CNC Tips & Techniques, Page: 1   2   3   4   5   6   7

CNC Tips & Techniques (p. 3)

How can I assign more than six program zero points on Fanuc controlled machining centers?

Fanuc (and most Fanuc compatibles) are quite limited when it comes to the number of fixture offsets that come standard with the control. While you can buy more fixture offsets as an option (Fanuc offers an extended fixture offset feature containing 48 total fixture offsets), there is a way to assign as many program zero points as you want with but one fixture offset! However, you do have to give up one important feature of fixture offsets if you use this technique. It is the ability that fixture offsets give you to separate program zero assignment from programs. With the technique we show, you'll be entering program zero assignment values into sub-programs (instead of fixture offsets). (Article)

How do you decide between conversational (shop floor) programming and programming off line?

While there may be some in this industry that disagree with me, my feeling is that conversational programming (like Mazatrol), while simple, should only be used if its application fits. Generally speaking conversational programming on the shop floor should only be used when there is no time to be preparing programs while the machine is running in production. That is, the machine must be down during the programming process. In this case, conversational programming makes the most efficient method to get programs prepared. Maybe lot sizes are very small and 90% or more of your jobs are new ones. Or maybe lead time is extremely short in conjunction with a high percentage of new jobs. These are common scenarios in contract shops (job shops) and tooling producing companies (or toolrooms in product-producing companies). (Article)

What is the difference between mean, nominal, and target dimensions?

The mean value of a tolerance band is always the value that is right in the middle of the tolerance band. For example, for the dimension and tolerance: 3.000 +/- 0.002, the mean value is 3.000. For the dimension and tolerance 3.002, plus nothing minus 0.004, the mean value is still 3.000. As it is for the dimension and tolerance 2.998 plus 0.004, minus nothing. A CNC programmer should ALWAYS program the mean value for every coordinate they specify in a CNC program. Other important tolerance naming includes high limit (3.002 in the examples above), and low limit (2.998 in the examples above). (Article)

How is G50 used to assign program zero on turning centers?

Frankly speaking, geometry offsets were designed to replace G50 for assigning program zero on turning centers (though they have nothing to do with spindle limiting). The only reason people use the G50 command today is that they have older machines that don't have geometry offsets (like you) or they don't understand geometry offsets. With G50, you include the distances from program zero to the tool tip at the time the G50 is executed. If, for example, you plan on starting the program from the zero return position, the G50 values will be equal to the geometry offset values (but positive). One major problem with G50 is that the machine must absolutely-positively be at the planned position when the program is executed. If it is not, the control will "believe" the G50 values and move accordingly - this is a major source of machine crashes - the operator activates the cycle with the turret out of position. Note that this can't happen with geometry offsets. The control will automatically take the machine's current position (relative to the zero return position) into consideration when making axis movements. (Article)

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