COMMON ABBREVIATIONS
TAP DRILL SIZES
Metric Threads - Coarse
Metric Threads - Fine
Imperial Threads - UNC/UNF
Straight Pipe Taps - NPS
Taper Pipe Taps - NPT
60-DEGREE THREAD FORMS
USEFUL FORMULAS
Speeds and Feeds
Machining Holes - Spot Drilling
Tapers
Control Memory Capacity
Trigonometric Chart
Index
| 1 | CONCEPTS OF CNC MACHINING |
Making a certain part (also called a workpiece) does not normally start at the CNC machine - it starts much earlier, at the design engineer’s desk. Engineering design means developing an intended part that is economical to make, of high quality, as well as a part that does what it is supposed to do - simply, to design a part that works. This process takes place in various offices and laboratories, research centers, and other places, including engineer’s imagination. Manufacturing process - CNC process included - is always a cooperative effort. Modern part design requires professionals from different disciplines, aided by a powerful computer installed with suitable design software, for example, SolidWorks®, Autodesk Inventor®, and many others, as well the venerable AutoCad® - one of the oldest and still very popular of the design group of application software. In simplified terms, engineering design starts with an idea and ends with the development of a drawing - or a series of drawings - that can be used in manufacturing at various stages.
For the CNC programmer as well as the CNC operator, this engineering drawing is the first source, and often the only source, of information about what the final part is to be. Typically, CNC programmer follows a certain process - or workflow - that can be summarized into a several critical points or steps:
▪Evaluate drawing
▪Identify material of the part
▪Determine part holding method
▪Select suitable tools
▪Decide on cutting conditions
▪Write the program
▪Verify the program
▪Complete documentation
▪Send program to machine shop
Keep in mind that this is not always the step-by-step method as it may appear to be. Often, a decision made in one step influences a decision made in another step, which often leads to revisiting earlier stages of the process and making necessary changes.
This initial stage is very important because it will influence all activities that are necessary to machine a part. Evaluating a drawing means finding a solution to a single question:
What is the best way to machine the part?
This single question will be the foundation for solutions of all other items in the workflow. One of the biggest challenges modern machining faces is not the lack of people who can master a CAD/CAM system but lack of people who know how to machine a part.
Knowledge of how to machine a part is the most important quality of a CNC programmer
An engineering drawing does not offer solutions - it only provides goals and objectives. Studying the features of the part, dimensions, tolerances, various relationships between features, quality requirements, etc., will largely determine the method of machining. All requirements of the drawing have to be met.
A single drawing is often not sufficient to provide all answers. Also having a drawing of a matching part or an assembly drawing, may be necessary in some cases.
Drawing - or even a set of drawings - offers the main source of information for both CNC programmers and CNC operators.
Part is made from a blank stock material specified either in the drawing itself or in another source. For programming and machining purposes, material should be identified by its type, size, shape, and condition.
Type of material is important when selecting tools, setup, and cutting conditions. Soft materials such as brass and aluminum will require a different method of machining than steels and space age materials. Size of the stock provides information about how much material has to be removed by roughing. Stock shape is important primarily for selection of the holding device (fixture).
Material condition may require special cuts, if necessary. Machining a forged steel requires a different approach than machining the same steel type as a round bar.
Shape of material comes in many different forms. Ideally, all material supplied for a particular batch of parts should be the same. That is not always the case; for example, bars that come cut-off into individual pieces should have but may not have the same rough length.
Part holding - or work holding - is a topic that can have its own book. From CNC perspective, the decision a CNC programmer has to make is to select such a work holding method that provides fixed, safe, and stable location for the mounted material.
Selecting tools is also part of the programming process, and experienced programmers often discuss the tooling possibilities with CNC operators. Selecting a tool for CNC machining means selection of:
▪Cutting tool holder
