Problems with part quality can boost your probe investment

The problem

In any modern machine shop, improving the quality of your milled parts requires metrology-quality data - and metrology-quality data requires machine-tool probing.

A machine tool probe is the best purchase you can make to improve part quality, develop automated part processes and understand the capability of your machines. But it isn't a silver bullet.

A probe needs to be introduced as part of a controlled process to make sure the problems that are causing poor part quality are actually fixed. And problems must be fixed in the correct sequence, or you'll just be pushing error around your process. You’ll get even more error if your probe and your probing processes are not set up and used correctly. Measurement data from a probing system that has not been set up correctly cannot be trusted. This ‘bad’ data can introduce error throughout your part process - which is exactly the opposite result you want from your investment in the probe.

To get real value out of your probing system, you need to engineer out the errors - just like squeezing the toothpaste out of a tube.

The solution

The following checklist outlines the steps you can take to minimise the errors in your metrology data:

1. Buy the best probe you can. As soon as you buy a probe you're in danger of losing tolerance in every step of your process. Buying a cheap probe will be a false economy if the probe doesn’t have high enough accuracy to prevent compound error affecting the tolerances you require. It's advisable to buy the most accurate probe you can, even if your required tolerances aren’t that tight at the moment.
2. Calibrate the probe accurately. Accurate probe calibration is critical on a 5-axis machine to avoid losing accuracy in the measured data. A 5-axis calibration should be taken around a 5-axis artefact and the first step is to set the artefact up correctly. MSP recommends you use a sphere as your artefact, and our patented AutoClock device can automatically calibrate the sphere position. You can then use our NC-Checker program to perform a full 5-axis calibration of your probe.
3. Check and correct your machine tool’s geometric movement. You need to be sure your machine tool is capable of moving to a location accurately. Any error in the machine’s position transfers directly into the probing results which, if used for setup, will affect your parts. The traditional approach to finding such errors is to take your machine out of production and use external equipment to check the machine set-up. However, with a product like NC-Checker, you can use your probe in a closed-door, lights out environment to perform a daily machine capability check to prove that the machine positioning is correct.
4. Create part probing programs - manually. You have choices for creating part programs - you can either create programs manually on the shop floor at the machine tool, or generate them automatically in your design office with your CAD system. The manual solution involves the operator programming directly into the HMI of the controller or using local programming products. The operator is unlikely to be a metrologist and so will most likely create simple probing programs that don’t get the most out of your probe. Programming directly into the controller usually involves the manual entry of tolerance or program information - which is a recipe for introducing human error. This is not a recommended solution for any automated production facility.
5. Create part probing programs - automatically. The automated solution (and the one recommended by MSP) involves generating probing programs using your CAD system in your design office. The NC programmers and designers know the critical areas of the part and all the design and tolerance data is available to them. As a result, the probing programs will have the correct metrology-sound methodologies built into their program creation and post-processing. They can be developed with your existing work-package delivery and run on a schedule as part of your automated factory system.
6. Check the condition of supply. You must check that there is enough stock material to allow the part to be machined by probing and measuring the stock at key locations. This is especially important if you are machining from castings, forgings, or additive parts.
7. Check and correct your part setup for machining. It is vital that the part is positioned accurately in the machine volume. Your probe can be used to measure the part in the machine and to check it's in the correct position and orientation for the machining program to run successfully. Using a system like MSP PerfectPart, you can also automatically update the controller with the calculated correction to the mis-alignment.
8. Inspect the part before it is unclamped from the fixture. The final step is to use the probe and a suitable probing program to inspect the part and create an inspection report before the part is unclamped from the fixture. This will determine whether the machine tool has machined the part correctly. If there is any induced distortion from the fixture this will be released back into the part when it is unclamped. If you don't inspect the part before you unclamp it, when you measure it in final inspection you'll not be able to tell where the error came from - the machine, machining or fixture or during the transfer to a CMM.

Next steps

Investing in a probe is the best thing you can do if you want better quality parts, factory automation and automatic control over your machine capability and maintenance schedule.

The only thing you might be missing is some expert help to maximise its true potential.