CAM NESTING TRICKS FOR WATERJET CUTTING

By taking part-geometry in the form of CAD files in and outputting NC code used to control a waterjet cutting machine, CAM nesting software automatically and efficiently arranges the required quantities of individual parts needed for optimizing sheets or plates of stock material.

A variety of specialized CAM software available today is developed independently from machine suppliers that provides a single software solution allowing companies to operate all brands of waterjet machines (and other processes), regardless of CNC controller type. Although use of material is still a core focus, nesting software now plays a central role in other areas: maximize part quality, reduce programming time and complexity, optimize manufacturing system productivity, provide detailed operator and management information.

For example, consider a few of these opportunities where programmers can provide a competitive advantage:

Quality. Different cut speeds deliver different edge quality when applied to a particular material and thickness. This is useful to maximize productivity and achieve the desired part quality. The width of the cut being made by the jet nozzle is called kerf. Generally speaking, the slower a jet nozzle moves across the material being cut, the wider the cut ? the larger the kerf ? it makes. In hard, thin material such as ½ in (12 mm) stainless steel this effect is negligible. But kerf can enlarge up to 0.005 in (0.1 mm) or more in thicker material or softer material.

Programmers can set the desired edge quality via the nesting program using either CAD line color to automatically set a specific cut speed, or by manually assigning a cut speed to the CAD file after it has been imported in to the nesting program. Also, the offset setting in the software is typically intended to correct for the cutting width of the jet. However, if the offset is manually set by an operator and based upon the average width of the jet, it may not fully account for any slight variations in the cutting width as the jet slows for corners or speeds up for straight cuts.

Dynamic Piercing. Being able to pierce materials without requiring a mechanically-drilled starter or pilot hole is a strong feature of waterjets, but there are some limitations. For example, when piercing thicker material the jet has a tendency to rebound directly into itself, reducing the effective power of the jet. Using nesting software to program the cutting head to move back and forth or in small circles during the piercing process, known as dynamic piercing, helps avoid this problem.

Ramping. To achieve optimum edge quality and part geometry, cut speed ramping is required. Programmers can vary cut speed for incremental CAD line segments, based on part geometry such as corner angle or radius size.

Clamping. Effective clamping of work materials is necessary in many cases. Forces imparted from the jet and movement from water in the tank below can cause the sheet or plate to vibrate or even float and move during cutting. Programmers can establish safe zones on the nest where clamps are used. These are not filled with parts and are avoided during cutting. Avoiding specific areas on the nest is also useful when cutting work material such as stone or leather, which typically contains defects. In some cases it is possible to overlay a digital picture of the work material to the nest area, making it possible to nest around defects to avoid scrap and wasted time.

Collision Avoidance. Avoiding nozzle damage from tipped up parts is important. Though some machines are equipped with crash protection, this does not really treat the root of the problem. By assessing a variety of factors, nesting software can determine a certain probability of a profile tipping up or warping on the sheet. With that knowledge, a cut path is automatically developed that moves the head around any profiles using a partial or zero head-raise, or directly over profiles with a full raise depending on which is faster. Also, intelligent lead in/out locations may ensure that the head always moves away from the previously cut profile, negating the need to create avoidance paths altogether.

Reports. Communication of the job(s) relevant information between programmer, machine operator, and management is another vital key to success. Nesting software reports can assist with material selection and preparation, scheduling jobs, tending to the job during cutting, etc. A variety of reports are typically available, often incorporating images of the nest, cut sequence, production time and cost per part and per job, etc.

TRICKS OF THE TRADE

Here are some tricks of the waterjet trade that operators and programmers alike should find useful:

? Utilizing the flexibility of nesting software to create different 'machine' settings in conjunction with 'tech tables' allows programmers to quickly generate tailor made NC code depending on the desired requirements from different material types and thicknesses. For example, when cutting expensive material such as titanium, programmers can maximize material yield by using common line cutting to get the biggest bang for the buck. This requires more attention from the programmer and operator, but the yield benefit is worth the extra manpower expense. Similarly when cutting thick material, cut quality is greatly improved by ramping into corners and, whereas using ramping on thin material may not be necessary and would increase overall production time.

? When use of tabs is undesirable due to a requirement for good surface/edge quality, using toothpicks inserted into the cut path allows parts to be firmly held in place without falling through the cutting table.

? Cutting material on top of good smooth plywood or other substrate minimizes the effect of backsplash against the underside of the material being cut due to the jet being deflected back off the grates/slats.

? Adding a plate around the front and side of the cutting table, then cutting it parallel with the machine axis, allows the operator to quickly set up sheets that are square with the travel of the machine without needing to double check the set-up and verify the alignment.

? Using a crop cut in the nest allows programmers to create defined crop cuts, save remnants and index the sheet against the previously made 'straight' edge at the front of the tank. This allows operators to cut sheets that are longer than the table is ordinarily capable of cutting. This maximizes yield and eliminates further operations such as shearing metal and extra loading/unloading of sheets. Indexing is quick and easy and minimizes operator fatigue and confusion, especially when reports are issued from the nesting program to graphically define the process.

? If the machine has a programmable Z axis, the full-raise feature can be utilized quickly and easily to avoid clamps and weights that are on the table when traversing.

? Nozzle crashes are often costly and stop production due to the damage it can cause to the cutting head, as well as the chance of moving the material partway through a cut and thereby scrapping some or all of the parts.

Using 'zones' to avoid clamps and other obstacles reduces these risks.

Derek Weston is the vice president for international sales and marketing for MTC Software, Inc., 22 West Main Street, Lockport, NY 14094, 716-434-3755, Fax: 716-434-3711, www.mtc-software.com.