Shop Solutions: Automated Microdrilling of Abrasive Parts
Hypertherm (Hanover, NH) was founded in 1968 by President Dick Couch and Bob Dean when they discovered an innovative way to increase the speed and accuracy of plasma cutting to levels that had not been achieved before. Radially injecting water into a plasma cutting nozzle created a narrower arc and became the foundation of Hypertherm's advanced plasma-cutting technology. Couch patented the radially injected water technique and unveiled Hypertherm's first plasma cutter, the PAC400.
Today, 40 years and 75 patents later, Hypertherm is an associate-owned company with nine buildings and 1100 employees. The company designs and manufactures advanced plasma-cutting systems for use in a variety of industries including shipbuilding, manufacturing, and automotive repair. Its product lines include handheld and mechanized plasma cutters and consumables, as well as CNC motion and height controls.
The water-injection technique resolved several issues that had plagued plasma-cutting technology from its beginnings 14 years prior to Hypertherm's innovation. One problem, the accumulation of dross, and a second phenomenon called doublearcing were virtually eliminated. The Hypertherm system, which uses only one gas, nitrogen, also eliminated the need to purchase and stock several different types of gas for cutting. End users also saw a marked improvement in nozzle life, because steam from the water helped to cool and protect the nozzle, reducing its wear rate.
Cutter nozzles and swirl rings made of lava are at the heart of Hypertherm's technology. The lava used is an abrasive material mined in South Africa in 13 x 13" (330 x 330-mm) blocks. The material comes to Hypertherm in boxes of 6" (152-mm) rods (or "bars") that are approximately 1" (25.4 mm) in diameter. All of the swirl rings are cut on a lathe, machined and micro-drilled, and then baked in an oven.
In its "green state," the lava can actually be cut with a knife, but even in its green state, lava is abrasive and extremely aggressive toward tooling. The fine dust produced in machining is particularly hard on microdrills, quickly filling flutes and snapping the tool. Making the drilling process even more challenging is the fact that the holes are often drilled at an angle, and that lava will grow during baking, changing tolerances. Holes that are slightly out of spec before baking can be even more so after baking. The density of the material can also change, not only from one blank to the next, but from one end of a 6" (152-mm) bar to the other. Finally, the more the product is handled, the greater the chance of chipping, breaking, and scrapping parts.
Several years ago, Hypertherm visited the EASTEC exposition, specifically looking for a way to automate the drilling process to increase volume, improve accuracy, and minimize attended operation associated with the production of swirl rings. Swirl rings were being produced manually, one part at a time on arbors and bench-top machining centers, by several operators working three shifts per day.
Hypertherm selected two Datron Velociraptor M8 high-speed machining centers with pick-and-place automation systems for untended part changes from Datron Dynamics Inc. (Milford, NH). These machines were installed to drill face holes, back holes, and radial holes ranging in diameter from 0.011 to 0.025" (0.28–0.64 mm) on all swirl rings in their product line. With installation of the Datron M8 machines, Hypertherm's process was transformed from manual to highly automated.
"Now, we're able to load 100 pieces into the Datron equipment, and they all come out ready to go into the oven. There's no handling," explains Tim McCarville, manufacturing process technician for Hypertherm's Lava Div. "To do the volume we're doing on the Datron machining centers today, the way we did it before, would take eight people with at least four people drilling all the time, plus two operators running the lathes. Now we have just two operators per shift running two lathes and loading one Datron VMC each.
"It just takes 5 min for the operator to load a Datron with a tray of 100 blanks and inspect the parts coming off. The pick-and-place system picks each blank off the lathe tray for drilling, and puts it on the bake sheet in a pattern that's ideal for controlled baking. After inspection of the first and last piece of a 100-piece run the entire sheet goes right into the oven."
To maintain product uniformity, Hypertherm has developed a process of constant measurement and testing of lava blanks. Density varies from one blank of lava to the next and growth occurs during the baking process. Each box of bars is tested, and machining variables are established for the QDCS (Quality Data Collection System) and lathe control. Through testing the bars and getting the growth properties for each lot, tolerances can be adjusted based on what will happen to the blank during baking. Operators simply enter the variables for a given lot of blanks into the Datron control and hit go.
"Drilling has to be very accurate, because any inaccuracy will be compounded by growth resulting from the firing process later," McCarville explains. "Bolt circles are going to grow, and the location can shift on the part. A radial hole can shift laterally on the part and open up the bolt circle as well as the drill hole. You've got three potential problems. So, if you're the slightest bit off on the drilled hole to begin with, it just gets worse in baking." Once the tolerances have been determined, only half of tolerance span is used during the drilling process. McCarville explains: "That keeps us just inside, so even if we use all one side or the other of tolerance at machining, we still have half our tolerance left to accommodate excess growth."
Hypertherm, which has embraced 5S methodology, relies on Datron's Automatic Tool Management (ATM) system in its ongoing efforts to minimize waste. The ATM is made up of three separate components: the tool checker, the tool changer, and the software. The tool checker is a mechanical sensor that measures tool length and detects broken tools. The tool changer is a rack or tray that has space for spare tools and sockets where the machine places broken tools before picking up a replacement. Operators can stock the rack with spare tools, thereby having a ready supply should tools break during untended operation. The software is a macro program that can be set up to run a tool check after executing a number of lines of code.
The ATM system gives Hypertherm the option of setting up for a piece count. McCarville explains: "If you have a batch of real grainy lava, you can change the program for where your drill failure is. By preemptively scheduling a tool-length check, you eliminate tool breakage and scrap in your process. I have three ranges that I check for, based on drill diameter. For 0.011–0.015" [0.28–0.38 mm], I'll check every 10 pieces. With a larger drill, every 20 pieces, and no more than 25 pieces regardless of drill size. Since the tool check is automated and fast, we can afford to do that, and we can't afford not to, considering the reduction in scrap. Now, that's not 25 holes, it's 25 completed parts, which is generally 150 holes."
The machine will shut down automatically and warn the operator with a flashing light if tool length or wear are lower than parameters set in the control. This is essential to waste reduction because these parts can't be re-drilled. If a worn or broken tool is detected and the part is put down on the baking sheet, it can't be re-drilled even if the part is deemed to be accurate thus far in the machining process. "If I can lose only one part, or better yet, set the tool check more frequently and lose none, that's huge," says McCarville. "That said, we don't want to check the tool after every part or even too frequently, because that's non-cutting time that is not beneficial to the process, and loss of time is also considered waste."
The M8 machines that Hypertherm purchased for micro hole drilling are versatile and have been employed all over the world for a range of applications requiring milling and engraving. "We have two Datron machines, and if one of them is not busy and our lathes are backed up, we can move some of their work, like drilling face holes, over to the Datron, and it can drill those holes as well as the back holes and radial holes. All the way around, these machines have given us a lot better throughput. Not only because the pick-and-place automation works so well for our application, but they can also take parts from another place in the process and still get the product through in a timely manner," McCarville concludes.
Waterjet Shop Cuts Tough Metals
When Jose Luis Arce's longtime employer decided to retire and close his waterjet shop in 2007, Arce decided to put his ten years of waterjet cutting, programming, and mechanical engineering experience to work by starting his own business.
The result was formation of Arro-Jet Engineering and Consulting (Camarillo, CA).
Arce bought his former employer's waterjet cutting machine from Jet Edge (Saint Michael, MN), and quickly established his company as a leading waterjet cutting shop in Southern California with very little advertising other than his Web site and his reputation for cutting complicated parts quickly and precisely. In addition to his job shop cutting services, Arce, who has a mechanical engineering degree, also provides engineering consulting services to other waterjet users.
Arro-Jet has grown rapidly. Arce installed a second Jet Edge waterjet in 2008, a 6 x 10' (1.8 x 3-m) high rail gantry machine. In 2009, he added a 30 x 30" (762 x 762-mm) Jet Edge Abrasive Machining Center waterjet system to cut small parts. Both high rail gantry waterjets feature multiple cutting heads for increased productivity. Systems are powered by three Jet Edge 60,000 psi (414-MPa) intensifier pumps, including 30, 50 and 150-hp (22, 37, and 112-kW) pumps.
"People just came to me," Arce says. "I haven't advertised. When I opened this business, I already had customers. People know me and know I have a good reputation and industry knowledge. Business has been steady. About 50% of my business is from people who knew me in the past, and 50% is new.">
Arro-Jet specializes in handling complicated jobs that other water jet shops can't handle, producing quality parts in a timely fashion. "If they want it tomorrow, they will have it tomorrow," Arce says. "I never turn a job away."
A wide variety of materials are cut using the Jet Edge waterjets, including stainless, aluminum, copper, titanium, Hastelloy, and laminated woods. The shop can cut plates up to 12 x 30' (3.7 x 9 m). Arro-Jet provides waterjet job shop work to anyone, but the majority of its customers are in the aircraft, chemical, and oil industries. The company also cuts a variety of artwork and signs.
"About half of our work is with titanium, which includes aircraft and industrial projects," Arce says. "Waterjet is the only way to cut Hastelloy, zirconium, and titanium for aircraft parts [odd shaped]. The waterjet process will not change the properties of the material. Industrial titanium grade 2-11 thin material can be plasma cut, but you will have to grind it to clean it up. Waterjet cutting will give you a nice clean cut that's ready to assemble," Arce explains.
"When cutting thick 6A 4VL [ELI] aircraft titanium, the concern is that the amount of garnet always has to be consistent during cutting. Otherwise, if the amount of garnet is too small or inconsistent, unlike steel, which just will not cut, titanium will cut in an undesirable pathway, increasing the chance of having to scrap the part. Titanium is a very hard material. Most of my processes involve roughing the part, leaving 0.060" [1.5 mm] for final finish machining."
The majority of the parts that Arro-Jet cuts are odd shapes that are nested on the plates to maximize the number of parts to save material costs.
Multiple cutting heads on Jet Edge waterjets have saved time and money by enabling parts to be tightly nested. As an example, one of Arro-Jet's customers needed to have parts cut from twenty 84 x 40" (2.1 x 1 m) 3" (76-mm) thick titanium plates valued at $50,000 a plate. The customer expected to get nine parts out of each plate, but Arce tightly nested the parts and cut ten parts from each plate. The customer received 20 extra parts that they had not expected.
"That's the reason they come back," he notes. In another example, Arce cut 6800 0.781" (20-mm) diam holes in a huge 0.5" (12.7-mm) thick stainless baffle for a heat exchanger, maintaining a tolerance of ±0.005" (0.13 mm).
"I random-checked the holes, and all of them were in tolerance," he says. "The precision is in the machine. My experience comes in during the repositioning. The design of the Jet Edge waterjet machines allows me to do any kind of plate," says Arce.
Arce also has successfully cut pipes with his waterjet system. "In cutting pipe, it's really important where the fixture for the part is going to sit," he says. "With the waterjet, you will complete a template that includes the degrees you will have to manually rotate the pipe. That template will be matched with the start point. I do not have a Dynamic Waterjet head. The movement of the Z [axis] will be manual, and in another case, when the cut has to be perpendicular to the surface like a dome shape, sometimes it's necessary to tilt the part."
When cutting laminated woods and Plexiglas, one of the ways to avoid delamination without drilling is to lower the pressure in incremental stages until it goes through to the material," he says. "Always consider that the garnet hits the parts in the moment of piercing. If just water hits the part, you will have delamination. One of the methods I sometimes use when I refer to wood, G10, or Plexiglas, is, in the moment of piercing, I put a 0.020" [0.51-mm] stainless sheet on top so the initial impact will go to this, and the Plexiglas or wood will not take the complete impact to eliminate delamination. This does not always work. In my experience, delamination is possible, and I prefer to drill at the start point. Wood delamination is very difficult to avoid. When I start the point, the customer is advised of the possibilities for delamination, and we agree where the best start point will be so the delamination won't be an issue," says Arce.
"Regarding Plexiglas, my experience is that when it is a thin material—say 3/8" [9.5 mm] or under, usually the pierce-surface damage is less than 1/4" [6.4-mm] in perimeter. So if the hole I'm cutting is bigger, it's not a problem. Sometimes, there are other ways; including drilling before piercing, piercing first at low pressure in all the start points, and then starting to cut like cutting G10 or fiberglass. I would put a thin sheet of metal or aluminum on the workpiece to absorb the impact of the water. Delamination is always a possibility."
Honing Fine-Tunes Valve Performance
Jim Brizzolara credits lessons learned on a Sunnen hone at his first job in a hydraulic-valve factory in 1966 with helping him understand how precision bore finish and geometry can fine-tune the performance of a valve.
Fast-forward to 2007, and Brizzolara, president and CEO at Hydra-Force Inc. (Lincolnshire, IL), has used that knowledge to help build the company he co-founded in 1985 into the world's leading supplier of cartridge valves and integrated circuit manifolds.
Brizzolara considers honing one of the company's core competencies, giving HydraForce products a quality edge in a market where cartridge valves are often considered commodities.
HydraForce uses nearly 36 conventional honing systems from Sunnen Products Co. (St. Louis) at its plants in the US and the UK. The Sunnen machines produce the final size and geometry of valve cages and other critical parts to 0.000050" (0.00127 mm) accuracy, and customize the surface finish to specs tailored for various mating components used in the products.
Precision size and surface finish on mating parts help eliminate leakage and ensure consistent performance on electrically actuated models under low-voltage conditions, often encountered with low batteries on mobile equipment.
Conventional honing takes a little more time than single-pass honing, but produces a better final product. The process typically involves a roughing operation on Sunnen ML-5000 Power Stroke machines and a finishing operation on CGM and KGM 5000 Krossgrinding machines. Cubic-boronnitride abrasive is used for roughing. Finishing passes typically use diamond-plated tooling on the Krossgrinding machines, which can control hole size to accuracies of 0.000010" (0.00025 mm). If Rk and Rpk surface values are critical, aluminum oxide or silicon carbide abrasives may be used on the ML machine.
Most valve cages start as 12L14 or 12L15 steel that is carburized to about RC 60 hardness after screw machining. Occasionally, stainless and other materials are used. "Any competitor can purchase screwmachined components from suppliers just like ours, but what sets us apart is our knowledge of how the final fit and finish on mating parts can be optimized with honing," Brizzolara explains.
"Conventional honing can really make a difference in how a product performs. It's not just the size control. It's the surface finish and crosshatch pattern you put on the bore of the cage. Our tight control of operations produces the fit, finish, and clearance of mating and sealing components that results in more efficient hydraulic circuit. Most machinists are pleased with a bore tolerance of 0.002" [0.0508 mm], while we are holding less than 0.000050" [0.00127 mm]."
For finishing operations on valve cages, conventional honing provides a number of advantages, according to Brizzolara. A conventional honing mandrel, which contacts almost the full length of the bore while the part reciprocates, corrects any geometric error (straightness, cylindricity) from screw machining, or distortion from heat-treating or stress relief. A single-pass honing tool, on the other hand, is tapered, so the small area of the tool representing final size tends to follow the path of the cage bore, making it less likely to correct a curved bore.
Conventional honing also produces a crosshatch pattern on the bore surface, while single-pass honing produces a helical pattern on the surface. "The crosshatch surface ensures a consistent full-length flow path for lubrication around the mating parts of the valve," Brizzolara says. "It's the same surfacing technology used in automotive cylinder bores, particularly in performance racing."
In addition to the crosshatch, HydraForce also measures and controls the surface roughness. "A superfine finish without crosshatch actually diminishes lubrication between mating parts, thus increasing friction," he explains. "We control Ra on our honing, and for certain parts will control Rk and Rpk [the mean height of the peaks protruding from the roughness core]. The valleys improve lubrication, but the peaks cause friction, which leads to sluggish operation." HydraForce customizes the surface roughness for optimum valve performance, based on the nature of the mating parts, i.e., O-ring, steel piston, etc.
Conventional honing also minimizes the need for part deburring. Single-pass honing produces more burrs in the cage cross-holes. "Single-pass honing mandrels tend to fold, tear, and push more material, while conventional abrasive tools cut the material, though speeds, feeds, and pressures need to be controlled to achieve the best result," Brizzolara says. "We document these parameters to develop best practices, stick to them, then try to improve them as we go on."
All of the honing machines at HydraForce have a two-fixture methodology for maximum in-cut time, allowing the operator to load one while the other is in the machine. Operators air- gage every part after honing, and lot sizes run from a few hundred pieces to several thousand. Honing is the only batch process in the plant due to cleaning requirements. Part diameters range from 0.097" (2.46 mm) to just over 1" (25.4 mm). Final product performance is 100% tested in one of 60 product-specific test cells.
Although honing is only part of a much larger picture that includes grinding, cleanliness, assembly methodologies, and testing, Brizzolara says it is certainly a key enabler for the HydraForce five-year product warranty.
This article was first published in the August 2009 edition of Manufacturing Engineering magazine.