Choosing the option of plasma cutting or waterjet cutting basically boils down to the customer’s applications, tolerance requirements, and budget.

Other factors can be evaluated, but for simplicity, focusing on the customer’s application, productivity, tolerance, and operating cost needs will steer the discussion to the right solution.

Application

The customer’s application(s) will include the type of material to be cut, intricacies of the finished product, cut quality desired, thickness of the material, if a heat affected zone (HAZ) is acceptable, “downstream” (secondary) processes, cutting speed (production rate), etc.

Plasma stats

  • Plasma cutting machines can cut a wide array of conductive metals, including mild steel, carbon steel, stainless steel, aluminum, copper, and other metals.
  • Plasma can be used on metal that is in any condition (rusted, painted, grated).
  • Most productive cutting process on steel from 1/4 inch to 2 inches in thickness.
  • Covers thicknesses ranging up to 2–3 inches, depending on the material.
  • Some grinding may be needed for slag removal around the heat affected zone (HAZ); High definition plasma minimizes the amount of slag.
  • Plasma cutting machines can be highly automated making it easy to use for the operator.
  • Fine cut quality is not as precise as a waterjet cutting solution

Waterjet stats

  • Materials commonly cut with a waterjet include steel, stainless steel, aluminum, textiles, rubber, foam, plastics, leather, composites, stone, tile, glass, food, paper…essentially can practically cut any material.
  • Waterjets typically are capable of cutting metal thicknesses up to 6 inches and up to 18 inches on most other materials.
  • Waterjet is a cold-cutting erosion process that produces no heat affected zone (HAZ) or mechanical stress (often a requirement in the aerospace industry).
  • Creates a smooth edge – no secondary processing required.
  • Able to produce intricate cut – complex shapes can be made.
  • Can be a messy cutting process.

Productivity

There are many factors to consider which can affect productivity such as any necessary secondary operations, operator competency, delays associated with piercing, ongoing maintenance requirements, time to replace consumables, complexity of the part, the nesting pattern, etc.

However, one of the biggest differences between a plasma and a waterjet cutting solution are their cutting speeds. In other words, productivity efficiency is equated with the number of parts that can be produced within a defined time period. Plasma cutting is much faster.

For example ... 

  • to cut 1/2-inch thick mild steel with plasma, the cutting speed (using 300 A) is approximately 155 ipm (inches per minute). 
  • to cut 1/2 inch thick mild steel with a waterjet using a 90,000 psi pump, the cutting speed will be approximately 6.9 ipm. 
  • a waterjet using a 60,000 psi pump, the cutting speed will be approximately 4.6 ipm. 

Using this example, to cut 275 inches of 1/2-inch thick mild steel with a waterjet using a 60,000 psi pump, it would take the operator approximately 1 hour to complete. Using a plasma cutting solution to cut 275 inches of 1/2-inch thick mild steel using 300 A, it would approximately take the operator less than 2 minutes to complete.

In addition, if there is a requirement to pierce the material (i.e. lots of holes), this can significantly slow down the waterjet speed to cut the part.

With that said, the waterjet cutting solution generally has a higher cost per part since it takes longer to cut each part.

Cutting tolerances

Tolerances are affected by many variables such skill of the operator, speed, torch height, material thickness, and most importantly the quality (accuracy) of the cutting machine. Therefore, here is a basic tolerance generalization to show the difference in the cutting process’s accuracy: 

  • Plasma: +/- 0.015" to 0.020"
  • Waterjet: +/- 0.005"

Operating costs

Operating costs for CNC plasma cutting machines need to into consideration the cost of consumables, power, gas, spare parts, and labor.  Operating costs for waterjet cutting machines need to into consideration the cost of water, power, wear parts, routine maintenance, garnet abrasive, and labor.

Plasma operating costs are lower than waterjet operating costs. Generally, it can cost approximately $15-$20/hr to operate a plasma cutting machine. Operating costs for waterjet cutting machines can run around $20-$40/hr. Both estimates do not include labor.

More maintenance is typically required on waterjets. High-pressure water and the abrasive material can accelerate fatigue in the system’s parts. As an example, nozzle parts typically are replaced around 50 to 100 hrs. Pump seals are typically replaced between 500 to 1,000 hrs.

Summary

Every individual customer has their own unique needs, desires, and what they value the most. However, for most customers, the key differences between plasma and waterjet cutting machines (for cutting metal) are in the cutting speed (productivity /parts produced per hour) and cut quality with no heat affected zone (HAZ). This is what typically moves them to one solution over the other.