CNC Part Programming Workbook

Frontmatter

Introduction

Abstract

CNC (computer numerical control) and CAD/CAM (integrated computer-aided draughting and manufacture) are essential elements in many industrial processes. The assignments in this workbook will provide a broad range of practical experiences which represent a valuable foundation to the successful application of this technology.

Bernard Hodges

General note

Abstract

The purpose of this workbook is to help you to learn how to work from engineering drawings of components which are to be machined and to produce part programs which incorporate the various commands and functions of a CNC system.

Bernard Hodges

How to use this book

Abstract

Each learning assignment in this workbook has a similar structure, to make its use as straightforward as possible. Information and guidance that is needed for the completion of the practical work is included with each assignment.

Bernard Hodges

Learning Assignment 1. Machine axes and datums

Abstract

Axial definitions represent the linear movements of machine slides, which may be assigned X, Y and Z depending upon the type of machine. When using vertical milling machines, X and Y represent horizontal movements of the table, whereas in turning the tool movements are X and Z. In either case, precisely controlled linear movements are achieved.

Bernard Hodges

Learning Assignment 2. Setting up a component datum and the tooling

Abstract

One of the tasks of the setter/operator is to position the component datum and the tools that are going to be used for machining a program. The programmer cannot always specify certain information such as the exact distance of the component datum from the machine datum, or the precise diameters or lengths of the tools to be used.

Bernard Hodges

Learning Assignment 3. Line format and program structure

Abstract

A CNC program is a set of instructions that is read by the machine tool computer system to control machine slide movements, tool movements and additional miscellaneous functions. There are two types of CNC programming: conversational format and word address format. Most CNC machines use word address format, mainly for historical reasons — many of the first machines used this type of format.

Bernard Hodges

Learning Assignment 4. Input and edit function

Abstract

When you have entered a program into the control and executed a proving run, you may find you need to edit the program. You may need to change the feedrates or the tool change positions, to correct errors in the positional data or to alter the values for the tool diameters and lengths. Within the program, you may need to add or delete a block or blocks, or to insert other programs into the main program in the form of sub-programs or macro routines.

Bernard Hodges

Learning Assignment 5. Work holding and tooling

Abstract

CNC machine tools frequently operate at greater feedrates and speeds than those used in conventional machining, so larger forces may be applied to the component and therefore to the clamping/holding devices. As the programmer, it is part of your task to decide what type of work-holding device is required for a particular operation. The following conditions must be met:

1

When clamped, the component must not be able to move in any direction, either linearly or rotationally. It is essential that the tool cutting forces are exerted against a part or parts of the work-holding device that cannot be moved (Figure 5.1). This can be difficult to achieve when machining complex components of an irregular shape, such as castings, and you may need to use more than one holding device for all machining operations to be executed.

 

2

The component must not ‘chatter’ or deflect as it is being machined; this would produce a poor surface finish, dimensional inaccuracy and possibly tool damage. This may be overcome by using steadies for turning or fixture inserts when machining thin-walled components.

 

3

The component must be positively located and precisely positioned within the work-holding device. The part program datum point (that is, the point from which the co-ordinates of toolpath are programmed), is generally the same as the component datum point; so if the component is not positioned accurately the required component shape may not be achieved. A grid plate with a holding device attached can provide positive location and position in several directions.

 

4

In order to avoid collisions, the toolpaths must not coincide with any part of the work-holding device or clamps. When proving out the program, you may find you need to insert moves in the program to ensure this condition is met.

 

5

The work-holding device must provide adequate location and support to achieve the geometric tolerances specified on the component drawing.

 

6

All information relating to the work-holding device(s) used and the setting of datums must be documented for the machine setter/operator.

 

Bernard Hodges

Learning Assignment 6. Toolpath calculations

Abstract

Before you can start to write a CNC part program manually, you may have to carry out various calculations to determine the tool centre line path in order to machine the required shape. These calculations may be needed to determine the intersect points of arcs and lines, arc centres and tangency points of lines on arcs or circles, and so on. If you are going to write programs manually, you will need a good knowledge of mathematics, trigonometry and geometry. These are not discussed in this workbook, since they are covered in the accompanying core book. If you feel you are on uncertain ground here, there are plenty of mathematical textbooks to which you can refer.

Bernard Hodges

Learning Assignment 7. Planning a CNC program

Abstract

When you are beginning to construct a program there are many factors that you need to take into consideration, and a systematic approach is essential.

Bernard Hodges

Learning Assignment 8. Canned cycles/macros

Abstract

All CNC machine tools have, as part of the programming facility within the controller, the feature of executing machine canned cycles and/or sub-programs or macro routines. These are built into the controller’s computer software and make it easy to program routine machining sequences. Such routines are drilling, tapping, spot-facing and area clearance for square pockets, circular pockets and boring cycles. The difference between the two is that a canned cycle is inherent within the control system and usually cannot be changed or re-programmed, whereas a macro is a separate program generated by the programmer and inserted within the main program or, in the case of some controls, stored apart from the main program text.

Bernard Hodges

Learning Assignment 9. Starting-up procedures for a CAD/CAM system

Abstract

Most computer systems use similar hardware for data storage, such as floppy discs and a hard disc, with printers and/or plotters for output of hard copy. They may, however, use different ways to input information and data, such as moving a tracker ball or a mouse, or simply typing commands using the keyboard. Various facilities may also be available for transferring CNC programs to machine tools, such as a punched tape/reader or a magnetic tape cassette.

Bernard Hodges

Learning Assignment 10. Operating parameters for a CAM system

Abstract

When you use a CAM system, you need to set up some additional parameters in the initial start-up procedure, over and above those required by a CAD system (discussed in Learning Assignment 9). These parameters relate to the ultimate generation of a CNC program to machine the drawn component and may include the following:

• the ata directory — the directory for storing drawing profile and CNC data files; this allows data for different types of work, or for work for different customers, to be separated and stored in their own directories on disc
• the machine directory — the directory for storing post-processors, backplot files and tooling information; this allows data for different CNC machines to be stored in their own directory, such as Fanuc OM for milling, or Fanuc OT for turning
• the tool library, either milling tools or turning tools
• the canned cycle call — an option to use the canned cycle calls for the CNC machine tool set in the machine directory.

Bernard Hodges

Learning Assignment 11. Using a drawing package

Abstract

The CAD front end of most CAD/CAM packages is usually a standard CAD system. Various graphical functions are used to draw circles, lines, arcs, points, tangents between circles, arcs through a point tangential to a line and so on. The main difference is in the way that you construct the geometry, since you only need to draw details like the profile shape or the hole positions. You do not have to incorporate features like hatching lines, dimensions or circles for holes into the drawing. You only have to specify the finished shape the tool has to produce, such as the profile shape, area clearance/pocket or points where holes have to be drilled, tapped or counterbored. Should you need to produce a fully dimensioned drawing to British Standard specifications, however, drawing features such as hatching and dimensioning are available on most CAD/CAM packages.

Bernard Hodges

Learning Assignment 12. Using a CAM system to produce CNC part programs

Abstract

The CAM section of the CAD/CAM package requires an input of geometrical data. This can be generated from the CAD front end of the package or imported from an external CAD system using IGES, DXF files or other suitable interfacing software. At this stage the important task is to determine the various toolpaths from the geometry of the drawing. To simplify this, delete or trim any construction lines and remove all dimension lines, hatching and so forth. Once you have defined the profile to be machined, select tools with appropriate feeds/speeds and program the cutter path around the desired profile. Store this information, and use it when required to produce a part program by post-processing into the relevant machine tool controller language.

Bernard Hodges

Learning Assignment 13. Machining pockets

Abstract

The procedure for machining pockets of any shape, with or without ‘islands’, depends on the CAD/CAM software that you are using. The software may operate on a pull-down menu system which requires the name of the previously defined pocket shape and the name of each ‘island’ within the pocket. Or a pocket clearance routine within the software may be used to define the shape of the pocket, including ‘islands’. In either case, the data entered is used to generate the cutter toolpaths.

Bernard Hodges

Learning Assignment 14. Machining slots

Abstract

The component in this exercise, shown in Figure 14.1, has six slots 6 mm wide x 10 mm deep to be machined in addition to the outer profile. When drawing the component show the slots as single lines, 25 mm long (that is, the centre line of the slots will represent the line that the centre of the cutter will take). Select the tool change position and define the toolpath for machining the outside profile as in previous exercises, using a 12 mm dia cutter.

Bernard Hodges

Learning Assignment 15. Complex profile with a spline

Abstract

The profile shown in Figure 15.1 is quite complex and for clarity’s sake letters are used to designate certain points on the profile. Their X and Y co-ordinates are tabulated at the top left-hand corner of the drawing, thus avoiding a mass of dimension lines. A spline (a series of points through which a smooth curve is drawn) is shown along the top edge of the component. Consult your manual and research the commands that your CAD/CAM system uses for drawing splines.

Bernard Hodges

Backmatter