Turret punch press:  Not just for thin material

Since its inception, Turret punch presses have come a long way over the decades. They progressed from the mechanical to the hydraulic to the electric servo drives introduced in the 1990s.

Servo-electric machines dominate much of today’s punch press market. But what if you’re processing carbon steel between 0.25 and 0.375 inches thick? You could call this thickness range a “thin plate.”

Servo-electric machines usually are rated up to only about 33 tons of punching force. When you’re punching 0.375 in, you move beyond the capacities of the electric servo punch press. It is here that the modern mechanical punch press fills a need.

Cutting Options

Cutting options abound in this thickness range. A CNC high-density plasma table with 130 amps of power generally can cut 0.25-in-thick steel at 150 in per minute (IPM), 0.375 in. at 110 IPM.

A laser, be it fibre or CO2, is another option, particularly if you’re cutting a lot of parts with a lot of contours. Lasers have so many power options that it’s difficult to quote a specific speed in a specific material. Regardless, if you process a lot of thin material, it may well be that, for your applications at least, nothing can beat a fibre laser’s speed. But as you cut thicker stock, the speed advantage isn’t quite as obvious.

Speed isn’t the only factor to consider. If you process precision parts sensitive to heat-affected zones, a waterjet is another option. You can expect a waterjet to cut at about 13 to 20 IPM on 0.25-in. material and at about 6 to 9 IPM on 0.375-in.-thick material, depending on the machine you have.

Opting for the Punching Press

If you’re planning to punch a steady diet of 0.25-in thick material, choose the press wisely, especially if you want your machine to last. Push the envelope of your machine’s rated capability, and the machine almost surely won’t last as long as expected. And if you’re punching in the realm of 0.375 in., you’ll need a punch machine with 45 to 50 tons of punching force, something only a hydraulic or mechanical punch press can offer.

Servo-electric drive systems have become popular for good reason: They’re extremely efficient. Some modern electronic servo-driven machines are rated up to 30 tons of force (able to punch up to about 0.25-in. material) and produce two hits with every complete revolution of the servo motor—one during 0 to 180 degrees of rotation, and the other from 180 to 360 degrees. The energy for the first hit comes from a pushing force, while the second hit comes from a pulling force.

Electronic servo-driven machines adjust their stroke based on the material and the tool in use, and they excel at forming. You can adjust the bottom dead centre (BDC) of the tool’s stroke to control the height of the form.

Note that you can still use form tools on a mechanical punch press. However, you do need to shim the tool to ensure you achieve the desired stroke depth for the form, test it, and record the settings in the setup log. If you don’t record the amount of shimming needed, you will need to test again when the job comes up next, which takes time and cuts into profits.
This shear punch can penetrate material with lower tonnage requirements. But it should never be used to bring a job within tonnage limits. As the edges wear, it essentially turns into a flat-bottom punch, which raises punching tonnage.

But if you’re working with 0.25- to 0.375-in.-thick material, it’s highly unlikely you’re making knockouts or forming louvres for airflow. If an enclosure with such thick material needs airflow, it probably has obround or circular holes or other kinds of openings. All these can be made in the mechanical punch press.

On your workbench or desk, find a pen with a clip that attaches to your shirt pocket. Hold the pen lengthwise, parallel to the table with the clip pointing straight up. That clip represents the eccentricity of your crankshaft on the old-style punch machines. As you rotate the pen in your fingers, the clip is at the bottom of its stroke; this is when the punch is through the material. On your machine, the flywheel engages the crankshaft to start rotating. When the eccentric mechanism is on top, the punch is at the top dead centre (TDC), after which it will rotate to the bottom.

Modern mechanical punch presses operate in largely the same fashion, except for a few key differences, the primary one being a modern control system. The punch press may be mechanical, but it’s still controlled electronically by a TDC controller, a permanent feedback system on the ram axis that ensures the system always knows where the ram is.

Basic Punching Factors

When the punch contacts the material, and the punch and die perform the initial cutting, the throat of the frame—be it a C-, J-, or bridge-frame machine—opens slightly. During this time, actual cutting occurs. If you were to look at the punched edge under a microscope, you’d see shiny portions at the top and bottom and a jagged edge in the middle. This is where the machine overcomes the material’s shear strength and the punch rips or bursts through.

With this stroke of the punch, all the rules of thumb we’ve been taught about punching come into play, including the clearance requirements between the punch and die, along with the punching tonnage calculations as published by your machine and tooling manufacturer. The higher the material’s shear strength (as with, say, stainless), the smaller punch perimeter you’ll be able to use to stay within the machine’s tonnage limits.

You need to balance this with your punch diameter requirements. From a tonnage perspective, you should never punch through the material with a punch diameter that’s less than the material’s thickness. Any smaller and you end up defying the laws of physics, and your punch is bound to break.

The smaller the punch, the more force goes into a small area. It’s a little like over-sharpening your pencil. Push down on a dull pencil point on a table, and it will take considerable force to break it. Push down on a very sharp pencil and within seconds the sharp point breaks.

When nibbling with punches, you should never have less than two times the material thickness under the punch. To do so could cause side-loading of your tools. Nibbling with a pitch less than the material’s thickness is like putting a power drill right on the edge of a piece of material. Pull the trigger, and it will walk right off the edge; before you know it, you’ll have tool wear because of side loading.

Also, watch for edge quality problems when nibbling. Whenever you punch large holes with a round punch, you will have scalloped edges, which can become especially apparent when punching thick stock. If you’re using a punch with shear, the scalloped edge will be more pronounced and might not be acceptable. To minimize this, you should use a flat-bottom punch. The downside is that this will require full blanking pressure, because you’re using the full bearing surface of the tool. The upside is that the flat-bottom approach will eliminate the heavier scallop you would receive from a punch with shear.

If scalloped edges are acceptable, using a punch with shear can certainly be a good approach, and it can lower the punching force. Still, it’s never a good practice to use the shear of the punch just to bring the tonnage of the hit within the limitations of the machine.

Material Handling with a Punching Press

When punching thick material, slug pulling usually isn’t a problem; gravity works to the process’s advantage. But in many other cases, you need to accommodate for physical realities when punching 0.25-inch-thick material and beyond, and this includes material handling.

When punching thin material, you have many part removal and handling options. You can go the conventional route and micro tab parts in place, to be shaken out later, or you could use speciality tools like V-line punches and dies that put a groove in the material. That groove allows parts to be hand-bent, eliminating a downstream forming operation. Parts also can be snapped apart, revealing an edge that requires no deburring.

With stock that’s 14 gauge and thicker, though, you’ll likely need to resort to traditional micro tabbing—and you need to make sure those tabs are sufficiently narrow and of the right number for each part. If you have 0.375-inch-thick material with two 0.010-inch. tabs on the corners of your parts, you don’t have a shake-and-break sheet; you have a bang-and-break sheet. Removing those parts will be difficult, to put it mildly.

You need to ensure you have tabs in enough places to hold the piece in place during the punching cycle, but you also need the tabs to be thin and sparse enough so material handlers can break out the parts efficiently. Determining the optimal tab size for your product comes from experience, and it is dependent on the sharpness of the tools and the tool radius. For instance, a rectangular part with sharp corners will require a different tab than a rectangle with radius corners. A common practice for using square and rectangle punches in thicker material is to add a small radius (0.010 in.) to the tools, so they don’t crack in the corners.

Heavy-duty punching machines should have heavy-duty work holders in good working order, with multiple clamps exerting significant force. This allows the machine to manipulate plates up to about 440 pounds, depending on the machine model you have. If a plate exceeds this, you can slow the material’s traverse speed. Of course, if you’re punching such a large sheet or plate, you won’t be breaking any speed records anyway. You should also lubricate the sheet, use heavy-duty tooling, and possibly use punches with additional back taper.

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