LinuxCNC - BBB - STEP-NC project

Hi

I just started working on a project where we are trying to build a 3 or 4 axis CNC controller to do research with for my university. I'm currently working on picking out electronics for this controller. Although we will not be using LinuxCNC (except maybe for testing) we will be using a Linux based machine. The project will be open source. The scope of the project entails creating a program that reads STEP-NC files and generates G-Code on the fly. Thankfully, I'm not a computer science wizard and I will not be as involved with the coding. :laugh:

Our initial plan is to use a BBB to run the software and communicate with the electronics. We have settled on steppers over servos because of the cost and complexity. We aren't trying to remove large amounts of material quickly so there was no need for high speed torque. We are trying to do HSM however, so we will need quick communication and processing between the software, electronics, and motors. Whatever electronics we use must be expandable to accept connections from various sensors. We will be adding sensors to the machine to feed into the software to update things like feeds and speeds.

The budget for the controller and machine is $8.5k USD. We are most likely going to buy a used stepper based CNC machine and remove its brain. We will probably be left with $2-3k for the controller.

My question is what should I use for electronics and what all do I need? We want to use a BBB or possibly a PC to run the software, and probably something nicely packaged like a Geckodrive for the motor drivers. What goes between the BBB and the motor drivers? What kind of processing is done by that board?

Thanks!
After some progress has been made on the project I will post again with cool pictures and updates.

This is pretty off topic as you say your not going to use LinuxCNC... the road will be hard trying to fit things together that don't fit. Go to MachineKit for BBB or something like that.

If it was me (I've converted several machines) I'd retain the steppers or servos and drives from the machine if they work and use LinuxCNC with a PC and Mesa 5i25 with 7i76 or 7i77 cards and be done with it.

JT

Ok, thank you for the information. Feel free to delete this post or move it. I just knew you guys know your stuff

all you ask has already been done !( apart from the step-nc conversion ) and i believe that has been done , although a long time ago , but please don't use toys with real cnc machines (BBB) , they will bite you and they dont cut it , or have the onboard resources if you pardon the pun , you'll spend more time on the software trying to resolve issues
than you will using the machine , especially with the budget you have , it would be a real waste of resources , you could do so much better .

BishopCNC15 wrote: The scope of the project entails creating a program that reads STEP-NC files and generates G-Code on the fly.

This seems silly. Why not skip the G-code phase and go direct from STEP-NC to motion control?

In theory you could replace the LinuxCNC G-code intepreter with a STEP-NC interpreter. The changes to do this are already included in the Machinekit project, I believe.

andypugh wrote:

BishopCNC15 wrote: The scope of the project entails creating a program that reads STEP-NC files and generates G-Code on the fly.

This seems silly. Why not skip the G-code phase and go direct from STEP-NC to motion control?

In theory you could replace the LinuxCNC G-code intepreter with a STEP-NC interpreter. The changes to do this are already included in the Machinekit project, I believe.

That's just what I assume happens inside the brain. You're right it's probably easier to cut out the G-Code entirely.

I looked into the Mesa Electronics boards. So if I use a 5i25 and 7i76 then my PC will still be communicating with my drivers with step+direction right? I've heard there's a bandwidth limitation doing things like this. Since the budget is fairly high, what about EtherCat or something similar?

The system will still need to run linux, so if it can run LinuxCNC and a machine that will be a good starting point for us.

Thanks

just sent you a message directly .

Their was an attempt at iso-14649 as an interpreter for what was then EMC2 back in 2008 , by Tom Kramer
here's a link , although you may need to modify to work with the current versions of Linuxcnc or Machinekit

references:
code.google.com/p/iso-14649-toolkit/
www.linuxcnc.org/index.php/english/forum...iso-14649-ap2-stepnc

BishopCNC15 wrote: I looked into the Mesa Electronics boards. So if I use a 5i25 and 7i76 then my PC will still be communicating with my drivers with step+direction right? I've heard there's a bandwidth limitation doing things like this.

With the 7i76, which is a dedicated step/dir board, yes. But there are many other options. The 7i77, for example, is designed to drive servos by analogue voltage command. Old-school but very effective inside a feedback loop.

My own mill, as a random alternative example uses analogue position feedback (Resolvers) into a Mesa 7i49 card, but the servo drives are 8i20 which take phase angle and current commands in a serial binary format.

Since the budget is fairly high, what about EtherCat or something similar?

That is certainly an option, and one that has worked well for others.

You could also look at General Mechatronics for a modular system with good LinuxCNC support.

You've stated that you are using steppers to avoid cost and complexity, but you also say that you want to do HSM.

As far as I can tell, High Speed Machining involves high speeds and high accelerations. Cutting forces are relatively low because of small tool diameters and high spindle speeds, but acceleration forces are high.

I would go with servos if I were you.

Ken

lerman wrote: As far as I can tell, High Speed Machining involves high speeds and high accelerations. Cutting forces are relatively low because of small tool diameters and high spindle speeds, but acceleration forces are high

I used to think that, then watched this video with a stepper-controlled Tormach mill. (jump to the 4 minute position)