I had a little accident with the cnc module today, and I figure I would share it with the forum just to show that things can go wrong and it’s not the best idea to leave the device unattended for extensive periods of time. Anyway, in stead of only sharing success stories, this time it’s an episode of “Failure Sunday”
Through my octoprint camera feed (which I continuously record through the home surveillance camera system) I was able to capture it going wrong. Quality is terrible, the video angle as well (I still intend to build my own enclosure and have it at a better angle).
At about 40 seconds in the video it starts to go wrong. A little triangle breaks loose, gets on to the spindle and everything just goes south.
video:
(reuploaded the video to youtube, onedrive didn’t seem to be able get the preview working)
Luckily I found it a few minutes after this (because I was occasionally checking the video feed) and it didn’t end up too bad. By the time I found it the spindle wasn’t turning anymore unfortunately, but it did came back to life after a reboot of the machine. And using an rpm counter it still spins at 12000rpm. I did a tiny little cut with it and it did seem to be doing it’s job. But will require some more testing and validation that it actually still works the way it should.
Important lesson: things can go wrong (and some point will, no matter how careful you are) Just make sure that you limit the consequences.
Always good to hear other people’s mistakes with the CNC module. On my end I jogged the end mill 10mm down into the work piece instead of moving it up to retract it.
Luckily it was a thin end mill so it just snapped in two and didn’t damage anything, but that wasn’t really what I had in mind when it comes to Snapmaking.
In my experience, I tend to learn more from mistakes than from successes. So that’s why sharing failures is important too
After doing some post-mortem analysis on the situation, I’m a bit confused. And as the video is not at a great angle, it doesn’t really help to pinpoint where it went wrong. It looks like it went wrong twice.
In the picture cutout A is the last one that was in the right place. For some reason it didn’t go as planned and cuts B & C still worked out, but they were already shifted. Then finally at cut D it went wrong completely.
What I think went wrong:
not using the right tool. I did use a 3.175mm bit (that has a 3.175mm shaft) but the cutting part of it is only 13mm long. And I was cutting 18mm thick stock. I have gotten away with this before, but it’s always a bit tricky as when you go deep enough the shaft is continuously rubbing against the walls. (and there is no cutting edge anymore)
Probably in the deeper part of cut A it has gotten stuck and missed some steps resulting in a shifted location. → Problem 1 which can be attributed to using the wrong tool. So a machinist error obviously.
But then in cut D, the part that came off wasn’t even the cut to full depth. So the tabs that were there held on as expected. at least 3mm of material is still there, it just sheared off completely. Again probably due to the tool not having a cutting edge that’s deep enough for this material, causing vibrations etc.
Possible solutions: get a longer bit or create a bit of a slope (which is possible for this part) so there’s enough clearance. (Or cut half way through, turn the part around and mill from the other side) but in this case that’s too much trouble for the purpose this part needs to serve)
I probably could get away with clearing the pockets completely, but that will take significantly longer and still results in the tool rubbing against the stock from one side, but it won’t be wedged in between anymore probably not causing the same problem anymore. But that doesn’t solve the problem.
Maybe put a slight slope on it and clear out the pockets completely.
Oof CNC mishaps… a few days ago something caught and ripped my entire plyboo board off my spoilboard (I was holding it down with double sided tape). This happened about 4 times, but I just kept adding more tape and reseating it to try again (insanity is trying the same thing and hoping for a different outcome, right?). The first 3 times, it just pushed the piece sideways so nothing happened except getting dislodged.
The fourth time, the entire board got pulled up into bit but instead of spinning the entire piece around like what happened with you, it basically just went on a rubbing frenzy and the piece started to smoke and char within seconds. Good thing I was sitting right there watching since I knew beforehand that this was the step where it might fail, so I turned it off immediately. Piece is useless now, but at least the bigger disaster was avoided.
(Note that I do have a smoke detector and a camera in case I’m not sitting next to it)
maybe go less agressive on your cuts then saana?
I did some Milling aswell ricently, got away with 2.5mm DOC into birch wood but the sound it made was horrible and i got reminded of checking/ limiting my stepdowns instead of expecting Freecad to do it for me. No broken mill though phew.
I have pretty good experience using painterstape and super glue. Painterstape on the back of your workpiece, painterstape on the spoilboard and superglue them together. But as you can see in the movie of my original post you have to make sure that all parts that can get loose are actually glued together
Really easy to remove afterwards. Should be similar to double sided tape though. Don’t know what would be the “strongest”
Depending on the type of failure, I actually prefer a broken bit in some cases
Having your work piece come loose - this seems to be the best way to start a fire with cnc. And/or damage your toolhead.
If you can clamp the piece you should clamp it. That’s by far the strongest way.
There are a bunch of different clamp fixtures on thingiverse. You should be able to find some that works. One way I’ve used for awkward pieces is earthquake straps. Use the regular screw holes on the spoilboard and strap the piece down.
If I have room I also like to set up some stops on the side of the piece that keep it from being able to move laterally.
If you can’t use clamps I think the blue tape/super glue is the next best method. Give it time to dry and/or use accelerator. If you can, add some stops on the side to make sure it doesn’t move. Hot glue will also work really well for this. It comes off fairly easily as is, but to insure you don’t damage your surface you can use some blue tape in between.
Generally though, especially with the limitations of the SM, if your work is coming loose you’re pushing it too hard and are going to end up with play in your linear modules.
-S
