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Cutting bolt-quality holes with plasmaHypertherm’s True Hole technology enables bolt-ready holes to be plasma cut, which rivals drilled or laser cut holes – and at a fraction of the cost. The technology links CNC nesting to a combination of specific parameters to do with amperage, material thickness and hole size. One of the key advantages, in addition to improved hole quality, is the automation of the process.

For years, plasma operators have faced what they considered their number one process issue: hole

quality. Despite the many benefits of plasma – including fast cutting speeds and low operating costs – plasma could not really create a bolt-ready hole.

Operators faced two main challenges. The first was edge quality. When cutting holes with plasma, a ding or divot would form along the edge of the hole (see Figure 1a). This formed an out-of-round hole and made the hole susceptible to stress fractures in high-strength applications. A second challenge was caused by a taper in the sides of the hole (see Figure 2). Because of a lag angle caused by the plasma jet, the top of the hole was often wider than the bottom. This taper created tolerance issues that forced fabricators to undertake secondary operations such as reaming or drilling to improve hole quality.

TECHNOLOGY DEVELOPMENTPlasma manufacturers were aware of these challenges yet it has taken years to find a way round them. The quest to solve plasma’s hole cutting challenges began in Hypertherm’s advanced engineering labs. After months of hole cutting and data gathering, our engineers did prove that excellent hole quality was possible with plasma, however, the list of equipment and parameters needed to achieve that hole quality was very lengthy.

The engineers found they needed the best possible

Author: Jim ColtHypertherm Inc

FINISHING PROCESSES

plasma technology, the best motion control technology, height control capability and an on-site CAD programmer to achieve excellent hole quality. They also needed a large number of set-up parameters and, to complicate matters, the data and parameters were different for every hole diameter, every material thickness and every plasma power level. So, expecting to find this level of expertise at every mechanised plasma user site was unlikely, and affordability would be outside the scope of users. The way forward required a large database that could be controlled automatically through computer aided manufacturing (CAM) software.

There were quite a few nesting software manufacturers that catered for CNC plasma cutting systems. These nesting packages were designed to make the part programmers’ jobs easier in cutting shops by automating the input of CAD drawing files and automatically positioning these parts on the steel plate to get the best plate usage. The more advanced companies had features that took more aggressive control of the machine code, allowing automated manipulation of feed rates, plasma on/off timing, torch height control functions, as well as inventory control of plate remnants. After a thorough search, Hypertherm decided to partner with MTC Software, a leading US developer of cutting CAM software. Through the sharing of propriety data and engineering expertise, Hypertherm and MTC were able to finish development of a

r Fig 1a,1b Cut hole showing ding/divot: a) normal plasma cut, b) True Hole

1a 1b

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This technology requires a HyPerformance Plasma such as HPR130XD®, HPR260XD, HPR400XD or HPR800XD auto gas system along with a True Hole-enabled cutting table, nesting software, a qualified CNC control and a high quality torch height control such as the Sensor THC or ArcGlide in order to coordinate properly. This technology is optimised for mild steel applications up to 25 mm with a 1:1 to a 2:1 diameter to thickness ratio.

The four steps involved in creating a True Hole are as follows:

` The operator imports a standard .dxf drawing file into True Hole-enabled software such as ProNest2010; the software automatically recognises the presence of holes. The programmer chooses the number of parts required, sets the plate size, selects the material thickness and the process power level (amperage for the plasma process). Multiple parts can be entered in many different quantities.

` The output .cnc file is then sent to the Hypertherm CNC, either through the network or a USB mini drive.

` The CNC prompts the system operator to check the consumables in the HPRXD plasma torch (there is a picture on the CNC screen with part numbers for verification) and also prompts the operator to make sure the right thickness and size plate is loaded.

` The operator merely pushes the start button as gases, amperage, arc voltage, delay times, kerf adjustment and machine code changes are all automatically set.

The ProNest software recognises hole sizes, plate thickness

r Fig 3 ProNest screen

r Fig 2 Definition of hole taper and bolt fit

this new hole cutting technology, now called True Hole™. This mutual development effort led to Hypertherm actually acquiring MTC in early 2009.

TRUE HOLE TECHNOLOGYTrue Hole technology is a specific combination of many parameters linked to a specific amperage, material thickness and hole size. One of the key advantages, in addition to improved hole quality, is the automation of certain parameter settings by the True Hole-enabled nesting software package and CNC including: process gas type, gas flow rates, pierce methodology, lead in/out techniques, cut speeds and cut timing.

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and plasma power level and gathers the data that was developed during in the R&D labs. Figure 3 shows a typical screen display. It then applies this data in a few milliseconds, modifying the machine code with corrections to make extremely high quality holes without any time or expertise required. This is a truly robust process that takes full advantage of advanced software, advanced CNC control technology as well as advanced plasma process technology, while making the operator’s and programmer’s jobs easier.

The sequence of operations to produce a hole are shown below and partially shown in Figure 4.

` Pierce in centre of hole using O2/air` Auto switch from O2/air to O2/O2 for hole cutting ` Auto adjust for kerf compensation from the shifting

process point` Applies semicircle lead-in` Auto sets optimal lead-in speed` Auto switch to optimal cut` Auto begins current ramp down` Current switches off while monitoring full speed torch

position` Auto applies optimal lead-out length` Auto switch for continuous cutting

RESULTS The end result is an up to 50% improvement in the shape of the hole (as defined by the cylindricity) and virtual elimination of taper and ding on holes with an equal diameter to thickness ratio. Figure 2 compare holes and bolt fit without and with True Hole technology. Figure 1b shows the cut surface with much reduced dings. Figure 5 compares hole taper (cylindricity is a measure of hole quality) for normal plasma, laser and True Hole technology for 9.5mm bolt holes. This is within normal diameter tolerances. At the 2010 EuroBLECH show in Hannover, Germany and the Fabtech show in Atlanta, USA. Hypertherm demonstrated this technology by combining a HPR260XD auto gas system along with the EDGE Pro® CNC, MTC ProNest 2010 nesting software, and the ArcGlide® THC. The system did not required operator intervention in order to produce plasma cut holes that rival drilled holes.

CONCLUSIONSTrue Hole technology is such that operators can now use plasma to easily and quickly achieve holes that rival drilled and laser cut hole at a fraction of the cost. MS

Jim Colt is Strategic Account Manager, Hypertherm Inc, Hanover, New Hampshire, USA.

CONTACT: jim.colt@hypertherm.com

r Fig 4 Cutting 25mm with True Hole

r Fig 5 Comparison of hole cylindricity