Vibrations during CNC

Does anyone have a simple way to reduce or eliminate the vibrations the CNC bit causes on a piece you’re milling out? We’re using C-channel for our side rails of our platform, and we’re milling out holes for the wheel wells.

Near the end of the fourth run for each rectangle we cut out, the middle piece that’s being cut out begins vibrating. It’s not too rough, but it does cause noticable (yet tolerable) imperfections in the cut. Do we have to live with this (and file it down afterwards) or is there a way around it?

Thanks in advance.

Slow down the cutting speed.

It’s a lot more complicated than that.

There are a great deal of issues that are all combinations of the type of cutter, depth of cut, feedrate, rpm, number of flutes, cutter geometry, etc… etc…

Clamp that part of the workpiece better.

I think what you are saying is that the waste part of the hole you’re making starts jumping around, right? If so, make the cut in more than one pass - almost all the way through to start, then using only a very light cut for the finish. Or exclude the middle of the sides at first, then take them last. Or clamp the middle of the waste rectangle (drill a hole and bolt thru to the work table)

And yes, slowing down the cutting speed (not the tool speed!) may help.

Shorter tools… but that only gets to be an issue when you’re using BIG tools (1"x4" FEM or something of the like).

Mainly, clamp better. That’s the biggest thing.


A bit confused here. When you say the middle piece that’s being cut out vibrates-does that mean you’re cutting the outer perimeter, and a chunk is falling down through the hole you’ve created?

If so, super simple fix. Spiral from the inside to the outside, and then you never have a big old chunk flopping around.

If the problem is just the leg of the C chattering, that’s going to be pretty much unavoidable most likely. You can try to minimize it though. Easiest thing to do would be to only conventional mill as it puts much less force on the workpiece than climb milling.

A bit confused here. When you say the middle piece that’s being cut out vibrates-does that mean you’re cutting the outer perimeter, and a chunk is falling down through the hole you’ve created?
– Correct –

Thanks for the tips guys, there’s alot of great stuff here.

We should be able to keep the middle pieces for scraps if I bolt the middle piece in the opposite corner to where the CNC ends the final cut. For now, we’ll try that on Saturday.

To clarify a bit more, we’re still very new to the CNC; the teacher in the school who’s an expert on it simply cannot make it to our meetings due to other things. So we’re slowly figuring things out based upon emails and cautious common sense.

The way we mill things right now:

  • Clamp the piece in place
  • Manually zero on a reference point
  • Automatically move the bit (via the “GOTO” button) to the first calculated point on the CAD drawing – we have to manully input the X, Y, and Z coordinates
  • Turn the spindle on, drill down into the metal
  • Set the speeds for cutting (not sure of the units so I can’t post the exact settings yet) – It takes about 30 minutes to cut a rectangle that’s 2.5" x 5" in 1/8" thick aluminum using a 1/4" bit
  • Use the “GOTO” button again to go to the next point in the CAD drawings we generated
  • Use the “GOTO” button three more times to finish the rectangle

So in the end it’s still not a fully automated process…yet…but it’s definitely an improvement in quality over what we’ve done in the past. We’ll have our own process perfected by the time we get the first rail done, and hopefully will have more guidance by Tuesday of next week so we can do the other rail faster, more autonomously, or with higher quality.

Don’t worry about keeping that piece for scrap; I would take a nicer finish with no scrap versus a small scrap of aluminum (that is probably worth pennies at most). If you start in the center and spiral out, removing all the material, you’ll fix this problem (as Cory suggested… and when it comes to machining, he knows his stuff).

What kind of machine is this? 30 minutes for that much material is extremely slow.

Is there any other wayt o clamp it down?:confused:

Carpet Tape+Pressure works well in addition to clamping.

Why do you even want to keep the piece for scrap? a 2"x4.5" or so piece of 1/8" Al is fairly useless. It’s almost not even worth the effort in my opinion.

One way to reduce vibration on a leg of a C-channel would be to take a heavy C-clamp and just clamp it to the leg. This will increase the mass, hence the inertia, of the leg, thus reducing the amount that it can move or vibrate.

That sounds like a good idea with the C-clamp. I would just make sure it was properly clamped/supported.

I’ve done something similar to this, but put something solid (generally a 1-2-3 block) into the C on either side of the cut, and depending on what I’m doing either clamping the legs down to squeeze the block, or just sticking it in the vise if the fit is snug enough.

what i do is just not cut all the way leave a 64 there to hole it then just punch the part out later and file the burs off

Yea, 30 minutes. After doing a bit of looking, I think the bit is either dull or isn’t designed for fast cutting. It looks like a simple 2-flute square end bit. Any recommendations? I know he has a couple of bits hidden somewhere so maybe I can get him to leave them out for me Saturday.

And I want to keep at least some of the scraps simply because this is only a prototype and the rectangles are perfect for fixing up some minor problems on an old bot.

What brand/model machine is this?

You’ve got the right type of end mill. 2 flutes with a square end will work fine. It may have been dull already, or you may have run it so slow that it dulled it just cutting out your pockets.

30 minutes seems absurdly slow. Even the smallest CNC mills I’ve ever seen (the little Taig/Sherline ones) wouldn’t take 30 minutes.

End of Weekend update:

A brand new bit was put on the CNC and we learned that the spindle speed had to be manually adjusted. So after some fine tuning of the machine and refinement of the drawings, the machine cut the rectangles in under 5 minutes each. We wound up bolting the middle rectangle on the opposite end from the final cut and it worked out beautifully.

Pretty sweet!

Now we’re off to scavenge parts off of an older bot to fix the '08 bot’s problems.