Teamwork takes black magic out of bending stainless steel

Cincinnati Incorporated listens, then enhances software for AUTOFORM press brake, enabling Moline Machinery to bend precision stainless components right the first time for baking systems.

Precision bending of stainless steel, especially in heavy gauge and plate thicknesses, can require special processing. For Moline Machinery, a leading maker of made-to-order baking equipment systems, special processing was killing productivity. "Fabrication used to be a real bottleneck in our manufacturing," says Mike Plaunt, manufacturing engineer. "When doing custom parts - one of this, one of that - you need them to be right. You don't want to have to tweak the control for each and every part. Bending used to be a black art that relied on the skills of our sheet metal mechanics. We did a lot of test bends and throw-aways."

Detailed empirical testing by Plaunt and a software enhancement by CINCINNATI for the AUTOFORM press brake have turned black art into science. "We've achieved our goal of making a precision sheet metal component right the first time," states Plaunt.

Aiming to raise bending productivity, the Duluth, Minnesota company in late 2002 received the first AUTOFORM fitted by CINCINNATI with a newly introduced high-speed, five-axis backgage, as well as Wilson quick-change precision tooling.

The five-axis backgage speeds processing of the multiple-flange bends typical on Moline machine components. It provides 3000 ipm positioning with ±.015" accuracy and repeatability on long Z1, Z2 left right axes, 1200 ipm positioning and 24" range in X1, X2 in-out axes with ±.001" accuracy and repeatability, plus 8" range at 300 ipm in R up-down axes with ±.005" accuracy and ±0.003 repeatability. The independent axes enable fast processing of multi-die setups where the operator works down the length of the bed. Closed-loop servo drives zip the gage fingers from bend to bend, then provide fast, precise lock-in of gage positions.

Despite those backgage capabilities, Moline found that dimensions on first piece, multi-bend stainless components sometimes fell outside specification. Investigations by Plaunt identified the cause to the AUTOFORM's control formula for calculating backgage positions. Discussing the situation with Todd Kirchoff, CINCINNATI product manager for press brakes, Plaunt suggested that the method used by the control to automatically calculate gage allowance, developed for air-bending mild steel, could be expanded to include a larger variety of materials and bending conditions.

Moline almost exclusively runs stainless in meeting food industry sanitary requirements, he notes, and stainless behaves differently in bending than mild steel. "The bends are more like a parabola than a radius," he says.

The accuracy of the gage allowance calculation, explains Kirchoff, is dependent on the ability to accurately predict the K-factor neutral axis location and the inside bend radius. These factors can change with different materials and bending methods (such as air bottoming vs. coining).

While the stainless steel vs. mild steel gage allowance error is small, the overall effect on Moline parts with multiple bends could add up to cause fit problems or throw a part out of specification. "Error stacking was a real problem," says Plaunt.

After listening to Plaunt's needs and suggestions, CINCINNATI designed a custom gage allowance table that allows users to customize the K-factor (location of the neutral axis) and inside bend radius used in calculations for different material types, thicknesses and tools. The result is more exact automatic gage allowance calculation for a wide variety of applications, says Kirchoff. "This software enhancement is now available to all CINCINNATI PROFORM, PC-control AUTOFORM, and Bend Simulation Module users," he says.

Plaunt carefully built his custom table by doing test bends and determining inside radii and neutral axis offsets for all Moline's various material and tool combinations. Still, when the software upgrade was completed and it was time to see how the data would pay off, "I was astounded," he admits.

He dashed off the following email message to Kirchoff: While not a thorough test of the upgrade, we have had some truly amazing results. When the service technician had finished the software install, my operator was about to leave for the day. I quickly looked to see what bottom die was still in the machine from the last job. To save time, only the parameters for just the die with ten gauge stainless were installed. Then we bent up a single scrap piece with a 2" flange, just to see how good it was going to work. No trial bends, no time for second chances - just put it in and bend. The resulting flange measured 2.000" on one side, 1.996" on the other. I was dumbfounded. Thanks for listening and for having the custom bending parameters included in the new software update.

Using his empirical bend data, Plaunt has standardized the drawing procedures in the engineering department. "Now what we program is what we get," he says. "With the new software, the machine control now conforms with the real world. The black magic about bending metal just went away."

Moline has seen real savings on scrap, especially with stainless prices going through the roof, he stresses. The company runs 304#2B grade stainless sheet in 10 to 16 gauge, and bends stainless plate to 1/2".

However, that's only part of the savings. "Accurate parts lead to accurate welding fit-up," he notes. "This is particularly important when making corner to corner TIG welds. We wish to minimize welding time, part distortion and unnecessary grinding. Close fit-ups result when the mating parts are accurate. Furthermore, if we send a part to assembly and it doesn't fit, the cost of rework, assembly disruption, or shipping delays can be far greater than the value of the parts," says Plaunt

The software enhancement fulfilled Moline's precision expectations in ordering the 230-ton AUTOFORM with 14 ft. bed. "It's a very accurate system for us," he says. "The angles are almost always right on and the flange widths within a few thousandths of nominal spec. In fact, material thickness variation now accounts for most of the differences," he adds.

"With the AUTOFORM's precision and Wilson quick-change dies, our productivity has far surpassed what the older, conventional press brake allowed us to do," says Plaunt. "The work just flows through now. We have the confidence to do our bending just-in-time, bringing us into line with the way the rest of the plant works."

Moline Machinery is a leading manufacturer of industrial baking equipment for production of doughnuts, sweet goods, pastries, cookies, pizza crusts and specialty breads. It specializes in automated systems and lines for volume production by wholesale bakers and food processors. A comprehensive product line covers everything from moulders, dough oilers, flour dusters and automatic panners to rotary cutters and frying systems.