CNC Machine Accuracy

CNC-Build believes this is one of the hottest topics out their for users and machine builders. Here is our take on it. First we need some definitions of some commons terms involved in this topic:

• Accuracy - this describes any deviation from perfect (nominal) dimensions. It is usually stated like +/-.002". We believe many companies beat around the bush with this number because there are so many factors that affect it.
• Repeatability - this describes how well your machine can cut to the same postion every time. It is also stated in terms like +/-.0005".
• Resolution - describes the size of your incremental moves. There are three types of resolution to take into account. Mechanical resolution is your machine's resolution based on the lead screws and stepper motors. A 1/4-20 lead screw combined with 200 stepp motors will have a resolution of .00025". Contoller resoltuion is the stepper mode that you chosse to use in your set up. Most stepper controllers have full, half, 1/4, 1/8, and 1/10 stepping modes. Software resolution is increment size that your CNC computer software allows you to have. The lowest I have seen on most hobby systems is .0001". Our preferred combination of resolution is a 1/2-10 acme/200 step machine using the 1/10 stepping mode from a controller and controlling it all with EMC2 software whcih has .0001" stepping increments. In this example we get down to .00005" resolution but our software trumps us at .0001" but that is probably trumped by the .001" capability of our hobby level machines.
• Backlash - is any slop in your lead screw system. The slop can be found in your drive nut to screw clearance, or in your thrust bearing mounts. This is easily tested by circular interpolating circles and measuring to check for a round hole. If it is egg shaped then you have a backlash issue.
• Lead Screw Error - this is the real issue with hobby machine accuracy. In a stepper motor cnc system we are trusting the stepper motors to move the distance we ask it to. If the steppers do their job and there are no skipped steps and there is an error then there is a high probability of lead screw error. Most manufacturers will list their lead screw error like .004" over 12". This means if you have a 3 foot lead screw you could possibly have .012" of error. One way to avoid this is to buy top quality lead screws which can be quite expensive. The other way which many major machine builders use is to map or compensate for the lead screw error. Mach 3 CNC software has a lead screw mapping feature in it for helping to get rid of this error. This is not the same as backlash compensation, this is different. You can also get similar results with any software by altering the number of stepper motor steps in your set up page.