CNC 101 "step by step"

[FLtenwheeler]

I know nothing about CNC and need an explanation on it. Are the following steps correct or am I making it to complicated.

First: you need a CAD program to make a drawing. Any CAD program that can export a DXF file will work. Correct?

Second: you need a CAM program to convert the CAD drawing into G code.
Does this program talk to the machine or do you need a driver program?

Third: you need motor drivers (little black boxes) for the type of motors you are using. Stepper or Servo.
Am I right in that all servomotors have a feed back function? Closed loop.

Fourth: you need motors that will provide the motion on whatever type of machine you are using.

In a 2½-axis machine: you have full control of the X and Y but only 2 positions on the Z. Mechanical stop out and mechanical stop in. Is that correct?

[GlennW]

CAD certainly saves on Trig calculations.
Cam is not a requirement. Push the buttons to program it!
Don't need DXF as DWG will work if not usig CAM.
2 1/2 axis is controlled X,Y with manual Z + or -.

[Bill Shields]

right on the CAD to generate the trig.

DXF is OK as long as the CAM package you have supports the DXF that you are creating for import. There are many flavors of DXF, and AutoDESK (DXF is their trademark) continues to enhance.

Basic lines and arcs are all that is needed, so you can 99% of the time strip the drawing down to just that and export / import it to a CAM system.
Many basic CAM systems also do more than adequate basci CAD, so you might not need to go 2 ways....

SOME CAD / CAM programs include communications software, some do not...varies with manufacturer. Since most modern controls are coming with ethernet connections AND because the CAD/CAM software people don't want to be involved with supporting communications nightmares, many of the programming packages have dropped RS-232 support (for sanity maintenance if nothing else).

if you have a machine that does RS232 communications, I have a software package that will talk to anything...after you get the cables correct.

You need more than motors, you need a box to read code and control the motors.

Servos, buy themselves are not closed loop, they REQUIRE closing of the loop...either by encoders on the motor/ball-screw or with a glass scale on the actual table (axis) being controlled. Some servis have the encoders built into the motor, some do not...but the closed loop function is NOT an integral part of servo motors, they are added on.

Steppers ASSUME that a specific number of commanded steps = a certain table distance...but if there is any backlash, you don't know where you are...OR if there is a big load and the motor doesn't actually STEP when told to...again, you don't know where you are.

Steppers can be closed loop, either with an encoder on the shaft (doesn't account for backlash) or with a glass scale on the axis (accomodats all manner of errors)....


correct on the 2-1/2 axis...MANUAL Z for the up / down. typically these things move to a point then STOP and wait for the operator to feed up / down the push the GO button.

They save some $$...or at least you think you are...until you realize that you cannot run them without standing there to move the tool up and down.

If you are planning on starting a program and letting the machine run until a tool change is required, you will be dissapointed..2-1/2 axis machines require constant baby-sitting.

Don't get me wrong, they are much better than spinning dials, but if you have the $$ available, spend it on the 3rd axis and be VERY HAPPY that you made a good choice.

My retrofit Bridgeport Series II came with steppers on X Y and Z (knee) and a pneumatic cylinder on the quill. You could set stops to make the quill go up and down to a specific point, or move the knee up / down by code control.

Whole thing went in the dumpster and a 3 axis control went on.

STILL, it doesn't have a tool changer, but I can get away from it for quite a while between tool changes if I am creative with my programming.

Also, I personally wouldn't have a machine without a closed loop system to tell the control where the table actually is. Steppers today are as good a servos (but not better)....but are only as good as the ASSUMED table position without the feedback.

Of course then there were the MOOG machines that had HYDRAULIC motion on the tables with positive feedback.

When they were right, they were OK. When they needed adjustment, they were nightmares....don't see too many of them anymore for just that reason.

[FLtenwheeler]

Hi Bill

Let me take it one step at a time.

Software

Cad to do the drawing.

Cam to convert the drawing.

Communications software to talk to the motor drivers. (black box)

These could be 3 separate programs or all bundled into one.

Correct?

Tim

[toastydeath]

I do not use CAD or CAM for my machining at work. I sit an the machine and program g-code at the tool, except for parts where it isn't economical. If you are doing fancy shapes, then yes, CAD/CAM is required.

The CAM software (or, in my opinion, you) generates a G-code program.

Then, you transfer it over to the control (or if you were making the program, it's already in the control if you programmed it at the machine). This is the computer attached to the machine that interprets the g-code and makes everything on the machine run. It is NOT a simple program. DIY control solutions include EMC2 and Mach - these run on your average desktop PC. Commercial contol solutions include Fanuc, Yasnac, Mitsubishi, etc. These are dedicated hardware built specifically to control machine tools.

The CNC then interprets the g-code into what it needs to do, and commands the servos or stepper motors to drive the axes and spindle, ultimately making the part.

Regarding the "three in one," you could do that with a "DIY" solution, but not with a commercial CNC machine tool or control. Doing three in one only makes sense to the hobbyist market.

[GlennW]

Perhaps if you were to disclose your intentions such as looking to purchase a machine, build a machine, retrofit a machine, etc. it would be somewhat easier to find answers to your questions in the proper context.

Glenn

[Bill Shields]

Tim:

Rather than guess or tell you what I or others do, I will answer your questions specifically as asked:

Yes, it could be all 3 in one bundle. Mastercam (for example) has all 3 (or did at the time I was writing code that they were using)....

I have (not currently) worked for a company that does CAD/CAM but will NOT (understandably) get into the communications business because it is such a nightmare. They would spend more time supporting communications than their software....

There are packages out there that are all 3 in one and capable of doing what you want, but I am not in a position to supply names of companies (don't think we should here anyway)

I have the comms package that you can have for free if you find something that does CAD/CAM but you need RS232 to communicate.

You can feel free to contact me off-line if it would be more beneficial to get right to the crux of the matter.

[dalee]

Hi,

One thing that I would like to correct in this thread. A 2 1/2 Axis machine isn't what has been described. If one of the axis needs to be manually controlled, it would be a two axis machine.

A 2 1/2 axis has all three axis under machine control, but only two of the axis can move simultaneously. Generally that would be x and y. With the z axis programed to move separately. But by issuing a G18 (xz), G19 (yz), or G17 (xy) the plane of movement can be "flipped" to provide easier programing. (Specific G-code may vary with software). While this seems like 3 axis control, it really isn't. The two active axis must pause and wait for the third axis to move before continuing on. Hence the "1/2" axis.

A 3 axis control would allow for all three axis to move simultaneously. And is pretty much the norm for PC control software today.

dalee

[Bill Shields]

A 2 axis machine is one where ONLY X and Y are recognized.

The industry (for whatever reason), has decided that a 2-1/2 axis machine is one where you can program in Z (for a mill) and every time a Z move is encountered, the control automagically stops and waits for the operator to do something before he pushes the GO button again.

The Z code, while not acted upon, provides the operator with a prompt for 'where to put it'. Some controls actually allow you to install a glass scale on that axis and will READ OUT the Z position - even though it isn't controlled...hence the 1/2 axis...

I know, it's weird, but that is how it is applied, accurate or not....

I THINK it was South West Industries (or something like that) that started it that way in the mid to laet 1980's

[Mike_Henry]

The industry (for whatever reason), has decided that a 2-1/2 axis machine is one where you can program in Z (for a mill) and every time a Z move is encountered, the control automagically stops and waits for the operator to do something before he pushes the GO button again.

I'd thought that 2-1/2D was defined as machining with simultaneous moves along only two axes at a fixed position for the 3rd axis. Do I have that wrong?

Mike

[Bill Shields]

You are correct from that standpoint...but you have to stand on the side of the table with the guys selling the 2-1/2D controls, who want to appear to be selling more than just a 2 axis control....so they call it 2-1/2 because it RECOGNIZES the 3rd axis...I know it seems whacky, but that is the way the industry seems to define this casel

Besides...any control that can move 3 axes can do 3D tool motions (at least I don't know of one that cannot).

Whether or not the program calls for ony 2 simultaneous axes is immaterial, it IS a 3D control. Even something as silly as ramping into a slot is 3D motion...

Point in fact, 3D contouring motion is easier to implement in a control than 2D with cutter compensation, and were I a control manufacturer....

Stop and think about it....with 2D contouring, you have to worry about all the possible cutter compensation cases (there are 14 of them that FANUC worries about IIRC).

With 3D, you just go point to point to point...the programming software does all the calculations for where the center of the tool needs to be.

All the control has to worry about is handling the massive program size..which is actually quite easy.

[geraldvowles]

Hi Bill

Let me take it one step at a time.

Software

Cad to do the drawing.

Cam to convert the drawing.

Communications software to talk to the motor drivers. (black box)

These could be 3 separate programs or all bundled into one.

Correct?

Tim,


IF you want to do your own drawings, the thing to keep in mind is that there is a VERY BIG learning curve increase going from 2D to 3D.

It seems to me that a lot of the woodworking and signmaking people use programs like those from Vectric. www.vectric.com that lean towards your buying pre-designed templates. Whereas, the metal guys lean more toward programs that give them much more control, although wizards & plug-ins are getting more common.

That's the way I understand it. Three programs are needed. as you outlined. It may be that there are some that bundle 2 or even 3 but you need each capability. An example of a controller program that is low cost but very highly regarded and well supported is Mach from Artsoft. You can actually use the demo for as long as you want for nothing. It does have a quite basic CAM plug-in called LazyCam included although the $75 Pro version offers more capabilities. I'm sure some of the high-end users will sneer but many claim it's the best, least threatening way to get your feet wet. - Stick with the locked down versions rather than the more bleeding edge beta versions. That's what I'm going to do anyway after months of digging. I imagine I'll then move up to a stronger CAM program once I know I'm going to stick with it.

Hope this helps a bit,
- Gerry


Tim