Training 101: Starting Out By Starting Right

The Piercing Process: Here are some tricks of the trade for creating smaller holes in thicker steel plate using a laser.

There are no secrets.

One subject that never fails to bring enthusiastic student participation in our training classes is the exercise of creating smaller holes in thicker steel plate using the laser. Users are sometimes frustrated by the task of cutting smaller sized holes with good results. Working with sheet metal, the long standing rule has always been: The diameter of the hole should not be less than the material thickness.

Unlike the laser, other technologies such as punching are limited by the physics of force versus surface area, thickness and tensile strength. All conspire to shatter and destroy small punches while attempting to produce small holes in thicker material. Plasma torches lack the required tolerance and finesse to produce any holes of real quality. Prior to laser technology, only drilling and wire electrical discharge machines (EDMs) had the capability and, at best, those methods were slow and required multiple setups, not to mention the broken wire and drill bits.

Fortunately, for the last 20 years or so, laser manufacturers have developed the capability to produce small holes in thicker material simply, cost effectively and relatively quickly. A successfully cut geometry always begins with a good pierce. The goal is to produce a pin hole with no splatter or crater; multiple piercing parameters discovered over the years can produce a small start hole.

A new advancement in drilling holes or piercing involves an optical feedback monitoring device that adjusts pulsing frequency and pierce time by “watching” the reflected light from the actual drilling process. During the piercing process a monitoring device senses the light reflected from the hole being drilled. The intensity of the light being received by the sensor adjusts the frequency of the pulse. The power is always on full. Lack of any light being received by the sensor switches the piercing operation off. This device produces consistently small quality through holes rapidly.

For the owners of earlier model lasers, it is still possible to create similar results if their machine is capable of producing a cycle that can control the application of power over time. A ramp cycle or a power cycle applies power during piercing, from around 10 percent (or whatever doesn’t splash the material) rising to 80 percent or so over 10 seconds, but at 10 pulses per second. The actual pierce time will only be a fraction of this cycle time.

The key to the success of this technique is the 10 pulses per second pierce, a rate that allows the material to cool slightly between strikes of the beam so the material doesn’t over heat. Using the same pulses per second during cutting helps keep the cut cooler also.

Set the focus down deep into the material and test drill a couple of holes to establish the total time needed to pierce through the material. Make sure to watch and see the beam break through the bottom of the material. Total pierce time using this ramp will probably be under two seconds. Again, be stingy with the assist oxygen. Less than 1 bar is usually all that’s needed.

Using this type of ramp produces a very good start hole; it just isn’t automatic like using the sensor above. These parameters are based on using a 3 kW to 4 kW laser cutting machine. A machine of smaller resonator wattage will need to be adjusted for time. Figure 1 shows a typical power cycle that ramps power over time.

The important parameter is the application of 10 pulses (10 Hz) per second. Minimal heat is applied to the material. Avoiding over use of the oxygen during the pierce will also help reduce heat and splatter. With a good start hole, cutting the geometry is straight forward (see the example in Figure 2).

As an example, program a small rectangle with a few .125 in holes. Move the pierce holes are in the middle of the shapes you are processing (see Figure 3). There is not a lot of room here for a longer lead in. The general cutting guidelines are as follows:

(1)  For .250 in thick steel, do not use much power. Maybe 400 watts 10 Hz, 1 ipm to 3 ipm for feed and around 0.6 bar of oxygen. Set the focus above the material surface to open the kerf and enable the oxygen to flow into the material.

(2)  For .375 in thick steel, use around 500 watts, 10 Hz, 1 ipm to 3 ipm for speed and approximately 0.6 bar of oxygen (9 psi). These are very gentle cutting parameters. The cutting speed is slow, but you can double the speed by doubling the Hz to 20, which will still give you reasonably good results. The best quality is achieved with slow speed and low frequency. However, you should experiment with the parameters and find what is best for your combination of resonator, lens and nozzle size.

(3)  For .500 in thick steel,increase the power by 100 watts or so to 600 watts, while still using a federate of 1 ipm to 2 ipm, with a pulse rate of 10 Hz. Use a little less oxygen. Although this may be really pushing the envelope, cutting is still possible, but the results are less acceptable. Usually the hole loses quality in the last 20 percent of the bore.

Now for the important question: How good are the holes? The outcome depends, in large part, on the material used (see Figure 4). There is no substitute for good material. We have found that hole diameters of 1/3 the material thickness and above are good and start declining below that ratio. And, if time allows and better quality is required, drive the geometry tool path twice.

This cutting technique should work on any manufacturers’ laser of sufficient wattage that can pulse at low frequency. Surprising, not much heat is absorbed by the material and case hardening is minimal. Correctly cut, these holes can be successfully tapped.

Students enjoy this exercise immensely. They always desire to learn the “secret” parameters that produce really well cut small geometries. There are no magical settings; just a few basic principles. New operators are always surprised and amazed with the laser’s ability to produce such precise work so simply and quickly.