In abrasive jet machining (AJM), a high velocity jet of abrasive particles accelerated in pressurized air (carrier gas) is made to strike the work surface. A nozzle is used to direct this jet towards the work surface maintaining pre-defined stand-off distance (SOD) and inclination angle. When abrasive particles thwack the work surface at high velocity, they remove material by micro-cutting (impact erosion). AJM is commonly used surface cleaning, such as removing oxide layer, smears, coating or colour present of solid surface. It can also be used for deburring and engraving hard brittle materials. Several process parameters of AJM process can be controlled precisely to obtain desired result in terms of dimensional accuracy, material removal rate and surface roughness. Typical process parameters include carrier gas pressure, flow rate of abrasive, abrasive particle size, mixing ratio, nozzle diameter, SOD and inclination angle.
Another similar age old technique is sand blasting, where fine sand particles are propelled to the work surface by pressurized air at a very high velocity to remove material. Mechanism of material removal (i.e. impact erosion) is same for both AJM and sand blasting processes. However, process parameters of sand blasting cannot be controlled so precisely, and thus it is less reliable for micro-cutting. It is suitable for cheap cleaning purposes, particularly in civil applications. Sometimes it is also used to make substrate surface rough before carrying out coating operation (a rough surface can hold the coating layer tightly). Various similarities and differences between abrasive jet machining (AJM) and sand blasting are given below in table format.
Similarities between abrasive jet machining and sand blasting
- Both abrasive jet machining and sand blasting processes utilize high velocity jet of hard abrasives (sand is one type of abrasive).
- Both use gaseous medium (commonly air) as a medium to carry abrasives.
- Impact erosion is main mechanism of material removal for both the processes.
- Both can cause air pollution if used abrasives are not ventilated properly.
- Both can cause health problems if proper protection is not utilized while operation.
Differences between abrasive jet machining and sand blasting
| Abrasive Jet Machining | Sand Blasting |
|---|---|
| Several abrasive particles, such as silicon carbide (SiC), alumina (Al2O3), glass beads, etc. are used to remove material by impact erosion. | Only silica sand grains (SiO2) are used to remove material by impact erosion. |
| Size of the abrasive particles and its flow rate are more or less uniform. | Sand particles usually have wide range of diameters. Its flow rate is also not uniform in all cases. |
| Process parameters and other influencing factors are regulated properly and precisely. | Process parameters and other influencing factors are not regulated carefully. |
| Quality and finish of the machined surface achieved by AJM are comparatively better. | Quality and finish of the machined surface are not so good. |
| It is costlier but precise, thus beneficial for industrial high quality finishing requirements. | It is cheaper and thus efficient for common civil works. |
| AJM is usually used for removal of coating and rust. It is also used to cut profiles or drill holes on hard metals. | Sand blasting is mainly used to remove rust and coating from solid surfaces. |
| Usually carried out in rooms with proper ventilation. Thus it is less hazardous. | Often it is carried out in open atmosphere, thus causes air pollution. Workers are also exposed to various occupational lung diseases, such as Silicosis. |
References
- Advanced Machining Processes by V. K. Jain (Allied Publishers Private Limited).
- Unconventional Machining Processes by T. Jagadeesha (I. K. International Publishing House Pvt. Ltd.).
- Nonconventional Machining by P. K. Mishra (Narosa Publishing House).