is a different story.

My mill has a knee already on it. It moves on the column.
the spindle is stationary ( except for a small z-axis mechanism that i want to eliminate, hence my post)

[Zumba]

I hear you. Is this the machine you have?

http://www.grizzly.com/products/G3102

[bill south]

Zumba;
My reason for looking for a driven nut design (on-going for a few weeks now with very little luck) is so I don't have to move all of that weight on my benchtop lathemaster ZAY series milling machine. I've installed a 3/4 inch ball screw in the column and driving with a 600 oz continuous servo. The balls wear out very fast and I have to replace them on every once in awhile. I have a second nut on order and will install a dual nut to help with the wear problem if I can't find a good, simple quill drive design.
Also by powering the head on a square column design, the head movement up and down the column is not very predictable. It jerks from time to time like it is sticking. I've tried every adjustment on the thing and it still sticks upon occasion. Also, I have both ends of the ballscrew achored to try to help this problem. However, the problem (whatever it is) is still there and rears it's ugly head at the most inconvenient time ususlly screwing up a good piece of metal.
Yes, driven quill is for me and I want to use a fixed ballscrew design. I saw a great setup somewhere but can't find it any longer. Somewhere out there in cyber space.
Any help will be appreciated guys. Lathmaster works great otherwise!
Good luck.
Bill

[bill south]

Hey Guys;
After looking at almost 1000 photos on my hard drive, look at what I found. Can we reverse engineer this thing??? Looks like a good design to me.
Wat ya thunk?????
Bill

[Zumba]

Cool photo. I like the design. Looks pretty simple. Here's how I'd do it if I were copying it... take my advice with a grain of salt.

1.) 3/4" ballscrew and nut. Ballscrew is turned down to 5/8" on one end. Ballnut typically has a 1-1/8 x 18tpi male thread.

2.) Blue cast iron mounted bearing: 1.5" ID.

3.) Steel cylinder: 1.5" OD at the bottom, steps up to 2.0" OD in the center, then steps back down to 1.5" OD at the top. 1" ID bore all the way through. 1.5" diameter outside threads cut at the bottom. Top of cylinder is bored out a bit more with threads cut or tapped to 1-1/8" x 18.

4.) Flanged Timing Pulley, maybe a 42 tooth .200" XL, bored out to 1.5".

5.) Servo Pulley, maybe a 14 tooth .200"XL for 3:1 drive ratio.

Note that mounted bearings are typically just simple radial bearings. I would much prefer a dual angular contact bearing mounted in my own housing. But I guess those mounted bearings are cheap enough that you can just replace them every once in a awhile and save some labor.

[handlewanker]

Hi Bill, looking at the design, in post #9, gives me a bit of doubt as to whether the offset drive would be strong enough to remain true without flexing under load.
I would be more inclined to look at a way to have two ball screws, one on either side of the spindle to balance the load.
Using a rotating ball nut is going to be an expensive way to do it, apart from being more complicated.
Looking at the attached photo, could you fit a stepping motor in place of that fine down feed handle?
In this case I would make sure that there was positive up pressure on the spindle all the time, ( like a bench drill) so that backlash didn't come into it, probably by spring loading or counterweight.
The knee drive mechanism is probably only suited for hand winding, due to the weight, and if a stepper and ball nut were contemplated for this situation then some form of relief to get the load off of the nut would help, probably a hydraulic cylinder that assists lifting but still keeps some down pressure on the nut.
At the end of the day which way do you want to go? Table lift or spindle drive?
Both methods require different designs. Spindle drive has short travel and table lift has long travel but large loads.
I could not imagine profile milling a job by lowering and lifting the table to be very accurate, as opposed to spindle or Z axis drive.
Ian.

[Zumba]

Originally Posted by handlewanker Hi Bill, looking at the design, in post #9, gives me a bit of doubt as to whether the offset drive would be strong enough to remain true without flexing under load.

I don't think the offset is that big of a problem.

The problem is that in the photo, the bracket that attaches to the quill is only 1/4" thick. Adding another ballscrew to the other side would help, but no more than thickening the bracket and using a larger ballscrew.

The real issue to consider is the quality of the quill. Most bench mills have pretty crappy quills. If your quill deflects as it goes through its travel, even the sturdiest drive setup will not be good enough. It would be like using precision ground ballscrews with crappy linear motion guides.