Numerical Control Programming
Chapter 5
The Preliminaries of Programming

N/C part programming, as with all manufacturing activities, begins with process planning: determining what has to be made and how it is going to be made (and perhaps who is going to make it). Determining what has to be made starts out with the examination of two very important documents:

1. the part drawing (obviously) and
2. the purchase order or similar document that specifies which version of which drawing to use, and may contain other information essential to the programming process. The purchase order is especially important in job shops, which are facilities that manufacture pieceparts and/or perform manufacturing operations for other firms on a contract basis.

The Part Drawing

The primary function of a part drawing is to describe the geometry of the workpiece. In addition to the workpiece geometry, a part drawing may also contain a great deal of information in the title block and in notes that the inexperienced programmer might overlook.

Workpiece Geometry

The part drawing must be carefully studied in order to visualize the entire shape of the workpiece. This can be a considerable task for a complex-geometry workpiece, but it is absolutely essential. It often helps to make a freehand pictorial sketch (isometric or oblique) of the workpiece to visualize the geometry. Then check the sketch against the part drawing to make sure every line, circle, pocket, slot, groove, step, projection, and hole is accounted for. If you are not absolutely certain, have your supervisor check the sketch with you.

Note the tolerance on each hole, both size and location, for the tolerance can determine how the hole is to be made. Locational tolerances of less than 0.002 inch may require a boring operation with a single-point boring tool to true-up the location. The hole will have to be drilled undersize to allow sufficient stock for a boring operation. Diameter tolerances of less than 0.003 inch usually require the hole to be reamed (or bored) to size.

Notes, Symbols, and Title Block Specifications

Review each part of the title block. Make certain that you have the correct part drawing and revision. Note the material specifications. Determine the tolerances that apply to angular, fractional, and decimal dimensions.

Read each note on the part drawing, both local notes and general notes. Ask your supervisor to explain notes that are not clear. Pay particular attention to such note specifications as:

• Surface finish.
• Concentricity specifications.
• Dimensions that have to be coordinated with mating parts.
• Screw thread specifications: diameter, pitch, direction, thread form, class of fit, etc.**
• Heat treatment specifications, which may have to be performed before finish cuts are made.
• Material specifications: a specific alloy from a specific manufacturer may be required.
• Fillet specifications.
• Deburring specifications: how much to break sharp edges, which edges to leave sharp, whether to countersink all holes (and chamfer angle and size).

The Purchase Order

Most manufacturing activities are initiated because someone wants to buy something, and that "something" has to be made. The purchaser issues an agreement to buy the article at a certain price if the article conforms to certain specifications. That agreement is called a purchase order. A purchase order is a legal contract. It is an agreement between a purchaser (your company's customer) and a vendor (your company). It provides that the customer agrees to purchase a certain quantity of one or more articles at a particular price provided they conform to specifications. The specifications typically consist of

1. the geometry shown in a particular part drawing;
2. modifications to the part drawing specifications that may be contained in the purchase order; and
3. other specifications contained in the purchase order concerning such factors as material source specifications, heat treatment specifications, the quantity of parts required, delivery schedules, and inspection requirements.

Many employers, especially job-shop employers, are sensitive about permitting employees to have access to the purchase orders they receive, for they list the price the customer is paying. Nonetheless, the part drawing by itself doesn't always tell the whole story. Many specifications may be contained in a purchase order that can have a profound effect on the product manufactured (and hence on N/C programming).

For example, the purchase order may specify the use of part drawing No. 12345, revision D (several dimensions changed), but the programmer is provided with the earlier revision C. The programmer spends 125 hours writing and debugging a program for revision C and then, when 500 parts are rejected by the customer, it is discovered that the wrong part drawing was used for programming. It may be poor management, but it is expensive for the company, frustrating for the customer, and exasperating for the programmer! Although it may not always be possible to gain access to the purchase order, it would be wise for the employer to at least furnish the programmer a copy with the sensitive information obliterated. Perhaps a diplomatically phrased suggestion from the programmer might help.

Specifications that are omitted on the part drawing or the purchase order do not give programmers carte blanche to do as they please. In the absence of written specifications, these factors legally default to what is called "standard industrial practice." For example, a customer who omitted the tolerance specification on the part drawing or purchase order would probably not need to purchase piece parts that are 1/4 inch undersize. A court would undoubtedly say that standard industrial practice applies--for example tolerances of plus or minus 0.001 inch for four-place decimal dimensions, 0.010 inch for three-place decimal dimensions, and 1/32 inch for fractional dimensions.

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Updated Jan. 24, 2002
Copyright © 1988, 2002 by George Stanton and Bill Hemphill
All Rights Reserved