Any discussion of machine tool error sources must begin with the 21 degrees of freedom of a typical 3-axis machining center, for example:
Each axis has the following seven errors, or "degrees of freedom":
The x-axis shown above has linear error, pitch, roll and yaw errors, x to z, x to y squareness errors and vertical straightness error.
Most, if not all these errors can be quantified with our Renishaw Ball Bar and Laser calibration equipment. Some errors can also be checked using granite squares, straight edges, and levels.
All machine tools have the above errors. Your problem, as a user, is whether or not the magnitude of these errors causes your parts to be machined out of tolerance, or if you cannot maintain your machine capability (cp, cpk).
For example, assume that your machine has just had a ball screw replaced. Is the ball screw the same length as the old screw? If not, the compensation tables that are resident in (most) CNC controls are incorrect, and may in fact add to the ball screw error.
A laser interferometer is needed to check and verify this error. After quantifying the errors and where they occur, a comp table can be generated to put the ball screw back in new machine specs.
Pitch, roll and yaw errors are errors of rotation. Depending on the tolerances you hope to hold on your machine, these errors may or may not be significant. On the x-axis, pitch and roll can be checked with the angular optics of the laser interferometer. Correcting these errors is usually done by correcting machine levels, but may involve more in-depth corrections. Yaw can be checked using straight edges, and is usually caused by bad or loose bearings or slide ways.
In the field of metrology, thermal changes are like having the flu. They are difficult to control, and it is difficult to predict what a thermal change will do to a machine tool. It is therefore advisable, if a high degree of accuracy is desired, to maintain the ambient temperature as closely as possible. Thermal changes inside the machine can cause detrimental changes in ball screw positioning, as well as changes in axis squareness. Many manufacturers place controls and special cooling inside the machine (i.e., cooled oil circulating around spindle bearings and through ball screws) to help minimize thermal changes. MAKINO has patented this technology. A user can help minimize thermal problems, once an understanding of how thermal changes adversely affect machine accuracy.
Other manufacturers use complex algorithms to change the machine positioning as the temperature changes, in attempt to control temperature changes and their effects on a machine.
High demands are made on machine tools today. Users want 24 hour per day availability, with high horsepower heat generating roughing ability combined with positioning capability in microns. Machine tool builders must compromise in order to give the user boring mill accuracy and roughing capability.
The problems we have are: (a) how can we quickly (so as to not use valuable production time) get a footprint of the machine performance? (b) How can we verify that the machine servos, ball screws and the machine squareness meet original machine specifications? (c) After a machine crash, how can we verify that machine alignments were not affected, and if the alignments were changed correct them before scrap parts are made? (d) How can we quickly verify that our new machine tool is within the quoted accuracy?
The answer is with the Renishaw Ball Bar. The ball bar check is performed on two axes at a time. It is quick, and produces valuable data and diagnostics, highlighting ball screw performance and backlash, machine squareness and servo response. By checking the machine periodically, trends can be spotted, which can help eliminate un-planned downtime.
The ballbar and laser interferometer can be used on most CNC turning centers as well as CNC machining centers. For further information on how Beckman Precision, Inc. can assist you, please contact us via our contact page. For more information on Renishaw products, click here.
