Why did this happen?


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  1. #1
    Activation process justicedavid65's Avatar
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    Default Why did this happen?

    Cutting 1/2" BB Ply with a brand new Whiteseide 2 flute down cut spiral carbide bit at 150ipm 18000rpm and the bit snapped in half? It snapped off right at the collet. I'm confused by this. I have cut at speeds quite a bit faster than this before with no issues and the bit was only about 1/8" into the plywood.

    Anyone have any ideas about why this happened?

    Machine is 1500x1500 C Beam extrusions with ballscrews and linear rails on all axes and a 2.2KW spindle.

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    Community Moderator ger21's Avatar
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    Default Re: Why did this happen?

    What size was the bit?
    If the bit breaks right at the collet, replace the collet.
    I'd also lower the rpm when cutting at 150ipm.

    Gerry

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    Activation process justicedavid65's Avatar
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    Default Re: Why did this happen?

    .25" bit. How would the collet cause, or contribute to the break. I was just so surprised by this, I would not expect a .25 bit to break in plywood at these speeds at all.



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    Default Re: Why did this happen?

    Cutting direction of the spindle?

    G540, Rack and Pinion Drives-X/Y axis, 1/2-Ball Screw-Z Axis w/THK HSR 25 Linear Slides, Steppers KL23H2100-35-4B, Power Supply-KL-600-48


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    Activation process justicedavid65's Avatar
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    Default Re: Why did this happen?

    HA, I know that's not it, I had trouble with that when I first got my machine together. Also, this was about 45 minutes into the cut and after the bit broke I picked it up immediately and it wasn't even warm, so I'm still confused on why it broke.



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    Community Moderator ger21's Avatar
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    Default Re: Why did this happen?

    If the collet is damaged, it can put extra pressure right at the edge. This, combined with any run out or other shock to the bit, can cause breakage. There were likely a combination of factors that lead to the break.
    If you don't replace the collet, there's a good chance another bit will break in the same place.

    Gerry

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    Quote Originally Posted by justicedavid65 View Post
    HA, I know that's not it, I had trouble with that when I first got my machine together. Also, this was about 45 minutes into the cut and after the bit broke I picked it up immediately and it wasn't even warm, so I'm still confused on why it broke.
    Post a close up picture of each of the broken emds.Did the bit break right where it was sticking out of the collet?



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    Default Re: Why did this happen?

    Ah, sorry I threw it away, didn't think to take pictures, but yes it broke right at the collet. In fact I had to push the part that was left out from the top because there was nothing left to grab and pull it out.



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    Quote Originally Posted by justicedavid65 View Post
    Ah, sorry I threw it away, didn't think to take pictures, but yes it broke right at the collet. In fact I had to push the part that was left out from the top because there was nothing left to grab and pull it out.
    More than likely the collet caused this problem, as ger21 said. Why? Imagine a burr around the inner edge of the collet scoring the bit. If you had a photo, I'd bet we would see a clean score line part way through the bit, followed by a rough surface where the bit snapped.



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    Default Re: Why did this happen?

    If it happens again with a new collet,you may have hit upon a combination of spindle speed and feed that is causing a destructive harmonic oscillation. There is almost certainly some sophisticated software that could model the machine and it's flexing.You would need a similarly sophisticated engineer to derive accurate results.



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    Default Re: Why did this happen?

    Thanks guys, I have a new collet on order, so I'm not going to run any 1/4" jobs this week until I get it, but I will run this same job again, just to see what happens.



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    Default Re: Why did this happen?

    There is always the possibility of a manufacturing defect in the cutter, it doesn’t happen often but it can happen even to brand names.

    Unfortunately you didn’t take photos as that certainly would have helped. I do have a question though, was this a solid carbide but or a carbide cutter with a brazed on steel shank? I actually think that most cutters these days are solid all the way up the shank but it use to be a real thing.

    As for the collet it could very well be damaged. A bit of inspection work should reveal it that is the case. Visually inspect it then put a long shanked end mill in the collet and measure run out. Excessive run out is a bad thing. Which brings up another question how much of the shank was in the collet when the cutter broke? With ER collets it is a good idea to have plenty of shank engagement in the collet.

    By the way don’t throw out the collet and in the future keep all broken end mills. The shanks on end mills are rather precise in diameter and can be very useful spacers. Beyond that they can often be used on other equipment when reground. They can be very useful on a lathe for example. If nothing else you might be able to trade them.



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