How to Build a CNC Mill

Manufacturing technologies took a major step forward when computers became able to precisely control the creation of complex shapes in materials. One such Computer Numerical Controlled (CNC) tool is the CNC milling machine. A milling machine is similar to a drill press, except the the work can move, and be cut, in three directions with respect to the bit, or end mill. This guide will describe the formulaic construction of the mill, providing the reader with the background needed to build his or her own CNC mill.

Things You'll Need

• Headstock assembly
• X-Y table
• Stepper motor assembly (or assmblies)
• Stepper motor controller
• Computer

Instructions

1. Plan the assembly. Know what tools will be needed when. Hobbyists and professionals employ two methodologies to construct a CNC mill. The first is to convert an existing mill to computer control. Here, the mill itself is already constructed, and the modification involves only the addition of motors and a controller.The second method is to start from scratch and construct the mill with the express purpose of serving as a CNC mill. The first method has the benefit of building on the precision of others, but generally requires more money and time to adapt someone else's design. The second method is typically less expensive, and can be engineered to the task at hand. However, the accuracy of the setup will depend greatly on the skill of the builder.If a pre-built mill is being converted, then proceed to step 4. If the mill is also being constructed, the continue to step 2.

2. Making the head stock. A head stock is the motor powering the mill and spindle which holds the tooling. There are a variety of methods of assembling the head stock, but they all require a solid mount for the motor and spindle and a way to transfer the power from the motor to spindle.The motor can be sourced from anything from a garbage disposal to a wheel chair. Minor cutting, plastics and soft woods, can be done by a motor up to 1/4 hp; harder woods and metals require a stronger motor up to 1 hp and beyond.The spindle is a metal cylinder that is mounted to a heavy, solid block. The spindle sits within the mount through ball bearings or bushings. If bearings are used, then the best option is to utilize taper bearings, as milling not only exerts vertical forces but also horizontal. The other functional part of the spindle is the collet or holder for the tool bit.The mount is a solid block which holds the spindle vertically above the table described in the next step. The block ensures that the spindle stays aligned at a perfect 90 degrees to the table, as well as holds the motor and gears/pulleys to transfer the power from the motor to the spindle.

3. Install a XY table. The XY table is a surface that is capable of moving forward and backward, as well as from side to side. There are many mechanisms to achieve this motion, but the hobbyist typically uses long hardware screws and bolts; the rotation of the screws causes the nut to move laterally in the X and Y directions. The table also provides a way to hold the material secure as the bit cuts into it. There are many sources for pre-built tables. These tables can typically provide an accuracy of 0.005" and beyond. With these tables, it is a matter of mounting the head stock to the table. The main requirement is to ensure that the spindle axis is perfectly perpendicular to the table plane.

4. Modify an existing mill. This is an easier method for constructing a CNC mill. Home shop mills can range in size from a small drill press to a large standing mill that is 7' to 8' tall. These mills have the benefit of built-in precision, as well as the safety features and mechanisms that come with professionally made products. However, these mills are not cheap. The smallest mill for our purpose would run at least $500, and floor based mills can easily cost $10,000 or more. That is the price of accuracy and stability.

5. Install stepper motors, as well as a controller for them. The main interfaces between the computer and the mill are stepper motors and a stepper motor controller. Stepper motors are special, in that one can easily specify how much to rotate the motor in small angular divisions. Via coupling hardware, stepper motors are secured to the table and attached to the rods controlling the X and Y of the table, as well as the depth Z of the headstock.The controller can be home-built from electrical components, but it can easily be bought; modern units, though expensive, are more reliable and accurate. The controller plugs directly into the computer and motors, and as the computer tells the controller how to move, the controller relays these commands to the stepper motors.

6. Find and install software. The software on the computer is what controls the motors and ultimately the CNC mill. There are many software packages available, the primary features of which should include: 3d input and output, automatic rastering of the design to mill, and numerous input and output formats. Some packages require a small registration fee, some a much larger fee. Others are free. Finding the numerous pieces of software requires further research. (See Resources.)

Tips & Warnings

• The headstock is a critical piece of equipment that must be built correctly, otherwise pain, death, dismemberment or injury can result.

• If you do not possess machinist skills, then do not attempt to build a mill, and instead start with one that is already built.

Resources

• Forum where people discuss all things CNC