Teaming up for productivity: Saturn, Sandvik boost throughput and double tool life at plant. (Auto Report).
Saturn Corp. sought greater productivity and cutting tool life In its gear-turning operations. So its Spring Hill, TN, operation teamed with cutting tool supplier Sandvik Coromant on a Productivity Improvement Program. The goal: to identify better cutting inserts and machining data. The changes included optimized high-feed inserts with wiper geometry, increased turning speeds and recommended changes in machining and tool-setting routines.
The results included 23 percent greater-average throughput on 26 different parts and a 50- to 100-percent increase in insert life, depending on the operation. In addition, the effort also standardized cutting tools for a 33-percent reduction in insert types, reducing insert inventory 50 percent. Scrap on setup dropped by half.
All told, the program changes generated savings projected at more than $63,000 a year.. .at literally no cost. And all of Sandvik's advice, testing, training and support were free.
Seeking a remedy
The rotational gears module in the Saturn Powertrain plant has 16 CNC lathes turning 28 transmission gear blanks from 50 mm to 140 mm diameter. Before gear teeth are cut, the faces and outside diameters of the blanks are machined to [+ or -]50 microns. Bores are rough cut, finish cut, and chamfered within [+ or -]25 microns.
All of the parts are machined from Type 5.12 0 low-carbon steel forgings with just 150 to 160 Brinell hardness. "It's a tough metal to machine because it's so soft and gummy," explains Craig Fincher, Saturn engineering support technician. Ordinary coated carbide cutting inserts failed quickly because of the stringy chips and rapidly accumulated built-up edge (BUE). "Over the years, we've optimized our cutting operations in terms of throughput--or so we thought--but we were still falling behind in tool life," acknowledges Fincher.
For example, ordinary coated carbide inserts for facing and boring the Type 1788 automatic transmission gear were failing after just 200 parts. With production running around 1,000 gears a day, the machine could shut down four or five times for tool changes. Each tool change took five minutes from shut down to test cut, and each test cut risked a scrap part. Long stringers from the soft steel also clogged the plant's central chip evacuator and sometimes shut down the entire manufacturing module for chip cleaning.
To solve their production problems, Fincher, together with Saturn engineering team leader Al Farley and process engineer Jim Alds, sampled cutting inserts from several manufacturers on one machine. Based on promising preliminary findings, they contacted Sandvik Coromant to see what could be done to remedy the troublesome tool wear.
"The ultimate goal was to increase productivity through tool life," recalls Fincher. "Along with that, we wanted to reduce scrap and downtime."
More good parts
Sandvik Coromant Productivity Teams conduct several hundred Productivity Improvement Programs a year. Experience shows such partnerships boost plant-wide machining throughput 20 percent on average and generate significant savings in machining time and maintenance, repair, operating costs and tooling inventories. Team leader Perry Birkner observes: "It's not necessarily longer tool life that saves companies money; it's more good parts in the bin at the end of the shift."
Sandvik representatives Birkner, Willie Brogley and Slade Blackwelder began the program with a broad survey of machining practices in the Saturn gear module. A quick look at machine controls showed that by today's standards, surface speeds and feeds were limited by old-technology coated carbide cutting inserts. For most cuts, the tool specialists suggested standardizing on high-feed Type CNMG 432 WM wiper geometry inserts with Grade GC2015 carbide to exploit two technical advances:
* First, by permitting higher feeds without sacrificing finish quality, wiper geometries could shorten cutting time and make more parts of the same quality per shift.
Wiper geometry on a turning insert incorporates an extended face after the cutting nose radius. The leading edge removes metal and produces a surface roughened by peaks and grooves. The extended face remains in contact with the work to wipe down or burnish away the peaks and leave a smoother finish. "As a result, a wiper insert at double the feed rate or surface speed removes stock twice as fast as ordinary inserts yet leaves a finish at least as good," says Brogley. However, to keep machine spindles synchronized and prevent production bottlenecks, Saturn manufacturing engineers elected to increase speeds without changing feeds.
* Second, Grade GC2015 coated cemented carbide is formulated to cut stainless steel with stringy chips much like those of the 5120 steel machined by Saturn. The carbide substrate is formulated to withstand high cutting temperatures, and the smooth chemical vapor deposited coating reduces BUE. The coating adheres strongly to the cutting edge despite high machining temperatures. Long-lived carbide inserts, turning at higher surface speeds, create manageable chips and recover time previously lost on frequent tool changes, thereby increasing machine utilization.
For boring and other cuts where wipers were inappropriate and GC2015 carbide was unavailable, the productivity team recommended general-purpose inserts and carbide grades. The Sandvik productivity team and Saturn engineers tested the recommended tools and machine settings on three gears turned on different machines.
On one 70mm diameter manual transmission gear, a twin-spindle machine rough and finish cut the face and OD of the forging in four distinct operations. Previously, cumulative cycle time for all the operations was 0.402 minutes. With the retooling, programmers were able to increase surface speeds about 20 percent, reducing cutting time to just 0.322 minutes. Tool life increased from 350 to 600 pieces per edge, despite a difficult interrupted cut on the slotted face. Together, the 20-percent reduction in cycle time and 71-percent improvement in tool life are equivalent to 298 additional parts per shift, or 25 percent greater productivity.
In another gear blank, measuring 80.25 mm diameter, the previous cumulative cutting time to rough and finish the face, bore, and OD of the part was 1.423 minutes. Afterwards, cycle time fell 32 percent to 0.967 minutes. Combined with another 71 percent increase in tool life and the resulting higher utilization on productivity increased a total of 47 percent, an additional 159 parts over a full work shift.
On a 67.1mm diameter automatic transmission gear with a deep counterbore, a combination of wiper-geometry and high-performance boring inserts cut cycle time 17 percent. Coupled with longer tool life, productivity increased 21 percent, or an additional 305 parts per shift.
In addition to the changes in cutting tools, Birkner suggested changes in machine programs. "Most of the speed changes were minor," observes Fincher, "and we made a few minor changes in tool paths." One facing operation, for example, failed to achieve the required flatness because of chips collecting in the previously machined bore. The productivity team observed the operations and saw the troublesome chips were not left by the boring tool but created by the face cutter. Sandvik representative Willie Brogley recalls: "Changing direction of the cut from right feed to left feed reversed the curve of the chips and stopped the packing."
The successful tests led to the broader application of Productivity Improvement Program recommendations to all the parts made in Saturn's rotational gear module. Saturn continues to implement and expand upon the recommendations. Also significant, the effort helped Saturn rationalize insert inventories. "The wiper inserts rough and finish at the same time on almost all the cuts," says Fincher. "So we're using fewer inserts and stocking fewer kinds of inserts. So far, we've been able to reduce our inventory from ten types to four." Sandvik Coromant, Fairlawn, NJ, www.RSLeads.com/204tp-271 or circle 271.