CNC Conversion of a Sieg X-3 Milling Machine

By Bob Berg


    Goal – To create a fairly accurate bench top CNC machine at fairly moderate price.


Here are some shop pictures:


The base mill is a Sieg X-3, this was purchased from www.lathermasters.com who was the only importer of this mill at the time.  Priced delivered was about $1,100.00. 

This also shows Shumatech DRO on top of the motor.  If your not going to CNC your mill, you need at least a DRO!


  This is my Harbor Freight 9X20 lathe with Quick Change tool post.

  The Windex bottle is filled with Denatured Alcohol I use it to clean EVERYTHING!





Once I received the mill I completely tore it down and cleaned every piece that was practical with a brass brush, rags and mineral spirits.  After the cleaning it was amazing how much grit and metal goo was at the bottom of the cleaning bucket!   The mill was then greased/oiled and then reassembled in its new home.   I used the mill for about a year for some small manual projects.  During this time I got tired at looking at numbers on the hand wheels and dealing with back lash.  I then built a DRO for it and fitted the X and Y axis with scales.  This single upgrade was time well spent, once you have a DRO you can never go back!     


I knew I wanted to convert the machine to CNC from the start, so I did a lot of research on the WEB for information about CNC conversions.  I began to look for components.  I first looked at stepper motors and then servos as many have said that servo performance will be better.  So I decided on the servo approach.   

I first located some very cheap servo motors from a link I found on www.cnczone.com.


These are Clifton Precision servo motors rated for 36VDC with a max torque of about 360 oz/in. 

These motors were purchased for about $10.00 each! 

However, they had no encoders on them, what do you expect for $10.00?  I purchased some 250 count rotary encoders from US Digital for about $40.00 each and then mounted them to the back of each motor – Now they are servos! 


 For about $50.00 each I have a nice set of servos that should work well.   I’m thinking that the power will be marginal at a 2:1 belt reduction ratio. I toyed around with 3:1 and finally settled on a 2.5:1 ratio.  If things don’t work out I can always change the ratio.

Pulleys and belts were purchased from Stock Drive Components and should be here in a couple of days (2/16/05).    


During the first major shopping trip I purchased 5/8 Rockford ball screw stock, ball nuts and bearings from www.industrialhobbies.com.  I decided that for the machine to be accurate I needed to eliminate as much backlash as economically possible.  And decided to go with the common 5/8 rolled ball screws and homemade preloaded ball nuts.    I purchased the preloaded ball nut plans from the web and the proceeded to build the components from the plans.   This was actually some of the most difficult parts to machine so far in the whole project.  I had to do some threading on the lathe, milling, boring and tapping on fairly small parts that were hard to hold.   I didn’t feel comfortable doing internal single point threading of the lock collars so I purchased a tap from Mcmaster.com for $50.00 for the tap, Yikes!  But it’s better then scrapping the part!




    Pictures of ball nut components coming soon!


Design the conversion

As I was about to start making chips and “design as I go”   I realized quickly that I’m going to make a lot of mistakes and I need to better plan the whole design before I go to town.  So I quickly taught myself SolidWorks and began to design the various parts of the conversion.  I was able to model everything in 3d.  This allowed me to see how things would fit and look.  It is like being able to create a virtual model of my machine before ever cutting a single chip or drilling any holes. I’m sure it has saved me many hours and dollars of mistakes!


This is the full assembly of the mill table with all the CNC components.


This shows the X-axis ball screw details.


The bad thing about modeling in 3d is that it is so easy to change something you are constantly

changing things and never reaching a “Final” design.   Also, there is no surprise when you finish

making a part, you know what it’s going to look like before you even start!


    X,Y & Z Axis Bearing retainers

These parts trap the bearing in the bearing plates so there is no axial movement.  The bottoms of the retainers are recessed .100” to the diameter of the bearing.  Based upon the depth of the bearing plate holes these parts give .015” of crush.  Not that it’s all used of course!


Machined the ball screws

I think this was the most difficult part to machine in the whole process, due to the fact that the ball screw is case hardened to 56C. 

When turning down the ball screw to fit the bearings I wanted to keep the journal end as concentric as possible in relation to the screw.  Once the ball screw was in the chuck of the lathe I placed a dial indicator in the thread grove and rotated the chuck by and hand and also moved the lathe carriage so that the dial indicator would follow the thread. I had to re-center the dial indicator via the lathe carriage to the low spot on the thread a little each time as I can’t follow the thread perfectly by hand.  By doing this I adjusted the lathe chuck so that the screw would run true by indicating off of the threads not the outside.   Once the screw was centered in the chuck I faced off the screw and drilled a pilot hole so that I can support the other end with a live center. 

The machining process was begun by taking a hand grinder and removing as much material by hand as possible

To try to remove the case hardening.  Be careful not to take too much off doing this method as it’s easy to do!

I then turned the rest of the diameter with a brazed on Carbide tool bit.  This worked pretty good once I figured how much of a cut to take.  I then finished the diameter with some sand paper.  The next step was to thread a portion of the screw for the locknut.  I used a 7/16-20 hand die to make the threads. I tried to use the lathe to single point the threads but something wasn’t working (don’t know what) so I did it by hand.



X-Axis with new ball screw installed with plates and motor mount attached. 

All plates are made from 1” aluminum stock.

I was able to obtain all the aluminum stock from the scrap bin at work!

The axis is now silky smooth, in fact I can move the table by just turning the shaft with

my fingers!  

I think I’m going to need to put some thought in keeping the ball screws clean. 

They are protected by the table, but I’m sure some chips will get in and gum up the works!



    Space Plate, Bearing Holder, and Motor Mount

I just need to bore the hole in the bearing plate and the Y-axis will be ready to go.


Here is the finished Y-axis bearing plate its mating parts and assembly (2/18/05)


    Y-Axis ball nut mount


Y-Axis with old acme nut removed and new holes drilled for new ball nut mount


Hint: in order to preserve the stock Y travel. I bored a hole about 1 – ½” deep in to the column here.  This allows for a longer ball screw as it can go into the column when the Y table is at its maximum.  This actually allowed me to gain more Y travel then stock.  With out this modification, I would have had less travel because the ball nut is longer then the stock nut. And you don’t want to let the screw go half way out of the nut or you will loose your balls! J  Ask me how I know!  Limit switches are important to keep that from happening!

Y-axis new nut mount installed.  The electronic device mounted on the left is a reader head from a harbor freight 6” electronic caliper. 

This is used for the Y-Axis DRO Scale.   When I put the Y-table back on the scale just slides back inside the head!



Z-Axis –

I still need to design how I’m going to approach the Z-axis.  I will most likely drill a hole in the top of the column and mount a bearing plate and motor mount for the ball screw on the top.  I will need to make a new mount for the ball nut.  I’m not sure if I will need to support the screw from the other end with a radial bearing. I’m also not sure if I will disconnect the hand wheel gear or make so that I can engage and disengage the hand wheel from the ball screw.  If I do use the hand wheel I will definitely need to support the other end of the screw.


Before conversion:



After Conversion

Here is the new ball screw, preloaded nut and mount installed!

I decided to support the screw at the opposite end.  I used the existing bearing holder, removed the thrust bearings and replace with radial bearings.  I have decided to leave the crown gear off so the hand wheel is now disconnected.  I don’t need it anyway!

Here is the pulley and empty motor mount on the side of the mill.

The plates are 1” alum.

The little white switch on the left is the upper Z limit/home switch



Here is the X axis ready to go!

And the Y


Just need to finish up some wiring!


The following is a list of all the “major” components that I used, approximate cost and source:



Web Site

Part Number


Approximate Cost

Stock Drive Products


A 6A 3-20DF03708

Motor Pulley -.200 (XL) Pitch, 20 Teeth, Aluminum Alloy Timing Pulley


Stock Drive Products


A 6A 3-50NF03710

X & Y Pulleys .200 (XL) Pitch, 50 Teeth, Aluminum Alloy Timing Pulley


Stock Drive Products


A 6R 3-082037

X Belt - .200" (XL) Pitch, 82 Teeth, 3/8" Wide, Neoprene Belt


Stock Drive Products


A 6R 3-102037

Y Belt - .200" (XL) Pitch, 102 Teeth, 3/8" Wide, Neoprene Belt


Stock Drive Products


A 6A 3-17DF03708

Z - Motor Pulley - .200 (XL) Pitch, 17 Teeth, Aluminum Alloy Timing Pulley


Stock Drive Products


A 6A 3-50NF03710

Z Big Pulley.200 (XL) Pitch, 50 Teeth, Aluminum Alloy Timing Pulley


Stock Drive Products


A 6R 3-102037

Z Belt .200" (XL) Pitch, 102 Teeth, 3/8" Wide, Neoprene Belt







Industrial Hobbies



5/8" Rockford Single Circuit Ballnut  (2 Per Axis)


Industrial Hobbies



5/8" 5 TPI Rockford Standard Grade Ballscrew (6 Feet!)


Industrial Hobbies



3 Gecko 320 Combo (Servo Drives)


VXB Ball Bearings


7201B 12x32x10

Angular Contact Bearings (2 Per Axis Back to Back)





Digital and Analog Panel Meters





Controller Box




BreakOut Board

Parrallell Break out board


US Digital



250 Count Encoders (For the servo motors) (3 Pcs)


Avel Lindberg



30AMP Toroid Transformer (32Volt)


CNC Zone.



Surplus Clifton Precision Servo Motors $10.00 Ea! See Link for info


Jeff Davis



Preloaded Ball Nut Plans - Also Sells Servo Motors that would be good for the X-3



 As of now I have completed the CNC conversion of the mill a few years ago and everything works perfect!  The Z axis has been converted to ball screw and has full range of motion and works great, the servo has no problem lifting and lowering the head.   I have since remove the hi-low speed change gears and now have a direct belt drive from the motor to spindle. I have also replaced the stock motor with a 3 phase 1/2HP Motor and a VFD from Automation Direct.

I’m currently running about 100 inch/minute in rapids.  I could get it to run faster, however for the size of the machine I don’t need it any faster.  Most of my cutting is under 20 in/min.  It has ton’s of power, able to snap 3/8 end mills and keep going like nothing ever happened!

The X travel is 16.6 inches and Y is 6.4”, this is more then stock!  Not sure about Z but it also has more then stock.