Here is some information on the different types of materials and components you will need to understand, in order to choose the proper components to retrofit or build your own CNC machine. This is a hardware picture index page with a brief description and a link to another page. Clicking on the pictures will take you to more info and links on that type of component.

   When I first started thinking about building my own CNC mill I though that the hard part would be making the electronics to control the motors, this turned out to be wrong. The hard part was getting all of the mechanical parts working right without slop and yet not binding or flexing under cutting forces. Many of the problems I encountered could have been avoided by buying off the shelf hardware but this approach is not cheap.

Mechanical Hardware Components

Ballscrews

Rack and Pinion

   This is a ball screw it is used to convert the rotary motion of your servo motors to precision linear, table or spindle motion. There are two grades of ball screws rolled and ground. The rolled type are less expensive and less accurate than the ground type.

   Rack and pinion is another option for positioning but like belt drive, it is not as accurate as a lead screw. The advantages of belt drive include increased speed and reduced susceptibility to contamination. When mounted with the teeth pointing down debris falls downward and out of the way.

Timing belt and cable drives

   Timing belts drives are very similar to rack and pinion drives, they work like a flexible rack. Belt drives are fast quite and tolerant of contamination.

  The top example shows two ends of the belt anchored to the frame while the motor moves with the carriage or table.

  The bottom example shows the motor being anchored to the frame and the belt moving with the carriage.

   If you drive the table using either a gear or pulley, mounted directly to the motor,  try and find one that has a low RPM and lots of torque. A motor turning at 1000 RPM with a 1” pitch diameter drive pulley would move the table at over 3000 inches per minute.

Linear Guide Rails

   Linear guide rails can make building a CNC machine as easy as cutting a couple of slots in flat piece of metal and drilling a few holes. Sounds easy enough but getting everything perfectly square can be kind of tricky.

Bearings

   The next problem to tackle is how to hold the ball screw without allowing axial movement. The most common types of bearing that are used in supporting the lead screw are radial and angular contact. The way that most mills are set up is to have one end of the lead screw held with angular contact bearings and the other end is floating in a radial bearing. You can get both types of bearings with ether balls or cylinders as the rolling elements. Balls are quieter when moving but the cylinders can handle more load.

Power Transmission Components

   You need to get the power from the motor to the lead screw and just using a solid coupler with a couple of set screws won’t cut the mustard. The problem is, it just about impossible to mount the motor with such precision that the bearings holding the lead screw and motor shaft will not bind when they turn.  Any binding between the lead screw and motor will damage the bearings and cause other problems as well.

   There are better choices than solid couplers for coupling the power from the motor to the the lead screw which include gears, chains and belts. These other choices also have some advantages over direct coupling, such as the fact that you can pick the reduction ratio that best suits your application. The only one of these three methods I will discuss is timing belts as the other two have some characteristics that I do not care for.