Ag Weldment to Casting Conversion
Dwayne Hammond was a newbie at Besler Industries Inc., Cambridge, Neb. He had just logged two weeks as draftsman at the agriculture equipment company when his boss tasked him with fixing a problematic component for its farm field harrow machine. The fabrication was too costly, too dimensionally inconsistent and took too much time to produce. Would a casting sow savings for Besler?
Hammond invited Smith Foundry, Minneapolis, which produces other castings for Besler, to work with him to design a single-piece component. Uninitiated in designing a part for casting, Hammond received a crash course on the process’s capabilities and constraints.
“I had not worked with casting before,” he said. “So we worked back and forth to get our draft angles and parting lines right.”
The problem part was a steel fabrication consisting of three flat pieces with two bends and a hole punched in each. The three pieces were welded together. Each fabrication required five minutes of welding. The component clamped onto long tubes of a harrow, which drags spikes over sod to break it up for better seeding. Each tube has two clamps, each harrow can have between three or four tubes, and a piece of Besler machinery can have up to four sections of harrows—a total of 40 clamps. Hammond calculated each weldment was costing the company $12-15 to make.
“It was a fairly expensive piece for us to make, and it didn’t really look that great,” Hammond said. “It worked, but it wasn’t clean and neat.”
Parting Line Position
Hammond consulted with Smith Foundry on material choice since the part would be converted from A36 mild steel to ductile iron.
“They told us the grade of steel they were using, and we conferenced with our metallurgist to come up with the cast product that met the physical properties of that grade of steel,” said Jim Pint, sales manager for Smith Foundry.
The metalcasting facility suggested 65-45-12 ductile iron, which exhibits 65,000 psi tensile strength, 45,000 psi yield strength and 12% elongation.
“There wasn’t a real high strength requirement, but they wanted high ductility, which is why we went with that grade,” Pint said.
With material selected, Hammond sent over his first drawing of the redesigned component. Smith Foundry responded with a redesign of its own that would make it easier (and less costly) to cast. But Smith Foundry’s version inhibited key features of the tube clamp. The main culprit to a quick and easy design: the casting’s parting line location.
The parting line is the line on a casting corresponding to the separation between the cope (top) and drag (bottom) parts of the mold. For ease of setting the core into the mold, Smith Foundry initially wanted to position the casting so the parting line would run through the center of the Besler Industries name on one of the flat ends and across the top hole (Fig. 1).
“Since I had never designed for casting, I would send them a picture,” Hammond said. “They would draw on that picture showing where the parting line would be, spelling it out pretty easily.”
The parting line in a mold is typically located at the largest cross section, and the patternmaker develops the pattern and corebox around it. Often, gates and risers are located on the parting line and will require removal via grinding, which may conflict with the customer’s requirements for the part.
Hammond determined the initial parting line location would not allow the redesigned component to include an important design element—a recessed diameter surrounding the top hole. Each clamp is bolted onto the tube through the top hole. Sometimes the clamp is positioned onto the tube so that a spike, which drags through the farm soil, must go through the hole instead of just a bolt. Besler found that the fabricated clamp often required modification to allow the spike to fit, adding expense and time.
“In some places, the threads on the spike would be a hair short,” Hammond said. “We decided to dish the hole down and put a recess there so we wouldn’t have to modify the spike every time.”
With the recess at the hole, the parting line location had to change.
“The way we wanted to make the part would have been better for [us] but not work for [Besler] at all,” Pint said. “We had to regroup and come up with an alternative coring design [with a different parting line location].”
Zero Draft With a Core
The tube clamp is cast with one core to produce the internal holes, including two 0.75-in. (1.9-cm) holes and a single 1.5-in. (3.8-cm) hole. Pint said that while the casting was fairly straightforward, it involved a bit of engineering to meet the holes’ dimensional requirements.
“The location of those holes was fairly critical,” Pint said. “Having it all on one core required controlled dimensional positioning.”
Further, the vertical face of the clamp, where the holes are placed, needed to be cast without draft so the part had a flat surface for assembly. Typically, draft or tapered angles are required on castings to allow the pattern to be withdrawn from the mold without breaking the edges.
Smith Foundry solved the zero draft requirement by forming the face of the casting with a core.
“[Hammond] wanted a part with no draft, which is always a challenge, but in this case it worked out,” Pint said. “In all the critical areas, we found ways to eliminate draft totally through coring.”
With this core design, the casting was repositioned in the mold so the parting line cut across the width of the part, out of the way of where the spike would run through the clamp.
“Once we got the design and parting line down, it was pretty problem-free,” Pint said. Initial samples were cast without difficulty, and the part went into production shortly after approval. Besler Industries is able to place the tube clamp casting directly into the assembly as a raw casting, without any secondary machining or cutting. The casting conversion costs 60% less than the weldment, according to Hammond, for an estimated annual savings of $8,000-10,000.
“I’m happy with the way it turned out,” he said. “The part came to the shop and worked. And it has a good, finished look.”
The casting also came with a bonus improvement. The original steel fabricated piece had a lot of play in the two outer bolt holes, which did not affect the part’s function but caused the assembly to rattle around, according to Hammond.
“The holes are much tighter now and don’t wobble,” he said. “It wasn’t a requirement, but it’s a nice benefit.” Metal