The Building of a Bridgeport Milling Machine
It's a Lot More Than producing a Simple Knee Mill.
A couple of years back, four Bridgeport machines were heavily damaged when they were dropped off a railcar in China. When the machines finally made it to their final destination, plant engineers removed the covers and checked the geometries of the machines. All were within tolerance. "I think that speaks a lot about our ability to make and build iron," says Bing Benson, Bridgeport VMC assembly manager.
While building its somewhat indestructible machines, however, Bridgeport Machines Inc., Bridgeport, Conn., faces two distinct challenges. On one hand, the company produces a commodity product that requires dedicated machinery and hours of hand work. But it also makes a lower-volume product line in batches using flexible machining systems.
The plant builds 1,800 Series I knee mills annually. Throw in another 700 EZTrak mills, and the company produces about 2,500 machines a year. In the same time frame, the company makes approximately 370 VMCs.
These lines are machined side-by-side, then assembled in different areas.
Many of Bridgeport's process improvements over the last year or so have focused on its knee-mill production. Bridgeport's first production step is spraying a 0.030-in.-thick filler on the knee mill's major cast iron components -- the column, knee, ram, turret, and J housing for the head -- to smooth out surface imperfections. The company then paints the components with the first part of two-part polyurethane coating to seal the castings.
"Painting before machining lets us build the depth of gloss that we want on our finished paint job," says Leon Tyrrell, group manager of standard products and EZTrak assembly. It is also more economical, cutting down on both the hazardous waste and a significant amount of labor time. "We used solvents to remove cutting oils from our components, then applied masking tape to all machine surfaces," remembers Tyrrell. "After painting, we had to remove and dispose of the masking tape with paint on it." Now, the company removes the paint in the machining process, so there is no need to mask or clean surfaces.
Painting before machining also minimizes problems in the assembly area. "We would build a machine and find geometries out of tolerance," recalls Tyrrell. "You would tear it apart and find 0.0002 in. of gummy residue from the masking tape ruining the tolerance. We've eliminated that problem."
Subsequent to painting, the castings are processed through a variety of large machining centers, primarily horizontal systems with two to eight pallets. As much as possible, Bridgeport uses its own standard products in the machine shop. However, the company has also built a few one-of-a-kind Bridgeport designs for certain operations. For instance, one such machine, built on the carcass of an old Kearney & Trecker horizontal machining center, makes T-slots for the Series I tables. This operation would normally require several conventional machining centers. Obviously, the machine is a crucial part of the company's Series I table cell.
Cell production is one way Bridgeport has improved operating efficiency over the past couple of years. In the table cell, for instance, tables travel only 8 or 10 feet for all operations but finish grinding. Finish grinding is not done in the cell, reports Bob Crossman, machine shop manager, only because machine vibration would have caused finishing problems.
The Series I ram also is made in its own cell, where it first goes through a milling operation to put on the 50° dovetails, then through a grinding operation. Another example is the cell where the 2J housing for both the Series I and II heads is made. In this case, the cell is comprised of six 5-pallet Yasda horizontal machining centers configured to run lights out for up to 12 hours.
The purpose of these cell configurations are two-fold, says Cross-man. First, they cut down on the time parts travel across the shop floor. And in doing that, the cells have slashed work-in-process (WIP) dramatically.
The cells also let Bridgeport use a Kanban system in its operations. As Ed O'Doy, vice president of manufacturing and operations, describes it: "Kanban is a concept put together by the Japanese in the Toyoda production system. Basically, you use visual aids to tell you when product needs to be made." The Kanban system, in fact, tailors nicely into Bridgeport's philosophy of continuous improvement. "It highlights the area where work needs to be accomplished, and if work is not needed, then personnel are assigned to other areas," says O'Doy. "That way, we don't build up a lot of inventory sitting around on the floor. We are trying to get away from batch-type production and into one-piece, flow-type production."
One area where the Kanban concept has helped production involves the leadscrews in the knee mills. At one time, leadscrews were produced in an area far from the assembly line. Now, they are machined in a cell adjacent to the area. This lets a machinist tell just by looking at a rack that he needs to make more leadscrews.
"At one time, work orders were on paper, and you would work to whatever amount the paper said," says Tyrrell. "Frequently, we would produce a 200-piece batch, then find it wasn't needed." By having the leadscrew racks and production next to the assembly area, Bridgeport reduced batch sizes from a couple of hundred pieces to just fifty. It also reduced WIP from over 2,000 pieces of bar-stock to approximately 400 pieces. "And that accommodates three or four different product lines and spare parts," adds Tyrrell.
Kanban also streamlined production in the quill cell, where all Series I and II quills are made. About two years ago, the company measured how far the quill traveled from where it was delivered as barstock to where it was handed off to the assembly line. The quill traveled about 1,700 feet, stopping for at least 20 operations.
To shorten that journey and reduce operations, the company configured two lathes, a manual horizontal machine, a Bridgeport VMC, a boring mill, and a grinder into what is now the quill cell. As a result, quills travel only 20 ft, and WIP has been slashed from roughly 1,700 pieces to 140. "Be-cause there is such a demand for quills, we used to do them in batches, which would literally queue up at each operation," says Crossman. "Just to keep the flow of parts moving, we would stack parts in front of every station."
The feel of a Bridgeport
After machining, Bridgeport components move into the hand-scraping department. The company believes that hand scraping is what gives its machines a distinctive feel. "As far as the Series I and II machines go, it's tough to describe it with words," says Crossman. "The only way to experience it is to grab the handles on the Bridgeport and feel the tolerances, the tightness, the quality of the machine. That sounds like hype, but it isn't."
Bridgeport hand scrapes bearing surfaces on all the mating surfaces of the iron -- on the back of the knee, the dovetails, the saddle, and the gibs -- to provide a customized, surface-to-surface fit. Surfaces are scraped to tolerances ranging from 0 to 0.0002 in.
Scrapers can spend hours working on components, such as the gibs, until the fit and tightness between the mating surfaces is up to par. "For the gibs, they want less than 0.0008-in. total rock, and between two readings, no more than 0.0002 in. of difference in the looseness of the machine," reports Tyrrell.
On the Series I machines, the department spends 8 hr hand scraping individual part details and matching components. Series II machines take 29 1 / 2 hr. At one time, scraping operations on the Series II took longer, but Bridgeport eliminated a couple of operations to shave some time off the process. "We identified areas where it's not as critical that they be done by hand scraping, and we now machine them," Tyrrell explains.
Once scraping is completed, Bridgeport puts the same serial number on matching parts. That way, when parts move into assembly and are taken apart for cleaning, assemblers have the serial numbers to maintain the match.
Off to assembly
After the assembly department disassembles major components to clean, it puts in leadscrews, brackets, handles, and heads. All the while, assemblers are checking machine geometries.
The idea, says Tyrrell, is to catch problems as a machine is going through the line. Inspection takes place before, during, and after each step of the manufacturing process, from machining through scraping and assembly. Although it may sound like a lot of unnecessary work, the company says inspecting parts along the way has eliminated rework and repair time and kept work flowing smoothly.
Among the checks done in the assembly area are those on the head and quill. On the head assembly, for instance, Bridgeport checks vibration, noise, and engagement of clutches in forward and reverse gears. It also makes sure that the downfeed on the machines is up to spec.
When the machine shop delivers the quills from finish grinding, assemblers fit the quill to the quill housing and assembly, maintaining no more than 0.0002 to 0.0004-in. fit between the quill and the housing. Once the quill is assembled and built onto the machine, the assembly department ensures that the runout is no more than 0.0002 in. of the quill in the housing and no more than 0.0005 in. when the quill is extended.
"Fit is everything," according to Andy McNamara, product manager for the company's VMC line. "Other companies do not hand lap or fit the quills to the degree that we do. Their quills might be cylindrically ground to size and then fit into a generic component that is bored to size.
"Many other mills fail," says McNamara, "where the quill and the quill housing come into contact. That's where the side cutting loads show up when you're machining and why quills bind up or machines start vibrating soon after you buy them. A good fit makes the machine more rigid and gives you longevity and accuracy."
After the final inspection, the painting department sands and masks the machines. It then sprays on a second coat of polyurethane to give the mills a glossy finish.
Over to the VMCs
Although Bridgeport makes far fewer VMCs than it does knee mills, production is just as involved. As with the knee mills, XV-Series VMCs start as cast iron components. The company doesn't use weldments, believing that cast iron gives better rigidity and vibrationdampening characteristics.
As with its knee-mill operations, Bridgeport has streamlined machining for its VMC components. The company produces all major components in batches, with two exceptions: the VMC column and head.
The column is the only major component that is common across the Bridgeport VMC line. Because of the quantity needed, the company makes them on a Bridgeport Portal machine, a bridge-style mill produced in the company's Leicester, England, facility.
Heads, too, are done on one machine, a Yasda 120 HMC. This system has helped Bridgeport cut out a grinding operation and combine six other operations. "At one point, we would grind the heads," says Crossman, "and now we do them on the HMC. We combine six operations across three pallets, and we take the finished parts out of the machine and straight to assembly."
The combined operations include roughing the bottom, which mounts to the column; roughing the quill bore; drilling the bottom and the quill-cartridge area; and finishing the bottom, the bore and the quill/spindle cartridge, and the mounting surface for the spindle cartridge.
Bridgeport uses a flexible Okuma VMC to machine the VMC bases, saddles, and, if necessary, columns. "We've been able to accomplish 0.0004-in. linear on that machine, which lets us produce the parts and ship them directly to the assembly line with no grinding," says Crossman.
Although Bridgeport has eliminated many grinding steps, it does grind its table tops. "We're the only company doing that," says McNamara. It grinds not only for aesthetic reasons but also to erase mill lines. "With mill lines, you pick up ripples when you indicate across, for instance, or you put vises or fixturing down," explains McNamara. "It will never be as flat, especially in overlapped areas." Mill marks can also adversely affect indicator results while commissioning the machine. At machine startup, tram and level are key factors in obtaining accurate results when machining.
Assembling the VMCs
Following machining, components go directly to the assembly department, where machines are built in place. Unlike the knee mill products, the VMCs do not stop in the scraping area. Any scraping needed is done during assembly.
The first step in the assembly process has assemblers laying out the base, leveling it to within 0.0004 in., and installing linear guides and ballscrews. They then check the machine for parallelism and height.
The XV-Series uses large-diameter ballscrews, double-anchored with axial thrust bearings, so the machine can handle high cutting loads and reach high acceleration and rapid traverse rates with little heat and wear.
The saddle is the next component to be installed. Saddles are pre-built, pre-tested, and prelubed to ensure that there are no leaks and that all lines are in their proper locations. After installing the saddle on the machine, the assemblers tie the ballscrews into the saddle and hook up all the oil lines in the proper locations.
Unlike the knee-mill components, the iron making up the VMCs is not serialized, so Bridgeport can mix-and-match parts. "The tight tolerances held in the machine shop let us put any saddle on any base, any table on any machine, any column on any head," remarks VMC assembly manager Benson.
His group builds the VMC's table separately from the base. Assemblers check the table for accuracy and hand scrape all the brackets necessary to "tickle them in," as Benson describes the minor scraping needed to get a 0.0004-in. overall tolerance.
The table is then mounted on the machine and checked for geometry from front-to-back and side-to-side -- less than 0.0005 in. on all checks. Once the assemblers establish the geometry of a table, they re-level the table again by putting the head column assembly on the base and take all the geometry readings again. At this point, they remove the head column and hand scrape the base to ensure a solid fit with no room for movement. "It is the only way to get the tolerances and stability that we need for a Bridgeport product," states Benson. "We don't resort to shims."
Once the head column is set, assemblers install and check the spindle, taking a 12-in., circular sweep off the table to check what should be less than a 0.001-in. reading. If necessary, they scrape the bottom of the head casting, not the spindle casting. Doing this lets users change cartridge spindles in the field, without scraping them first. "Just spindle-out, spindle-in, touch-and-go, and you're on your way," says Benson.
After the assemblers lock down the geometries on the iron, they add accessories and configure the machine to the customer order. "Then we put the machine on an overnight cycle," Benson says. "After the cycle is completed, we do a quick geometry check to make sure nothing has moved -- and all tolerances are 0.0005 in. or less on all axes, so that doesn't leave you much room for error."
At this point, the machine is turned over to quality control, which spends five hours doing laser and ballbar testing. After the machine checks out, it is returned to the assemblers, who pin all axes. "We have pins in all our brackets to ensure that no ballscrews can move and there are no alignment problems," says Benson. "If you ever have to change anything in the field, everything is pinned for you."
Finally, the department installs all covers and perimeter guards and finishes with a series of safety, sound, and other checks..