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Cold heading cranks up profits.

If you manufacture cranks for every major bicycle company in the US and Canada, you must supply rugged units and lots of them for the lowest possible cost. That's why Thun Inc, Clarksville, TN, turned to cold forming its range of bike cranks on a special progressive header from National Machinery, Tiffin, OH; the tooling came from Form Flow Inc, Wayne, MI.

Cold-forged cranks were first produced about 15 years ago, immediately replacing hot-forged units. Cold forming wasn't done on a header, but on presses.

In 1979, Thun bought National's 204 ML progressive header; they call it the Monster, Figure 1. With 15" of available die kick, the header is one of a rare breed. Other long work that has been cold headed on this size machine includes MacPherson strut rods, certain collapsible steering gear parts, and some of today's automotive tie-rod ends that can extend a foot in length.

Crank'em out

Thun uses hot-rolled bar stock for its bike cranks. The stock is straightened, mechanically descaled, a lubricant is applied, and then the metal is drawn and sent into the Monster.

Through a progression of four die stations, Figure 2, two journals are upset in the stock, and so is the collar, which eventually will fit against the sprocket. The arm is extruded smaller (to reduce weight), and the keyslot is extruded during cold heading.

After heading, there are a number of secondary operations: upset and flatten the pedal eyes, bend the workpiece into the form of a crank, drill and tap the pedal eyes, and weld on a tug pin.

With cold forming, there's no need for deburring operations. Finishes from 8 to 15 microinches rms are common (hot-forged surfaces range from 125 to 250 microinches).

When Thun went from hot forging to cold forming on the header, for example, they virtually eliminated polishing that cost 50^/crank. Moreover, in the header, there's no flash line where dies come together, as with hot forging.

When hot forging, stock for a crank had to be heated until it was cherry red. With cold heading, the slug requires no preheating--a great energy saver.

Extruding the key slot, by the way, also reduced manufacturing steps. Previously, the slot was milled. This extra operation threw up fine burrs along the edge. Then, when the cranks were chrome plated, the burrs built up with plating, causing fit problems in assembly. Thanks to cold forming, there are no longer any burrs.

Accommodating change

Thun makes cranks in four different lengths, for a variety of applications. Ten-speed bikes, for example, have a longer crank than exercise bikes, children's bikes, and BMX racers.

The header can be set up for each type of crank with a tooling change that typically takes two people about 1 hr. Each die weighs 50 lb.

"The header's versatility is fantastic," says Thun's General Manager, John W Himes. "Our customers are always coming up with something better looking and different. One recently developed a new model bicycle and discovered a problem with the usual crank we supplied them. They found that the 7/8" journal on the adjusting side didn't carry far enough toward the collar. When they put on the cone, it was just hanging on by a few threads. We easily lengthened the journal in about two days, thus making a running change at minimal cost."

Other benefits

Cold forming simply compacts or bends the metal grain, actually improving the structure. This means that less costly materials often can be used because strength is increased greatly. For instance, cold forming can give low-carbon steel (e.g., 1018) the strength of a medium-carbon grade (i.e., 1040) by making the grain flow follow the part contour.

Strength also can increase in nonferrous materials. Those most commonly used for cold heading are brass, copper and nickel alloys, bronze, and aluminum alloys. Others, which are commonly used for impact extrusion, include lead, tin, zinc, magnesium, and titanium.

Design for cold forming can effect great savings in material as well. A cold-headed part is made of stock that usually equals the diameter of the shank, while larger parts are headed to conform to the largest portion. Consequently, scrap loss in cold heading may be only 1 to 3 percent, while that in traditional machining or forging may run up to 75 percent.

Tooling tips

Mastering the Monster was by no means an overnight success. Thun's selection of Form Flow for tooling followed a year and a half of difficulties with other tool suppliers. "When you have a machine tool that costs about $1.5 million, it must run," notes Himes. "The tooling must be intricately compatible so workpieces move across the machine stations and come out like they should."

"At first, Thun asked us to become involved in building the back-up tooling and improving on the initial design to get better tool life," remarks Larry Cocke, Form Flow's general manager. "Then, with the job running as free of bugs as possible, an on-going program was established.

"In selecting tool materials, we chose some of the newer matrix-type steels. These have a more even carbide distribution. The material is M4, but offers the toughness of M2. Therefore, it has better shock resistance, but is easier to grind than conventional M4 steel."

He continues, "We increased tool life using segmented dies to a greater extent than usually is found in the industry. These dies consist of segments held together in a case with tapered screw plugs. The dies are less likely to break than a solid die because, in effect, they already are broken.

"Also, we used tapered dies where experience suggested they would be desirable. These dies are narrower in the front than the back. As a die insert moves from the back to the front of the die case, it doesn't have to be pressed the full length of the bore in the case, like a straight die does. In a tapered case, the insert is dropped in place and may only have to be pressed 0.125" or so, depending on the taper.

"This minimizes wear and makes for a more effective press fit at the front of the case, where it's usually needed.

"Whenever we are given the opportunity to design a die from scratch, we work for a specific ratio between part and insert size. This ratio, in turn, is relative to case size. Our insert sizes often may be smaller than those of other tooling suppliers, but it gains better case support.

"We are very particular about heat treating dies," stresses Cocke. "Cases, for example, are done in a vacuum heat treat, rather than a salt bath. They're allowed to cool naturally, rather than being quenched.

"Final grinding is usually to a four-microinch finish. This isn't for appearance, but for function. A scratch could split a heading die much like glass breaks at a scratch line. Following final grinding, dies may be stress relieved, and punches sometimes are coated with TiN and TiC."

For more information about cold headers from National Machinery, circle 540. For more about cold-heading tooling from Form Flow, circle 541.
COPYRIGHT 1985 Nelson Publishing
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Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Thun Inc. management
Publication:Tooling & Production
Date:Oct 1, 1985
Previous Article:Flexforming expedites automotive prototype sheet-metal parts.
Next Article:Forging trends update.

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