Lot of Basic Questions from a Newbie

[mvadu]

Hi I am a software engineer by profession. I also studied some electronics in my graduation. I can do lot of stuff in a Comp, few stuff with Microchip PICs, but mechanics is a new world for me .

Only Last week I got introduced to fascinating world of Home Made CNC by visiting http://www.mr-bean.co.uk. Though first I though “It’s not my cup of tea”, after visiting http://www.cnconabudget.com and http://www.crankorgan.com/brute.htm I though of building one on the same design. But I have some basic questions which you senior people need to answer.. be patient to read till end..:

1. Since a stepper motor is Open Loop motor, how computer (or the CNC controlling software) knows that a command is successfully completed by the machine? For example if the controller asks the CNC machine to move the spindle down to drill 0.5” deep hole how it knows that its finished? What is stepper stalls because the job material is very hard?

2. In all the designs I have visited I could see a limit switch and a home switch but none say anything about the wiring part. If steppers are controlled by stepper motor controller where these switches will go? What happens when these switches are closed? If it simple stops the stepper from rotating further or it even informs the computer that the job table reached the extreme end? If it only stops then what happens if I try to mill a job of say 8” (in X axis) but my machine supports only 6” movement, so when the x axis tries to move behind 6” it closes limit switch. What will be the response from the controller?

3. As per my understanding movement of the table along the leaner guide depends on pitch of the guide screw. And its thickness as well. So the distance the table moves/revolution of stepper is a variable. How does that taken in to account? Is it at the controller hardware or we have some thing to configure in the CNC software?


Don’t blame me for such a long set of questions.. be patient once I get answers for these I have few more… actually for the last few days I am getting dreams of my own CNC, al that I could see is a PCB milled by my machine.. but I was so happy in that dream that I could not notice much info about the machine so I have to ask you guy’s only..


Note: Mods.. I am not very sure whether this the right place to ask these questions, if not then please move this thread to proper location (other than Trash Bin )

[in2steam]

First off welcome, cnc is an intersting place between the realms of mechincs and electronics. Your questions are none that have not been answered before I suggest in the future before you ask them that you use the forum search functions as there many areas to cover. But I will answer them to the best of my knowledge and what you are planning on doing.

1. The control has no way to know if the stepper has not completed a move, you answered the question yourself its "open loop". This is the biggest difference between servos and steppers, on most low cost low power systems such as taigs and sherlines steppers are more then sufficent. The stepper control has no way to know if the lead screws broke either so you are reading into the issue more then needed I think. This other then the intial setup is not a normal worry if you run the machine in its normal parameters.
2. This depends upon what kind of control, interface board, and software you are using. Some setups the the limits and home switchs are not even needed but you should IMHO have them for saftey reasons. The software will almost always stop you from exceeding the limits of the table. The wiring could be done directly to the control board in a couple cases but most often the wiring is attached to a break out board. You may wire a limit switch to the control board in such a way that it shuts it down if the "hard limit" is exceeded.
3. The control software takes care of the lead on you machine, in can be fine tunned with measuring insturments while setting up. The motor control strictly controls the motor and depends upon the software to preform its task. The thickness of the lead(or axis) does not effect the movement only the teeth per inch/mm.
You may want to vist artsoft they make Mach 2/3 series of cnc control software and are excepted as one of the leaders in the homeshop area. They have alot information on there product and cnc in general in there instructions.

chris

[mvadu]

First off welcome, cnc is an intersting place between the realms of mechincs and electronics. Your questions are none that have not been answered before I suggest in the future before you ask them that you use the forum search functions as there many areas to cover.

Thanks for the reply Chris.. You are helpful. I know that there is a search facility, but i was not sure on what to search.. (Those were little big questions you know)

By looking at your reply it seems that every thing is handled by the CNC software running on the PC. (The problem with me is I have not yet got my hands on such software.. so bit confused)
Do you mean CNC controller hardware will not send any feedbacks to the system with respect to machine health, limit switch closing etc.

Actually I am planning to do my own Stepper controller, if it need not to send ant thing back to system then its job will be just to turn the stepper in desired direction for desired time as instructed by the software through Parallel port. Am I right here

If I am right then why some designs use PICs in controllers when every thing can be driven by Parallel port only

[epineh]

Mvadu, the electronics on the machine will send signals back to the controller if a limit or homing switch is activated, usually a breakout board is used to terminate the limit and homing switches, also can provide terminals to send signals to relays to turn on the spindle, dust extraction etc from the controller PC.

The steppers on the other hand do not send any feedback back to the controller, and the method is to make sure the steppers are not loaded to the point of stalling and move where they are supposed to, it might not sound the best but it works quite well so long as the motors are sized properly.

Servo's do have feedback but the added complexity makes them more expensive , though drives are getting cheaper and appearing more and more in hobby machines.

Not too sure what you mean by your last question, usually the drives with micro's on them are servo drives, that need a little more"brains" to work, these might be what you were talking about ?

Russell.

[mvadu]

Not too sure what you mean by your last question, usually the drives with micro's on them are servo drives, that need a little more"brains" to work, these might be what you were talking about ?

Russell.

Thanks Russell,

Actually i got similar info about limit switches few minutes back from here http://www.solsylva.com/cnc/limits.html.

My last question is not about servo controllers. (actually I know the different type of motors and difference between Servo and a Stepper, I am a graduate in Electronics, Physics and Electronics )

I am actually referring to this schematic http://www.fromorbit.com/projects/pi...4_0_schema.pdf. This one is used by Mr-Bean of UK (http://www.mr-bean.co.uk/step_drive/p1/index.html). This circuit uses LMD18245T, and when i compared both the schematic and data sheet of LMD18245T it seems PIC is used as a source for current controller of LMD18245T.
can any one confirm it?

This guy has got a simpler servo controller as well..

[in2steam]

Yes you are correct most of the handling is done vis the software on the smaller home machines. This replaces in what the past used to be hardware controlled on larger older machines.

Typically a stepper control used in these systems sends a step and direction signal and thats it no feedback. Russel slightly misstates that the machine electronics will send a signal back if a limit switch changes state whereas the signal normally goes directly from the switch to the BoB(break out board) or the computer parallel port, it really depends upon the equipment used.

The use of a BoB prevents serious damage to your PC should there be an accident or a short as the stepper drives are high power, its possible to drive the systems directly through the parallel port however I don't recommend it, it also decreases safety and flexibility. These boards allow you to use otherwise pins that would not be usable with direct hook ups. You need to remember in a machine environment that metal chips and coolant get everywhere so you stand a very good chance of shorting something out.

I recommend you check these sites:
This is the mach controller software with detailed instructions and download able demo so you can get your hands on some software. Their instructions will answer many of your questions as they cover not only there software but also machine design in general and BoB's

www.machsupport.com

Gecko manufactures stepper and servo drives they are considered one of the best.

www.geckodrive.com

Chris

[epineh]

Russel slightly misstates that the machine electronics will send a signal back if a limit switch changes state whereas the signal normally goes directly from the switch to the BoB(break out board) or the computer parallel port, it really depends upon the equipment used.

Chris

It was late, and I was including the BoB as a part of the electronics... that is my excuse and I am sticking to it

Also check out EMC2 for a controller, it is Linux (Ubuntu) based and FREE !!!
It is also very good, you could use it to build a servo drive simply as it will read the encoder pulses directly and calculate PID values in the PC, all you need is a "dumb" H-bridge driver stage and you are in business... well for that side of things anyway

There is also support for using FPGA based cards to get extra IO and functionality, I am thinking of getting either a MESA electronics 5i20 anything IO card or a Pluto-p card to retrofit my router, the Pluto-p is a parallel based card while the MESA is a PCI card, might be worth a look for you.

Russell.

[mvadu]

I recommend you check these sites:
This is the mach controller software with detailed instructions and download able demo so you can get your hands on some software. Their instructions will answer many of your questions as they cover not only there software but also machine design in general and BoB's

www.machsupport.com

Gecko manufactures stepper and servo drives they are considered one of the best.

www.geckodrive.com

Chris

Thanks Chris.. for the links. earlier also you had given ArtSoft links, but i never went in to Mach docs, I was looking for some files with a more generic names, Only after this message that i went and opened Mach-3 user guide, its detailed beautiful document. Thanks again. I will spend some time reading this before I come back to you..
I must say.. You guys are really helpful.. I am looking forward for your help till I have my own CNC running on my table.

[epineh]

Make sure you post a build log for to see with lots of pics

Russell.

[mvadu]

Ya.. Man.. I think this weekend I will go out to hunt for acme threaded rods for guide screw. The one's i saw on e-Bay or way too costly (more than 100$ which means more than 4000 Indian rupees, which is nearly 15-20% of my monthly income.. I f I spend that much for a 15" steel rod my wife is gonna kill me )
For sides and base i am planning to use Plastic boards (the one used for veg/meat chopping).for leaner guide i will use small ball bearings bolted to the plastic sides rolling over bottom plastic.(like X-Axis bearing will run over Y-Axis side walls) Lot of plans in mind.. hope to finish... God.. please help me out..

[epineh]

I just used threaded rod for my leadscrew, I managed to find high tensile for the Y and Z axis, but couldn't get a piece long enough for the X, so I just used plain old threaded rod. I ran a die nut up and down a few times to take off any sharp bits and it works like a charm

I might eventually upgrade to ballscrew's if I can find some cheap enough but maybe later.

My build log is here if it is any help to you :

My Steel Frame Router

I also got heaps of HDPE plastic, same kind of stuff as breadboards, but this stuff is black, about 10mm thick, got it from a plastic manufacturing shop, they make all kinds of stuff with it, water tanks etc. I got heaps of offcuts for next to no cost

Good luck !

Russell.