The History of Single Point Diamond Turning Part 2 with Dr. Jeff Roblee

In part one of this video ASPE Fellow, Dr. Jeff Roblee, talked about the early days of diamond turning starting with the precursor company to Precitech - Pneumo Precision. In part 2 of the video Jeff talks more about his time at Precitech and recent advances in the field of diamond turning and optics manufacturing in general. 

Transcript:

Savannah: Hi. I'm Savannah Jones, and today we're speaking with Jeff Roblee, a divisional Vice President of Technology at Precitech in New Hampshire. Besides increased complexity and shapes, what else can you share about new products and additional and tighter performance requirements?

Jeff: More recent applications for freeform optics are for augmented reality or mixed reality goggles, and that's pushing a lot of even crazier freeform shapes that need higher form accuracy.

But the surface quality and how you define a good optical performance has evolved over the years as well. And so we normally focus on surface finish, which you need for keeping down scattered light and any optical surface can't be rough. It has to be an optical finish. So we focus on surface finish and then normally form accuracy. So figure accuracy - how much peak to valley deviation is from the ideal over the whole surface. It also comes into infrared optics too.

We get performance requirements in terms of waviness. And so in terms of spatial frequencies on the surface, high spatial frequencies are roughness, low spatial frequencies are form error. Then we get this middle zone, mid spatial frequencies, a lot of people call it waviness.

And that's driving a lot of the -- and there's different error sources for -- on a diamond turning machine for all three types of errors. And that's somewhat we're focusing on now and our new fast tool servo technology is typically well suited to address some of the new waviness specifications on optics for, augmented reality, Okay?

Savannah: Jeff, while everything continues to evolve, from your experience, how has Precitech Innovation such as spindles and slides helped customers to ensure they can consistently and routinely turn out high quality components.

Jeff: So the slides moving at low velocity are floating on a film of oil. The high speed spindle is the highest heat source on the machine, and we have to worry about temperature and thermal expansion as one of the biggest error sources we're trying to watch. So that has to be an air bearing, supported bearing. And they all have direct linear motors or rotary motors. There's no mechanical contact, nothing wears as far as the mechanical part. It's just covers and things like that. And so we keep pushing to make the bearings stiffer, lower vibration to get better surface finish, and then a better accuracy.

And we push it to the new limit with our newest fast tool servo, which is an air bearing guided five millimeter travel device with the voice coil actuation. Again, no moving components. But it requires better positioning technology with the scales, as well as how you control the motors. And the electronics controlling motors and measuring position have improved a lot over the years.

And the piece that's driven that is high speed computing. That's actually come down way down in price. In the early days when I started, you had to build your own computer because you couldn’t buy a commercial one that had enough power to do what you needed. And so that's driven the cost way, way down the new electronics in particular and improve the accuracy because we have very, very high positioning resolution down into the picometer range, tens of picometers we're talking about. It's mind boggling that we can position down at those levels.

Savannah: What about instances where the parts change in weight from run to run? How do the slides and spindles respond to changing loads?

Jeff: Well, the dynamics of the slides It's a whole system. It's a structural loop from the work piece, through the slides, through what's supporting the tool. Anything that affects that structural loop, and inertia changes will do that, will change the dynamics of it. And to get really high performance, we have to fine tune the servo loops controlling the position of each of the axes. When you change Inertia, it will change anything has a natural frequency.

If you have a very fine tuned servo, you tune it with particular natural frequencies in mind. And if you have a lightly tuned servo, it doesn't matter so much, but you pay a penalty in poor surface finish and form error. And so we've been working on new technology to automatically detect changes in inertia and natural frequencies. And adjust the servo gains to keep the same servo performance as you change the mass or the inertia on the slides or spindle.

Savannah: Well, with that in mind, how does Precitech help those customers to rise to the challenges and overcome those complexities?

Jeff: Oh, I also run the applications group here and the very experienced team there in applications. And had faced a lot of machine challenges over the years. And they remain available to assist customers in unique applications. We develop a lot of software. We have one called DIFFSYS. It's a computer manufacturing software.

We can go right from an optical design file and create a tool path that will run on our machines and try to simplify the process as much as possible going from a design to a finished optic as smoothly as possible. We continue to work on that. We've added more ease of use features on our controller since we do our own control system, machine monitoring, we’ve got a whole wealth of machine diagnostic tools that I think are unique to the industry that give feedback to the operator what's going on with the machine in the process, if they've got a dull tool or something's abnormal, they'll flag that and tell them.

So we're always trying to allow customers to create better and better quality optics with lower and lower skilled operators. That's sort of the dynamic. A real skilled operator can get a lot out of even a poor quality machine, but there are very few of those around these days.

Savannah: So do your customers enjoy peace of mind, thanks to Precitech products, and the reliability?

Jeff: I'll give you an example. We measure the outer straightness of all of our slides. And it being an oil bearing slide way, the slides are made out of Dura-bar and, you know, high form of cast iron on a granite base is very, very stable. It doesn't change with time. So this out of straightness, which We'll go back maybe years later and recheck to see if anything's changed. And almost invariably, it's within twenty five nanometers, fifty nanometers of the errors that it had previously.

And so we make all the slide components, particularly the spindles, very robust. In case you do have an accident, everyone has an accident, where the tool crashes into the machine. The machine can be quite robust against that. And because we build our own spindles here too, we can repair them readily if something happens, like they lost air pressure, or got the spindle contaminated from dirty air, for instance, we can turn that around very quickly and less often in less than a week's time to keep customers' machine time down. We have also of course monitors. For instance, there's a loss of air.

We have a tank on the machine that will keep the spindle running until has time to stop, just self contained in the machine. So you can cut the airline and still not cause permanent damage to the machine as an example. Those are some of the things. Got a lot of safety interlocks as well.

Savannah: Jeff, how else does that instill confidence that your machines achieve the level of performance that your customers expect?

Jeff: Well, we do have some features on our machine. We can do on tool process control where we can actually do some levels of inspection of the part on the machine while it's still on the machine. That gives some immediate feedback if something's gone wrong, like, for instance, a worn tool or damaged diamond tool. It's nothing to do with the machine, but it can affect the quality of the part. You can detect that early on just change the tool out and recut again with minimal lost time. We have on machine optical inspection devices we can put on the machine inspect optics right on the machine as well.

We have a whole range of machine monitoring, looking at following errors, vibration levels, And while the machine is cutting and running, a fast Fourier transform, looking for vibration levels on the machine. We could set levels to flag if something is exceeded during a cut, like, for instance, if someone bumped the machine during a cut or someone even slammed the door, or you had somebody drop something heavy in the next room that can couple through. We can detect that in the following error of the machine. We could flag the operator that something happened. They should just take another cut.

Savannah: We started out discussing diamond turning and I’d like to revisit that before we wrap it up. Minimizing the sensitivity to vibration is really important for a diamond turning machine. How does Precitech do this?

Jeff: As I spoke earlier, the key magic of diamond turning. It's a very deterministic process. We call it. It's wherever the diamond is placed with respect to the surface. That's where the surface is created because it's a very sharp cutting edge, low coefficient of friction. The metal cuts very cleanly and smoothly with the diamond tool.

So that means that whatever vibration level is on the machine, the diamond will work like an inverse record player, if you remember records, ages ago. Any vibration level, either the work piece vibrating or the tool vibrating will create a surface texture corresponding to that vibration. And that mainly affects surface finish. And that's been the biggest trend I would say over the last twenty, thirty years is the improvement in surface finish. And that's all tied down to better server control and better control vibration levels on the machine. And so these days, it's pretty common. We can get following errors on the order of a nanometer. And this can drive surface finishes down as low as a 0.2, 0.4 nanometers RMS. It's two to four angstroms RMS.

I never dreamed you could achieve those kind of surface finishes in diamond turning when I first started my career. And some of that's related to stiffer components, the oil and air bearings, tighter, better servo loops. But fundamentally, there's always some vibration sources either on the machine, fans and other things. We try to remove and get off the machine. But the dominant source left over is vibration that's coupled in through the ground.

And a real critical element is how to isolate the machine as best as possible from the ground vibration. And that's where we work with our sister company. So part of our our same business unit in AMETEK is TMC, based in Peabody, Massachusetts. And they're world leaders in vibration isolation technology. And in its interaction between the isolation and the frame and support design of the machine.

And they’re also experts on weldments and much more expert on how to build structures that sit well on their isolators and mate well with their isolators to minimize how ground vibration gets through to the critical part of the machine, where the diamond tool and the workpiece are. So there's a whole range of vibration isolators - pneumatic, isolators, they make active ones with piezo, active leveling with a special valves to help control and cancel vibration. They'll even have vibration cancellation platforms like noise cancellation headsets that can be employed. And how they're integrated with the structure support of the machine is critical too. And then we get working directly with the engineers at TMC. We can get an optimized performance that's really helped drive down the vibration levels on the machine and helped us get to these new levels of surface finish.

Savannah: Jeff, you are a wealth of information and knowledge, and been in this field for a long time. What keeps you interested in the industry and in Precitech?

Jeff: I've been in this industry now for forty five years. I can't believe it. And I still learn something new every day. And it's part of the discipline of precision engineering, just bringing in a lot of engineering disciplines, you know, you have to be really good at mechanical engineering, materials, electronics, electrical engineering, servo controls, and you had to bring this together as a system to get working together So you have all these different challenges in all these different areas.

And if you make improvements in one area, that error source goes down, then another one becomes the dominant one. And every day, it's a new challenge. And I learned something new every day, and it's just I'm still intrigued with this after forty five years being in this field. I'm still challenged every day and learn new things every day, and it's kind of amazing because it's at that level where everything's important.

It's not dominant by one or two error sources. But, so that just interacting with all these different specialists and bringing all these technologies together to get a strong functioning system in the end. That's real exciting for me, you know, from a technology perspective and keeps me going, and I like working here.

Savannah: I agree. Always learning is a great way to live and enjoy your life. Jeff, before we go, what would you say to someone who wants to be a part of Precitech?

You want to be challenged and have something interesting and different to do every day and interact with people, experts in a lot of different fields, bringing technologies together, And then, you know, working with satisfied customers. There's lots of challenges working with service and applications customers in the field.

People are coming up with new uses of our machines every day, really. I get demands for that and try to figure out how to respond to them, and so it's never boring.

Savannah: Jeff, thank you so much for your time and your insight on Precitech. I appreciate it.