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October 10, 2011

Dear Subscribers,

Welcome to Issue 88! 

We’re just recovering from our flurry of fall school orders. It was especially busy this year (which is why this issue is a bit late). More and more schools are using our curriculums to help them teach CNC classes. And our Setup and Operation curriculums – those aimed at entry level shop people – are growing in popularity.

This issue contains information on a variety of topics – we’re back to our normal format in this issue. I hope you enjoy it.

Mike Lynch

IN THIS ISSUE
Product Corner: Custom macro B and NCPlot
Instructor Note: Axis polarity can be tough to explain because...
Manager's Insight: Changing methods to accommodate new features
G Code Primer: Some G codes that have run their course
Macro Maven: Using offset information to make programs more intelligent
Parameter Preference: Initialized G codes
Safety First: Eliminate distractions that cause lapses in attention

Product Corner: Custom macro B and NCPlot

We’ve been marketing this great software program since March of 2007. One of the things that initially attracted me to NCPlot was its ability to handle custom macro B commands. NCPlot can show tool motions for almost all functions of custom macro B. About the only exception is certain system variables, like those related to machine position, that contain values related to the current state of the machine tool.

It frustrates me to some extent that eyebrows are often raised when I tell people that NCPlot can plot custom macro B programs. It seems we’re not doing a very good job of marketing since so many people don’t know that NCPlot can help verify custom macros. So this short blurb is intended to help get the word out.

Not only can NCPlot show tool path generated by custom macro B programs, it has some program verification tools that are especially helpful when verifying custom macros. Expression Calculator, for example, allows a user to get the result of an expression written in custom macro B format. This helps confirm that the expression is correct. And the Show Variables function lets you see the current values of all variables as programs are being executed. They even allow you to see the values of local variables for the various nesting levels when one custom macro calls another – a feature that even most Fanuc controls cannot do.

If you haven’t already, check out NCPlot – especially if you need to write custom macro B programs from time to time. You can download a fully-functional fifteen-day trial version here:

 

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Instructor Note: Axis polarity can be tough to explain because...

In almost every CNC class I teach, at least one student struggle with the polarity of each machining center axis. The reason for this has to do with the fact that – with some axes – the cutting tool does not move along with the axis. With C-frame style vertical machining centers, for example, it is the table that moves to form the X and Y axes. With the Z axis, at least, the cutting tool does usually move along with the axis – either a quill motion or headstock motion. This makes it easier to understand polarity for the Z axis.

While not always entirely successful, it helps (especially when teaching programming) to ask students to visual the polarity of each axis as if the cutting tool is moving along with each axis. This relates nicely to how a programmer will view a drawing. It forces students to view polarity as the machine does – from the perspective of the spindle nose.

When looking at the machine correctly – from the front (Y minus side) of a vertical machining center – tool motion to the right is X+. Tool motion away is Y+. And tool motion up is Z+. For Z, it is simple since the tool does move along with the Z axis.

After presenting this, I ask students which way the table must move in X for a plus direction motion. Unfortunately, I often hear a response of “negative” – or “minus”. I explain that the choices for an answer are to the left or to the right. Then they usually get it, responding with “to the left”. So I reiterate that in order for the tool to have a motion to the right, and since the tool does not actually move in the X axis, the table must move to the left.

I repeat the process for the Y axis. I ask which direction the table must move in order for the tool to move in the plus Y direction – for a vertical machining center – the direction away from you if you’re standing in front of the machine. And of course, the correct response is “toward you”.

The situation is made more complicated when students must work with different kinds of machines. With a gantry style vertical machining center, the cutting tool does move along with all axes. With a horizontal machining center, you must imagine that you’re standing on the headstock side (again, looking from the perspective of the spindle nose and cutting tool) in order to correctly interpret polarity. For a sliding headstock turning center, cutting tools remain stationary in the Z axis – the workpiece (bar) moves to form the Z axis. And Z minus bar movement forward – coming out of the collet.

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Manager's Insight: Changing methods to accommodate new features

This field of computer numerical control is constantly evolving. New features and functions are coming along on a regular basis. Though most new features and functions will in some way enhance the way a CNC machine is utilized, there can be some down-sides related to implementing them – especially if your company owns (older) machines that do not have the functions.

I was recently in a company that has used CNC machines for many years. Indeed, they used this kind of equipment even before computers were an integral component in the control – when these machines were called NC machines.

Control manufacturers have long made they’re controls backward compatible, meaning new controls can accept programs written for controls that they have made in the past. But this is a one-way street – programs written for new machines that take advantage of new features – cannot be run in older machines. This provides a degree of compatibility among machines.

For this reason, the company I visited elected to ignore many helpful features that make CNC machines easier to utilize. Decimal point programming, arc size for circular motion with an R word, and even tool length compensation with G43 are among the features they elected not to use in order to maintain compatibility among older and newer machines. What makes the situation even worse – from a machine utilization standpoint – is that, over time, many of the older machines have been replaced. The very reason why many new features were not used does not even exist any more.

Admittedly, compatibility among machines is a very important factor in any CNC environment. But as a manager, you must ensure that it is not taken to extremes. As new features become available, you must weigh the benefits of maintaining compatibility against the possibility that the machine could be better utilized if newer features are used. At some point – like when old machines are replaced – you must have your people bite-the-bullet and start taking advantage of newer features.


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G Code Primer: Some G codes that have run their course

There are several G codes that are rarely used. Some are options, meaning you’ll have to pay extra to have a machine equipped with them. Others simply don’t apply to your application. And yet others are Fanuc’s “first attempt” at handling a problem or function. Other methods or G codes have been subsequently added that replace them. G codes in this third category will be the topic of this article.

Fanuc controls are – for the most part – backward compatible. This means that program written for older machines can be run in newer machines without modification. While it is unlikely that you’ll have any need for them, it may be helpful to understand why they were created and why they’ve been replaced. We’ll go through them in numerical order.

G22 and G23 – Interference zone check (G22 instates and G23 cancels). These G codes allow the programmer to set up a zone into which a cutting tool cannot move. While they are still active, over the years I haven’t seen very many CNC users that actually use them. It is quite cumbersome to determine the values that set up the interference zone (specified within the G22 command) – and this zone must be determined for every program that is to be protected. For turning centers, this problem is further compounded by the fact that each tool requires its own G22 instating command. While these G codes have not been replaced by anything better, again, they are seldom helpful.

G27 – reference (zero return) position return check. This is a testing command to ensure that the machine is at the zero return position at the end of a motion. If G27 is included in a motion command that sends the machine to the zero return position, the machine will perform at test. If the axes included in the motion command including the G27 are at the zero return position, the related axis origin lights will come on and the machine will continue. If an axis is not at the zero return position, the machine will stop and go into alarm state. G27 was helpful on turning centers prior to when geometry offsets became popular for assigning program zero (over twenty years ago) – when G50 was used to assign program zero. G27 could help a programmer determine that the machine was where it was supposed to be and that wear offsets were appropriately canceled before the next tool’s G50 command was given.

G29 – return from reference (zero return position). G29 will cause a motion that moves through the intermediate position specified in the most recently executed zero return (G28 command). This can actually be dangerous, especially when cutting tools are not run in the same sequence in which the program is written – like when a cutting tool is re-run by itself after the program completed a cycle. Nothing has been developed to replace G29. In my opinion, it was never a very helpful G code.

G44 – tool length compensation minus. G43 has become the universal G code (among Fanuc controls) for tool length compensation. G44 works just like G43, except the polarity for the value specified in the tool length compensation offset register must be reversed. If, for instance, you use tool length compensation as I recommend (tool length is the offset value), you know that with G43, the offset value must be positive. If for some reason you wish to enter the offset value (again, the tool’s length) as a negative value, G44 would correctly instate tool length compensation.

G45 – G48 offset expansion and reduction. They are named as follows:

  • G45: offset expansion

  • G46: offset reduction

  • G47: offset double expansion

  • G48: offset double reduction

G45 was used for tool length compensation prior to G43 (about thirty years ago). The four of them were used together in Fanuc’s first attempt at cutter radius compensation – again, before G41 and G42. While there may be some times when you wish an axis motion to expand or contract by the amount of an offset (which G45 can accomplish), most CNC users have no need for these G codes.

G50/G92 – coordinate system setting. G50 is for turning centers – G92 is for machining centers. These G codes were required prior to geometry offsets on turning centers and fixture offsets on machining centers. They were quite cumbersome to use, requiring the machine to be in a specific position before a program could be run. If it was not, the most likely result would be a crash. Note that G50 is still used on turning centers to set a maximum spindle speed.


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Macro Maven: Using offset information to make programs more intelligent

Custom macro B provides the ability to read and write offset data from and to a CNC program – and this has been the topic of many past articles in this newsletter. We’ve shown many applications for reading and writing offset data – most have been related to facilitating the way that setups are made or confirming that offset data is correct.

I was recently in a company that had a rather unique problem, one that could be solved by utilizing this ability to read offset data. It involved cutter radius compensation. As you know, cutter radius compensation lets you use a range of cutter sizes – and the operator simply enters the radius of the cutter currently being used into the cutter radius compensation offset. The machine, of course, uses this data to keep the tool the appropriate distance away from the programmed surface.

With most applications that use cutter radius compensation, the range of cutter sizes is relatively small. If the programmer intends to use a 1.0 inch diameter end mill, for instance, the setup person will use a cutter that is close to – likely slightly smaller than –1.0 in diameter (smaller would be for a sharpened cutter). The small deviation in cutter size (from the planned cutter size to the actual cutter size) isn’t usually enough to warrant a recalculation of cutting conditions – that is – spindle speed and feedrate.

The company I recently visited has a special problem in this regard. They use slotting cutters that range in diameter (even for a given job) from 3.0 to 4.5 inches. With this large range in cutter sizes, a change in speed and feed are important when going from a small cutter diameter to a large cutter diameter – and vise-versa. So the setup person and operator were actually recalculating cutting conditions and editing the program whenever a slotting cutter was replaced.

As you have probably determined by now, having access to offset data provides the ability to eliminate the need to recalculate speed & feed and edit programs. We simply changed the program to perform the speed and feed calculation based upon the value that is in the cutter radius compensation offset register.

Here’s how. Say the cutter’s radius is placed into offset number 31. For many Fanuc controls, the system variables that provide access to offsets range in the 2000 series. #2001 gives access to offset one, #2002 gives access to offset two, and so on. So system variable #2031 gives us access to offset number thirty-one. (You must confirm system variable numbers for offset access based upon your control model in the custom macro section of the programming manual.)

So we replaced the S word and F word with these:

  • S[3.82*120/[#2031*2]] (Running at 120 sfm)

  • F[3.82*120/[#2031*2] * 0.021] (Running at 0.021 ipr)

Since #2031 contains the cutters radius, we must – of course – double this value to come up with the cutter’s diameter for use within the calculations.

Note that larger cutters tend to have more teeth, meaning the inches-per-minute feedrate may also change based upon the cutter size. Though this criteria is not included in our example, it could be if you know the number of teeth for the different cutters that can be used. It would be relatively easy to determine which cutter is being used – along with its number of teeth – with a series of IF statements that test the value of #2031.

Other times when accessing offset data can make programs more intelligent

Eliminating cutter radius (or tool nose radius) compensation – Use the value in the radius compensation offset register during motion commands. This will eliminate the need for G41 and G42 – and the potential for cutter comp. alarms.

Use the T register on turning centers for more than tool nose radius compensation. By knowing the type of tool being used (turning tool, boring bar, etc.), you may be able to move to a more efficient safe index position for turret indexing.


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Parameter Preference: Initialized G codes

Many modal G codes are automatically set to a default setting during the machine’s power up. Indeed, this is the definition of an initialized G code. It can often be helpful to know that parameters control the initialized state of many G-code-controlled functions. While machine tool builders do a pretty good job of setting defaults for G codes, they aren’t perfect. It is possible that you won’t agree with their initial settings.

For example, one machining-center-using company I know of uses a right angle head for almost all jobs they run on a vertical machining center. This device points the cutting tool in Y minus direction (instead of Z minus), so almost all machining is done in the YZ plane instead of the XY plane.

Though this is the case, the company found that the initialized state of plane selection was G17 (XY plane). With a little digging, they found the parameter that controls this function and change it so that the XZ plane is initialized. The appropriate plane selection, of course, is important for circular motion, cutter radius compensation, canned cycles, and other important CNC functions.

Whenever you don’t agree with the choices made by the machine tool builder for initialized states, it is likely that a parameters will be involved with changing them. Admittedly, finding the appropriate parameter with most Fanuc controls can be challenging. My suggestion is that you begin by looking at the G code’s section of the Fanuc programming manual. The parameter related to the initialized state of plane selection, for example, is mentioned in a note at the end of the plane selection section of the Fanuc programming manual.

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Safety First: Eliminate distractions that cause lapses in attention

Setting up and running CNC machine tools requires setup people and operators to focus on the tasks at hand. A high degree of concentration is often required to keep from making mistakes that can cause wasted time, damage to machines, and/or injured people.

Most companies have come up with rules that attempt to limit such distractions. A few obvious examples include rules that limit the use of cell phones, music playing systems, and any other activities (like eating and drinking) that impede concentration, coordination, hearing, or vision. While distractions caused by many activities and devices are pretty obvious, others are not.

Consider, for example, certain work related distractions. Just about any time a person must break out of their train of though to do anything opens the door to forgetting where they left off when it comes time to continue. Think about times when operators and (especially) setup people must stop what they are doing to answer questions, take a phone call, or go and get a needed component. Though the distraction may be related to company business, it is still a distraction – and can cause the same problems as distractions that are not related to work.

I remember one company in which I was accompanying the lead setup person during a walk from a machine he was setting up to the tool crib – a walk of about 100 feet. During this walk, he was stopped five times. Another setup person needed advice. Two operators had questions about their machines. An inspector needed to tell him about a job that was having problems. And, almost comically, the shop manager wanted to know when his machine would be in production. A walk that should have taken about a minute took almost a half hour. The fact that he remembered what it was he was going to the tool crib for in the first place was admirable.

My suggestion in this regard is to set up a working environment that minimizes the potential for interruptions – especially when the task being accomplished is especially critical – like a hot job – or in any way dangerous. Don’t let people be interrupted until the completion of the task.

 

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The Optional Stop newsletter is published quarterly by CNC Concepts, Inc. and is distributed free of charge to people subscribing to our (email) distribution list and to those downloading it from our website (www.cncci.com). Information is aimed at CNC users and instructors teaching live CNC classes. All techniques given in this newsletter are intended to help CNC people. However, CNC Concepts, Inc. can accept no responsibility for the use or misuse of the techniques given.

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