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June 23, 2008

Dear Subscribers,

Welcome to issue 76 of The Optional Stop newsletter. We appreciate your continued support.

It’s hard to believe I’ve been publishing this newsletter for nearly twenty years. Over the years, we’ve received countless suggestions and comments that have made this newsletter better. And again, we appreciate your interest.  So don't hesitate to let us know your thoughts and ideas!

In this issue, we’ve included a custom macro for face milling on a CNC machining center. It actually serves two purposes. First, it’s a great little macro – and should be helpful to anyone that performs face milling operations. But second, it should make a good way of testing the custom macro features of our NCPlot software product.

Enjoy!

Mike Lynch

IN THIS ISSUE
Product Corner: NCPlot helps custom macro B users
Instructor Note: Explaining the importance of knowing your machine
Manager's Insight:  Do your people understand the implications of on-line and off-line tasks?
G Code Primer: When are plane selection commands required?
Macro Maven: A face milling custom macro
Parameter Preference: Two Parameters related to measurement systems
Safety First: When not to use a dwell command!

Product Corner: NCPlot helps custom macro B users

NCPlot has been the topic of previous Product Corner articles. We’ve explained that NCPlot is a tool path plotter that you can use NCPlot to verify G-code level CNC programs. But it’s such a great tool path plotter – easy to use, effective, and flexible – we haven’t said much about some of its other important features. Some of the more neglected features are related to custom macro B users – so if you use custom macro B on a regular basis, read on – you’ll surely find some points of interest in this edition of Product Corner.

Our first point is that NCPlot allows the tool path plotting of programs that contain custom macro B commands. Very few tool path plotters can plot custom macro B programs – and none we know of are available at the price of NCPlot. Just about anything the custom-macro-B-equipped CNC machine will execute can be executed in NCPlot. (About the only exception has to do with certain system variables that provide internal control manipulation – like the offset setting and input/output signal system variables.) This means you can use NCPlot to verify custom macro B programs in the same way you can use it to verify normal G code programs – and in turn – eliminate the machine downtime normally needed for custom macro verification.

If you’re incorporating custom macro commands in the main program, as is commonly the case with part family applications, simply load the program into the NCPlot editor and execute it. It will display all motions – including those affected by your input variable arguments.

If you’re developing separate custom macros and activating them with a G65 (or G66) command, as is commonly the case with a user created canned cycle applications, you load all programs into the NCPlot editor, with the main program first. When you activate the tool path plotter, NCPlot will start at the first command in the editor (the main program’s first command) and work through the program in the normal manner until an M30 or M02 (end of main program) is seen – at which point it will stop.

In both cases – and as is the case when plotting normal G code programs without custom macro commands, NCPlot will show the destination point for each motion in all axes. This is especially helpful for spotting mistakes, as would be the case when a calculated position is incorrect.

Like the CNC control, when a G65 or M98 (custom macro or subprogram calling command) is executed, NCPlot will jump to the beginning of the subprogram or custom macro and execute through the M99. It will then return to the main program and to the command after the calling M98 or G65.

You can, of course, modify the values of any input variables (in the G65 command or at the beginning of the part family program) to confirm that the custom macro will work for all conditions of the call statement. When you find a mistake, you can use the single step function to step through the program just as you would on the CNC control with single block.

Speaking of mistakes, NCPlot provides much more help for diagnosing problems than your CNC control does. With the “Show Variables” feature activated (found in the “Calc” menu), you’ll be able to see the current state of all variables in a dynamic fashion. This means you’ll be able to single step through the program – one command at a time, including calculation and logic commands – and see the changing values of each variable as you execute the program.

On the “Show Variables” page, you’ll also see the custom macro source command as well as the CNC command that it renders. For example, the source command may look something like:

  • X[#24+#1-#20/2] Y[#25+#7/2-#1] R[#1-#20/2]

The CNC command it renders may be:

  • X3.25 Y2.25 R0.75

NCPlot also provides an “Expression Calculator” (again, under the “Calc” menu) to help you develop and verify your custom macro calculation commands. You’ll be able to see and confirm the results of calculation commands before you place them into your custom macro programs.

One last important custom-macro-related feature we’ll mention is “Macro Translator”. This valuable feature gives you the ability to convert custom macro B programs into normal G-code programs. This means you can take a saved custom macro program (or main program that calls custom macros) – with all input variables set the way you want them – and have NCPlot create another program (and save it) that contains only hard-and-fixed CNC commands (no custom macro commands). You can, of course, then use NCPlot to verify the motions in the created G code program. In essence, this is like having a self-created computer aided manufacturing (CAM) system that creates G code programs from your custom macros.

You may be questioning why this is such an important feature – and when it could be helpful. Admittedly, at first glance this may not seem like a very important feature. While not everyone will have need of this feature, there are at least two times when this feature is priceless.

The first has to do with machines that don’t have custom macro B. You probably have a collection of very helpful custom macros – maybe even some that do things not possible (or easy to do) with your CAM system. Examples might include thread milling, taper tread milling, grooving, and pocketing. But if one or more of your CNC machines does not have custom macro B, of course, these custom macros can’t run in them.

With NCPlot, you can still use your custom macros even with machines that don’t have custom macro B. Simply call up the program in NCPlot’s editor, and use “Macro Converter” to convert it to a standard G code program that can be run on machines that don’t have Custom Macro B.

While this does eliminate the on-the-fly ability to quickly modify programs at the machine, for applications for which you have no other feasible way to create the G code, “Macro Translator” can be a life-saver.

A second time “Macro Translator” can be invaluable is related to custom macros that are weighed down with calculation and logic commands. You know that any time you make the control think, it takes time. The control doesn’t actually think, of course, but what it does during the execution of calculation and logic commands does resemble thinking.

Consider a taper thread milling custom macro that requires the control to calculate each point through which the thread milling cutter will move on its path to form the spiral motion needed for taper thread milling. With the resolution set to one degree, 360 motions will be necessary. If resolution is set to 0.1 of a degree, 3,600 motions will be necessary. If set to 0.01, 36,000 motions will be necessary. And so on.

Even with current model CNC controls, at some point the machine will get bogged down, not being able to keep up with the required motions at the desired feedrate. This will caused increased cycle time and possibly inadequate surface finish on the threads – and may keep you from using custom macro B for this kind of application.

With “Macro Translator”, you can have NCPlot convert your taper thread milling custom macro (or any calculation- and logic-heavy custom macro) to normal G code containing only simple G01 commands to form the tiny motions around the tapered thread. There will be no thinking for the control to do and it will execute the motions without dwells or delays – and of course – at the desired feedrate.

If you consistently use custom macro B, you may find that the custom macro B features of NCPlot alone justify its purchase price of only $299.00. When combined with everything else it can do, NCPlot is an outstanding value!

By the way, we provide a face milling custom macro in the Macro Maven article of this issue of The Optional Stop. It makes a great way for you to test the custom macro features of NCPlot. Simply download the trial version of NCPlot from our website and then copy and paste the main and custom macro programs from this newsletter into the editor of NCPlot.

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Instructor Note: Explaining the importance of knowing your machine

I’ve often said that machinists make the best CNC programmers. A machinist already knows what they want the machine to do – it’s a relatively simple matter for a machinist to learn how to tell the machine what they want it to do.

It can be frustrating for instructors to teach people that have limited – or no – basic machining practice experience to setup and run CNC machine tools – let alone to teach them how to write programs. I equate this to trying to learn how to fly an airplane without a basic understanding of aerodynamics and flight. Just as the aspiring pilot must understand how an airplane flies, so must an aspiring CNC person know what a CNC machine is designed to do.

I like to begin by explaining that there are some CNC machines that have been designed to replace existing equipment. These tend to be the easiest machines to learn because it’s possible that the student has a working knowledge of (if not hands-on experience with) the conventional machine being replaced. The newcomer can draw on this previous knowledge when learning the CNC machine.

Two great examples are CNC machining centers and CNC turning centers. Machining centers, of course, have been designed to replace drill presses and milling machines. Turning centers have been designed to replace all kinds of manual lathes, including engine lathes, turret lathes, and screw machines.
Since most people have at least seen a drill press – if not the opportunity to work with one – it is pretty easy to introduce students to CNC machining centers.

Explain that like drill presses, CNC machining centers have the ability to perform all kinds of hole machining operations. I’ll test the waters at this point in an attempt to learn how much students know. I’ll ask what kinds of hole machining operations must be performed on workpieces.

Almost everyone – including novices – will first name drilling. Almost everyone has drilled a hole or has seen it done. So I’ll explain that one very common operation performed on CNC machining centers is drilling. I may mention the kinds of drills available (twist drills, spade drills, inserted drills, etc.).

I’ll push to find out if they know what other kinds of hole machining operations can be performed and why they are required. At the completion of this discussion, I’ll be sure to have mentioned reaming (to improve hole finish and size after drilling), boring (to straighten holes and improved finish and size after drilling), tapping (to machine threads in holes), and counter-boring (to open up a current hole to a larger diameter to a specified depth).

I’ll also see whether any students can name other operations that can be performed on the kinds of machines that a CNC machining center is replacing. Someone may know about milling operations – and this will open the discussion. We’ll cover the reasons why milling operations are required as well as what kinds of milling operations can be performed (slot milling, pocket milling, face milling, etc.).

We’ll then do the same for turning operations performed on CNC turning centers, as well as a variety of CNC machine types (like CNC turret presses, press brakes, plasma cutters, water-jet machines, and vertical E.D.M machines). The point of this, of course, is to stress that entry level people must know their CNC machines are designed to do. It’s likely that you’re only trying to cover one type of machine in your class, so you must eventually narrow the focus to address processes performed on your machine in detail.

Before ending this discussion, I do like to point out that there are certain types of CNC machines that have been designed to perform new and unique processes. These ground-breaking machines are doing things not previously possible before the advent of CNC, and include CNC wire E.D.M. machines, laser cutting machines, coil winding machines, and soldering machines. These machines tend to be more difficult to learn because there is no conventional type of machine being replaced – and no chance that the student has previous experience. Not only must the student learn the CNC-related functions of the machine, they must additionally learn the processes that the machine has been designed to perform.

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Manager's Insight: Do your people understand the implications of on-line (internal) and off-line (external) tasks?

What may be obvious to one person may not seem so to another. This is why – as a manager – you must understand and be willing to teach good machine usage practices to setup people and operators.

When it comes to setup, an on-line (internal) task is one that adds to the amount of time a machine is down between production runs. Indeed, setup time is the sum-total of all on-line tasks. Examples of tasks that are commonly performed on line include mounting workholding devices and loading cutting tools into the machine.

In similar fashion, on-line (internal) production running tasks include any tasks that add to the length of time it takes to complete a production run. And again, production run time is the sum-total of on-line tasks.

While these two statements are at the heart of any setup or cycle time reduction program, I’m not going to dive too deep into setup and cycle time reduction principles. Instead, I simply want to relate some pretty obvious tasks that setup people and operators commonly perform on line that could easily be performed off line. While they may seem obvious, I’m often surprised as I walk the shop floor in many companies. If you carefully watch your people, you may be too.

In one shop I visited, for example, I was watching an operator run a vertical machining center. When the cycle ended, he removed the workpiece from the vise. Then he cleaned it off with a rag, picked up the deburring tool and began deburring. About three minutes later, he put it into the completed workpiece bin. Only then did he go back to the machine and clean the vise. He then picked up the next workpiece to be machined from the raw material basket. But before loading it into the machine, he cleaned it and de-burred it with a file. Finally, he clamped it in the machine vise, closed the door, and started the next cycle.

What’s (obviously) wrong with this picture? Everything just described was done while the machine was down. They were all on-line tasks. Yet four of these tasks (cleaning the completed workpiece, deburring the completed workpiece, cleaning the next workpiece, and deburring the next workpiece) could have been done off-line – while the machine is in cycle. This assumes, of course that the cycle time for the job is longer than the time it takes to perform all the related tasks, which in this particular case, it was.

As you walk your own shop floor, do you ever see setup people and operators performing tasks on line that could be done off line? Unless your company has been involved in setup and/or cycle time reduction programs, I’ll bet you do.

A setup time example includes gathering components for the next setup during a lengthy production run. If the setup person waits until the current production run is completed to begin gathering, all of the gathering time will be on line. If gathering is done for future setups while the machine is still in production, setup time will be reduced by the time it takes to do the gathering.

Whenever you move a task from on line to off line, of course, you reduce the time it takes to complete the setup or production run. While this may sound like a very basic statement, I’m always amazed by how often I see tasks that could be done off line being done on line. In many cases, just a little explaining from the management side can have a big impact on productivity.

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G Code Primer: When are plane selection commands required?

There are three basic plane selection commands:

  • G17 – XY plane selection

  • G18 – XZ plane selection

  • G19 – YZ plane selection

When you first power up a Fanuc-controlled CNC machining center, G17 is automatically selected, meaning the machine will be in XY plane selection mode unless you select a different plane by commanding G18 or G19.

There are several CNC functions that are affected by your plane selection choice. But frankly speaking, almost everything commonly done on a machining center requires the selection of the XY plane, so there aren’t many times when you’re required to switch planes. But let’s discuss a few of the times when plane selection is necessary.

Circular interpolation

As you know, G02 and G03 are used to specify clockwise and counter-clockwise circular motions. When milling in the XY plane, as is commonly required, it is the X and Y axes that are moving during the circular motion. And by the way, we evaluate the direction (cw or ccw) by looking at the motion from the plus side of the uninvolved (Z) axis.

Again, most circular motions require the X and Y axes to be moving to form the motion. But consider placing an end mill in a right angle head, maybe one that points the tool in the X minus direction (tool facing to the left on a vertical machining center). In this case, a circular motion will require a YZ motion and prior to making such a motion, the YZ plane (G18) must be selected. If the right angle head points the tool along the Y axis, the tool will be machining in the XZ plane, and G19 must be commanded prior to this motion.

Another time plane selection must be considered with circular motions is when using a ball end mill. It’s possible that a circular motion with a ball end mill will be in the XZ or YZ plane, and the appropriate plane selection G code must be commanded prior or within the circular motion command.

Canned cycles

Most holes are machined in the Z axis with CNC machining centers. That is, the hole centerline coordinates are specified with positions along the X and Y axes. But again, consider placing a hole machining tool (drill, tap, reamer, etc.) in a right angle head that is pointing the tool in the X minus direction. This tool will have the ability to machine holes along the X axis – and believe it or not – using the appropriate plane selection command (G18 for YZ plane selection in this case) will allow you to use canned cycles (G81, G82, G83, etc.) to machine the holes. This dramatically simplifies the task of programming.

When this is done, the canned cycle-related letter addresses will change in meaning. If YZ plane selection is chosen (drilling along X), The hole center will be specified with Y and Z. The rapid plane will still be specified with R. But the hole bottom position will be specified with X.

What about odd angles?

With G17, G18, and G19, the planes are, of course, ninety degrees apart, which is why these commands can be helpful with right angle heads on standard three-axis machining centers. But these commands won’t help with other planes – those that are not at right angles with the three axes of the machining center (X, Y, or Z).

There are, however, machines that have the ability to machine in any plane. Five axis machining centers can machine in planes other than just XY, XZ, and YZ. For this reason, most five axis machining centers come with a special feature called user-defined plane selection. With this feature, the programmer can define any plane – and still use circular interpolation, canned cycles, and other coordinate manipulation features (like rotation and mirror image) in any defined plane.

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Macro Maven: A face milling custom macro

Suggested by Glenn J. Doutrich Sr. of Inside Development Systems, Inc.

I’ve had several requests recently for this custom macro – so, here it is! This custom macro performs a face milling operation and allows the percentage of overlap to be specified. It provides for two methods of milling, climb milling (assuming a right hand cutter is used) and zig-zag milling in both directions.

The letter address M specifies which type of milling will be done. Leaving M out of the G65 command – or setting it to M1.0 will cause the machine to climb mill.  Setting M to M2.0 will cause the machine to zig-zag mill back and forth until the milling is complete. This custom macro assumes the X length of the workpiece is longer than the Y length. That is, it makes passes along the X axis only.

The illustration shows the variable meanings:

Face milling macro variables

Here is an example calling program followed by the custom macro.

  • O0001 (EXAMPLE MAIN PROGRAM)

  • N005 T01 M06 (4" FACE MILL)

  • N010 G54 G90 S500 M03

  • N015 G00 X0 Y0

  • N020 G43 H01 Z2.0

  • N025 G65 P1000 X0 Y0 Z0 C0.15 M2.0 U40.0 V33.0 D4.0 Q20.0 F15.0

  • N030 G91 G28 Z0 M19

  • N035 M30

Line N025 calls the face milling custom macro. X and Y specify the lower left corner. Z specifies the surface to be milled. C specifies the amount of clearance for the cutter (approach and escape distance). U and W specify the length and width of the part to be milled. D specifies the cutter diameter. Q specifies the overlap percentage (percentage of cutter diameter for overlap). F specifies the feedrate.

  • O1000 (FACE MILLING CUSTOM MACRO)

  •  

  • #120 = #7*[[100-#17]*.01] (effective cutter size)

  • #121 = [2*[#7-#120]] + #22 (TOTAL STOCK INCLUDING OVERLAP)

  • #100 = 1 (CURRENT PASS [COUNTER])

  • #101 = FUP[#121/#120] (NUMBER OF PASSES)

  • #102 = #25+#22+[#7-#120]-#7/2 (CURRENT Y)

  • #103 = [#121-#7] / [#101-1] (Y STEP AMOUNT)

  • #104 = #102-#103 (TESTER FOR "ZAG MOTION)

  •  

  • IF[#13 EQ 2.0]GOTO 20 (TEST IF ZIG-ZAG)

  •  

  • (CLIMB MILLING)

  • N1 IF[#100 GT #101] GOTO 99

  • G00 X[#24-#3-#7/2] Y#102

  • Z#26

  • G01 X[#24+#21+#3+#7/2] F#9

  • G00 Z[#26+1.0]

  • #100 = #100 +1 (STEP COUNTER)

  • #102 = #102 -#103 (STEP CURRENT Y POSITION)

  • GOTO 1 (GO BACK TO TEST)

  •  

  • (ZIG-ZAG MILLING)

  • N20 IF[#100 GT #101] GOTO 99

  • G90 G00 X[#24-#3-#7/2] Y#102

  • Z#26

  • G01 X[#24+#21+#3+#7/2] F#9

  • IF[[#25-[#7-#120]+#7/2]-0.02 GT #104] GOTO 25 (TEST IF ZAG PASS IS NEEDED)

  • G00 G91 Y-#103

  • G90 G01 X[#24-#3-#7/2]

  • N25 #100 = #100 + 2 (STEP COUNTER)

  • #102 = #102 -#103*2 (STEP CURRENT Y POSITION)

  • #104 = #102-#103 (TESTER FOR "ZAG" MOTION)

  • GOTO 20 (GO BACK TO TEST)

  •  

  • N99 G00 Z[#26+1.0]

  • M99

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Parameter Preference: Two Parameters related to measurement systems

As you probably know, current model CNC machines can be used with two measurement systems – the Imperial (inch) measurement system and the Metric (millimeters) measurement system. A parameter setting controls which measurement system is automatically selected at the machine’s power-up, meaning you can have the machine default to your measurement system of choice.

The parameter that controls the current (and initialized) measurement system can be found on the setting page and can be modified without activating the parameter write enable (pwe) function. You need only be in the manual data input (MDI) mode to change this setting.

There could be a second, little known, parameter that is related to your measurement system choice. It determines what happens when you switch measurement systems. This parameter controls whether a true conversion of all data will take place (this is the desired setting) or whether the decimal point for all values will simply shift one place. If available, you’ll find documentation about this parameter in the measurement system selection section of your Fanuc Operators manual.

Again, the desired setting causes a true conversion of all numeric data, including position displays, tool offsets, and fixture offsets. On the position display page, for example, say a value of 10.0000 inches is currently shown for the X axis. When you switch to the Metric mode (by setting parameter or by commanding a G21), the X position display should be updated to 254.000 mm. If instead, it shows a value of 100.000 millimeters, the decimal point is simply being shifted one place to the right.

The true conversion is especially important if you’ll be running jobs in both measurement systems. When you go from job to job, it is likely that at least some of the tooling from the previous job will be used in the next job – after the measurement system change. Having the control convert all values between the measurement systems is important. It will keep the setup person from having to re-measure tools used in the previous job.

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Safety First: When not to use a dwell command!

As you know, G04 is used to specify a dwell – or pause – in your CNC program’s execution. With Fanuc controls, the format for the dwell command can vary based upon what character you choose to specify the period for the dwell. The commands

  • G04 X0.5

  • G04 U0.5

  • G04 P500

all specify a half second dwell (assuming dwell is specified in time and not the number of spindle revolutions).

While the dwell command is necessary when you wish to relieve tool pressure – as would be the case just after plunging an end mill into a surface and before starting to mill a pocket in XY – there are certain times when you should not use a dwell command.

The first time has to do with programming around machine problems. I’ve seen, for example, programmers that include a dwell command in their programs to allow time for the coolant system to kick in. Maybe there’s a bad check-valve in the coolant system – and the programmer wants to allow time for the coolant to be flowing at its maximum before allowing the machining operation to start. While this may be a reasonable temporary fix, the better long term solution is to fix the machine. In my opinion, this is an inappropriate use of the dwell command.

While it may be inappropriate, at least it isn’t dangerous. And there are times when I’ve seen some pretty dangerous applications for the dwell command. In one instance, for example, the programmer was trying to provide time for the operator to polish the workpiece during the CNC cycle. To this end, they included a twenty-second dwell in the program right after the finish turning operation. During this dwell, the operator was supposed to open the door and perform the manual polishing operation.

The obvious (and dangerous) problem with this technique, of course, is that if the operator didn’t finish the polishing during the twenty seconds, the machine would continue anyway, indexing the turret and bringing the next tool into position for the next machining operation. And of course this would be very dangerous for the operator.

Never use a dwell command to allow time for the operator to do something during the CNC cycle. Always consider the worst that could happen if they don’t finish what it is you intend them to do during the dwell. If it is at all dangerous, don’t use the dwell technique.

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