What's the difference between sand blasting and abrasive jet machining?

Sand Blasting

Sand blasting is a general term used to describe the act of propelling very fine bits of material at high-velocity to clean or etch a surface. 

Sand used to be the most commonly used material, but since the lung disease silicosis is caused by extended inhalation of the dust created by sand, other materials are now used in its place. 

Any small, relatively uniform particles will work, such as steel grit, copper slag, walnut shells, powdered abrasives, even bits of coconut shell. Due to the dangers of inhaling dust during the process, sandblasting is carefully controlled, using an alternate air supply, protective wear, and proper ventilation.

The first sandblasting process was patented in the US in 1870. As a cleaning method, it is often used for priming a surface for the application of paint or a sealant.

A sandblasting setup usually consists of three different parts: the abrasive itself, an air compressor, and a blaster nozzle. For etching and small object cleaning, a workstation to hold the piece of glass is also needed, as is some sort of collector to gather up excess dust. Sandblasting is primarily used for two somewhat different applications. The first of these is to clean a surface of anything that may be clinging to it. The second is to either etch or carve designs or words into glass or a similar material.

By launching small bits of abrasive at the surface at a high speed, all imperfections are knocked loose and can then be easily washed off, creating an incredibly smooth surface upon which to lay the new layer of paint. Sandblasting may also be used for such projects as cleaning the hulls of ships or large structures such as the Golden Gate Bridge.

Abrasive Jet Machining

Abrasive Jet Machining, also known as micro-abrasive blasting, is a mechanical energy based unconventional machining process used to remove unwanted material from a given work piece.
The process makes use of an abrasive jet with high velocity, to remove material and provide smooth surface finish to hard metallic work pieces.

Schematics of Abrasive Jet Machining 

Dry air or gas is filtered and compressed by passing it through the filter and compressor.

A pressure gauge and a flow regulator are used to control the pressure and regulate the flow rate of the compressed air.
Compressed air is then passed into the mixing chamber. In the mixing chamber, abrasive powder is fed. A vibrator is used to control the feed of the abrasive powder.

The abrasive powder and the compressed air are thoroughly mixed in the chamber. The pressure of this mixture is regulated and sent to nozzle.
The nozzle increases the velocity of the mixture at the expense of its pressure. A fine abrasive jet is rendered by the nozzle. This jet is used to remove unwanted material from the work piece.

When an abrasive particle (like Al2O3 or SiC) having sharp edges hits a brittle and fragile material with a high speed, it makes dent into the material and lodges a small particle from it by a tiny brittle fracture. The lodged out or wear particle is carried away by the air or gas. The operating elements in AJM are abrasive, carrier gas and the nozzle as schematically shown in the following Figure

The distance between the nozzle tip and the work surface has great influence on the diameter of cut, its shape and size and also rate of material removal. The following Figure shows the variation in the diameter of cut with change in the stand off distance (SOD). It is evident that the SOD changes the abrasive particles spreads (i.e. covers wider area) on the work surface and consequently increases the diameter of the cut.

Abrasive jet machining is best suited for machining brittle and heat sensitive materials like glass, quartz, sapphire, ceramics etc.

It is used for drilling holes, cutting slots, cleaning hard surfaces, deburring, polishing etc.