Tool plunges off toolpath

My toolpath in VCarve Desktop is to cut a 1/6 inch channel in 1/4 inch brass. The 3D preview in VCarve looks perfect. The CNC machine executed it perfectly once. Now, partway through the cut the tool plunges through the material into the wasteboard and breaks the endmill. Any idea what could be going wrong? I would like to upload the VCarve file and the toolpath file, but as a new user am not able to.

Thanks for any guidance here!

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I would prefer to systematically eliminate possibilities such as problems with the VCarve files, before speculating. However, here is a possibility that seems to fit the randomness of the problem. I had the game controller on and set aside. I also was using bluetooth-connected headphones for noise cancellation. Could bluetooth signals from the headphones (like skip song) be taken for random commands by the controller?

Hey Rod,

This reminds me of this.

This is unlikely.

You have Trust Level “New” . But you can reach Trust Level “Basic” very easily by:

  • Entering at least 5 topics
  • Reading at least 30 posts
  • Spend a total of 10 minutes reading posts

If you reached “Basic”" this way, you can:

  • Use all core Discourse functions; all new user restrictions are removed
  • Send PMs
  • Upload images and attachments
  • Edit wiki posts
  • Flag posts
  • Mute other users

You see, the restrictions quickly go away if you continue to use the forum.

Thanks Aiph5u for the link.

  1. I couldn’t replicate the bluetooth effect so that can be ruled out
  2. I did an “air carve” with not tool and did not observe an aggressive plunge out of the prescribed path, so, presumably the toolpath is OK.
    So, the current hypothesis is that the tool came loose.

So I have a new question. Were my feeds and speeds too aggressive for the material?

Brass 360-H02
Spindle speed: 17,000 rpm
Feed rate: 40in/min
Plunge rate: 10in/min
Endmill: 4 flute 1/4in
Passes: 3 at 0.0417 in.

Hey Rod,

if this is the case, shouldn’t you have been able to see how far the broken end was still in the collet?

Wow, a lot of imperial to convert. At first sight such numbers say nothing to me (I would have to convert them).

Also I just was called for dinner! On the web, there are a lot of tables for cutting parameters, e.g. this one here that includes brass as material (PDF). Good tables for metal are also in the book ISBN 9783758510717.

What do you think of the cable problem theory? Especially since Onefinity lacks strain relief on their cables and the Molex pin crimps are not strong enough to redeem this fault.

I think it is a good theory. But since the machine was able to execute the piece perfectly in softwood, I decided it did not apply to my case.

Here is the finished piece in brass.

Lessons learned:

  1. Crank the the toolbit super tight if you are working in soft metals
  2. Try making the piece in softwood first if you are still learning the software, the machine etc. and if you don’t want to waste (very expensive) brass

The Europeans are right about metric. But I inherited my Dad’s machinist tools and everything is in Imperial.

Thanks so much Aiph5u!

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Hey Rod,

Yes, tightening the collet is important. Do you use a spindle with ER collet?

But cable problems with Onefinity CNC come up relatively frequently in the forum and often the machine behaves strangely then. The problem can appear from one moment to the other and then disappear (with a workpiece ruined in between).

PS: Europeans, lol :slight_smile:

For brass, you’d want a brass-specific end mill. You need a solid core and dual flutes (vs 4). A 1/8" with a 1/4" shank is good because it will have less resistance but the 1/4" shank will keep it from deflecting and breaking. Also, keep the bit length short - not much longer than you want your max cut. That will help keep it from breaking as well.

Also, some lubrication is useful - either a wax for simple sheet brass to something like plumbers lube from McMaster for thicker carves.

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OK, I will make strain relief for the cables.

I have the collet which came with the Makita RT0701C. I will get an additional 3/8 inch collet so I can use some of my Dad’s old bits.

I also have a set of collets for a Taig lathe. They look like they fit in the same taper as the Makita. The opening of the cap screw would have to be machined a tiny bit wider to fit the collets properly. Do you suppose that would work? I imagine this could be useful for drilling small holes.

Thanks for the ideas!

Thanks Jim,

As much as possible I would like to try to use what I have got. I was hoping the 3/8 inch shafts would give rigidity – enough for the 1/4 and 3/8 examples? I also have some odd sizes on 1/4 inch shafts like 3/16 and 5/32.

I dub my drills for brass because of the problem of catching. My thought was that there should be enough rigidity and constraint with this machine that dubbing milling bits is unnecessary. Thoughts?

I like the idea of wax – seems like it might be more adherent and less messy that the tapping lubricant that I have been using.

Here is some of the collection. Any idea if the rasp-like ones in the upper left could be put to any good use?

You’re probably fine with what you’ve got for the design you showed. It’s shallow and you’re taking it easy with your settings. Good sharp carbide end mills are the way to go. Brass isn’t “sticky” like aluminum so the extra flutes aren’t as critical (a 4 flute end mill on aluminum will have serious chip issues as they’ll try to stick to the mill).

One change I would make is to reduce the stepover to get a smoother finish on the base of your groove. You could do that for all 3 passes or add a separate last pass with a tool defined with the reduced stepover. Which way to go will depend on which takes more time overall.


I have had similar problems with the tool crashing, either from a problem with Z or X.
Two different problems that I have caught, I have found the machine zeroing an axis while cutting a path.

  1. Chips were hitting the touch screen, and triggering buttons on the screen. I relocated the screen to prevent this.
  2. The router power wire was crossing over the Z axis wire, right on top of the router. I rerouted the wires, so the Z axis wire goes over the left side and back to the controller, and the router power wire goes over the right side and continues to the right.
    Additionally, make sure the router and any vacuum motors are not plugged into the same circuit as the controller and the monitor.
    I plan to add a UPS (universal power supply) for the monitor and the computer.
    FYI #2 fix above has resolved my issues.
    Hope this helps!