Ball Screw Conversion

T.I.T.

www.tomsinstitute.com


Conversion of a Grizzly G1005 Mill/Drill to Preloaded Ball Screw CNC

The Quest For Zero Backlash

Follow the links to pictures and captions as you read.

I decide to convert the mill/drill to have zero backlash in at least the X and Y axes. Again, at the lowest possible cost. I accomplished this with ball screws and preloaded ball nuts. This was my first experience building anything with ball screws so I did a good bit of research before I started.

For the ball screw, I decided to use the nominal 5/8" size for both fit and cost ($1.09 per inch). The screws and nuts a readily available from MSC, McMaster Carr, and the local bearing outlet. Pricing was about the same from all sources when you use my employer’s discounts.

There is a large price difference between preloaded ball nuts ($140) and non-preloaded ball nuts ($21). My research led me to one CNC hobby site that was selling plans to build your own preloaded ball nuts from two non-preloaded nuts. I paid $7 for these plans. The plans were of good quality and provided an elegant design to preload the two nuts with 18 pounds of preload. The "store bought" preloaded nuts have over 100 pounds of preload. The plans required you to disassemble the nuts (tiny balls everywhere, and tedious reassembly) and machine slots in the hardened body. A couple other pieces with a lot of detail and special threads also had to be fabricated. What the plans made me realize is that a preloaded ball nut is just two nuts with some sort of spring between them pushing them apart, and something to keep them from rotating relative to each other.

With this in mind, I came up with a simpler solution to making my own preloaded ball nuts. I did not want to disassemble the ball nuts or deal with machining the hard body. My design uses a coil spring compressed between the two nuts after they are threaded onto the screw. A piece of channel is then placed over the two nut bodies and secured in place with setscrews. The end result is a preloaded ball nut with about 50 pounds of preload. It is about  ¾" longer than the one in the purchased plans. I could be shortened by about  ½" by machining the mounting threads off the second nut, but in my installation it would make no difference.

Ball Nut Y.htm

Ball Nut X.htm

I wanted to do my conversion without the hassle of picking up the mill/drill. To disassemble the table from the top down, I first ground away a small area on the under side of the table casting. This allows the table clear the X axis nut and to be slid entirely off the ways. With the table removed, you have access to the socket head screw holding the Y axis nut in place. Detaching the Y axis nut allows you to slide the Y axis slide off the machine base. You must thread the Y axis screw out of the nut in order to remove the entire assembly from the base.

Grind for disassembly.htm

I thought I was going to have an interference with the new Y axis ball nut and the base so I extended the cut out in the base where the Y axis nut travels. I turns out I did not have an interference with the nut, but the extended cut out allowed me to install my new ball screw and nut assembly as one piece. This eliminates having to thread the ball nut onto the screw while the nut is installed under the slide. Whenever threading a ball nut onto a ball screw, there is a chance of all the balls falling out. See my tool below that helps prevent this time consuming event.

Base SLot Extension.htm

Nut Install Tool.htm

My design of the ball screws and ball nut mounts is fairly straightforward. I reused the existing thrust bearings and bearing mounts. The screws have a nut and jamb nut arrangement for taking up play in the thrust bearings (a feature lacking in the original screws). Some new hubs were made to fit my original timing belt pulleys, and the motor mounts were re used.

I order to machine the ends of the ball screws, it was necessary to soften them a bit.   I wrapped a damp rag around the screw to protect the usable threads, and heated the end to be machined to a dull red and allowed it to air cool.   The screw could then be easily cut with carbide tooling.

Screw Assemblies.htm

I decided to make put a ball bearing in the shaft support of the X axis screw on the drive end.   The original design has no bearing in this support.

X Shaft Support Bearing.htm

I have had a lot of fun with the whole CNC and ball screw conversion project and would highly recommend doing it if you have any interest.   As you can see from my project, you don't have to be an expert at anything, and you don't have to spend thousands of dollars.  

The end result is a CNC mill/drill that can mill out a ¼" diameter circular pocket with accuracy of about +/- 0.001".   This is fine for my tinkering.

See some First Cuts with zero backlash.

Here are some additional photos of interest.

  

 © Copyright 2003, Tom's Institute of Technology.

 

  

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