G-code question

[throbscottle]

I'm lining up my first ever couple of projects (well, parts of the same project) to cut on a very cheap "Woodpecker" engraver. One of the files is converted from .DXF created in a cad program (front panel designer) to g-code by importing into bcnc. All of the z movements are going from z3 to z0 and back. I think I should be seeing some negative z values for where it cuts. All this is still somewhat alien to me. Bcnc is set up with stock etc. Any idea what's going on? My best guess is fpd is set up to produce crippled files that won't cut ('cos they want me to buy their CNC service) but it just seems a bit improbable.

 

[Ollie78]

Not familiar with those particular programmes.
Do any of them provide a 3d preview at all, or even a 2d preview.
In v carve it gives you a demo cut, and uccnc will show a toolpath drawing.
You could download the free version of Fusion 360 which has full preview of cutting.
Or maybe easel is free or carbide 3d its shapeoko one or maybe xcarve.

Ollie

 

[throbscottle]

Bcnc (which actually writes to the CNC machine and provides gcode editing) gives previews from various x/y/z and iso angles. But it's quite clearly going between the safe height and surface. There's no difference in depth between a shallow engraving and a hole.
I'll see if I can try those other programs though. Might reveal something.

 

[worn thumbs]

Is it possible that the stock is set up with the underside at z0? It is quite possible for a machine to work in such a configuration.Can you advise us of the material thickness abd the retract height you were expecting?I've never seen or used bcnc but I can understand Gcode.A 3D view of the part plus toolpath would be a real help.

 

[Yojevol]

Have you set the z datum as the material top surface? Have you specified your required depth of cut
Brian

 

[throbscottle]

Ooh, I don't know. I with have to check these things!

 

[Ollie78]

Is it possible that the stock is set up with the underside at z0? It is quite possible for a machine to work in such a configuration.Can you advise us of the material thickness abd the retract height you were expecting?I've never seen or used bcnc but I can understand Gcode.A 3D view of the part plus toolpath would be a real help.

This is a good point, if Z zero is set to the bed rather than the top of the work piece it could cock it up.

 

[throbscottle]

I think I've figured it out! Nothing to do with z0 being in the wrong place.
It seems bcnc doesn't correctly insert cutting depth when importing .dxf files. Anyway I know what to edit now, I can add the depths in manually.
Thanks for the help!

 

[Yojevol]

2.48am This doesn't bode well
DXF is being recognised as a 2D drawing. Other formats are used for 3D, eg, SFX (I think that's one). Bcnc may recognise z dimensions in such a format but you would still have to input desired number & depth of cuts.
If you're cutting right through your material make sure you've got a spacer underneath to avoid damaging the table. I expect a warning will be thrown up if that is a possibility.
Brian

 

[throbscottle]

Yes, we have got into very bad habits at my house!
That would explain everything though. I just assumed the dxf file would include the cut depths, I ddn't realise it's just a 2d format.
Don't worry, it's got piece of ply on there!
Just had a quick look at it vcarve - same result, nothing showing in the 3d view.

 

[throbscottle]

Ah, found what I'm supposed to do. There is a "cut" section that lets you set up the options for each cut. So I don't need to edit every block.. phew!

 

[worn thumbs]

If you have Vcarve,can you just use bcnc to transmit the Gcode?All you need to do is locate a compatible post processor from the large selection and the 3D visualisation in Vcarve will show you what the machine will do.Always run the visualisation until you reach the point where you can confidently predict that you will get the correct outcome-and even then you will get the occasional nasty surprise.

 

[throbscottle]

Bcnc has all that, I think it does everything. What I hadn't realised is that the DXF file doesn't contain any actual cutting information - because it was generated by a program which is essentially an order generator for a commercial CNC company, I had assumed it would contain everything. But it doesn't, it's just a line drawing. I keep using fpd though because it's pretty good for what I want.

So my workflow is now, open the file in bcnc, select all the objects with the same cut either in the GUI or code editor, switch to the CAM tab, set the profile for inside/outside/line/pocket etc, which cutter to use, then set the actual cut parameters like feed rate, number of passes etc, check what it will look like (I get 3x 3d views and 3x 2d views) as you say, go back to the code editor and optimise the cutting sequence and add my tool change and any other bits I don t know about yet.

I was bamboozled at first because I could see how to set the parameters, but not how to apply them. When you know, you know. Not knowing what I should be expecting to happen didn't help.

Then I can switch to the forget-what-its-called tab, set up the machine and actually run the job. Not bad considering I didn't know any of this last week!

So my first trial run was going really well on a piece of scrap acrylic, until the computer went to sleep, had to reconnect, re-home, delete everything it had already done and run the job again. The tool change is something I'm going to need to learn to do properly though. I know what's supposed to happen but it seems a bit of a black art! Anyway I bodged around it, then found just how wrong I'd got the parameters for the cut outs. More farting around resetting, deleting, redoing...

Then I found that my cheapie Woodpecker, while in principle it's a good idea to have a spindle coupler instead of a chuck if you only ever have 1 size of shank, well in practice it's a little large. Engraving it will do, but cutting goes wrong. So I've now got a collet chuck on order from AliExpress.

Well anyway sorry for the long rant, I'm just so pleased I've got this far with it!

 

[Yojevol]

Bcnc has all that, I think it does everything. What I hadn't realised is that the DXF file doesn't contain any actual cutting information - because it was generated by a program which is essentially an order generator for a commercial CNC company, I had assumed it would contain everything. But it doesn't, it's just a line drawing. I keep using fpd though because it's pretty good for what I want.

So my workflow is now, open the file in bcnc, select all the objects with the same cut either in the GUI or code editor, switch to the CAM tab, set the profile for inside/outside/line/pocket etc, which cutter to use, then set the actual cut parameters like feed rate, number of passes etc, check what it will look like (I get 3x 3d views and 3x 2d views) as you say, go back to the code editor and optimise the cutting sequence and add my tool change and any other bits I don t know about yet.

I was bamboozled at first because I could see how to set the parameters, but not how to apply them. When you know, you know. Not knowing what I should be expecting to happen didn't help.

Then I can switch to the forget-what-its-called tab, set up the machine and actually run the job. Not bad considering I didn't know any of this last week!

So my first trial run was going really well on a piece of scrap acrylic, until the computer went to sleep, had to reconnect, re-home, delete everything it had already done and run the job again. The tool change is something I'm going to need to learn to do properly though. I know what's supposed to happen but it seems a bit of a black art! Anyway I bodged around it, then found just how wrong I'd got the parameters for the cut outs. More farting around resetting, deleting, redoing...

Then I found that my cheapie Woodpecker, while in principle it's a good idea to have a spindle coupler instead of a chuck if you only ever have 1 size of shank, well in practice it's a little large. Engraving it will do, but cutting goes wrong. So I've now got a collet chuck on order from AliExpress.

Well anyway sorry for the long rant, I'm just so pleased I've got this far with it!

Well done, the learning curve is steep at this stage.
My CAM system is 20 years old and has a very rudimentary simulation capability so I simulate the cut by running an 'air' cut, ie, set the z datum a few mm above the workpiece and set it going.
Brian

 

[worn thumbs]

I agree that much progress has been made in a short time. I believe a lot more could be made if you seek out a couple of good youtube tutorials for Vcarve.It is quite easy to create the entire programme and all you have to do with bcnc is establish the part datum and send the file.

You may need to add the tools you have to the tool library and having done so,the speeds and feeds will stay there.You also get access to things like ramp entry and tabs for small parts.It will even locate joints missing from the boundary of shapes and help to fix them.Then you can run a 3D visualisation to see what will happen.We won't even mention nesting for now.

At the current rate of progress,you could be a superstar machinist by the end of the month.

 

[throbscottle]

In about 1985 I was shown how to use CNC as part of an engineering course. BBC micro ran a desktop lathe - we made a chess pawn I think. Pretty sure we had to enter the gcode directly via the keyboard! So we learnt how it's all coordinates. Never touched it since until now.
Thanks for the confidence, but I don't think I'll ever be a "superstar machinist"!
I did download V Carve last week. Took one look and decided to persevere with what little I knew about what I'd got. I'll revisit it now I know a bit more.
Then reflash and possibly modify the Woodpecker's grbl board and see if I can get some spindle speed control. Scary prospect, but it's either full speed or off at the moment, which is quite annoying. Extra circuit board will be needed...