Stamper converting machined parts to stampings

A.T. Wall Company, Warwick, R.I., is a high-speed stamping and tube redraw facility, servicing the sensors, medical, telecom, aerospace, and automotive industries. The company is known for its ability to form glass-to-metal sealing.

An electronics OEM approached the stamper about making a universal pressure sensor. The company's stainless steel pressure sensors had been machined, and the OEM wanted to look into the possibility of converting them to single-sized stampings to save production costs.

The challenges of stamping a previously machined part out of 1/8-inch-thick stainless steel were demanding, but the potential rewards were great.

"On large volumes, we can make 100 to 120 pieces of this part a minute on a press, which is far quicker than it can be machined," said John O'Brien, corporate sales, A.T. Wall Company. The stamper applied its design expertise, material knowledge, and careful monitoring of tool wear to achieve the conversion in what became a four-year-long learning project.

"The customer had been looking for cost-down efficiencies because this product was entering a very competitive market. Even in lower volumes, we can stamp them for 65 percent less than the machined version," O'Brien said. First-year volumes are modest but could grow to several hundred thousand depending on its market acceptance, he said. "The more product lines our customer can get this sensor into, the more cost savings they'll realize."

Problems and Challenges

Tooling Costs."When you're talking with customers, the biggest hurdle you have to overcome is justifying the upfront stamping tooling cost which typically is much higher compared to machine setup costs. The tool itself is 42 inches long and has 14 working operations and 26 total stations," O'Brien said.

Converting machined parts to stampings"There's really no such thing as soft tooling for a 1/8-inch-thick stainless steel part like this," he continued. "You really need a lot of pressure to move this material around. It's not like you can work with a little bench press to form it. You have to make the real thing.

"Your customer really has to trust in what you're saying and in your ability to deliver," O'Brien said. "We spoke to the customer about the inherent problems and there were some naysayers who looked at the part and said, 'You can't stamp this.'"

Some of the skepticism derived from the differences between machining and stamping, and some from the material properties of the hard, thick stainless steel.

Forming Differences. Engineers have to look at designing a stamped part rather than a machined part very differently, O'Brien said. "Forming and flow of material is very different than plugging in your X-Y-Z axes and machining it and getting a part."

Material Properties. One material property the stamper had to contend with was work hardening. The thickness of the material required a lot of tonnage (120 tons on a 150-ton press) to make the part. "As you strike and move stainless steel around, it work-hardens very quickly, which makes it difficult to pierce without having detrimental effects to the material," O'Brien said.

Evaluating Design Differences

"First you look at the part design, and the material thicknesses, and determine whether you have the ability to actually blank the part out of the strip without having all the profiles become distorted. You think, 'How am I going to do this?'

Some problems that are inherent with stamping do not exist with machining. "There's no material flow in machining," O'Brien said.

'Brien said that the issues discussed with customers initially are related to meeting and holding customers' critical dimensions, and added, "It is our job to understand customer needs and for our engineers to imagine how this can be designed. Nothing at this point would necessarily be show stoppers. When we're working directly with engineers in the initial phase of project development, design issues can often be designed around."

Baby Steps."The first step in designing the tool is identifying the characteristics of the raw material," O'Brien said. "You want to make sure that you have identified incoming raw material temper and thought about your coining and draw work. Because the material work-hardens quickly, the order in which profiles are produced becomes extremely critical to the success of your design concept.

"We looked at this part. Several operations are going on—drawing the well, coining the weld projection, and piercing eight precision holes through the outside of the part. The operations must yield a smooth surface for glass-to-metal sealing.

"A lot of baby steps go on in the beginning. Our first step, without really worrying about the final dimensions, was just to punch holes through 1/8-in.-thick stainless strip that was milled to the correct width. The purpose was to measure the ID surface of each hole to ensure that the break or tear at the base of the holes still was suitable for sealing.

"Then we fed the strip through some basic forming steps and took readings to determine how much hardening took place."

Forming Challenge Areas

Well Thinning, Flatness. One of the first operations the company tested was the well's wall thickness, O'Brien said. "We weren't sure, once we drew the material down, how much it would thin from the point of entry.

One of the biggest concerns we had is that the well is under a tremendous amount of pressure in its final application. When you machine a part, you can control material thickness, but when you're stamping a part, material tends to flow."

Another challenge was achieving very tight parallelism and flatness tolerances. "When you're doing all this forming on a part, it can have a tendency to look like a potato chip. That's an extreme example, but there are stresses inherent in the material that cause it to distort. Being able to maintain parallelism is very critical to its final sealing assembly.

The customer pointed out critical dimensions and explained how not holding the tolerances affected the sensor's accuracy. Even the angles leading into the well and the flatness at the bottom of the well are critical to the part's function.

"There's no magic to producing and holding critical customer dimensions," O'Brien added. "It is a matter of maintaining clearances between die and punches and monitoring the stamping operation closely." The stamper measured critical dimensions about every 1,000 pieces for close compliance.

Small Hole-to-Material Thickness. One of the reasons it took years to develop the part is the difficulty the company encountered when piercing small holes through thick material. A stamping rule of thumb is for the ratio of hole diameter to the material thickness to be no less than 1-to-1. "It would be far less challenging if both the hole and material were close to 1/8-inch," O'Brien said.

"In some cases, we're attempting to pierce a 0.0415-inch hole through 0.125-inch material," he said. "In this step, we had to thin the material without work-hardening the strip." A.T. Wall engineers developed a method to move some of the material away from the area without work-hardening it, and then pierced the remaining thickness with a separate punch (see lead image). The customer was able to compensate for the extra material movement. "This process is one of the most critical breakthrough designs that allowed our engineers to keep the project moving forward. Working with a flexible group of engineers on both sides kept the drive going to complete the project," O'Brien said.

Burrs. The eight holes are glass-sealed, so they have to be burr-free. To achieve this, the stamper pierces and shaves the part's holes. First it pierces through the material with a smaller-diameter punch to remove the mass of the material, then comes back through and takes a few thousandths off all the way around to clean the edge. "It looks smooth and shiny inside. Not even a machined part would look like that," O'Brien said.

Precise Weld Projection

A weld projection is a feature commonly used to weld sensors and electronics parts. When it is welded to another part, the projection becomes molten and forms the bond between the two parts. The weld projection looks like a pyramid with a flat top. Its sides have to be at a determined angle, because those angles dictate the amount of material that will become the weld. A set current is passed through the projection during welding; variation could compromise the sealing of the part.

Blanking the finished part out of the strip can be a real challenge to maintaining critical dimensions. The weld projection has a tendency to lean outward after this stage, and this needs to be countered.

"We had to do a lot of experimenting to make sure that the weld is formed properly and that it maintains the proper height," he continued. The stamper makes the projection in three steps in a progressive die. First, material is pushed in two directions to form the pyramid shape. In the final step, they qualify the height and give it a flat top so the mating cover has a qualified area to meet.

Knowing Your Trade

"Not everybody in the world is interested in turning machined parts into stampings. The project wasn't a slam dunk by any means," O'Brien said. "Some parts must be machined, but knowing where stamping fits can provide a competitive advantage to my customer. Becoming a resource for our customers, rather than a vendor, gives my company a leg up on our competition.

"It really comes down to knowing your trade and working with stainless steel enough to know what to expect when the coining process begins. For every action, there's a reaction, and you have to have an idea as to how to interpret the clues given after each hit," O'Brien said.

"It's giving our customer's engineers a real clear idea of what's going to happen upfront rather than coming back and asking them for forgiveness later. That goes a long way," O'Brien said. "The openness and mutual respect we have with our customer really allowed a lot of 'out of the box' thinking, even though the part changed many times from its original machined drawing."

O'Brien said that although the project was difficult, he would welcome another like it.

"A.T. Wall Company has built its reputation around difficult-to-process parts and materials. We are always looking to work with customers that have challenging and difficult designs."