SOLIDWORKS Design Approach · Part 19
Manufacturing a part is the culmination of product design. Numerical control is how a geometric model becomes controlled motion — and the design decisions that make that motion cheap were taken long before the program was written.
Context
A wide variety of manufacturing processes exists, and the manufacturing engineer — the process planner — selects a suitable one based on the part material and the design tolerances. That role carries deep knowledge of manufacturing processes, and with it the standing to request design changes that simplify manufacture and reduce cost.
The classic illustration is worth repeating because it is so easily missed at the design stage: machining a flat-bottomed blind hole is harder and more expensive than machining a conical-bottomed one. The first needs a milling operation with a flat end mill; the second needs a single drilling operation with a standard drill. On the model they differ by almost nothing.
Type of production matters too. Mass production makes hundreds or thousands of identical parts, and its economic advantage comes from setting up an operation once and amortising that setup across many units. Job-shop and low-volume work carries setup cost per batch, which changes which design decisions are expensive.
Hardware
Machining machines are known as machine tools, and one machine may perform several operations — a milling machine can mill, drill, ream, bore, tap and thread. The common structure is worth knowing because it explains most of the constraints a programmer works within.
Fundamentals
Controlling the position and motion of the cutting tool while it removes material follows well-established concepts.
An axis is a degree of freedom along which a cutting tool can move, and the number of axes determines what kind of surfaces a machine can cut. In three-dimensional space there is a maximum of six degrees of freedom — three translations and three rotations — but a machine tool may have more than six in total. Two-axis, three-axis and multi-axis machines all exist, with multi-axis machines sometimes reaching ten degrees of freedom.
The tool moves along two axes simultaneously. A lathe is the archetype: Z along the workpiece length and X into the workpiece to remove material.
Three simultaneous translations of the tool. The half axis is an indexing axis of the turret — the tool changer that holds several tools and swaps them automatically, eliminating manual tool changes between operations.
Additional rotational degrees of freedom, and sometimes auxiliary translations. Some vertical machining centres have flexible heads that tilt side to side and front to back for machining access.
Operations
Performed on a lathe, and used to produce axisymmetric parts — the revolves of the CAD world. The workpiece rotates while a single-point tool moves parallel to the axis of rotation. A lathe also performs drilling, boring, tapping, facing, threading, polishing, grooving, knurling and trepanning, each with a differently shaped tool. Workpieces are held in three-, four- or six-jaw chucks, in collets, or between centres.
Produces cylindrical holes. Blind drilled holes have a conical end matching the drill point. For through holes the drill must be fed far enough that the conical point clears the far face, ensuring full diameter throughout the hole length — which requires workholding that keeps the drill from striking a fixture face. Spotting drills mark the centre before drilling; centre drills produce a conical start for a larger drill.
The most common and versatile operation, performed on a milling machine or machining centre, and capable of almost any shape. Machines may be horizontal or vertical, with vertical the more common. Operations include slot, face, pocket and side or contour milling; in slot milling the cutter is a flat end mill cutting on its end and periphery.
Removes material by spark erosion between two conducting surfaces — an electrode and the workpiece, which must be electrically conductive. Two types exist. Sinker or plunge EDM uses a shaped electrode plunged into the work. Wire EDM uses a travelling wire as the electrode, cutting a profile through the part. Both run submerged in a dielectric fluid.
EDM cuts geometry that a rotating tool cannot reach: sharp internal corners, deep narrow slots, hardened material after heat treatment. It is slow and therefore expensive, so a design that requires it should require it deliberately. A generous internal corner radius that permits milling is often worth more than the aesthetic gained by a sharp one.
Design for manufacture
Designers should consider not only functional requirements and force analysis but the implications of their design across the whole product life cycle — design, manufacture, assembly, disassembly and disposal. Several established foci address this: design for assembly, design for manufacture, design for anything, concurrent engineering, product data management and product lifecycle management. All of them try to move change earlier, when a change is still only a change on paper.
Automated manufacturability checking implements a subset of this as rules, validating a part and flagging areas where manufacture may be difficult or production cost excessive.
Programming
NC programming applies to machines fitted with an NC controller. The controller reads a program, executes it, and uses it to control the motion of both the machine table and the cutting tool. NC machining is more accurate and faster than manual machining. Programs are written by machinists or designers using CAD models and NC software, which may be a module within the CAD/CAM system or entirely separate software.
Programs are written in G-code and M-code, an ANSI/EIA standard language, although some post-processing is normally needed to accommodate proprietary features and differing commercial interpretations. The benefit of a standard language is that programs transfer readily between controllers.
A program is a sequence of instructions, one per line, and each line is called a block. A block begins with a block number, conventionally prefixed N, followed by one or more code words. Each code word is a letter followed by digits, and is followed by the data it requires. A controller executes one block at a time.
Writing a program requires the context of how a machining operation is actually performed: first define the toolpath geometry, then instruct the tool to move along it, adding the auxiliary instructions the operation needs — rapid positioning, coolant on and off, spindle control, tool changes.
Broadly, G-codes are preparatory functions governing motion and geometry — rapid traverse, linear interpolation, circular interpolation, plane selection, units, coordinate offsets, canned cycles — while M-codes are miscellaneous machine functions such as spindle start and stop, coolant control, tool change and program end.
The standard defines the language; controllers interpret dialects of it. A program that runs correctly on one machine may crash a tool on another because a canned cycle, a coordinate offset convention or a tool-length compensation behaves differently. Always post-process for the specific controller, and always verify the result before cutting.
Toolchain
CAM software may run standalone or integrate with the CAD system as an add-in; where integration is wanted, the integrating version must be installed rather than the standalone one.
Automatic feature extraction works by recognising manufacturing features — holes, pockets, slots, bosses — in the geometry. A model built from clean, well-named features with standard hole sizes is recognised accurately. A model of imported dumb geometry, or one where every hole was cut with a general extrude, forces manual feature definition and loses most of the automation benefit. This is one of the more direct returns on the modelling discipline covered in Parts 04 to 07.
Series
The SOLIDWORKS Design Approach series works through computer aided design as an engineering discipline, from first principles to manufacture.
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