Member postings for john constable

Posted by john constable on 14/07/2018 20:33:59:

Thanks, for all that Martin. It's been very useful. It's also my first look at real g-code!

Posted by Neil Wyatt on 14/07/2018 20:40:40:

You can disable the temperature settings. Many printers don't have heated beds, and these rigs are often used to make dual head printer/router machines.

Neil

Posted by Martin Connelly on 14/07/2018 18:41:02:

The problem of low torque stepper motors is if they get a small kick when a tool cuts a shaving or bit of swarf they can miss steps. Once out of position they keep on merrily moving things around in the wrong position until there is human intervention or the program stops having produced something that is not quite right. You don't get these types of force with 3D printers, there is no high speed tool spinning around.The other thing is you may be underestimating the amount of force needed to push a work piece and tool together for the cutting action to take place. These motors when used with 3D printers are often driving fine threaded studding which gives some mechanical advantage. Is this what you were thinking of using instead of ball screws? They are not used to push a cutting tool into a work piece either. The Gcode for what you want is trivial don't sweat over it but do get suitable equipment for what you propose otherwise you could be wasting money.

Gcode for a barley twist

G01 X0 Y0 Z0 A0 F50 (Move to starting position. Tool over the centre of the work piece at one end and just touching the work piece. Use A=180 for a second start 180° around from the first and add 180 to the A value below ie 360+180=540)

G01 Z-1 F25 (Move the tool down 1mm into the work piece)

G01 X250 A360 F50 (Move the tool 250mm along the work piece whilst rotating the work piece 360°. Change 250mm and 360° to suit your requirements.

G01 Z2 (Lift the tool away from the work piece)

G0 X0 A0 (Rapid move back to X=0

G01 Z-2 F25 (Lower the tool 1mm more than before to do a second cut)

Repeat with decreasing Z until you have got to the depth you want.

Martin C

Posted by XD 351 on 14/07/2018 11:20:16:

Wouldn't it be easier to run a proper cnc controller like mach3 ? It would be seamless with something like fusion 360 cad/ cam which you can get for nothing on startup .

You can buy cheap 4 axis engraver / router units that just need to be connected to a pc running mach3 that may do what you want

I would build the electronics to be able to handle much more powerful motors - even if you don't need them now you may in the future . Nema 17 motors and the ramps controllers/ drivers are pretty gutless at around 2 amps max .

Posted by SillyOldDuffer on 14/07/2018 10:09:50:

"So GRBL is a piece of software - like an OS - that runs on the arduino board and controls the controller board" - exactly right. It translates G-code commands into the motor steps needed to move a tool or print head to an X,Y,Z coordinate.

grbl will respond to commands typed into a character terminal, this is the best way to test your configuration to prove the motors respond correctly to simple commands. For production work that simplicity gets tedious, hence the GUIs. They do the same job but - once properly set up - are slicker, more helpful and whatever else the designer has thought of.

What the GUI's do varies a bit, but there look to be three levels of sophistication:

• An improved version of a character terminal, tailored to support 3D, with easier connection.
• A 'sender', allowing lists of g-code commands to be sent from files, providing access to folders, with buttons allowing grbl to be reset, jog, or otherwise positioned.
• A 'sender with previewer', these can display the tool-path on screen, allowing the operator to see obvious mistakes before sending them to the machine.

A picture's worth a thousand words, this example is grblgui. (I've not used any of them and can't comment on installation, ease of use, or bugs.)

Do the GUIs generate code from instructions you give it or do they convert from a CAD image?

As far as I can see none of the free GUIs convert from CAD, you either type the code in by hand, or load it from a file. The file is usually created by something else; either a conversion program, or by the drawing package itself. Jason's post shows how Fusion360 does it.

A superficial read suggests OpenSCAD can't produce g-code directly, rather users export the model as a DXF file and run that through a DXF to G-code converter. At this stage it's worth mentioning a potential booby trap! The g-code files needed to print a 3d object are additive, that is the object is built inside-out by depositing layers until the shape is finished. The g-code instructions needed to mill the same 3d object are subtractive, that is the tool removes material from a solid block until the shape is finished, outside in. grbl doesn't care, but it's possible that software written specifically for 3D printing might not support milling and vice-versa. Your lathe is subtractive and the g-code you send to grbl needs to be that way round. It may be necessary to try a few alternatives to find what suits you best.

Dave

Dave

Posted by Martin Connelly on 13/07/2018 11:23:51:

It shouldn't be too hard to spoof a few signals to use it as is, my biggest worry would be the torque available from the motors. 3D printing doesn't put much load on a motor so small holding torques don't cause a problem. You may need to up the holding torque by either replacing some of the motors with something beefier or by fitting a planetary gearbox on some of the stepper motors. You will only find out if you need more torque when you have got it all set up and start to cut something. I would work on the basis that as the spindle will not need high rpm for your application there would be no problem putting a 10:1 ratio gearbox on the spindle motor from the off. It may be cheaper than a bigger stepper with its own power supply and driver unit. These motors are 40 N.cm which is the same as 0.4Nm. Since people converting mills and lathes to CNC will be looking at steppers or servos with holding torque up to something like 20Nm you can se where my concern is coming from.

Martin C

Posted by SillyOldDuffer on 12/07/2018 22:34:58:

Looks like a good way to start John. It already understands G-code (grbl). It also solves the user interface design issues I was wittering about in the other thread because all that is done on a PC. The Arduino gets its instructions off an SD-Card, which keeps that side simple.

My only tiny worry would be unwanted side-effects because it's a 3D printer driver. For instance the firmware might make a fuss if the bed & extrusion heating stuff is left disconnected - the motors might wait forever until a non-existent printing part was up to temperature. Shouldn't be a showstopper - I think it's all open source and hackable.

Edited By SillyOldDuffer on 12/07/2018 22:35:40

 

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