Product Corner: NCPlot - More Than A Tool Path Plotter

NCPlot allows you to plot tool path from G code level programs.  The text window of NCPlot is like the internal memory of the CNC control.  If you have subprograms being referenced by your program, just load them - or copy and paste them - into the text window.  When a reference is made to a subprogram, NCPlot will know where to look.

Tool path display is both common and unique.  Like most tool path plotters, dotted lines are used to represent rapid motions while solid lines represent cutting motions.  Color changes can be used to reflect motion type (G00, G01, G02, or G03) or tool changes.  But unlike many tool path plotters, you can click on an element in the tool path display and be told all about it (position, length, motion type, etc.) and the CNC command that causes the motion will be shown in the text window.  You can even measure distances between elements, a feature normally found only with more expensive program verification systems.

NCPlot also allows you to plot tool path for custom macro B programs.  Again, after quickly loading the main program and custom macros into the text window, you'll be able to see the motions your custom macro will generate.  There are even some helpful features built in to NCPlot to help you write and verify your custom macros, including a quick reference manual for custom macro functions, an expression analyzer to help you verify arithmetic expressions., and an easy way to step through calculation and logic commands one-by-one.

Using the dxf-to-G-code feature, you can even import drawings from computer aided design (CAD) systems and have NCPlot create the G code motion commands needed to machine the imported shape.   And the text-to-G-code feature lets you create G code for engraving on your workpieces.

We no of no other software at the price of NCPlot ($299.00 with student pricing available) that comes close all that NCPlot can do.  Check it out now!

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Instructor Note: How Self-Paced Training Materials Can Support Instructor-Lead Classes

We offer products for both types of training - self-paced and instructor-lead.  Our CD-rom Courses and Self-study Manuals are designed for self-paced training while our Instructor's Curriculums are designed to help instructors teach live CNC classes.  We often receive questions from instructors regarding which method is best - or at least - which method we recommend based upon their particular situation.  In this short article, we provide some of our thoughts.  We'll describe how our materials are currently being used to make certain points.

Self-paced training

As the name implies, students complete this kind of training at a pace that suits their allotted time, schedule, and aptitude. What the name doesn't imply is that with self-paced training, the student is often left completely on their own to learn. In many cases, they will view videos, read, and do exercises and assignments without the help of an instructor. A facilitator (most commonly an
instructor) should be available to direct them and answer questions, but for the most part, the student learn on their own.

The main benefit to the student is that it allows them to study at times when it best suits them. They learn on their schedules, not a school's schedule. This, lets people fit their education into their busy lives. Frankly speaking, there aren't many more benefits to self-paced training that I can think of.

Students with good motivation and aptitude for the subject matter do well with self-paced training, and won't need much help as they go through the material. Unfortunately, people with lesser motivation and aptitude levels may struggle with self-paced training materials. Most materials - like books and videos, can only present the subject matter in one manner. It cannot "roll with the punches" when students have special needs.

Self-paced (also called "open in / open out") training has allowed schools to offer training in a subject matter even with a small number of students. And it has allowed schools to meet the needs of students that have limited time to spend at the school. Frankly speaking, some programs probably wouldn't exist if they couldn't be offered as self-paced training programs.

Self-paced training can be time consuming for the facilitator when many students are participating in the same course at the same time - and especially when they're having problems. The instructor may have to explain the same concepts over and over - in a one-on-one manner with students. This may not be the best use of an instructor's time.

Instructor-lead training

In this training environment, much of - if not all of - the material is presented by an instructor in a classroom setting. Students are commonly asked to do reading assignments and other exercises as homework, but the majority of learning occurs in the classroom.

The main benefit to this kind of training is that the student has the ability to question the instructor at the moment they don't understand an important point. They also hear other student questions, ensuring that they understand the concepts being presented. Based upon students' questions, the instructor gets immediate feedback - and can elaborate when students are having problems.

Our suggestions

Ideally, you should be able to offer both training options and apply whichever is best to each situation. For example, one instructor I know of - a technical/vocational high school teacher that also provides adult education classes - told me that he doesn't want the high school students to use the self-paced training materials. Through experience, he has learned that high school students just don't have the motivation to do well with self-paced training - at least not in his school. So he presents live classes with lectures to get the subject matter through to his high school students.

On the other hand, his adult education students do have the motivation to learn well from self-paced training materials - and he incorporates them into his adult education classes.

More specifically, this instructor is using our CNC curriculum materials to teach live classes for his high school students. He is using our CD-rom courses to provide the materials for his self-paced adult education classes.

How self-paced training materials can support live training classes

Combining the two training methods can provide the best of both training worlds. Instructor-lead classes will provide students with a good foundation. But some students may not be able to retain all important points in a classroom setting. If self-paced training materials are available that parallel the instructor's lectures, a student can easily review course material without having to bother the instructor.

In similar fashion, if a student misses a lecture, they'll have to make it up in one fashion or another. If an instructor is teaching the same class multiple times during a training period (seldom the case with manufacturing classes), the student can simply sit in on the lecture in another session. But if the class is only presented one time per semester, they will probably come to the instructor for help. This means a duplication of effort as the instructor explains the material a second time for this student.

Again, if self-paced training materials are available that parallel the live presentations an instructor makes in class (as is the case with our CNC curriculums and CD-rom courses), the student can view only the segment of the self-paced course that they missed.

Many of the schools using our CNC curriculums to teach live classes also have the matching CD-rom course. It's kept in the school library, the computer lab, or in a computer in the CNC lab. Since this course includes the same material as is presented in our CNC curriculums, students can easily learn and review from the CD-rom course when they need to.

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Manager's Insight: Calculating Machining Time For Any Machining Operation

It is often necessary for CNC people to determine how long machining operations will take to perform. You may be trying to determine which of two or more processes will be used to machine a workpiece - or you may just be wondering how long a machining operation will require to complete.

Frankly speaking, the formulae related to calculating machining time are pretty simple to understand and use. Indeed, many manufacturing people have incorporated them into spreadsheets (like Microsoft Excel) - or they have programmed their calculators to include the related formulae. Here is the most important formula:

• Time (minutes) = length of motion in inches divided by motion rate in inches per minute

That's it - no problem, right? You simply divide the length of the motion required for machining in inches by the inches per minute feedrate. The metric equivalent is:

• Time (minutes) = length of motion in millimeters divided by motion rate in millimeters per minute

We'll be using the inch mode for the rest of the discussions in this article.

Example 1:

Say you must drill a 1.0 inch diameter hole. The hole depth is 0.75 and you intend to use an approach distance of 0.1 inch. The intended feedrate is 7.0 inches per minute. When we divide the motion distance (0.85) by the feedrate (7.0), we find that the time needed to drill this hole is 0.12143 minutes.

How many seconds is this? We obviously need to be able to convert decimal minutes (0.12143) into seconds. Here's the formulae:

• 1 second = 0.01666 minutes

• Time in seconds = time in minutes divided by 0.01666

When we divide 0.12143 by 0.01666, the result is 7.2887 seconds (just over 7-1/4 seconds). So we now know how long it will take to drill the hole.

In order to use the formula, of course, you must be able to determine the feedrate in inches per minute (ipm). Most machining data handbooks, however, provide feedrate in inches per revolution (ipr), meaning you must first calculate the spindle rpm and then calculate the inches per minute feedrate. But speed recommendations are usually given in surface feet per minute (sfm). This speed is how much workpiece material will pass by each cutting edge during one minute. Here are two more formulae, based on speed being recommended in sfm and feedrate in ipr.

• rpm = 3.82 times sfm divided by diameter (the tool diameter in our case)

• ipm = rpm times ipr

Note that for some tools, the recommendation for feedrate will be in "per tooth" fashion, meaning you need to know the number of cutting edges (inserts, flutes, or teeth) there are on the cutting tool. This is commonly the case for milling operations. So we need to add yet one more formula:

• ipr = ipt times number of cutting edges

Example 2:

Say you need to determine how long it will take to rough mill a 3.0 inch long slot with a 0.75 diameter, four flute, cobalt end mill. The three inch motion distance is the total motion length, including feed-on and feed-off distances. Based upon the material you are machining and the kind of machining operation you are going to perform (rough milling), the end mill's manufacture recommends a speed of 90 sfm and a feedrate of 0.002 ipt.

• First, determine the speed in rpm: 3.82 times 90 divided by 0.75 is 458 rpm.

• Next determine the inches per revolution feedrate: 4 times 0.002 is 0.008 ipr.

• Next, determine the inches per minute feedrate: 458 times 0.008 is 3.664 ipm.

• Finally, determine the time required in minutes: 3 inches of motion divided by 3.664 ipm is 0.8187 minutes.

• To determine the number of seconds, divide 0.8187 by 0.01666 - this comes out to 49.141 seconds.

Fixed diameter machining versus changing-diameter machining

Note that it is quite easy to apply these formulae to machining center machining operations since the cutting tool diameter does not change during the machining operation. This is the case for the vast majority of cutting operations, including milling cutters, drills, taps, reamers, and just about any tool you use in a milling machine or CNC machining center. Again, the diameter being machined does not change during machining.

But do note that there are some operations during which the diameter being machined will change during the machining operation. Consider, for example, a rough turning operation on a CNC turning center that requires multiple passes to be made. The feature called constant surface speed will cause the spindle speed in rpm to change based upon the diameter being machined. For rough turning, this means you must calculate a new rpm and inches per minute feedrate for each rough turning pass.

Example 3:

Say you need to rough turn a 4.0 inch long diameter down from 1.0 inch to 0.75 inches, taking two passes (0.125 inch each). One of the passes will be at 0.875 and the other will be at 0.75. And each pass will be 4.1 inches long, including the approach. For the material being machined and the machining operation being performed, the cutting tool manufacturer recommends a speed of 400 sfm and a feedrate of 0.011 ipr.

Again each pass must be calculated separately. For the first pass:

• rpm = 3.82 times 400 divided by 0.875, or 1,746 rpm

• ipm = 0.011 times 1,746, or, 19.206 ipm

• time = 4.1 divided by 19.206, or 0.213 minutes (12.785 seconds)

For the second pass

• rpm = 3.82 times 400 divided by 0.75, or 2,037 rpm

• ipm = 0.011 times 2,037, or 22.407 ipm

• time = 4.1 divided by 22.407, or 0.182 minutes (10.924 seconds)

As you can see, the calculations are no more difficult to make - there are just more of them. One per roughing pass.

Calculating time for finish turning and boring operations done on a CNC turning center are also more complicated. To do it perfectly, you must treat each segment being machined separately. For this reason, many quoting people will try to come up with an "average" diameter on which to base the rpm calculation. This allows the to more quickly come up with a pretty accurate machining time.

Diameter changes while machining

There are even CNC turning center operations that require the diameter to change even while the cutting tool in engaged with the workpiece. The two most common are facing and necking operations (including cut-off operations). If constant surface speed is used (as it should be), the speed in rpm will accelerate as a facing tool moves toward the center of the workpiece. Again, most estimators will try to come up with an average diameter in order to quickly determine approximate machining time.

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G Code Primer: G41 Versus G42

G41 and G42 are used with both machining centers and turning centers for similar purposes.  With machining centers, the feature is called cutter radius compensation.  With turning centers, it is called tool nose radius compensation.  In both cases, one of G41 and G42 is used to specify which of two conditions exist related to how the cutting tool is related to the workpiece.  G41 specifies a "tool left" condition while G42 specifies a "tool right" condition.

While these G codes are commonly used, newcomers to CNC often have problems determining which one to use for a given application.  For machining centers, there is a pretty easy way to determine which one to use if you know the difference between climb and conventional milling.  You simply picture a right hand milling cutter (spindle running forward) as it machines the surface in question.  If the milling cutter is climb milling, G41 must be used to instate cutter radius compensation.  If the milling cutter is conventional milling, G42 is used to instate.

Unfortunately, there is no such thing as climb or conventional milling with turning operations, though I know of turning center programmers that will visualize the small radius of a turning tool or boring bar as a tiny milling cutter and use the technique just described to determine which G code to use (this works nicely!).  But if you don't know the difference between climb and conventional milling (commonly the case with lathe people), another way to distinguish between G41 and G42 must be found.

The technique we recommend is: Look in the direction the tool will be moving during its machining operation and ask yourself which side of the workpiece the tool is on.  If the tool is on the left side of the workpiece, a G41 will be used to instate.  If it is on the right side of the workpiece, a G42 will be used.

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