Abrasive Delivery System

Autores do Artigo: Shripad Chopade, Sagar Kauthalkar, Dr. Pushpendra Kumar Sharma.

ABSTACT: Abrasive water jet machine tools are suddenly being a hit in the market since they are quick to program and could make money on short runs. They are quick to set up, and offer quick turn-around on the machine. They complement existing tools used for either primary or secondary operations and could make parts quickly out of virtually out of any material. One of the major advantages is that they do not heat the material. All sorts of intricate shapes are easy to make. They turns to be a money making machine.

I. Introduction

A machine shop without a water jet is like a carpenter without a hammer ultimately. Sure the carpenter can use the back of his crow bar to hammer in nails, but there is a better way. It is important to understand that abrasive jets are not thesame thing as the water jet although they are nearly the same. Water Jet technology has been around since the early 1970s or so, and abrasive jets extended the concept about ten years later. Both technologies use the principle of pressuring water to extremely high pressure, and allowing the water to escape through opening typically called the orifice or jewel. Water jets use the beam of water exiting the orifice to cut soft stuffs like candy bars, but are not effective for cutting harder materials. The inlet water is typically pressurized between 20000 and 60000 Pounds per Square Inch (PSI). This is forced through a tiny wall in the jewel which is typically .007” to .015” diameter (0.18 to0.4 mm).This creates a very high velocity beam of water. Abrasive jets use the same beam of water to accelerate abrasive particles to speeds fast enough to cut through much faster material.

II. Abrasive Delivery System

A simple fixed abrasive flow rate is all that's needed for smooth, accurate cutting. Modern abrasive feed systems are eliminating the trouble-prone vibratory feeders and solids metering valves of earlier systems and using a simple fixeddiameter orifice to meter the abrasive flow from the bottom of a small feed hopper located immediately adjacent to the nozzle on the Y-axis carriage. An orifice metering system is extremely reliable and extremely repeatable. Once the flow of abrasive through the orifice is measured during machine set-up, the value can be entered into the control computer program and no adjustment or fine-tuning of abrasive flow will ever be needed. The small abrasive hopper located on the Y-axis carriage typically holds about a 45-minute supply of abrasive and can be refilled with a hand scoop while cutting is underway.

2.1 Control System Fundamental limitation of traditional CNC control systems.

Historically, water jet and abrasive jet cutting tables have used traditional CNC control systems employing the familiar machine tool "G-code." However, there is a rapid movement away from this technology for abrasive jet systems, particularly those for short-run and limited-production machine shop applications. G-code controllers were developed to move a rigid cutting tool, such as an end mill or mechanical cutter. The feed rate for these tools is generally held constant or varied only in discrete increments for corners and curves. Each time a change in the feed rate is desired programming entry must be made. A water jet or abrasive jet definitely is not a rigid cutting tool; using a constant feed rate will result in severe undercutting or taper on corners and around curves. Moreover, making discrete step changes in feed rate will also result in an uneven cut where the transition occurs. Changes in the feed rate for corners and curves must be made smoothly and gradually, with the rate of change determined by the type of material being cut, the thickness, the part geometry and a host of nozzle parameters.The control algorithm that computes exactly how the feed rate should vary for a given geometry in a given material to make a precise part. The algorithm actually determines desired variations in the feed rate every 0.0005" (0.012 mm) along the tool path to provide an extremely smooth feed rate profile and a very accurate part. Using G-Code to convert this desired feed rate profile into actual control instructions for the servo motors would require a tremendous amount of programming and controller memory. Instead, the power and memory of the modern PC can be used to compute and store the entire tool path and feed rate profile and then directly drive the servomotors that control the X-Y motion. These results in a more precise part that is considerably easier to create than if G-code programming were used.

2.2 Pump: Intensifier pump: Early ultra-high pressure cutting systems used hydraulic intensifier pumps exclusively. At the time, the intensifier pump was the only pump capable of reliably creating pressures high enough for water jet machining. An engine or electric motor drives a hydraulic pump which pumps hydraulic fluid at pressures from 1,000 to 4,000 psi (6,900 to 27,600 kpa) into the intensifier cylinder. The hydraulic fluid then pushes on a large piston to generate a high force on a 2.2 Pump: Intensifier pump Early ultra-high pressure cutting systems used hydraulic intensifier pumps exclusively. At the time, the intensifier pump was the only pump capable of reliably creating pressures high enough for water jet machining. An engine or electric motor drives a hydraulic pump which pumps hydraulic fluid at pressures from 1,000 to 4,000 psi (6,900 to 27,600 kpa) into the intensifier cylinder. The hydraulic fluid then pushes on a large piston to generate a high force on a small-diameter plunger. This plunger pressurizes water to a level that is proportional to the relative cross-sectional areas of the large piston and the small plunger.

Crankshaft Pump: the centuries-old technology behind crankshaft pumps is based on the use of a mechanical crankshaft to move any number of individual pistons or plungers back and forth in a cylinder. Check valves in each cylinder allow water to enter the cylinder as the plunger retracts and then exit the cylinder into the outlet manifold as the plunger advances into the cylinder. Crankshaft pumps are inherently more efficient than intensifier pumps because they do not require a powerrobbing hydraulic system. In addition, crankshaft pumps with three or more cylinders can be designed to provide a very uniform pressure output without needing to use an attenuator system.

Crankshaft pumps were not generally used in ultra-high pressure applications until fairly recently. This was because the typical crankshaft pump operated at more strokes per minute than an intensifier pump and caused unacceptably short life of seals and