What is CNC Turning?
Turning is the process of rotating a part and applying cutting tools to it. As the part rotates layers of material are removed until the desired part dimensions are met. Turning is performed on a machine called a lathe or turning center. Lathes can turn external diameters and internal diameters as well as taper and face work pieces. They can also tap, broach, drill, ream, perform hard turning and boring. Turning machines excel at creating round shaft like parts or spherical surfaces using a technique called spherical generation. CNC turning machines work with a wide variety of materials. Plastics, woods and aluminum are classified as soft materials. Materials like steel, stainless steel and titanium are considered hard but can still be machined to high precision.
CNC (computer numerical control) turning differs from CNC milling, on a CNC lathe the workpiece is spun and the machine tools travel on the x-axis whereas on a mill the tool rotates and stays stationary as the part is moved around it. A motor known as a spindle rotates the workpiece at varying RPM. The RPM depends on the parts dimension, cutting technique and material. Larger diameter parts will have a faster surface speed. Smaller parts need to be turned at higher rpm to meet the same surface speeds. Once the part reaches the proper cutting speed machine tools are applied.
Machine tools apply a cutting edge to the surface. As the part rotates the cutting tool removes material layer by layer. As the diameter decreases the tool is adjusted inward keeping the same cut depth. It will also move on the z axis, which is parallel to the part.
Specific cut depth, workpiece RPM and movements on the z axis are often referred to as speeds and feeds.
The standard CNC turning process consists of rough turning to remove material quickly. After which the turning center repeats the process with a finishing tool. In a production setting a parting tool may be used to separate the workpiece from bar stock. Facing tools create smooth surfaces perpendicular to the z axis. High pressure coolant is supplied to cool the tools and the work piece as well as remove any swarf.
What are Turning Centers?
CNC lathes can be equipped with tool turrets and protective enclosures making them into turning centers. Tool turrets have bays which are equipped with various machine tools which can be quickly swapped between. Common CNC machining drills of assorted diameter, reamers, and taps are equipped. Knurling tools, grooving and threading and other specialty tools are equipped according to the job requirements. “Live tools” (powered tools) can also be used allowing off axis drilling or flat creation. Turrets provide turning centers the versatility to perform various processes often completing entire operations in one machine. On longer parts live centers are used; these support the weight of the part and reduce the possibility of deflection or lost tolerances to centrifugal force.
Multi spindle machines:
Multi spindle turning centers have gained popularity, these use extra spindles on different axes. Multi spindle turning centers offer enhanced production capabilities as cycles can be done without the need for secondary equipment. The main spindle still rotates the work pieces; the second spindle supports longer parts or allows 2 parts to be machined at a time.
CNC turning centers largely automate the turning process but there is some prep before the workpieces are set in the machine.
The part starts as a 3D file in a computer aided drawing (CAD) program. It’s then moved into a Computer Aided Manufacturing (CAM) program where tool pathing is created. Tool pathing is the instructions the machine has to follow in order to complete the part. Once the engineer dials in the paths and runs a simulator he optimizes the code and eventually exports it. Skilled engineers use templates to quickly move through standard processes.
GCODE is a code made up of different coordinates or moves. A letter followed by a number which corresponds to the machine function followed by a few numbers for axis movement, spindle rotational speed, tool change etc.
The GCODE is then uploaded to the turning center. Machine specific offsets are configured in software or at the machine as they may have slight variations. Tools are inserted into the machine turret in the corresponding tool number. The required chuck is equipped, accessories and slot in the part. We let the machine do it’s thing while our hand hovers over the emergency stop button. Once the first part completes we check it’s measurements and adjust the GCODE accordingly. When we are satisfied we move into production.
An operator loads a workpiece into a work holder called a chuck. Chucks use 3 or more jaws, which attach to the chuck and are adjusted to securely hold the part. Workpieces can be gripped from the outside or inside. On specialized parts soft jaws are used; these are machinable jaws that can be made specific to the desired shape of the part. The part or workpiece is secured to the chuck either manually or by a pneumatic auto chuck. It’s convenient to use a foot pedal, stepping on it will open or close the chuck.
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