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Saturday, July 13, 2013

CNC mill conversion - Part 1

Machining the ball-screws

A side project I started working on, loosely connected to building the airplane, is converting my mill into a CNC mill.

After much playing around with the idea, I finally decided to do it!

This will eventually allow me to manufacture more complex metal pieces for the airplane, like brackets, adapters, etc. I have no pressing need for it at the moment, but I'd like it to be ready by the time I get to that point.

Working on the CNC conversion is a valuable distraction, as I tend to get fed up with anything fiberglass after a big layup, and find the challenge of precision metal carving somewhat refreshing. Conversely, machining difficult parts like these ball-screws leaves me yearning for the loose tolerances of composite construction once again, and the building cycle repeats over and over many times, keeping my interest level higher, longer.

I have looked at many different conversions for my X2 mill, and the one I settled on is from Daniel Kemp. I was initially leaning toward spending more money for a kit from "CNC Fusion", but I didn’t like the offset Z axis screw and motor, apparently they have caused a few issue for some people.

One drawback of my chosen design is that Daniel no longer manufacture kits for it, so all you have to go by are the plans. On the other hand, this is also a perfect learning opportunity, and it will allow me to modify things where needed to suit my machine.

Sounds like a familiar theme?

For those who are not sure why CNC could be useful, watch the next video and let your imagination take off.

Just imagine the possibilities...

I will not be making aluminum foot-pegs, but you get the idea, almost anything becomes possible with proper use of CNC.

Machining is of course all about precision, and the biggest offenders are the lead-screws that drive 99% of manual mills. These lead-screws introduce backlash due to the small gap between the thread of the screw and the thread of the nut, which allows for movement, but reduces precision. It would be physically impossible to create a lead-screw without backlash, because the friction with the nut would be too high to allow for relative movement. 

Typical lead-screw 

Nut cross section

The solution to this problem is to use ball-screws. With ball-screws the contact between the nut and the screw is made by tiny metal balls running in a race. The friction generated by the steel balls running in the race is much lower the friction generated by the thread of the nut sliding over the thread of the lead-screw, so we can have achieve tighter tolerances, and nearly zero backlash, and still be able to operate the mill by turning the screws.

Typical ball-screw (top one)

Ball-nut cross section

Ball-screw in action

This is why a nearly universal step in CNC conversions is to replace all lead-screws with ball-screws. Any conversion plans that doesn’t address this area, and retains the original lead-screws, will be plagued by the same backlash that existed before the conversion, and it is not worth the trouble in my opinion. This is the main reason why I rejected the "fignoggle" design (although it seems they are now starting to address this major drawback).

So, it seemed fitting to begin talking about my conversion by addressing this critical component, the ball-screw, upon which all machine movements will rely, and where the foundation of precision are laid. 

I purchased my ball-screws and nuts from Roton (#59321), and I had them cut to length for an extra charge, but there is still a lot of work to be done before they are usable.

One of the ball-screws as it arrived from Roton

Here’s the plan of action for the ball-screws...

The main issue I ran into while machining the ball-screws, is the fact that the ball races are hardened steel, and they required specialized cutters. I wore out every type of cutting implement I had (High Speed Steel, Carbide, and Crobalt), getting very poor results, while the lathe almost shook itself apart, spitting flames and swarf so hot it turned electric blue.

Worn out Crobalt cutter trying its best against hardened steel :-(

Thankfully, I did finally receive a suggestion from a machining forum about using a tool post grinder to cut through the tough stuff, so I bought one from littlemachineshop.com and started grinding away.

Initial grinding of the ball-screws

Final grinding to exact dimensions. "Look Ma', no heat!"

Freshly ground surface

Man... what a difference that made! Like night and day! The only problem being the abrasive dust going everywhere, requiring a complete lathe teardown and clean up. No sweat!

The next step was machining the thread onto the shaft.

Single point threading on the lathe

Here’s a before, during, and after shot of the ball-screws being machined...

Ball-screws before, during, and after machining.

... and the mostly finished X and Y axes ball-screws...

Ball-screws with bearings, nuts, and electric motor couplers.

Close up of the X and Y ball-screws and accessories.

I decided to leave the stems a little long where they connect to the electric motors, until I can fit them in place later.


  1. The approach followed in this blog is truly amazing with the detailed information on ball screw machining along with the video. Thanks for sharing such a valuable information.