Producing Projectiles on Multi-spindle Automatics

Here's how one Illinois-based supplier is meeting the needs of an airbag manufacturer by turning stainless steel projectiles that are part of the inflation activation system. The production rates are high and the need to maintain quality is similarly demanding.

Har Technologies (Harwood Heights, IL), a precision machining company, started out in the aerospace industry. It produced components that were used by a company that made explosive devices that are used to eject pilots from their cockpits. For the most part, the machining that was performed was done on single-spindle (nowadays, of course, CNC) turning centers.

Har CEO Jeffrey Lampert surveyed the market opportunities that his company had. Given the experience with the aerospace components, he looked into another explosive safety device: automotive airbags. "We knew we could do a good job on air bag components because we understand that any mistake can cost a life," Lampert says.

However, getting this work would require a significant investment in new machinery. People would have to be trained. And the approach to the process would have to be completely rethought. All of this because they would need to make a switch from single-spindle machines to multi-spindle turning machines.

That is, among the air bag components is a projectile that breaks through an Inconel burst disk. Once the breakthrough is effected, gas begins inflating the airbag. The projectile continues into the gas chamber, and initiates another reaction to start the gas generator. The generator burns propellant that adds to the gas and completes the inflation.

The projectile is made of 17-4 stainless steel hardened to 40-45 Rc. Overall, the part is about 3/4-in. long and 1/2-in. diameter. It tapers to a sharp point. The critical OD tolerance is ±0.00075 in. Another critical dimension is on an O-ring groove; it is 0.030-in. deep and 0.062-in. wide; its tolerance is ±0.0025 in.

"The difficulty we have making the projectile on a single-spindle automatic," Lampert explains, "is the cut-off that needs to be removed. An alternative would be to turn it on a sub-spindle machine, but the machine utilization cost would be very high for such a simple function. So, from a price standpoint, to be competitive we had to consider going to a multi which makes the part complete, picks it up, and then finishes the cut-off. No secondary operations are necessary. It's ready for shipping."

However, there was one non-trivial problem before all of this happened: No one in the company had a clue about how to run a multi-spindle machine. They do conduct a comprehensive apprenticeship program at Har, but there was never a need to learn about multi-spindles. So when they went out looking for equipment, Lampert advised his people that it was necessary to find machinery that had sufficient capability to virtually run on its own without a great deal of operator assistance or interference.

Four multi-spindle automatics were purchased from a vendor. The machines didn't meet the conditions of the purchase agreement. They were returned to the vendor.

Next, they got in touch with Tornos Technologies U.S. Corp. (Brookfield, CT). Two years ago Har purchased a model SAS 16.6 multi-spindle automatic that features six spindles, six cross slides, and six end slides. "Now we have eight," Lampert says.

"Tornos has more capability than we need for any job we have—or will have," Lampert comments. "That is important to us because our mode of operation here is that parts are not made by machines, but they are made by a process. Once we have a viable process, our people only need to follow it exactly, and we are going to make good parts. The machines hold size very well, so our operators never have to worry about the parts being out of tolerance—aside from tool breakage and other such occurrences."

But what about the issue of the lack of experience with multi-spindle machines? Lampert answers, "Tornos technicians taught us how to run them; they supported them; they serviced them; they had parts when we needed them."

Before the first multi-spindle machine was installed at Har they ran the projectile on two-axis CNC lathes with sub spindles. A secondary operation on a manual lathe was needed to remove the cut-off. The output: 60 parts per hour.

Now each of the multi-spindle machines produces a part, complete, at a rate of 360 per hour. An additional benefit: one six-spindle machine with a bar loader takes up much less floor space on the plant floor than six single-spindle machines.

Each of the machines is equipped with an IEMCA PRA 40 bar loader stocked with 25 12-ft. bars. This constitutes one day's work (i.e., two 12-hour shifts). In operation, the process goes like this:

•Position 1. A cross slide carrying a facing tool faces the workpiece and an end slide works on the end of the piece.

•Position 2. The carrier indexes the part to this position for OD grooving by a cross slide and partial hole drilling from an end slide.

•Position 3. The hole is drilled to final depth.

•Position 4. The pointed end of the workpiece is formed. The hole is reamed to its final diameter and then flat bottomed.

•Position 5. The point is finish turned from a cross slide.

•Position 6. The point is cut off, supported by the pick-off spindle. As the piece is cut off, the pick-off spindle retracts and a back-finishing tool comes into place. With the pick-off spindle and finishing tool, the point is finalized and sent down the part chute.

The cycle time is 12 seconds.

Once each hour, SPC data is reviewed at a nearby gage station. A minimum Cpk of 1.67 must be maintained.

Observes Lampert, "It's an interesting tradeoff between single-spindle and multi-spindle machines. With a single, you create the part geometry with the cutting point. That means you can create anything you want and change the specs easily with a couple of entries in the computer. But it also means you will run the job more slowly than with a machine that is doing five or six operations at once each time it indexes. For high-volume production of relatively simple parts, multis must be considered."