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Thread: Cut length of bits questions

  1. #11
    Join Date
    Dec 2008
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    Diamond Lake, WA
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    Quote Originally Posted by skintigh View Post
    Thank you Steve!

    Don, it sounds like I could have save a ton of work doing it your way. I've spent 10-20 hours so far puttying and sanding all the spots that tore out from the bottom of my porch bracket cuts, and damage from tabs. Just to clarify, you are plunging the compression bit in 3/4 deep into the plywood and rough cutting in one pass? Wow. Though I was cutting pretty fast at the end -- I got it down from 45 min per bracket to 15, and was cutting cleanly at 324 IPM, or 360 IPM but with 2 flaws, always in the same mirror-image locations. It was uncanny. I don't understand why. Actually, I took pictures, maybe someone here can tell me why...



    It made a ga-BANG noise when it did that, but the rest of the 2'x4' sheet was cut cleanly. I thought a chip just got caught and it was a fluke. Then it happened again in the exact same spots, so I went back to 324 IPM.
    I don't plunge the bit in 3/4" I plunge it .25" to get the bottom flutes to clear the surface quickly (otherwise you'll get veneer chip out on the top, because the bottom flutes are up cut), then I ramp down to the cutting depth. You shouldn't plunge a bit then start cutting. You should always ramp your cuts. I've had the best results that way.
    Don
    Diamond Lake Custom Woodworks, LLC
    www.dlwoodworks.com
    ***********************************
    Life is not a journey to the grave with the intention of arriving safely in one pretty and well preserved piece; But to skid in broadside, thoroughly used up, worn out, bank accounts empty, credit cards maxed out, defiantly shouting "Geronimo"!

    If you make something idiot proof, all they do is create a better idiot.

  2. #12
    Join Date
    Dec 2007
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    I always use a lead in lead out when I use a combination bit.

  3. #13
    Join Date
    Jun 2018
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    38

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    Quote Originally Posted by Chuck Keysor View Post
    Seth, on the vector drawing of your bracket, zoom in on the location where you got the little bit that chipped out, and see if you have some small, knot in the vector, or lots of nodes......... The fact that the same thing happened a second time would suggest there is something in the drawing used to generate the tool-path. Chuck
    I looked at 3 revisions of my design last night, and unless I was looking at the wrong spot everything seemed fine. No knot, just some curved vectors an inch or 2 long. It looked as good or better than other spots that had no problems.

    I do recall that the spot where it messed up happened as the mill was being lifted out of the cut, and it may have also been near/on a tab (I might be able to check that tonight). I had sped up some parameters like jog, and may have tweaked ramping or plunging parameters slightly based on online info. But the only thing I changed to make it NOT take a bite out of that curve was to lower the cut speed by 10% from 360 to 324 IPM, 18,000 RPM, I think it was a 3/16" end mill and pass depth was 0.195" or maybe a hair different than that.

  4. #14
    Join Date
    Dec 2008
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    One thing you might want to try is to establish a different start point in your project. You could also try lead in/out ramps in your project so that you don't actually start or stop on a specific line in your project. This could help prevent the type of chip out you are experiencing.

    What I would suggest is to get yourself some MDF or foam and practice on that. Lot less expensive then BB and much more forgiving with experimenting. I used foam and MDF when I was starting out to get a feel for what the machine can and cannot do, plus reduced/eliminated broken bits during experimentation. Practice with in/out leads, etc. I found my spindle speed for cutting just about everything on my machine to be 13K RPM. Plywood and softwood feed speeds are 6IPS for climb cuts. Hardwood, I slow down to 5IPS. Final conventional cut is done about .5IPS slower. This gets more cutting surface in contact with the material which helps to lesson or remove spiral marks left by spiral cutting bits.

    Since you're just starting out, you need to figure out what is right for YOUR machine. Remember, if a bit is screaming while cutting, it is hungry. You need to either slow down spindle speed or increase feed speed. However, when a bit goes into corners it will slow down and scream in the corners. This is OK. As long as it's not screaming during full speed runs. With my machine, I've learned what the proper feeds and speeds are and cutting noise is completely drowned out by the noise from my vacuum table motors.
    Don
    Diamond Lake Custom Woodworks, LLC
    www.dlwoodworks.com
    ***********************************
    Life is not a journey to the grave with the intention of arriving safely in one pretty and well preserved piece; But to skid in broadside, thoroughly used up, worn out, bank accounts empty, credit cards maxed out, defiantly shouting "Geronimo"!

    If you make something idiot proof, all they do is create a better idiot.

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