CNC rotary tables are a key to automated machining. Before getting into the specifics of CNC rotary tables though, a brief discourse in the development and terminology of CNC machining will help from this point forward.
CNC is an acronym for Computer Numerical Control. It is the direct descendant of numerical control (NC) technology; think punch cards and magnetic tape. In turn, NC technology was invented to improve upon cam and tracer machine control in use at that time. Cam and tracer machining required construction of a template to trace and that interposed a costly, time-consuming step that had significant potential for human error.
Rotary Tables Defined
A rotary table is, as its name suggests, a table that rotates very accurately in concentric fashion around its center axis. In the high-accuracy, high-cost category of rotary tables, accuracy can mean radial runout – the amount of deviation from perfectly centered while rotating on the center axis – of 0.00004 of an inch and angular accuracy – degrees of rotation about the center axis – of 0.0002 of an inch. For the purpose of dampening vibration, the body of a rotary table is normally made of one of the “gray” iron grades. The rotating table plate is typically hardened, alloyed steel that is machined and rotary ground.
Purpose of CNC Rotary Tables
For automated machining, the entire purpose of CNC rotary tables is to provide a fourth axis. In addition to the normal axes of X, Y and Z, a CNC rotary table adds a rotational axis, known as the “W” axis, to increase machine accuracy and reduce costs. For example, using a CNC milling machine alone to make a stop sign-shaped piece would require moving the cutting tool around the piece to cut the vertical and horizontal sides and then cutting the angled sides by moving the cutting tool on a 45 degree angle as it is removing material. This requires eight machining passes. Each operation introduces a cumulative dimensional error for the sides parallel each other. Using a CNC rotary table for the same part means positioning the cutter in X or Y one time while rotating the part in 45 degree increments for each horizontal or vertical pass. Because it is cutting on only one side of the cutter in either X or Y, any machine tolerance error, tool wear and normal tool deflection from the forces of machining are reduce by half.
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