Did A Parameter Setting Cause That Problem?

As you probably know, parameters control many functions of a CNC. Just about every CNC function has at least one related parameter setting—everything from setting initialized states at power-up to determining the specific procedure required for powering down is controlled by parameter settings.

Columns From: 8/20/2008 Modern Machine Shop,

Click Image to Enlarge

Mike Lynch

Mike Lynch

In many cases, parameters have become a safe-haven for control design misjudgments. They give the control manufacturer a quick and easy way to modify how a control function works. Control manufacturers may, for example, initially think that all of their users would like to monitor positioning movements on the position display page without considering cutter radius or tool-nose-radius compensation (monitoring programmed coordinates). After enough users complain, the control manufacturers add the desired function (in this case, the ability to monitor compensated coordinates) and use a parameter to specify which of the two (or more) available position-monitoring methods is to be used. It’s a quick fix, and it is easy to select when a user requests it.

Control manufacturers will set a default value for each parameter. When you install the machine, the parameter will already be set in the way the control manufacturer thinks the majority of their users will want it set.

It is important to know that many such parameters exist.They give you a choice as to how a given CNC function will work. Unfortunately, most control manufacturers don’t publish a list that relates only those CNC functions that have selectable features (other than the complete list that includes all parameters). Therefore, end users are on their own to find them.

Just knowing that these parameters exist may be enough to help most CNC users. The next time the machine behaves in an undesirable fashion, at least you’ll know enough to consider the possibility that a parameter change may improve the situation. However, you’ll probably need help from the machine tool builder or control manufacturer when it comes to determining whether the related function is controlled by parameters—and, of course, which parameter is involved.
Some control manufacturers do a pretty good job of providing parameter documentation, not in the parameter documentation, but in the specific sections of their instruction manuals that show how to use CNC features. Fanuc, for example, provides parameter documentation within a series of notes at the end of each feature’s description (though not all of them are documented in this fashion). The custom macro section of the Fanuc manual, for example, provides many such notes.

When a given CNC function is not behaving as you expect or desire, the first thing you should do (prior to contacting your machine tool builder) is read the function’s description in your control manufacturer’s instruction manual. You may find the related parameter and instructions detailing how to set it to make the function behave as you want it to.
For example, say you’re using the chip-break peck drilling cycle (G73) on a machining center. With this cycle, the drill feeds a specified depth, retracts a bit (to break the chip) and then pecks again. It repeats this process until the hole-bottom is reached, at which time the drill retracts all the way out of the hole.

As you watch this cycle on the machine, you notice that after each retract to break the chip, the drill takes a long time before it contacts the material. Each hole is taking much longer to drill than you think it should.

When you look in the control manufacturer’s instruction manual, you notice that a parameter is used to control the retract amount (the Fanuc manual will specify which parameter it is). Upon checking this parameter’s setting, you find it to be set to what you feel is an excessive amount (usually a 0.003-inch retract is enough to break the chip). By reducing the value of this parameter, you reduce the retract amount and get the G73 cycle to behave as you think it should.

Unfortunately, not all CNC function-controlling parameters are documented in this manner. The only way to find all of them is to study the parameter documentation for a given control. What is also unfortunate is that the documentation in the parameter section of most instruction manuals is minimal and difficult to understand, and you may think you’ve found the parameter that controls a given function when you have not. Whenever you’re in doubt, you must contact the machine tool builder or control manufacturer to get a better explanation for the parameter in question.

Comments are reviewed by moderators before they appear to ensure they meet Modern Machine Shop’s submission guidelines.
blog comments powered by Disqus
MMS ONLINE
Channel Partners
  • Techspex