Five-Axis Machining -- Simply More Productive
Many shops associate five-axis machining with complexity. While the five-axis process can and does produce some amazingly complex workpieces, there are real advantages in applying it to less severe geometries. Here's how one midwestern job shop is using a two-machine, five-axis manufacturing cell to simplify part production.
Calling Remmele Engineering's plant 20 (Monticello, Minnesota) a job shop might be considered a stretch of the commonly held definition. But that's exactly how Remmele views their repetitive batch manufacturing division. Plant 20 is one of five Remmele manufacturing facilities located around Minneapolis. It's responsible for producing parts in small-to-medium lot sizes, within a 24-inch cube, that are ordered on a regular basis.
Every job shop, regardless of size, has to make parts to make money. The more good parts they make--the more money. In high production environments, there are thousands or millions of parts to "process practice" on. Very high efficiency can be achieved over such long runs.
Not so for small-to-medium lot size producers. They must get it right fast and move on to the next job. Finding better ways to "move on" to the next job is how contract shops win in a very competitive market.
One of those better ways that Remmele has found is five-axis machining in a cellular environment. Rather than making their machining processes more complex, this technology actually simplified production on many jobs. To find out about what Remmele is doing with the new cell, we talked to Bert Casper, vice president of marketing, and others.
For many job shops, the production and quality requirements that customers routinely expect today would have been considered extreme only a few years ago. Remmele, and shops like them, are trying to stay ahead of these manufacturing trends. They see their customers needing reduced lead times with higher accuracy and fully expect pressure to continue in these areas. Implementing five-axis machining is one answer they've found to keep ahead of the trend.
Traditionally there has been a solid line dividing shops that do five-axis machining and those that do not. Recent developments in machines and controls have begun to blur these lines.
Part of the reason for this blurring is that five-axis machining increasingly falls within the financial and technological reach of more shops. Prices for five-axis machines have come down and continue to drop. Programming for these machines has benefited from computer advances in both processing speed and ease of use.
Most shops run jobs that would run much better with five-axis capability. Because the cost of five-axis machines is now less, shops that have them are running more jobs across the machines. Parts that were once considered too simple for five-axis work are being re-evaluated because the burden rate for the machine has come down with the price.
A job might need only a draft angle cut on a mold wall or hole drilled on an angle. Rather than put the work on a second operation machine or refixture, five-axis can be used to finish it complete.
Sometimes there are jobs with feature relationship tolerances such as parallel surfaces, concentricity and angularity specifications. To maintain relationships between these features, access to five sides of a workpiece in one setup is not only handy--it saves time and money.
For milling surfaces, five-axis allows the programmer to keep the tool nose normal to almost any surface. This is especially useful in mold-and-die work in which a flat nose endmill can be used instead of a ballnose. More metal is cut per pass, and scallops can be reduced or even eliminated on the milled surface.
Remmele's plant 20 got its five-axis capability this year. This new cell is among the first in the nation to incorporate five-axis machining centers.
Machining Centers--The Nucleus
The heart of Remmele's five-axis manufacturing cell is two Mazak H-630 horizontal machining centers. These are identical machine tools capable of handling any material that is assigned to them.
In some two-machine cells, one machining center is dedicated to ferrous materials. These machines are specified with options designed to cut iron and steel efficiently. The second machining center may be ordered with options for processing aluminum and other nonferrous materials.
Remmele opted to specify both machines with capability to process any material. Flexibility is their reason. In a job shop, sequence of repetitive orders is not material specific. One week, all orders may be for aluminum and the next for steel. "That's the nature of the business," says Mr. Casper. "Identical machines, fully capable of producing any customer workpiece, allow us to keep our processing options open."
This decision has required Remmele to compromise a little on its choice of coolant. Like most job shops that run a variety of materials, coolant selection must be very general-purpose rather than specific. "With a good, general-purpose coolant coupled with a 750-psi through-tool delivery system, no coolant problems have been seen so far," says Mr. Casper. As for mixing different kinds of chips, "you change the chip cart when you change material."
What Kind Of Five-Axis?
Remmele's machining centers are configured with a trunnion table, rotary axis arrangement in front of the spindle. There are other configurations available both from Mazak and other builders. These include nutating or universal heads, tilt/pivot spindles, various A/B-axis setups and others.
In general though, five-axis horizontals fall into two types. They either incorporate A-and B-axis motion at the spindle or at the workpiece. An A/B actuated spindle moves in these axes independently of the workpiece. In the case of Remmele's machines, the work is manipulated around the spindle using what is essentially a two-axis pallet handler.
Selection of a five-axis configuration is application specific. Proponents of spindle-actuated motion argue that moving the mass of the workpiece induces accuracy problems. Tilt/rotary table users come back saying that an articulated spindle can't take the heavy cuts. There are advantages and compromises to both configurations.
Remmele's primary advantage in an A/B-axis table arrangement versus an actuated spindle is flexibility. According to Mr. Casper, "these machines must be capable of cutting any material we throw at them. Our spindles are 50 taper, driven through a two-speed transmission. Both have a high-end speed of 15,000 rpm for cutting aluminum. The low speed gives us enough torque to take heavy cuts in tougher materials without stalling the spindle. Many of the articulated spindle designs offer a high-speed range or low speed but generally not both."
To secure the trunnion and rotary table during heavy machining operations, a brake is applied to the rotary axes as an augment to the servomotor. The trunnion is counterbalanced to offset workpiece loads.
Process knowledge gained using the company's three-axis machines for five-axis work is also a justification for the A/B table configuration. Any five-axis work done at plant 20, prior to the installation of the new cell, was set up using a bolt-on A/B-axis table. "It's conceptually similar to the five-axis machine we bought," says Mr. Casper, "but operationally very different."
In one case, moving a job from an A/B table equipped, three-axis machine to the five-axis saved Remmele over five hours setup time. "Setup for that configuration was horrendous, compared to what we have now," says engineer Jonathan Guck. "With integral five-axis you simply put the part on a pallet. There's no hoisting, hooking-up, or squaring-in of a device. It's always ready to go."
Picking The Jobs
At Remmele, a team of six is responsible for selecting which existing jobs go to the five-axis machines and which new jobs will be processed there. The team represents various manufacturing disciplines including operators, project managers, manufacturing engineers and programmers, who work together to see that the right jobs are quoted and the quotes reflect reality.
Because Remmele is a job shop, selecting the right workpiece for five-axis process is critical. It can be the difference between profit and loss on a run of parts. On the other hand, maximizing an asset like a five-axis cell is also a priority.
Several factors enter into the decision to put a job on the five-axis machine or process it some other way. Some jobs are "no-brainers." For example, workpieces with sculpted or contoured surfaces almost automatically go to the five-axis machine.
Other jobs require further evaluation. Run time is one of the criteria. Analysis of a workpiece includes how many steps it takes to machine. A workpiece may not have any five-axis contours, but total processing time due to five-sided access may tip the scale to the five-axis machine.
Conversely, if a three-axis part needs six sides machined, and it can be processed in two handlings on a three-axis machine, there may be no advantage to running it across the five-axis machine. It will take two handlings on either machine to do all sides.
Remmele's plan for the five-axis cell is to simplify workpiece processing. If three steps can be reduced to two--or better, one--then five-axis is the machine of choice. Otherwise, if a job's running fine on a three-axis machining center, leave it be.
Remmele keeps a kit of 80 tools resident on each of the machining centers in the cell. That's out of a possible 160 tools available on each machine.
Team members--project managers working with manufacturing engineers--write five-axis programs around the 80 resident tools. They try to use as few specials tools as possible. Five-axis has helped in this area because of its ability to present the workpiece at various angles to the tool.
"We need fewer angle-head tools," says Mr. Guck, "because the A/B-axis configuration can tilt the work so we don't need to tilt the tool like we do on a three-axis machine. Tool management is simplified because more standard and fewer special cutters can be used."
Even using a resident tool program, with 320 pockets between two machines, keeping track of which tool is where is still an issue. To help the cell operator do this and to monitor tool life, a chip is imbedded in each tool shank.
A tool presetter, located next to the cell controller, reads the programmable chip and feeds tool dimensions, location and offsets into the machine's CNC. It also apprises the programmer, through DNC (distributed numerical control), of which tools are actually in the machine at a given time.
Feed Me! Keeping the work flowing into the machining centers is accomplished physically by a 20-pallet storage and retrieval system built on the front end of the cell. The pallets are 630 mm square. The brain of the cell is a 150-MHz RISC CPU from Digital Equipment Corporation. Its primary function is to schedule work in and out of pallet storage and channel programs from the shop DNC system.
Scheduling is carried out using a three-tiered priority system. Level one operates from a relative priority that is assigned to each pallet using a number from one (high) through ten (low). Physical location of the part on the storage rack is the next level of decision making for the scheduling system. Process priority is the next level. If, for example, two pallets have parts requiring a set one operation and a set two, sequencing of the process is programmable so all of set one can be done before set two or variations.
Each pallet has a programmable chip on its side. Data on the chip tells the cell controller which fixture is on the pallet, which job is on the fixture, and where the pallet is located in storage. When a particular pallet goes into the machine, a probing routine ensues to double check that pallet fixture and workpiece match up with the current run program. "This is especially critical for automatic, lights-out operation," says Mr. Casper.
If a problem is detected (for example, a specified tool isn't in its location), the system will automatically go to the next job in queue. An alert on the cell controller will inform the operator what happened.
Keeping In Touch
Remmele has taken DNC beyond just downloading programs from a host computer to a machine tool CNC. It's operated as a real-time two-way communication system between engineering and the shop floor. Changes to program or process made anywhere along the communication channel are immediately available throughout the system.
A dedicated 486-based PC is within reach of all operating stations in the shop. Setup instructions, gaging requirements, tool sheets and SPC data are all on-line and accessible to the whole shop. The new five-axis cell is on this DNC system.
The Simple Truth
Investing in technology to expand the scope of a shop's capability is how the business grows. While five-axis is often associated with complex workpieces and extended support staffs, both of those notions are becoming obsolete.
As Remmele and others have discovered with application of five-axis machining, it really helps them do a better job for simpler parts, and at the same time, giving capability to process complex work.
The demands on shops that make parts for others is not likely to go away. More likely they will get worse. If for no other reason than simplifying existing processes, five-axis machining is worthy of consideration for the job shop.
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