Copyright Goodheart-Willcox Co., Inc. Chapter 9 CNC Mill Programming 181 block is also not intended for a close fit into another assembly. To meet these requirements, saw the 3″-wide material about 6.1″ long, then machine both ends to 6.00″. In the hole position, the tolerances are a little closer. Those toler- ances reference the top-left corner of the part. It is a good idea to establish the work coordinate system (WCS) origin—the point where both X and Y values are zero—at the same corner on the top-left edge. In the Cartesian coordinate system, this would be in quadrant 4, or the X+, Y− quadrant. An important feature to this part’s engineering is the hole diameter, which has a very close tolerance of .500″/.501″. This tolerance callout actually tells you how to machine this feature. There are a variety of ways to make a hole in a part, including drilling, milling, reaming, and boring. A drilled hole must be spot drilled first and then drilled, resulting in a hole no closer than ±.003″ of a specified size that is not perfectly round or accurately positioned. A milled hole is accurate in position and roundness, but due to machining technique, these holes can have some taper from top to bottom, or a cylindricity issue. A third option is a reamed hole, which must be spot drilled, drilled under- size, and then reamed. A reamer is considered a hard tool, or a tool that only cuts on the sides and must enter through a preexisting hole. For the hole in Figure 9-1, an adequate machining technique would be to drill this hole .485″ in diameter and then ream with a .5005″ reamer. The last option is to bore this hole, accomplished by drilling a hole and then using a boring head to machine a round, close-diameter hole. Boring achieves the best results—holes accurate in cylindricity, position, and diameter—but it is slower than the previous options. Boring this hole is not the most cost- effective operation, but it is sometimes required to meet print specifications. In communicating where dimensions are established and which features are most important, the Figure 9-1 print tells us how to machine this part. In sum, use 3″ stock, cut it to 6.1″, place the part in a vise where both ends can be machined, establish the top-left corner as the origin of the WCS, use an end mill to cut to 6.00″, and then spot drill, drill, and ream a .5005″ hole. By carefully examining the print, a good machinist gains a much better understanding of the designer’s intent for a part, which can then inform the best machining process. 9.2.2 Part Workholding After reading and understanding the full print require- ments, a machinist must decide how to hold the part. Holding the part securely during the machining process is critical, but machinists also need to have access to machine as many features as possible in a single hold- ing to reduce setup and overall cycle time. For some projects, workholding will be as simple as placing a piece in a vise, others will require the production of a specialty fixture, and some projects may require spe- cialized devices like magnetic or vacuum chucks (see Figure 9-2 and Figure 9-3). The objective of workhold- ing is to maximize machining and minimize setups or operations. inTer_b/Shutterstock.com Figure 9-2. A magnetic chuck used to secure ferrous metals in place. Goodheart-Willcox Publisher Figure 9-3. A vacuum chuck that uses negative air pressure to secure the workpiece.