♦ Return-to Machine Zero
In manual mode, the CNC operator physically moves the axes to the machine zero position. The operator is also responsible to register this position into the control system, if necessary. Never turn off power to the machine, while the machine slides are at or very close to the machine zero position. Being too close will make the manual machine zero return more difficult later, after the power had been re-stored. A clearance of 1.0 inch (25.0 mm) or more for each axis from machine zero is usually sufficient. A typical procedure to physically reach the machine zero position will follow these steps:
1. Turn the power on (machine and control)
2. Select machine zero return mode
3. Select the first axis to move (usually Z axis)
4. Repeat for the all other axes
5. Check the lighted in-position indicators
6. Check the position screen display
7. Set display to zero, if necessary
Mainly for safety reasons, the first selected axis should be the Z axis for machining centers and the X axis for lathes. In both cases, either axis will be moving away from work, into the clear area. When the axis has reached machine zero position, a small indicator light on the control panel turns on to confirm that the axis actually reached machine zero. The machine is now at its reference position, at the machine zero, or at the machine reference point, or at home - which-ever term is used in the shop. The indicator light is the confirmation for each axis. Although the machine is ready for use, a good operator will go one step further. On the position display screen, the actual relative position should be set to iiero readout for each axis, as a standard practice, if it is not set to zero automatically by the control. The POS button on the control panel selects the position screen display.
PART REFERENCE POINT
A part ready for machining is located within the machine motion limits. Every part must be mounted in a device that is safe, suitable for the required operation and does not change position for any other part of the job run. The fixed location of the device is very important for consistent results and precision. It is also very important to guarantee that each part of the job is set the same way as the first part. Once the setup is established, the part reference point can be selected.
This vital reference point will be used in a program to establish the relationship with machine reference point, reference point of the cutting tool and the drawing dimensions. The part reference point is commonly known as a pro-gram zero or a part zero. Because the coordinate point that represents program zero can be selected by the programmer almost anywhere, it is not a fixed point, but a floating point. As this point is selectable, more details can be covered - after all, it is the programmer who selects part zero.
♦ Program Zero Selection
When selecting the program zero, often in the comfort of programmer's office, a major decision is made that will influence the efficiency of the part setup and its machining in the shop. Always be very attentive to all factors that are for and against a program zero selection in a certain position.
In theory, the program zero point may be selected literally anywhere. That is not much of an advice, although true in mathematical terms. Within the practical restrictions of the machine operations, only the most advantageous possibilities should be considered. Three such considerations should govern the selection of program zero:
♦Accuracy of machining
♦Convenience of setup and operation
♦Safety of working conditions
Machining Accuracy
Machining accuracy is paramount - all parts must be machined exactly to the same drawing specifications. Accuracy is also important consideration in repeatability. All the parts in the batch must be the same and all subsequent jobs must be the same as well.
Convenience of Setup and Operation
Operating and setup convenience can only be considered once the machining accuracy is assured. Working easier is everyone's desire. An experienced CNC programmer will always think of the effect the program has in the machine shop. Defining program zero that is difficult to set on the machine or difficult to check is not very convenient. It slows down the setup process even more.
Working Safety
Safety is always important to whatever we do - machine and part setup are no different. Program zero selection has a lot to do with safety of the machining operation. We look at the typical considerations of program zero selection for vertical machining centers and lathes individually. Differences in part design influence the program zero selections as well.
♦ Program Zero - Machining Centers
CNC machining centers allow a variety of setup methods. Depending on the type of work, some most common setup methods use vises, chucks, subplates and hundreds of special fixtures. In addition, CNC milling systems allow a multipart setup, further increasing the available options. In order to select a program zero, all three machine axes must he considered. Machining centers with additional axes re-quire zero point for each of these axes as well, for example, the indexing or rotary axes.
What are the most common setup methods? Most machining is done while clamped on machine table, in a vise or a fixture mounted on the table. These basic methods can be adapted to more complex applications. CNC programmer determines the setup method for any given job, perhaps in cooperation with the machine opera-tor. CNC programmer also selects the program zero position for each program. The process of selecting the pro-gram zero starts with drawing evaluation, but two steps have to be completed first:
Step 1. Study how the drawing is dimensioned, which dimensions are critical and which are not
Step 2. Decide on the method of part setup and holding
Program zero almost presents itself in the drawing.In any setup, make sure all critical dimensions and tolerances are maintained from one part to another. Drawing dimensions not specified are usually not critical.
The simplest setup on a machine table involves support for the part, some clamps and locating surfaces. The locating surfaces must be fixed during the job run and easy to be measured from. The most typical setup of this kind is based on the three pin concept. Two pins form a single row and the third pin is offset away at a right angle, creating a 90° angle setup corner as two locating surfaces - Figure
Since the part touches only one point on each pin, the setup is very accurate. Clamping is usually done with top clamps and parallels. The left and the bottom edge of the part are both parallel to the machine axes and perpendicular to each other. Program zero (part zero) is at the intersection of the two locating edges.
The three-pin concept is common for virtually all setups, without using actual pins. If a part is mounted in a vise, there are similarities. The vise jaws must be parallel to or perpendicular with the machine axes and the fixed location must be established with a stopper or other fixed method.
Since a machine vise is the most common work holding device for small parts, let's use it as a practical example of how to select program zero. Figure below illustrates a typical simple engineering drawing, with all the expected dimensions, descriptions and material specifications.
When selecting a program zero, first study the drawing dimensions. The designer's dimensioning style may have flaws, but it still is the engineering drawing. In the example, dimensioning for all holes is from the lower left corner of the work. Does the program zero of the part suggest itself?
For this example, there should be no question about programming the reference point anywhere else except at the lower left corner of the part. This is the drawing origin and it will become the part origin as well. It also satisfies Step I of the program zero selection process. The Step 2, dealing with work holding device selection is next. A typical setup in a special CNC machine vise could be the one illustrated in Figure.
In the setup identified as Version 1, the part has been positioned between the vise jaws and a left part stopper. The part orientation is the same as per drawing, so all drawing dimensions will appear in the program using these drawing dimensions. It seems that this is a winning setup - yet, this setup is actually quite poor.
What is missing in the decision is any consideration of the actual overall size of material. The drawing specifies a rectangular stock of 5.00 x 3.50. These are open dimensions - they can vary ±.010 or more and still be acceptable.
Combine any acceptable tolerance with the vise design, where one jaw is a fixed jaw and the other one is a moving jaw, and the problem can be seen easily. The critical Y axis reference is against a moving jaw.
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