A) It improves the accuracy by a factor of 40, from 20 ppm to 0.5 ppm (parts per million). So for instance, over the length of a 40 inch table, without compensation, the measurements can be off by 0.000800 inches (40 inches X 0.000020). With compensation, the measurements improve to within 0.000020 (40 inches X 0.0000005).
Q) How often is too often to run a ballbar trace?
A) You decide. Renishaw runs a ballbar daily on each machine as part of a thirty minute daily maintenance break. The machines are productive for 20 hours out of the day, and only need one operator per seven machines.
Q) Does the Renishaw laser work differently than the range finder I take to the target range, or hunting?
A) Yes. A big practical difference to the user, is that with a range finder, you can interrupt (block) the laser beam for a moment and continue once the blockage is removed. With a Renishaw laser (which uses something called interferometry) if the beam is broken once a measurement is started, the measurement must be restarted. This is because the interferometry keeps track of (counts) how many “wavelengths” of the laser beam an optic is moved by. If the beam is broken, the circuitry loses count, and that is why it must be restarted. With a range finder, you are sending out a known pulse stream (or coded message) periodically on the laser beam, and simply timing it’s return to the range finder and calculating the distance based on the delay in the signal being returned.
Q) Why use valuable time on my machines to run a ballbar trace?
A) Preventive: Track trends that if left unchecked will eventually be a contributing influence in creating rework and scrap.
A) Diagnostic: This is for a machine that you already have concerns with. A ballbar trace will provide error contributions from twenty conditions that produce a less than ideal trace. (By the way, even an awesome machine in great shape will exhibit a “less than 100% perfect trace – this is a measure of how sensitive a ballbar trace is). The software provides the worst offending conditions – about four usually, that account for the majority of circular deviation.
Q) Should you “fix” all of the largest sources of machine error detected in a ballbar trace?
A) It depends, but typically no. Often you can bring your machine back within an acceptable tolerance with only attacking one or two sources of error.
Q) How do you know which sources to fix?
A) Pick the ones that both are the least expensive/disruptive to fix, and still restore the machine back to the desired performance.
Q) How do you know how much fixing a particular error source will help?
A) The software allows you to take your actual machine’s data (from the ballbar trace) and simulate the impact of say cutting one of the error sources in half. In a practical sense, you do not have to reduce an error source to zero, just need to not be so huge.
Q) What are some of the benefits of this simulation?
A) For example, let’s say one of the error sources indicates a worn ball screw. Before spending the money on parts for your machine and the cost of downtime, wouldn’t it be nice to know if a ball screw replacement would be sufficient to restore your machine?
A) Continuing with the same example, let’s say that “Squareness” (a frequent offender in ballbar traces) is an issue. Addressing this, often entails adjusting the supports between the machine and the concrete pad beneath the machine to remove/reduce the twist in your machine. So typically no expensive parts needed. Please notice I did not say this is a simple task. – Replacing a ball screw is more deterministic in how long it will take.