With its design-for-manufacture system, metrology supplier Renishaw reduced cycle times and improved productivity aiming for maximum efficiency
By Patrick Waurzyniak
By refining its design-for-manufacture (DFM) system for small machined parts, metrology supplier Renishaw plc (Wotton-under-Edge, Gloucestershire, UK) has honed a system for producing nearly unlimited product changes to machined parts for probes used in coordinate measuring machines and other measurement equipment.
With the RAMTIC (Renishaw's Automated Milling, Turning, and Inspection Center) production system, the company runs its highly automated manufacturing virtually unmanned, resulting in substantially improved cycle times and process efficiency. Under development for several years, the RAMTIC design for manufacture/concurrent engineering system has been further developed and embedded in the CAD system with design constraints available to product designers via the company's intranet web site. The RAMTIC system now uses 25 Mazak FJV250 VMCs, and the company produces about 250,000 parts per month with about 800 machine setups for production of 5000-6000 active part numbers and a production mix of roughly 80% aluminum and 20% stainless steel parts.
Process design improvements on RAMTIC allowed Renishaw to achieve high-quality, high-speed machining with standard Mazak VMCs that are capable of milling, turning, and inspection. Renishaw can run the RAMTIC systems unmanned for 140 hrs a week, while integration of machine calibration, in-process inspection, and process stability all combine to help the metrology manufacturer achieve high quality levels and reduce cycle and lead times.
Renishaw uses the RAMTIC line in milling, drilling, boring, and turning operations that run untended for more than a day at a time, deploying flexible build-to-order scheduling with its MRP system. The system includes using probing techniques with traceable standards, innovative fixturing, and automated work and tool-handling carousels.
According to Mark Buckingham, principal manufacturing engineer in Renishaw's Manufacturing Services Division, a machine upgrade was one of the keys to improved productivity. Renishaw replaced Mazak AJV VMCs with 10,000-rpm spindles with 25,000-rpm FJV machines, enabling helical interpolation at 8 m/min versus 2 m/min on the old machines and maintaining tolerances of 20µm or better.
On the RAMTIC line, aluminum and stainless parts are machined and deburred before undergoing black anodizing in-house to lower lead times. The system features product carousels that include all the parts and tools necessary for manufacturing, and on-machine inspection with artifact comparison is employed to track thermal expansion.
Lean manufacturing concepts comprise some of Renishaw's RAMTIC approach. "A true lean system, to me as an engineer, means it's a pull system," Buckingham says. "The RAMTIC line is well-suited to be included in a pull system. It's got the small batch sizes, it's got the zero set time, and you can use kanban tickets, if you like, to trigger batches. If you look to what we strive for, which is getting rid of setup time, absolutely minimizing the floor-to-floor time of our components, and reducing lead times to an absolute minimum, those are extremely valid lean concepts, but it's part of a push system.
"What we've developed is an extremely integrated system, integrated right from design, whereby you can minimize the lead time through that system, which is absolutely what you need for a lean system. So if you took the RAMTIC system out of its environment, you couldn't imagine one that could be any leaner."
With the RAMTIC machines, Renishaw tried to eliminate anything non-value-added, and now generally gets 140 hrs a week running per machine, notes Buckingham. "If you took the standard Mazak vertical machining center as your datum and called that a productivity output of one, we have calculated that the RAMTIC version, when we first developed it, was 2.3 times as productive. So in any given week, you would get 2.3 times as much out of it. The JFV version is three times as productive as that, so you've got a multiple on a multiple there."
High-speed machining. With the FJVs' 25,000-rpm spindle speed and much faster interpolation, Buckingham says cycle times typically are less than half of those on the original AJV systems. "The parts that we programmed originally with the AJVs, some of those have been taken off and re-programmed for the FJVs, and we've achieved a halving of the cycle time, which is turn is 21/2 times more than it was on the original vertical machining center.
"We've embraced true high-speed machining techniques," he adds. "We use HSK63 toolholders rather than BT; they're much less heavy, so inertia is far less of an issue. Heat-shrink holders don't have moving parts, and are intrinsically balanced."
Installing the Mazak FJV VMCs also eliminated the need to buy more machines and use up valuable factory floor space, as well as additional people to run machines if the systems couldn't run unmanned. "RAMTIC runs unmanned almost all the time," Buckingham says. "The only manned input is the man coming to the machine when the red light comes on, flipping the carousel up, docking another carousel, and pressing the go button. That takes about 15-20 minutes, and happens about once a day.
"You have to factor in planned maintenance and the odd bit of downtime, which does happen," he adds. "The machines are being flogged very hard."
With the company's formal continuous improvement project, Renishaw has invested a lot of engineering resources into understanding the failure modes and removing them, Buckingham says. "It's all very detailed, common-sense stuff, but it makes a heck of a difference. Swarf gets into some very strange and awkward places. If you talk to anybody that was trying to get a machine tool to run unmanned for a long period of time, swarf is the number one enemy."
The RAMTIC process starts at the kitting station, where product carousels are created with aluminum or stainless stock loaded on a universal artifact that has generic part features and is made of the same material as the workpiece. Calibrated at 20ºC with traceability, the artifact stays within the machining environment, subject to the same thermal influences as the part and enabling Renishaw's thermal compensation and traceability in workpiece measurement. "It's the key to our ability to run unmanned on RAMTIC," Buckingham says. Lead times also are shortened by automated in-house tool regrinding, which Buckingham says saves about £200,000 annually.
Using a Unigraphics CAD/CAM system, Renishaw part designers generally produce no drawings but instead import 3-D models directly into the CAM system. The product carousels include all the tools needed for the part machining operation, and a database keeps track of the tools. One of the key aspects of Renishaw's concurrent engineering development also is eliminating use of boring bars in order to run unmanned, Buckingham adds, so every bore uses an endmill to interpolate.
On the carousel, the general layout features six hemispherical components on the fixture, which is called the RAMTIC table furniture and consists of the tailstock, the core, and an indexer. The precision indexer was developed specially for Renishaw and it allows indexing to 24 stations in 15º increments. "We needed that to get to all of the 90 and 45º angles that we needed," Buckingham explains. "The most important part of the specification for that was the absolute accuracy of index that they would guarantee, and they guaranteed about two arc seconds, which is phenomenal. The reason it's so important is that a lot of our components have tight geometric tolerances.
With the universal artifact, Renishaw gains precision and quality as well as traceability. "This is really the policeman in our system," Buckingham says. "It's the standard feature reference really, and we call this one a universal artifact because we do, in certain instances, go even one stage further than this. We actually do go all the way to having a master component, so for very precise machining, what you would see instead of that is maybe six components, which have been calibrated.
"The key thing about it is it's made of the same material as the workpiece, so it grows thermally at roughly the same rate as the components. It's calibrated at 20ºC on a standard CMM, and that allows us to do thermal compensation, and allows us to claim that we have measurement traceability as well. It's put on the machine tool, which is then used to measure the same features on the artifact, so we can understand what errors the machine tool introduces to the measurement. We store that data, we sort of error-map the machine using the artifact, and that you can use to calibrate the probing system and all the subsequent measurements.
"Calibration is absolutely critical to our ability to maintain quality on our RAMTIC line," states Buckingham. "We've got very detailed and regular planned maintenance schedules on the machine, because one of the things I get into quite often is talking to customers here about calibration, and a lot of people are under the impression that a machine tool is a very stable environment. If you looked at the thermal growth charts that we do on the Mazaks, the Z axis in particular, if you came in on a cold winter morning and warmed up the machine, the Z axis grows by about 80 µm in about an hour or so."
Buckingham credits several factors with Renishaw's RAMTIC being able to achieve its 20µm tolerance targets. "The secrets of our ability to hold tolerances are partly the artifact comparison, very detailed planned maintenance schedules, and regular ballbar checks. We have a series of people in the shop that are trained in that. We call them 'ballbar doctors,' and I think it takes about 10 min to run a ballbar check on a machine tool. If someone suspects that the machine is drifting out of calibration, the first thing that would happen is, the ballbar check would be run. If that showed drifting into the yellow zone, a maintenance engineer would be called, and he would tweak the servos to give the circularity number a downward push. If that didn't work, what would then happen is that we would do a laser interferometer calibration."
With concurrent engineering, Renishaw's DFM system also incorporates the company's Gage-Point Philosophy, which is a way of conveying design intent in a language which is directly translated into process changes, with analysis or interpretation, notes Buckingham. An integral part of the design process, this system is embedded in Renishaw's component engineering.
"Gage-Point Philosophy really allows us to do away with drawings, because what happens is that the designer and the production engineer will sit down together, decide which features on the model are critical, and then together put some inspection points on that model," Buckingham adds. "And those inspection points are given a whole series of attributes, which becomes a text file associated with the model, which then gets passed to the CAM system. The CAM system then has the ability to interrogate that file, turning that into a series of probing moves on the machine and eliminating any manual intervention into the generation of a process-control program."
The Renishaw rules are now incorporated into its DFM Guidebooks for all preferred classes of machines. Initially issued as paper documents, the company now makes these product design rules available electronically to engineers on its intranet.
"The RAMTIC line is really just the last element of a very integrated concurrent engineering system," Buckingham says. "What we wanted to do was rationalize what at the time was an enormous range of different tooling manufacturers, different tooling configurations, and different types of tools that we had, so we set up a standard tooling library, with a greatly reduced set of tools."
On the Renishaw Design-For-Manufacture web site, engineers access the standard library of features available for any particular product. According to Buckingham, today Renishaw's entire suite of components can be made with about 70 different tool assemblies, as opposed to about several hundred tool assemblies used about 10 years ago.
"We've whittled it down dramatically over time," he says, "and it's a direct result of this component engineering. Since 80% of manufacturing costs are designed in, once the design is finished, your ability to influence costs, as a manufacturing engineer, is very, very limited. But cost is not the main driver--for us, it's lead time.
This article was first published in the May 2005 edition of Manufacturing Engineering magazine.