Converting a Mini Mill to CNC

Harbor Freight sells a pretty good small milling machine.  As it comes from the store it is a manual machine.  You turn cranks to move the cutting tool around.  You can make simple precision parts in steel and aluminum.   I’ve started a project to upgrade this mill to CNC.   This means adding motors to each axis and interfacing the motors to a computer.  There must be more than a dozen ways to do this.  After spending some time looking at what others did I decided on a general plan.  Then I modeled the mill and extra parts that need to be added in Autodesk’s Fusion 360.  Below is both a drawing and a render of the over all design. (Click an image to see it fill size.) I will go into details and the decision process the led to this design later.


There are a few things you can see in the above images:

  • There is a large screw raising up from the top of the machine.  This is a ball screw that is fixed firmly to the milling machine’s head.   In this design the screw is fixed and a motor turns the nut.  The screw is very close (2.5 mm) to the vertical dovetails and lifts the he’d without adding a turning force
  • The stock hand cranks are left in the stock position.  This way the mill can be used in manual mode if the motors are disabled
  • The stepper motors are (mostly) out of the operators way

The z-axis design might be a little hard to understand if you have not seen a rotating nut design before.   Normally the Minim Mill head to raised up and down using a rack and pinion system with the rack mounted to the column.  In this design the rack is removed so the head is free to slide vertically.   Think of the ball screw as if it were a rope.  The rope (screw) is used to rain and lower the head.  Pull the rope and the head moves up let it go and the head falls.   We use the nut to “pull” on the screw.  The nut is held firmly in place and rotated by a stepper motor.   See the image below:

rev b v28 z axis


Compared to the z-axis the X and Y axis are both very simple.   The stock lead screws are used (at least in this first revision of the design.)  Referring to the image below you can see that the X isis motor is directly connected to the x-axis lead screw.  A flexible coupler is used and the motor is held in place by the existing screw holes.  The y-axis motor is also connected with a one to one drive ration but uses a pulley.  The primary purpose of the pulley is so that the motor can be mounted on a “out of the way” location.

rev b v28XY axis

What’s next?  Before I begin cutting metal I need to fin tune a few of the parts, create a bill of materials.  Then I will make each part using a 3D printer and check it for fit.  Once satisfied with the design I will start cutting metal.  I will start with the Z axis

I will post periodic updates and document the entire process.  The plans will be available online to anyone who wants them.

The latest design files are available in to places

  1. On the Autodesk Fusion 360 cloud.   Use this link to access the design files.  You can use the interactive viewer to see a low resolution image from any angle and zoom it and even “explode” the mill to see all the parts and how they go together.  There is also a download link with the ability to convert the design to other CAD system file formats.   But only the Fusion design is here
  2. All the files associated with this project are archived at at Renders, 3D models, standard parts library, word processor documents, you name it , it is there (or will be there)
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