Selective Laser Sintering (SLS) is an additive Rapid Prototyping (RP) technique that uses a high power laser (like CO2 beam) to fuse small particles of plastic or metal powder into desired 3D shape. The laser selectively fuses powdered material by scanning cross-sections generated from a 3D digital description of the part (from CAD data) on the surface of a powder bed.

After the first cross-section is sintered, the powder bed is lowered by one layer thickness so that the laser beam will sinter the next layer of part. The process repeats until the complete geometry is sintered from powder.

SLS RP technique can be used for wide range of materials like polymers (nylon, glass filled or with other fillers, polystyrene), metals like steel, titanium, alloy mixtures and composites. Green sand also can be used in this process. However, these powders will be specially prepared by manufacturers to suit a particular SLS machine.

This is most powerful RP technique to form complex geometric shapes and even the assemblies. Almost any geometry which can be modeled in CAD can be produced physically.

SLS does not require support structures unlike other RP processes such as Stereolithography (SLA) and Fused Deposition Modeling (FDM) due to the fact that part being built is always surrounded by unsintered powder all the time. The part can be directly taken out from powder after the sintering process is completed. The remained powder can be reused in machine with appropriate combination with unused powder.

In Hyderabad, Andhra Pradesh, India, the Department of Mechanical Engineering of Osmania University has procured a SLS machine recently from EOS Gmbh.

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Nesting the parts while machining using CAM

While working on a recent CAM project using CAD model, we found few interesting techniques. In the first job, we requested the operator to use a 1/4″ Ball nose cutter of 1″ long. We were expecting there could be some tool flexing because for the first cavity profile cut. But he used almost 1 1/2″ long cutter. Finally, once the machining is done there is a clear cut draft left on the part which is almost 3 degrees. Also, there was not enough overpass of ball nose at the bottom of the part leaving a radius there. It was not too bad other than doing some manual cleanup of the part after machining.

In our second batch of CAM, we nested four parts as shown on image from two separate CAD models. So, we have to make a single pre-machining block to make two separate parts of quantity two each. Also, we tried to machine only up to 4 mm around the part using 1/4″ cutter. This saved us almost 50% of machining time as we eliminated machining unneeded stock. Also, we ensured the cutter overhang is only 22mm just more than nose overpass. This didn’t cause any tool flex and squeak.

Nesting the Parts example

 

Another interesting thing is after doing the rough cut with 1/4″ cutter, we lost the datums where we set original zero. So, for finish cut we have not changed X and Y coordinates. But, asked the operator to reset Z with respect part top surface which was intentionally left unmachined before. This helped us in quickly running the finish cut using 1/8″ cutter without much time on reference point set.

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