CMMShop Blog

Well, it`s that time again.  World Metrology Day 2016!  May 20’th...a day where we celebrate the signature of the Metre Convention way back in 1875.

To be honest, I didn’t know much about the history of this momentous day until I did some research myself, and it’s really quite interesting...if you’re a Metrology geek like me.  I’ll save you some time and give you all a somewhat brief history here.

The SI unit we all use today dates back to 1215, when the Magna Carta (Latin for The Great Charter) defined the standards of measure which were to be used throughout England.  I’m sure you don’t have hours to read the full story so sticking to the milestones, we’ll just jump forward almost five centuries to 1707, when England and Scotland united into a single kingdom, and the Scots agreed to adopt the same units that had been in use by England for almost 500 years...then, some time later (in the Eighteenth century) Peter the Great, Czar of Russia, adopted the same standards to facilitate trade.

??? Did you know that abuses of measurement units was one of the causes of the French Revolution?  Who knew?

In 1789, a National Assembly was held, where one of the items on the agenda was the reform of measurement units.  What came out of this was the introduction of the metre and the kilogram and the formation of the metric system, leading to the eventual manufacture of prototypes.

Between 1850-1870, many other countries adopted the metric system including Spain, several South American republics and many of the Italian and German states.  The Netherlands, however,  had already adopted the metric system back in 1817.  Another milestone, in my opinion, was the International Postal Union adopting grams as their unit of measure for weights of letters in 1863.

Jump back slightly to sometime in the 1860’s, when it was discovered that there was not only wear evident on the standard metre bar, but that it had a tendency to flex during use, which cast doubt on the reproducibility of the metre and the kilogram.  Napoleon III took this opportunity to try and create his own standard and invited scientists from around the world to attend a conference in Paris...which proved to be bad timing.  Two weeks before the conference, the Franco-Prussian War broke out, France was defeated, Napoleon went into exile, and the newly unified nations of Germany and Italy adopted the metric system.  One problem though...the prototypes of the metre and kilogram were under the control of the Third French Republic.  Problem?  Nah.  The new republican government re-issued invitations and in 1875 scientists from thirty European and American countries met in Paris.  The main focus was the replacement of the existing metre and kilogram, since the existing standards were deemed unreliable and may not be the same as when they originally adopted the system in 1799.  Apparently, French politicians feared the metric system would be rejected, since the existing metre was 0.03% (300 µm) shorter than the design length.  I’m still scratching my head as to how they measured 300 microns back then...  Anyway, in the end, it was decided that the new standards be manufactured as closely as possible to the values of the existing artifacts.

The new prototype metre was the same length as the original one, but the newly manufactured bar had an X cross section rather than rectangular, to reduce the tendency to flex while in use.  In addition to this, it was made a little longer and lines were engraved exactly one metre apart to eliminate the wear issue on the end faces.

The official metre was retained as the international standard until 1960 when, get this, it was redefined in terms of the wavelength of the orange-red line of krypton-86...wait, what?  Seriously?  Apparently, all you need is the equipment to heat a sample of krypton-86...OK, no biggie, I think I have some in my crawl space somewhere. Then, just look for the reddish-orange line produced...simple so far, right?  Oh ya, there it is...I see it.  Next, get out your calculator.  Final step...just multiply the width of that orange-red line by 1,650,763.73 ...and voila, there’s your metre!  Simple enough, right?

Now, before you start questioning how the heck someone came up with that...come to think of it, didn’t the government dabble in LSD experimentation around the same time?  No matter, it’s since changed...phew!  This definition for the meter only lasted until 1983. Scientists then decided to define a meter by how fast light travels in a vacuum. Now this makes more sense to me!  This system is even more exact than the one based on krypton-86.

What about the kilogram you ask?  In 2014 after the 25th CGPM (General Conference on Weights and Measures), the prototype kilogram was still in use. It is expected to be replaced by a new definition within the next few years, though not before the next CGPM in 2018.

Now that we’re somewhat up to date on the evolution of the metric system, you can see just how how important the standardization of a common unit of measure was, and is today.  The science of measurement plays a pivotal role in almost everything we do in today’s society.  Everything we rely on and take for granted in today’s society would not be possible without a standardized method of weights and measures.  Try to think of something that doesn’t rely on this system in some way...I bet you can’t :)

How many times have you spent hours struggling with vices, blocks, clamps, and even glue, in an attempt to create some sort of rudimentary CMM fixture to hold the part you need to inspect? If you just rolled your eyes, then read on.

Having spent the past 20 years programming CMM's and inspecting an untold number of parts, I've got loads of experience with part fixturing applications and have often been frustrated trying to find a suitable method of restraining a part for inspection utilizing "bits and pieces" from the tool shop. More often than not, a custom built CMM holding fixture is either not in the budget or is an afterthought, having been ignored during the planning stages. Now the CMM programmer has to figure out some way of holding the part, which is not always simple, and they're usually under the gun to get the part inspected after every other stage in the process has eaten up all the time. 

 

I hate to paint a picture with one broad stroke but it's been the truth in the vast majority of cases over the years. The inspection process is typically an afterthought and once parts are available, and all the time has been used up, the CMM programmers/inspectors are expected to pull results out of their...uh...hats, let's say hats. And, of course, it needs to be done today.

The CMM programmers reading this will relate. The inspection process is an experiment of sorts, and, as with any experiment, there are variables which must be controlled in order to obtain accurate and reliable results. Perhaps the most critical of these variables is the method you'll use to restrain the part, and the fixture is the key to replicating your experiment. This is your starting point and it needs to be right.

Proper part fixturing requires the part to be restrained securely, while still leaving all features to be inspected accessible to the CMM probe. This requires that the six degrees of freedom be controlled but a CMM fixture only needs to hold the part securely to accomplish this, it does not need to hold the part on the datums in most cases. If you decide to use the fixture to locate the datums, it's no longer just a holding fixture, now it's a gauge, and needs to be very accurate to replicate the datum points. In fact, the preferred method is to leave the datums accessible so you can create the datum alignment using the CMM. It's especially useful if your fixture locates the part in a repeatable manner so that a manual alignment is not required for each part. 

 

Now, at one end of the spectrum, you've got a custom built, dedicated CMM holding fixture that will do the job perfectly. On the opposite end of the spectrum, you've got your vices, blocks, clamps, and of course, the glue brothers....crazy and hot (you just rolled your eyes again). In the middle somewhere, lie the modular CMM fixture kits, which have always been intriguing to me but seemed a bit pricey at several thousand dollars for a starter kit, and so I've always gone to either end of the spectrum. Having co-owned a contract inspection lab and gauge/fixture shop in the past, I'm quite familiar with how much it costs to build CMM holding fixtures and it's usually been cheaper to build a CMM fixture than to have a modular CMM fixture system at my disposal...until now.

I was recently introduced to the FixLogix modular CMM fixturing system and it impressed me with both its versatility and price point. Fixlogix has taken what already existed on the market, improved upon it, and offers it a fraction of the cost. Now there's a kit that would normally run you well over $4000, available for less than $1200 in many cases. The justification just got much easier. A gauge/fixture shop won't get far with your $1200 building a dedicated custom fixture, and it's just that, dedicated. If you have any engineering changes to the part, you may need to fork over even more money to modify the fixture, not to mention lost time. Don't get me wrong, dedicated fixtures absolutely have their place, I'm merely pointing out their limitations when compared to a versatile modular system.

 

The Fixlogix system utilizes a T-Slot system for positioning and fastening components to the plate, allowing for infinite adjustability in multiple axes, not restricting you to a grid of tapped holes. The plates are constructed from anodized aluminum extrusions, which are just as rigid as a solid gauge plate, but at a fraction of the cost. Another innovation I was thoroughly impressed with is the FixLogix spring clamp, which due to its unique design, allows for near zero clamping force while still locking the part in place. This is extremely useful with flexible and delicate parts and cannot be accomplished using the typical spring clamps available on the market, which require you to push the clamp onto the part to achieve the desired clamping pressure, resulting in a significant force on the part surface. In addition to these innovations, FixLogix also utilizes stand-offs as a base for many different components such as rest buttons, conical pins, hard stop pins, jack screws and magnets, reducing the number of dedicated components required.

If you've read this far, you're clear I'm biased towards FixLogix. Of course, I could have written extensively about all of the other systems on the market, but having done my own research and spent the past 20 years programming CMM's, I figured I would save you from tedium and focus on what I believe to be the best all around modular CMM fixturing system available on the market today. In fact, I was so impressed with the system I now offer the FixLogix system as part of my turn-key CMM program packages.

What are the key factors to consider when choosing a CMM?
This is a big question. First, CMMs are very versatile, but you need to be clear about what the CMM’s function will be. For instance, will it be in a lab inspecting first-off parts or on the shop floor checking production parts 24/7? Will it be checking simple parts or parts with complex geometry?  Will it be checking precision machined parts or castings or plastic moulded parts, etc. What are the tightest tolerances you’ll need to inspect and what types of tolerances (i.e. form, position and orientation)? Will the speed of the inspection or cycle time be an important factor? The difference between CMMs has a lot to do with the materials used rather than the basic design, which doesn’t seem to have changed drastically in decades. Modern CMMs are built with lighter materials and smaller, more powerful servo motors allowing them to accelerate to scary speeds (hint: never lean over the bed of your CMM while a program is running to have a closer look at the part. You could end up the victim of a 500mm/sec CMM head-butt, or worse, impaled by the styli.). I’ve personally experienced the head-butt, although not at 500mm/sec, but the styli impaling was a story told by a CMM calibration technician I know.

Materials such as aluminum and ceramic are used to lessen the thermal effect but for different reasons. Ceramic has a very low coefficient of expansion and is therefore very stable throughout temperature fluctuations. Aluminum, while having a much larger coefficient of thermal expansion, reacts to temperature changes rapidly and in a linear manner and therefore can be compensated for accurately and in real time. Having said all that, it still seems the biggest advancements in technology have been made in the area of probing systems and software.

As far as probing systems go, there are many to choose from so I’ll just touch on the main categories. There are two distinct methods, single point probing systems and scanning systems. Single point probing systems take individual “hits” on a given surface whereas scanning systems stay in contact with the surface being measured, gathering hundreds or thousands of points in single sweep. The big difference is point density and how much of the surface you can sample. Behind the probing system is the probe head, which can be fixed or articulating, and is one of the most important factors to consider.

A fixed head means you’ll be relying strictly on the probe build-up (you may need to get creative here) to reach the features to be measured whereas an articulating head gives you the flexibility of hundreds or thousands of probe positions, allowing you to reach many different features with a single probe. There are articulating heads that lock every 7.5° and 2.5° and more recently, a head with infinite possibilities. Another key factor to consider is whether to get a manual indexing head or a motorized head. If you’re only measuring 2D parts and a simple “down” probe is all you’ll need, then of course the motorized head may not be necessary, but if you’re measuring more complex 3D geometry, then I would absolutely go for a motorized head. Having a motorized head on a CNC or DCC machine allows you the freedom to run parts completely un-manned. Take the same CMM and stick a manual indexing head on it, now you’ll need to do some babysitting when running programs that have more than one probe position involved.

Equally as important as the probing system, is the software you choose. Off the top of my head, I can think of at least a dozen different CMM software packages on the market but it seems the lion’s share of the market consists of the big three OEM’s; Browne & Sharpe (Hexagon), Mitutoyo and Zeiss. At Quality Inspection Technologies, we use all three. Being a contract inspection lab we come across all sorts of applications and I like each software package for different reasons and wouldn’t dare pick a favourite (at least not here).

These days you can mix and match CMM manufacturers and software. For instance, my Mitutoyo CMM also runs with PCDMIS and could potentially run with Calypso if I so desired. My advice would be to take a couple of sample parts, typical of what you will want to inspect, and have each potential OEM create a short sample program in your presence, so you can witness how user friendly it is. Take a few features you consider to be your biggest inspection challenges and compare how each software package is able to handle them. While all packages do the simple stuff very well, each has their own special features the others may not have and could make all the difference for your application. This should help you conclude which software is the best fit for your application.

In the end it’s the combination of the machine, the probing system and the software that you’ll base your decision on but it’s a good idea to look at each factor individually as well.

Where is the best place to install your CMM?
In an ideal scenario, your CMM belongs in a separate room that is temperature controlled, free of dust (as much as possible), with a clean and dry compressed air supply, and as far away from the chips, coolant and vibrations of a typical manufacturing floor. Not all scenarios are ideal, which is why CMM manufacturers have answered some of these challenges with such features as real-time temperature compensation and air cushion vibration damping systems, which are extremely important in these less than ideal scenarios. Some pretty rugged shop floor CMM’s exist on the market with completely enclosed air bearings and guideways that can handle the dirt and dust of a manufacturing environment that would ruin a traditional CMM in no time. Some shop floor CMM’s have even done away with air bearings altogether. In either case, there’s nothing to prevent dirt and oil from collecting on the styli so it’s good practice to regularly clean the styli to ensure reliable measurements. There’s also a breed of ultra high accuracy CMMs which call for a clean-room type of environment to produce the sub-micron accuracies but these are the exception and not nearly as common.

Calibration: Is it a given that CMM’s are calibrated once per year?
While a one-year calibration cycle is pretty typical, there are periodic checks that should be done to maintain confidence throughout the year. It’s also a good idea to perform a check after a particularly hard collision; they do happen and it’s imperative to know whether it has a negative impact on the accuracy of the CMM. A periodic check can be as simple as running a program on a retained sample part or test block through a program or as involved as something like the Renishaw MCG (Machine Checking Gauge), which checks the volumetric accuracy of the CMM and is sometimes part of the annual calibration process. It’s best to run your periodic check directly following the annual calibration so you can establish a baseline from which you’ll compare the results from subsequent checks.

You could make the argument that running periodic checks would allow you to extend your CMM’s calibration frequency but there’s more to consider than just maintaining accuracy. In addition to calibrating your CMM, your calibration technician normally performs routine cleaning and preventive maintenance and believe me, I’ve seen CMMs that look like they are cleaned once a year at best, and that’s probably done by the CMM calibration technician. Given the extreme accuracy of modern CMMs and the tight tolerances we rely on them to inspect, choosing to extend your calibration cycle to save money, in my opinion, is a risky proposition.

What key advancements in CMM technology do you expect to see in the next 5 years?
Alright, so don’t get me wrong, I love CMMs and I don’t expect they’ll ever be obsolete, but there have been some extraordinary advancements in technologies such as laser scanning, white light scanning, camera systems and computed tomography (CT). Laser and white light scanners can rapidly scan complex geometry for comparison to nominal data or for reverse engineering and have accuracies reaching 0.0005 in. or better in ideal conditions. Camera technology overall has blown up over the past 10 years leading to advancements in photogrammetry (taking measurements from photographic images) and handheld probing systems that rely on camera tracking systems, soon to leave the portable arm in the dust.

Probably the most interesting emerging technology is computed tomography. Originally developed for the medical industry, it’s been further developed for use in metrology and has the ability to measure internal features, hidden passages, wallstock, defects, porosity and density.

I’m sure we can all agree on one thing. Delivering a quality product is priority #1…or at least that’s what we say and we really mean it! Then why is it that so often the people charged with the huge responsibility of determining the ultimate quality of the final product, are lacking the proper training required to carry out this duty? I’m referring specifically to CMM inspectors.

Let’s consider that the vast majority of CMM inspectors out there have received their training on-the-job. If you’re one of the very lucky few, you were trained by someone who really knows their stuff and has been at it for a while. If you’re one of the unlucky, you received little or no training and are left to learn on your own. Sure, you can get training in the specific CMM software you will be using, but that alone is like learning how to operate a car and having no idea what any of the road signs mean…a recipe for disaster for sure.

A CMM is an inspection tool, but unlike most inspection tools, the results obtained on CMM’s are not so clear-cut and very easy to misinterpret. It’s easy to select a measurement from the software, probe a few points and attain a number, but is it the number you’re looking for? What exactly is the drawing asking for? Did you take the appropriate amount of points, in the appropriate locations? Apply the correct calculation for the geometry being measured? Filters? You get the picture. There are so many more factors to consider in being confident in the values attained.

Whether you fall into the lucky or the unlucky group, you perform your duties to the best of your abilities. “You know what you know and you don’t know what you don’t know,” operating inside a bubble and you’re point of view is all you have. You can imagine how nerve-racking it is for a CMM inspector to make a call on whether parts conform to specifications or not, when there is an internal conversation of uncertainty running in the background.

Running my own Metrology company since 2000 has provided me access to literally hundreds of companies in the manufacturing industry and it became apparent that CMM inspection, in so many cases, has become a self-taught skill. The danger in self-teaching is there is usually no one more experienced to learn from, and although one of the ways we learn is by making mistakes, you can’t learn from a mistake when you don’t know you’ve made one. Having access to someone more experienced is invaluable when learning a new skill.

If you read my first blog “Operating Inside of a Bubble,” where I set out to paint a picture of the life of a CMM inspector from my perspective, you now have a context for this conversation.

Some industry experts are forecasting a skills shortage. Some say we’re already experiencing one. On the surface, a skills shortage would pose problems. In a skills shortage, where would employers get the people who possess the skills they require for any given position? If an employer thinks along the lines of only people with skill “A” may succeed at job “A” then yes, a shortage in people possessing skill “A” would pose a problem…and you’ll miss out on some really extraordinary people by operating within these boundaries.

Given the opportunity, the appropriate resources and access to historical knowledge, it really comes down to finding the right person for the job, not necessarily the person who already has a given skill and experience—who, by the way, may also be entirely the wrong person for the job.

A few years ago I was introduced to the Kolbe A Index by a friend and business associate. The Kolbe A Index measures a person’s instinctive method of operation (MO), and identifies the ways he or she will be most productive. I took the short test, as did both my business partners and several other people I do business with. What I got from all this is that all the training and experience in the world doesn’t necessarily mean you’ll excel in a given position if the requirements of the job are not aligned with your natural method of operation or how you process information. So what if employers were used the Kolbe A Index or something like in their search for the right person for the job. In this scenario you can see that it’s not so much a potential skills shortage that poses a problem, rather the way an employer perceives it.

There are literally tens of thousands of talented people in Ontario alone looking for work. Skills shortage you say?

You may be wondering how this so-called skills shortage relates to our CMM community. Consider that one of the responsibilities of this community is knowledge management and that by sharing what we know, we create a resource for future generations. Sounds a bit melodramatic, I know, but please read on.

As I alluded to in my previous blog, the sharing of knowledge is something engrained or “hard-wired” in all of us and has always been part of what it is to be human, but really it goes much deeper than that. Civilizations have always depended on the passing down of the history, knowledge and skills necessary to not only survive, but to thrive and evolve in the environment they live.

Take a leap with me here and consider that our community of CMM inspectors is similar to a civilization in that without the passing down of history, knowledge and skills, how do we evolve and advance? The answer is we don’t. In fact it’s difficult to even sustain at some level since there is little in the way of “common” knowledge. We essentially keep re-inventing the wheel for ourselves, each wheel slightly different from the next. With little in the way of formal, standardized training and certification for CMM inspectors out there, how do we effectively perpetuate these skills? In my opinion, it’s up to this very community to do something about it.