The machine tool vertical is a good example of the latter – an industry where accuracy is critical to assuring performance – because there really isn’t (no matter how small) any margin for error. But before we look in more detail, let’s remind ourselves that there are plenty of examples of inaccuracy in our daily lives, and whether you realize it or not you’ve probably experienced many of them first-hand. Indeed, you accept them as if accuracy wasn’t important.
When has this happened? Well, next time you’re out in your car, look at the speedometer. You may be surprised to know that it’s less accurate than you think. Likely, as you hit the accelerator and your speed increases, the numbers you see on the dial become more and more theoretical.
If you think you’re a responsible driver
Why? Well, your car’s manufacturer never intended you to drive as fast as you are now, least of all on winding country or city roads in less than perfect weather! He didn’t calibrate your speedometer for that. Now maybe on a racetrack with a tail wind (perfect testing conditions), those speedometer readings you see would be very close to true, but if you want your car’s engine to work for tomorrow morning’s commute, you might be better off avoiding putting that claim to the test.
The point is, there’s a gulf between lab or factory measurements which are contrived in “ideal conditions” and what happens in the real world when those measurements are called into practice. The real world is where – to continue the automotive metaphor – the rubber hits the road. And unfortunately, the real world is where we live. Put simply, in a frictionless vacuum, accuracy in measurement can almost always be achieved. But who drives their car to work (or operates pretty much any other machine) in a frictionless vacuum?
What it says on the tin: time to get frustrated
If you develop measurement equipment for a living, you’ll be familiar with this story and you’ll recognize that we’ve reached the point in this blog where you allow yourself a wry smile or alternatively shake your head in frustration! You’ll know exactly what we’re saying. And that means you’re familiar with the sinking feeling you get when reading the claims on all too many equipment many spec sheets.
It’s why lab measurements don’t impress us and shouldn’t impress anyone too much either. If you want to know how well any piece of equipment performs, you need to test it in the environment where it is supposed to be working, under operating conditions.
The milling tool example – inside the machine
Let’s look more closely at this assertion using milling tools as an example. Most measurement equipment for milling tools is designed to be used around or in the machine itself. But from the perspective of assuring accuracy, this environment is an engineer’s nightmare. Oil and coolant coat everything in (and out of) sight. Metal chips fly in all directions. Even a traditional tape measure will curl up in fear at the mention of being used as a source of accurate metrics.
Inevitably, this means when it comes to measurement, most often the task will be done outside of the machine. Vernier calipers, v-anvils, and micrometer measurements are the tools of the trade here, all pragmatic choices when you have the luxury of time, and the operation allows it. If you do the task correctly, you can basically operate in lab conditions and just read the specs to know the degree of accuracy you derived from your measurement equipment. Right? Well, that’s the theory.
Hairnets and lab coats (really!)
In practice, unfortunately, don’t be fooled! Unless your colleagues and peers are walking around in white lab coats and hairnets (in which case you may actually be at an avant-garde rap concert and not in a laboratory at all), you’re taking measurements in conditions that are far from ideal.
For example, a micrometer has a written spec of 0.00005″ accuracy if you buy a good quality one with a ratchet (to minimize user errors). It can have an impressive precision of 1.3 µm. Yet, when using it for measuring tools, few if any machinists would be surprised to see variations up to four times off its scale. Why? Well, the short answer is: you’re not using it in a lab. The milling tool itself has a fine coating of oil, and the air around you has a finely dispersed fog of dust and particles floating in it. Dust particles can easily be 10µm/0.4 thou or up to ten times this size. A fingerprint is between 20-50µm or up to 2 thous thick. In short, we have a problem!
So, what’s the answer?
With this reality in mind, what should you do? Of course, the basics: you keep your tools clean and your equipment under covers. You also repeat measurements until you can see a pattern emerging. And you make sure you measure the diameter of every cutter.
Do this and you have a starting point for accuracy; you will know the repeatability of your conditions. Then, ask another person on the shop floor to do the same thing you’ve just done. Manual measurement means that you need to take the individual operator into the equation too.
For contact measurements, you also need to recalibrate your tool(s) regularly. Make sure that repeatability is even throughout the range of the device. And replace components before they get worn out. Even the best tools haven’t achieved immortality! And a side note – when measuring milling tools with contact measurements, be gentle. Carbide is very hard and brittle, and tools and measurement equipment might break in inexperienced hands.
When measuring from outside of the machine, there is also a further built-in disadvantage – a tool will enter the machine and there will be a discrepancy in the fit between the chuck and tool holder. Plus, there are effects from the machine in action to take into account.
Spinning an endmill up to 4.000 rpm or perhaps 14.000 rpm or even higher affects the tool and holder as well as the machine itself. The best way to know these effects is to measure the milling cutter at full speed. You’ll find many microns are getting lost on their way to the machine.
In the next blog, we’ll look at the other end of the spectrum: measuring from inside rather than outside the machine. That approach to measurement throws up plenty of its own issues, too. But the over-riding point in both cases is the same. Achieving accuracy is not a simple task. To do it successfully, you need to know what you probably don’t know yet. Unknown unknowns, as has famously been said before.
About Conoptica
Conoptica has been providing high tech camera-based measurement solutions for the metalworking industry since the 1993. We make sure that you can access key, quantitative data about your products and tools.
Conoptica is also the market leader for measurement equipment in the wire & cable industry. We are working to create a generational shift for inline measurement equipment in the machine tool industry.