Basic CNC Machine Programming Concepts
CNC CAD- CAM part programming provides the utilities that allow manufacturers to bring products to market faster and more affordable than ever before. The process of machining with CAM includes the use of “Toolpath”. Toolpath is the displayed machine path that a cutting tool will take in the process of actually machining a part from a block of material or stock on a CNC machine. The stock can be Aluminum, Steel, Titanium, Plastic, Wood or many other types depending on the application. There is 2D toolpath and there is 3D toolpath. 3D toolpath is not flat and has a 3 dimensional surface contour. An example of this can be seen in how automobiles have become more and more curvy and aero dynamic over the past 20 years. You may notice that consumer products have taken on a “Modern” look that is not as prysmatic as things were 20 years ago. This is directly related to advances in CAD- CAM software technology and the ability to design 3D models in computer aided design ( CAD) and the technology of CNC machining ( CAM).
Roughing and Finishing Toolpath
“Roughing” toolpath is typically used in the initial phase of machining as it allows the machinist to remove large amounts of material with a large end-mill cutting tool. Roughing is used in CNC machining to save time and maximize cutting. Basically, the concept is to rapidly remove the maximum amount of material in the shortest amount of time while producing minimal wear on tooling. CAM technology delivers many types of roughing strategies that are categorized into 2D & 3D machining as well as 4th and 5th Axis toolpath types. These may include:
- Drilling, Tapping and Boring
- Plunge Roughing
Each of these types typically will have a list of parameters that include toolpath linking, Lead-Ins and Lead-Outs for the tool, Processing or Grouping features to optimize the machine program for cutting sequence, Cutting Depth variables, Tool Feeds and Speeds information and much more. All of this information is translated mathamatically into a numeric code language called “G-Code”. This is where “NC” programming comes from.
“Finishing” toolpath is used after roughing to drive a tool across the remaining material and remove what is left behind in an effort to produce a smooth near-polished surface. There are several finishing toolpaths in existence such as Slice Planar, Slice Spiral and Slice Radial type cutter paths. There is also Equidistant Offset Contour toolpath that is also known as “Constant Scallop” or Constant Cusp” toolpaths which all do the same thing. These finishing cutter paths in CAD- CAM are considered “Semi-Finishing” toolpaths and straight “Finishing” toolpaths. The Equidistant Offset Toolpath is powerful and highly efficient in CNC machining as it maintains the exact cutter step-over for the size of the tool regardless of the surface complexity. Thia type of cutter path is used with very high tolerance step-overs to produce a near polished surface finish for tool and Die, Mold Making and other 3D machining applications. You can typically find this level of 3D machining toolpath in advanced CAD- CAM products that are used in Mold-Making, Aerospace, Medical device manufacturing and other types of industry segments depending on the applications
What is “REST” Machining?
“REST” machining was initially developed to come after a roughing or finishing toolpath opweration and clean up all of the material that was remaining. The idea for the name came from “Machine the REST” of a part. Generally, in CAD- CAM, machining operations are loaded into a “Job Tree” within the software interface one toolpath machining operation at a time. Higher level CAD- CAM combines machining operations to include a combination of strategies and even adds operation functionality within a machining strategy. This hybrid strategy concept is perfect for machinists that need to reduce cycle times and become more efficient in their programming workflow. This would be a Rough-Rough/REST combination or a Finish-Finish/REST combination. Therefore, the CAM programmer loads a roughing operation and then a REST roughing operation into the CAM Tree. Then a finishing strategy is loaded into the tree as well as a REST finishing feature. Each operation is fully editable within the tree and is associative to the part CAD model. That means that if a change is made to the part model, all of the toolpaths are automatically updated as well. This is a huge time saving aspect of modern CAD- CAM technology and is highly sought after.
Again, REST finishing allows the programmer to enter the initial finishing tool diameter. The final REST finishing tool diameter will be smaller and then the software automatically knows how much material is removed in the initial stage. Then the software applies toolpath automatically to machine the “REST” based off of the REST finishing tool diameter. It’s that simple. Once all of the toolpath is generated by the CAM system, the software should have built in simulation allowing the programmer to sit back and simulate the toolpath and watch the part being cut out of the material. This allows the programmer to visualize the entire process, calculate machine cycle times, catch tool and tool holder gouges or collisions and a lot more. Simulation is very important in the CNC machining process as it saves the programmer from experiencing costly mistakes that can happen.
The CAM machining stages in programming parts with CNC software are simple.
- Face Mill (optional)
- Pre-Drilling & Drilling operations (optional)
- REST Roughing
- REST Finishing
- Pencil Mill
The roughing and finishing “REST” machining part of the process in 4 and 7 are very important aspects of CAM programming process and is available for both simple and complex CNC part making. CAM programmers will benefit from rersearching REST machining for implementation into their CNC manufacturing processes.