Advanced Manufacturing Processes (AMP) are developed with an aim to satisfactorily cater the need to modern days’ industrial demands of improved quality and miniaturized products. Non-Traditional Machining (NTM) processes are a subgroup of AMP that follows subtractive approach to gradually remove materials from the workpiece in order to impart desired shape, size and finish. Such processes can utilize different types of energy in their direct form. On this basis, NTM processes can be categorized into several groups, such as mechanical energy based processes (AJM, WJM, AWJM, USM), chemical energy based processes (CHM, lithography), electro-chemical energy based processes (ECM, ECG), thermal energy based processes (EDM, LBM, PAM, EBM), etc. It is worth mentioning that EDM is sometimes considered as either electrical or thermal energy based process as it utilizes spark to remove material by melting and vaporization. Each of these processes have distinct benefits and limitations and thus is suitable for specific cases.
Both abrasive jet machining (AJM) and ultrasonic machining (USM) are mechanical energy based processes where material is removed by impact erosion. In abrasive jet machining (AJM), hard abrasive particles are mixed with highly pressurized gas at a pre-defined mixing ratio and the mixture is then allowed to come out via a nozzle in a form a narrow jet. This nozzle converts the pressure energy of the mixture into kinetic energy and also directs the high velocity jet towards the workpiece. This abrasive jet possesses enough kinetic energy to realize material removal by erosion (due to the impacts of hard abrasives). On the other hand, in ultrasonic machining (USM), an abrasive slurry (abrasive particles suspended in a liquid medium) is delivered in the small gap between a vibratory tool and the workpiece. The tool is given a particular shape based on the intended profile and it is vibrated at a very high frequency with low amplitude above the workpiece. Here also the abrasives in the slurry erode material. Various differences between AJM and USM are given below in table format.
Similarities between AJM and USM
- AJM and USM both are considered as modern manufacturing processes (MMP) or non-traditional machining (NTM) processes. However none of them is hybrid process.
- Both of them utilizes only mechanical energy for material removal
- Material removal takes place due to impact erosion in both cases.
- Both of them require abrasives for realizing material erosion. In AJM, the high velocity jet actually contains abrasive particles mixed with clean dry air. In USM, the slurry that flows in between the tool and workpiece contains water based abrasive.
- Abrasives used in both are processes are also similar. Common abrasives include alumina (Al2O3), garnet, silicon carbide (SiC), boron carbide (B4C), etc.
- Both the processes are independent of electrical and thermal conductivity and chemical inertness of the workpiece material. However, process capability relies on several mechanical properties like strength, hardness, etc.
- None of them is associated with the burr formation, excessive heat generation and related thermal damages.
Differences between AJM and USM
| Abrasive Jet Machining (AJM) | Ultrasonic Machining (USM) |
|---|---|
| A very high velocity (100 – 300m/s) jet of air-abrasive mixture is utilized for machining purpose. | In USM, a high frequency (>20kHz) but low amplitude (<0.1mm) vibration is utilized for machining purpose. |
| The high velocity jet is allowed to directly strike the workpiece. | The vibration is not directly imparted to the workpiece. Instead, vibration is imparted on an abrasive slurry that flows through a narrow gap between a shaped tool and workpiece. |
| It does not require any specialized tool. Intended profile can be cut by moving the worktable and rotating the nozzle (if required). | It mandatorily requires a physical tool having inverse profile of the intended shape. Accordingly, shape of the machined feature relies on tool profile. |
| It requires an air compressor cum drier to deliver highly pressurized clean air. This air is then mixed with abrasive. The pressure energy is converted into kinetic energy by the nozzle and thus high velocity jet is obtained. | It does not require any compressor unit. Instead it requires a magnetostrictive transducer (to generate vibration) and a horn (to deliver vibration at intended amplitude). The tool is mounted at the tip of horn. |
| AJM offers comparatively higher material removal rate (MRR), in the order of 5 – 10cm3/s. | MRR is very low in USM, typically around 0.005cm3/s. |
| AJM can be used for ductile and brittle materials; however, it is preferred for hard brittle materials due to the risk of abrasive embedment in soft materials. | USM is commonly used for machining brittle materials, especially those which are electrically non-conductive (like glass, ceramic, carbide, etc.). |
| AJM produces extreme noise during operation. Abrasive grits are sometime scatter out of the chamber. So proper protection is mandatory for the worker. | USM is relatively safe in this scenario. |
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).