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Thread: Compression bit selection

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  1. #1
    Join Date
    Aug 2018
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    7

    Default Compression bit selection

    I have been searching on here and centurion cutters seem to be very poplular. I want to try some just cant decide on the plunge end or the FEM(flat end mill) end. Any suggestions? I have been using whiteside 1/4" compression bits in my 2.2kw spindle so far but want some 3/8" cutters.

    On a side note what would be a suggested feed/rpm, cut depth for a 2.2kw spindle on a machine similar to a prt standard? Its actually a cnc routerparts pro with nema 34s and an atc spindle. Right now I can run a single pass in 3/4" maple plywood at 350ipm and 13,500rpm with the 1/4" compression. I want the 3/8" for a better edge finish.

  2. #2
    Join Date
    May 2014
    Location
    MA
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    I use those same bits and I really like them. Do one pass like you're doing now, but leave an onion skin and make the part about .005 larger (you can do this in Aspire). For the last pass come in the opposite direction and tight to the part (no offset) and you'll get great finish quality on plywood.

  3. #3
    Join Date
    Dec 2011
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    Piedmont, SD
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    728

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    Point #1 Having plunge end is in harmony with the plunging of the bit into material. Unless you ramp your cuts, Flat end mill will be stabbing straight into material, heating faster and hastening it's demise. Centurion refers to this geometry as a mortise compression bit.

    Point #2 - What Eric said, though I recently switched to 3/8 for general plywood cutting, simply to reduce number of passes. On a 4x8 sheet full of cutouts, this makes a noticeable reduction in processing time. As for better edge finish, I think you'll find as I did that the difference is not very perceptible. Others will note 1/4 bit allows you to squeeze more parts on each sheet. I've not had any such problems since up-sizing to 3/8, though sometimes that 1/8" wider kerf can add up.

    As for brand, I've been happy with Centurion, especially with pricing. Recently purchased a 3/8 of theirs and equivalent in Onsrud, and they appear almost identical. Time will tell if any difference in longevity, though they are both the "extended" / "Max Wear" varieties. Note part numbers in image.

    Pay attention to length of up-cut tip on your compression bits. Image shows a regular 3/8 compression at the bottom, with the Onsrud and Centurion above. Note the top two have half the length of up-cut tip. This is ideal for plywood, as you're less likely to plunge too shallow, resulting in tear-out of veneer , or worse yet, melamine face.

    .375 compression comparison.jpg


    Jeff

  4. #4
    Join Date
    Dec 2008
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    Diamond Lake, WA
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    A very important thing to consider with compression bits is that the upcut portion will cause chipout until it gets below the surface of the plywood. Because of this, you need to plunge a compression below the depth of the upcut portion then start your ramp. I've worked with 3/8" and 1/4" compression bits. I cut hundreds of sheets of plywood a year. The 1/4" does allow more parts per sheet, but at the cost of speed. You can run faster with a 3/8" bit because there is less bit flex. But, if your machine is not REALLY tight, the 3/8" bit can cause more machine flex. I compensate for this with 1/4" bits by making my first pass a climb leaving a 1.32" skin. The second pass is a normal direction cut. I plunge the bit 1/4" down then ramp all the way thru the material cutting 1/32" thru. This leaves a very clean edge ready to go straight to the edgebanding machine and the second cut is dead on the line.
    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.

  5. #5
    Join Date
    Jan 2004
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    iBILD Solutions - Southern NJ
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    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  6. #6
    Join Date
    May 2014
    Location
    MA
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    Brady, how much better are those than something like a Onsrud, Whiteside or a Centurion? I tend to get a long life out of my Whiteside compressions... Is it just the time that they last, or is the cut quality that much better?

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