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Precise carbon fiber cutting: mill-ends get regularly broken after ~10 cm

uuth

Plastic
Joined
Apr 27, 2022
Hello, I have a small CNC mill which I normally use for cutting wood like hard maple, without much problem. I often use small bits of 0.6 .. 1.0 mm.

This time I tried a 2 mm carbon fiber plate instead. It is fixed on a soft foamy plastic and fully submerged in water. The spindle rotates at its maximum of 12000 RPM. Carbon steel bit diameter is 0.8 mm.

I started with a small feed of 35 mm/min, depth step about 1 mm. It was a simple outline. Seemed to work smoothly, until the bit broke after about 10 cm. I lessened the feed to 20, then 10 mm/min. Did not help, the bits keep breaking after 5 ... 15 cm of feed. These are the same end-mills which I use for hard woods. I put one broken piece under microscope, near a new bit:

w2.jpg

The broken piece is to the right, the new bit to the left. They both look sharp and indistinguishable, as far as I can tell.

What could be the source of the problem?
 

Conrad Hoffman

Titanium
Joined
May 10, 2009
Location
Canandaigua, NY, USA
Carbon fiber is extremely abrasive. It will even wreck carbide cutters, but more slowly. High speed steel doesn't stand a chance. It may not look dull but only a little and you're done because it isn't cutting anymore, just pushing.
 

uuth

Plastic
Joined
Apr 27, 2022
So in fact, the broken bit is somewhat dulled, not enough to be seen on the photo but enough to cause a drag? I need a 0.8 mm bit, so the choice is limited. Would those cheap diamond-covered bits, for about one dollar a piece, work better? Or rather these "titanium coated carbide milling cutters" https://www.aliexpress.com/i/32907009831.html ?
 

TeachMePlease

Diamond
Joined
Feb 11, 2014
Location
FL
Your options are only limited if you insist on buying crappy tools. Plenty of quality carbide manufacturers make .8mm endmills. Here's the Harvey endmill coated in amorphous diamond, which I believe should be good for cutting carbon fiber: Harvey Tool also available as a 4 flute here: Harvey Tool or a 4 flute AlTiN coated: Harvey Tool or 2 flute AlTiN coated: Harvey Tool or, split the difference and get a 3 flute: Harvey Tool
 

uuth

Plastic
Joined
Apr 27, 2022
Sounds like its not fixed rigidly and is just going to bounce around and wreck your bit.

I was not precise. It is a quite rigid composite plastic-wood which is foamy inside but solid on the surface and cannot be dented with a nail. I do not know how it is called. The machined carbon plate was warmed, then covered with a thin layer of hot glue, then pressed to the foamy composite.
 

uuth

Plastic
Joined
Apr 27, 2022
Not sure about hot glue. Might build up on the cutter and break it.

The cutter never makes it to the hot glue before it breaks. It is also a high-temp hot glue which is relatively hard at room temperature and in case of wood, does not seem to pose a problem. Not sure about carbon. I cannot exclude that higher temperatures would make the glue sticky enough to break the cutter, even if everything is underwater. But as I said, it was not the case here as there was never a physical contact.

As an unrelated note, I wonder if the flutes hitting a very hard material at regular intervals do not create any kind of resonance which creates fatique in the cutter. Possibly an endmill this small should have a much higher rpm but the spindle runs at its max already.
 

mhajicek

Titanium
Joined
May 11, 2017
Location
Minneapolis, MN, USA
As an unrelated note, I wonder if the flutes hitting a very hard material at regular intervals do not create any kind of resonance which creates fatique in the cutter. Possibly an endmill this small should have a much higher rpm but the spindle runs at its max already.

That resonance is what we would call "chatter". An experienced ear can tell by the sound. To avoid it, use a proper chipload (feed per tooth), minimize cutter stickout, and make your setup as rigid as possible.

In addition to being harder, and thus remaining sharp longer, carbide is also significantly stiffer than steel, which helps reduce chatter.
 

uuth

Plastic
Joined
Apr 27, 2022
I just want to do a welcome to the board uuth.

Carbon fiber and wood are very different as you are now seeing.
Is this the highest mag on your microscope?
Bob

Thanks Bob.

This is a stereo microscope and has only one magnification. But it was hard to make a sharp photo. When looking throught the scope directly, the image is much sharper. Despite that, I see no wear on the broken bit.
 

uuth

Plastic
Joined
Apr 27, 2022
That resonance is what we would call "chatter". An experienced ear can tell by the sound.

The endmill does produce occasionally a sharp buzzing for few seconds which then slowly subsides.

As of the stickout, all the 0.8 mm endmills for composite suggested here seem to have a max flute length of 2.4 mm. Mine has 5 mm, useful for wood but useless for the 2 mm carbon plate.
 

DavidScott

Titanium
Joined
Jul 11, 2012
Location
Washington
As others have said, your cutting tools are the problem. Solid carbide is good enough unless you are getting into hours of production. A big problem I have had with diamond coatings is the sharp cutting edges get rounded so the tools don't work as well from the start.
 

Comatose

Titanium
Joined
Feb 25, 2005
Location
Akron, OH
You don't want an endmill. You want a PCB router bit. They're designed for cutting composites, they last forever and they're cheap. .8mm isn't that small in PCB land. $5.50 each in quantity. Can't beat that.

Products - Diamond Cut - Midwest Circuit Technology

You want a fishtail end geometry and probably downcut for this.

They like a lot of RPM. We run them in our PCB router cutting fiberglass at 50krpm and our CNC router cutting carbon at 24k.

For carbon fiber prototyping, I use carpet tape over mdf, then hold the mdf down to the spoilboard with vacuum. In production, gaskets vacuum fixtures.

But yeah, you want a PCB router bit, not some expensive diamond thing from Harvey. This is a common application if you know where to look.
 








 
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