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I want to use a round-over bit on a CNC router after cutting a profile with a 3mm (1/8") bit. So the round-over bit needs a shank thinner than that. I could only find shanks with 6mm (1/4") diameter.

Do round-over bits with shanks thinner than 3mm (1/8") exist? If not, what is the physical limitation that prevents it?

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    $\begingroup$ Probably the strength they need. $\endgroup$
    – Solar Mike
    Feb 24, 2019 at 19:22
  • $\begingroup$ Why not do the round over cut as a secondary operation by hand with a hand held router with a ball bearing round over bit ? $\endgroup$ Jul 20, 2020 at 0:53
  • $\begingroup$ @WilliamHird Yes, I tried that. I screwed the piece to a wooden board, rounded it with the hand router, removed it, and sanded the bottom of the piece that had the screw. Although the result was acceptable, my pieces are small and in the hundreds, so it was too much work and I was looking for something more automated. $\endgroup$
    – emonigma
    Jul 20, 2020 at 9:17
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    $\begingroup$ In that case you will have to have a very custom round over bit with a 1/8 shank made. It would have to have a very thin , low mass head of special design. You could get away with it because round over bits do not have remove much material (low cutting force on the 1/8 shank.), and if you further reduce the cutting force by making many light passes. So to summarize, think BIG bucks for a custom HSS bit and some programming to make several light passes. $\endgroup$ Jul 21, 2020 at 19:28
  • $\begingroup$ @WilliamHird I found one bit and was waiting to regain access to the CNC, which is barred during the pandemic, and have now posted a potential solution. $\endgroup$
    – emonigma
    Jul 24, 2020 at 15:54

2 Answers 2

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Depending on the material, likely strength doesn't allow smaller ones. Regardless, depending on shape and gap width, you can do this with a larger tool, by driving it multiple times over the edge, with the tip in consecutive passes going through paths lying on a quarter-circle of radius equal to tool radius minus desired fillet radius. (if the tool has a flat tip, you need to add that flat tip radius to the path offset.)

Say, the upper-right shows the bit cross-section (well, half of it). With consecutive passes with tip offset at positions on the red line you achieve a fillet of a much narrower radius (blue overlaid by yellow). The only problem is if there are narrow gaps or the depth doesn't allow to lower the tool deep enough.

enter image description here

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  • $\begingroup$ Good hack. My problem is that for the material and pieces I am cutting, I have to use a 3mm CNC bit, so I cannot fit a larger round-over bit on the profile cut by the 3mm bit. $\endgroup$
    – emonigma
    Feb 25, 2019 at 16:55
  • $\begingroup$ @mmorin: Consider a couple of conical tools of various angles instead. The result won't be nearly as nice, but better than nothing. $\endgroup$
    – SF.
    Feb 26, 2019 at 8:40
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I found this CNC bit that seems to do the job:

1pc 6mm Shank Classical Round Nose Point Cut Wood Router Bit Tungsten Cobalt Alloy 2 Flute Wood Milling Cutters Woodworking Tool

CNC bit

I have not tested it because I don't have access to the CNC during the pandemic.

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