CNC Tips and Techniques. Peter Smid
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There is a good chance that the drawing from which you get to make a part has originated in CAD software. Nothing fancy, just a standard, everyday-type 2D drawing — two, three views, dimensions, and some text — that’s all. However, just because the drawing has been prepared in CAD does not mean it is without flaws and even errors. Let’s look at this fictional drawing and focus on a few details relating to numbers (dimensions, actually).
Focus 1 — Metric or Inch?
What units are used in the drawing? If you have no clue, don’t assume — make sure! More drawings are now prepared in metric units than before, but they are not always prepared properly. The magic word METRIC somewhere in the corner of the drawing may just not be there to tell you. Here are some ways to tell (but verify it later, anyway).
The first clue is the drawing scale and the second clue is the dimensions, their nominal values. If your specialty is producing small parts and you see the part length as 50, chances are you have a metric drawing. If you are used to seeing dimensions such as 1.625 or 0.75 (inches), and there are no dimensions that small, chances are you have a metric drawing. If the scale is correct, it solves another piece of the puzzle. Another clue is the number of decimals used in dimensions. If the drawing follows proper standards, a metric drawing should have whole numbers without the decimal point. So, our length of 50 mm will be shown as 50 between the dimension arrows, not 50.0. This is how it should be, but not always how it is.
Which brings us to yet another clue, and that is how decimal numbers are expressed. Because the metric system uses 1/1000th of a millimeter as the smallest unit, you should never see more than three decimal places in a metric drawing — and even those are rather rare. Sometimes you have to convert dimensions if most of them are in one unit, but another (a thread or tap size, for example) is in the opposite unit. It can be messy out there, but that’s the price we pay for being behind the rest of the world in adopting the metric system.
Focus 2 — Tolerances
A related item that could cause trouble — tolerances — is also directly related to a CAD system. Let’s stay with inches for this example. The drawing you have just received is basically a rectangular part, with overall dimensions shown as 5.0000 x 3.0000 x 0.5000. What is this? A few decades ago we would have been taught that the number of decimals places indicates the precision of tolerance. For example, x.xxx would be ±0.001 (or whatever the internal company standards were then). So now we have a part that has four decimal places on all three overall dimensions! That’s not all — there is also a surface finish of 125–250 required on the length and width. Because this dimension makes no sense, we have to look at the real culprit behind this.
First, throw out the old notion of this tolerance being implied by the number of decimal places. Yes, it is still done and, yes, it is still wrong. Separate from the fact that the notation breaks all ANSI or ISO standards, it is wrong because it is not reliable. It is not reliable for at least one reason (there are several more) — because a CAD software defaults to four decimal places for inch drawings, and to three decimal places for metric drawings. This can develop some bad habits in the CAD/CAM community. Of course, the software does allow for changes to any number of decimal places, but it is the role of the CAD operator to choose the proper settings.
Focus 3 — Why Is Metric Better than Inch?
Note the question again. It does not ask if metric system is better, it assumes that it is better, and asks why it is better. Before I get too many e-mails challenging my question, let me state for the record that I am specifically referring to CNC machines, not to any personal habits of buying a one-pound T-bone steak at your supermarket (although I could argue that one, too). CNC machines can be set to either inch mode or metric mode through the part program or at the machine control itself.
What is important here is to understand something called the minimum increment of the control system. In plain language, it means the amount of the smallest motion possible within each unit selected. For typical CNC machining or turning centers, the amount is set to 0.0001 of an inch or 0.001 mm, known as one micron. Now, say you want to make an offset adjustment at the control of 0.00004 of an inch. You just can’t unless you program the part in metric. The amount of 0.00004 of an inch is equivalent to 0.001016 mm, which is a motion possible to make, as one micron. The machine will move by the equivalent of 0.00003937 inch, close enough to pass all inspections. Designers out there — take a note!
 | CNC Programmer/Operator —Should One Person Be Both? November 2004, updated February 2013 |
When it comes to jobs in the CNC field, two types of positions within a plant or machine shop are usually identified: CNC Programmer and CNC Operator. Many small or even mediumsized machine shops do not have the resources or even the need to keep each position separate. Instead, they combine them both into a new position, commonly known as the CNC Programmer/Operator. The first apparent and most obvious benefit is saving money on wages or salaries. However, there are additional benefits and the inevitable disadvantages that are worth considering before creating such a position.
Advantages of CNC Programmer/Operators
First, let’s look at the advantages. Involving one person in the larger process of making a part provides better understanding of that process. If problems do occur, solutions are often easier to find. Programmers who also run the CNC machine are more motivated to do a better job using their own program. It is a common requirement for part programmers to know how to machine a part, but it is not normally required of CNC operators to know all details about how to develop the program. Good CNC operators do not always understand the programming part and cannot always offer program-related solutions.
In a combined position, this lack of understanding is eliminated. Programmers also frequently interact with engineers and customers; operators usually do not. Good programmers also interact with machine operators and seek their ideas. Again, the combined position may offer solutions to design or manufacturing process that only skilled machine operators could think of.
Disadvantages of CNC Programmer/Operators
There are also disadvantages to this programmer/operator concept. The initial appeal of saving money on remuneration may be offset in the loss of production time. It is virtually impossible to make a program and at the same time operate a machine. To do both equally well takes not only a skilled and dedicated person, it also requires a suitable job. If a part has to be changed on the machine every two minutes or so, the programmer cannot sufficiently concentrate on the work of developing the next program. A part that takes twenty minutes to machine offers a little more flexibility.
Every shop manager knows the term machine utilization — that the road to profitable business is running CNC machines as much as possible, producing quality parts. There are always some delays involved when using CNC machines. First, it is the setup time, including the fixture, tooling, and control settings. Next, the first part has to be verified. Then when the machine does finally start production, there is always the necessity of changing parts, replacing dull inserts, modifying offset values, etc. Adding more idle time because the programmer has to prepare the program can be very costly; over a period of time it can cost more than having two skilled workers taking on individual responsibilities.
Thus far, this overview has applied to a single CNC machine. What if there are two, three, or even more machines in the shop?
Three other disadvantages of the combined position should also be considered very seriously. The first is quite simple. If the programmer stays home sick, so does the operator (and vice versa). The second disadvantage is related. What if the programmer/operator quits? Of course a suitable backup is an ideal solution, but the reality is that there will be a gap in the production somewhere; even if the gap is temporary, it still represents time lost. The third disadvantage relates more to certain traits in human nature and does not apply to everybody. Some programmers who also run the machine may have the tendency to cover up