Faster than just-in-time

Stema Punch & Die rough and finish grinds punches made from extremely hard material, such as M2 tool steel, on its Studer S32cnc punch grinder.

Some of the punches Stema Punch & Die produces require O.D. tolerances of ±0.0003 in., and its S32cnc grinder holds tolerances to ±0.0001 in. for profiles up to 3 in.

Just-in-time (JIT) manufacturing is critical to stay competitive for most shops, but it's particularly true for punch-grinding firms. This is because job turnaround times that were once 3 or 4 weeks have dwindled to a few days. And in some instances,-turnaround may be less than 24 hr. Such tight delivery schedules force shops to pump out work at a faster-than-JIT pace, and to do so takes a variety of flexible equipment, including advanced CNC punch grinders.

For Stema Punch & Die in Cambridge, Ont., the latest in punch grinding is a Studer S32cnc from United Grinding Technologies, Miamisburg, Ohio. This punch-grinding machine, which joins Stema's two other CNC punch grinders, not only meets production needs but also provides Stema with a degree of flexibility for handling hard materials such as M2 tool steel.

Mostly, the Studer roughs and finishes punch blanks ranging in size from 0.125 to 3 in. in diameter and 2 to 6 in. in length. "In some cases," says Stefan Lorbach, founder of Stema Punch & Die, "we have to do secondary operations, but these usually involve EDMing additional profiles, threads, or keyholes. Still, about 80% to 90% of our punches are roughed, finished, and ready to go when they come off the Studer."

In addition to flexibility, Stema relies on the Studer's ability to hit and hold close tolerances. Lorbach reports that most of the punches require an O.D. tolerance of ±0.0005 in. However, some must be held within ±0.0003 in.

"The Studer easily handles a ±0.0001-in. tolerance," points out Lorbach, "and a 1.00-in. ground profile, which is basically standard. But it also tackles 2.500 to 3.000-in. profiles for custom punches, giving us an additional competitive advantage."

Programming the Studer also provides flexibility and increased throughput for Stema. The machine control stores process parameters for many routine jobs and makes changeover from job to job as easy as calling up a part number on its Fanuc 16i touchscreen, letting the machine adjust itself. For its tricky parts or new, complex ones, Stema uses the machine's built-in routines for specific needs and then stores them for the next time the part comes along.

As much as it would like to, Stema doesn't automate its punch-grinding process. " Cycle times are short," says Lorbach, "and to grind punches through the night would require-a robotic loader to handle the blanks. And even with that in place, we would probably work on only six parts or so at a time, which would take just 35 to 40 min. So, rather than automate the punch-grinding process, we focus on making fixtures that are easy to change over, letting us save time that way."

The shop also saves time by re-evaluating its established processes. Lorbach notes, for example, that some of the punches coming off the Studer, in particular square or oblong ones, require matching die buttons produced by wire EDM. In these instances, programming the part often means entering as many as 86 different instructions, a process that takes twice as much time as needed to make the part.

"We were producing such a part in 6 min," says Lorbach, "but programming the EDM took 12 min. So we gave our software guy some profiles and asked him to rethink the process. We now program the same operations in a little over a minute."

This same sort of process evaluation is ongoing between Stema and UGT. Programming technicians at UGT often work on software routines for Stema to reduce setup times and produce parts quicker.

Organized chaos

According to Stefan Lorbach, founder of Stema Punch & Die, the Studer S32cnc lets his shop keep up with a production schedule that makes just-in-time look slow.

Due to the nature of its business, Stema Tool & Die cannot afford the luxury of building to inventory.-Every punch it produces is special in diameter, length, points, profiles, and surface finishes. The only hint of routine in the shop's chaotic schedule is prepping punch blanks, which are made up in advance. Beyond this operation, says Lorbach, everything else about Stema is custom and on-the-fly.

Rarely does the shop run jobs as a batch or a family-because the part mix is too high. "We may do two oval, followed by six round, followed by four square or rectangular punches," comments Lorbach. "And if it's not the mix, then it's the delivery requirements that make running batches next to impossible."

Even when a volume opportunity of 50 to 100 pieces does occur, there may be 10 of one type punch, 8 of another, 4 of another —all of differing lengths, diameters, and profiles. So, the idea of running production does not translate well into the operation.

According to Lorbach, the shop's machine-tooperator ratio is 2:1, 3:1, or more. This is critical, he adds, for flexibility and cost containment when working with an unpredictable schedule.

Operators often use one machine for roughing, another for finishing, and a third for something different like machining a flat on a part. By operating this way, Stema doesn't constantly change over machines for different parts. And if a customer needs one or two pieces in an emergency, the shop doesn't have to interrupt a process to dress a wheel or a radius. "Constantly starting and stopping to accommodate rush jobs," says Lorbach, "wastes time and money, which is why we have so many different types of machines."

Staying sharp

Proper punch sharpening requires a grinder, such as DCM Tech's PDG system, that feeds in small increments, delivers plenty of coolant, and feeds automatically.

Stema Punch & Die delivers punch tooling quickly so its customers can stay productive. However, it is the responsibility of those customers to maintain the tooling to continually produce quality parts and get the best value out of the tooling. Proper and regular sharpening maximizes tool life and increases production.

Tool effectiveness decreases at an increasing rate as a tool dulls. A dull tool doesn't punch a clean hole and creates a burr on the backside of a part. A general rule of thumb is to sharpen the tool before it's necessary to remove more than 0.010 in. of material to restore a sharp edge. A tool with 0.100 in. of grind life that is sharpened regularly makes far more hits than one sharpened less often. Since, regular sharpening requires less material removed, tool life is longer.

An additional point to remember, says Mike Anderson of DCM Tech, a grinding machine manufacturer in Winona, Minn., is that improper sharpening methods can damage tooling and shorten punch life as much as not sharpening often enough. Grinding, he says, is the only way to establish a new edge on a tool.

However, running an abrasive wheel across a tool's face creates friction and heat, which, if excessive, negatively alters the metallurgy of the tool steel from which it's made. The result is rapid dulling or tool breakage.

To keep heat generation in check, a grinder should feed in small incremental steps of 0.0003 to 0.0005 in., deliver plenty of coolant to the tool, and include an appropriate grinding wheel and a reliable automatic feed. One machine that fits this description is DCM's PDG Punch and Die Grinder.

Its rotary action and automatic feed control remove 0.010 in. of material in less than 4 min without burning tools. PDG's are easy to run and accommodate universal fixturing to sharpen all brands of tooling. According to Anderson, shops using the DCM sharpening process typically report a 30% reduction in replacement tooling.