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You’ve heard of round-over router bits, but how about round-under bits? Round-under bits allow you to form the bottom edge of a bull nose without turning the material over. Combined with the matching round-over bit, you can form a full bull nose. There are two reasons you’d want to use this bit: Either the material is too heavy and/or awkward to turn over easily, or you want to form the edge with a countertop in place.

A few companies make these bits, but we’ll look at Amana Tool’s offerings. They sell three round-under bits with 1/2″, 3/4″, and 1″ radii for solid surface materials. The largest bit, the 1″ radius, is so large that you need to make sure to keep it spinning under 14,000 RPM. So you won’t mar the finish, they use their Ultra-Glide Delrin sleeved ball bearing guides rather than steel ball-bearings guides.

Pricing for the bits starts at $100 for the 1/2″ radius bit and up to $150 for the 1″ radius bit.

Round Under Bit [Amana Tool]
Street Pricing [Google Products]
Via Amazon [What’s This?]

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9 Responses to Round-Under Router Bit

  1. Mrten says:

    Festool has a whole bunch of these (of course) but if you have to ask for the price you can’t afford them.

  2. John says:

    @Mrten… I love it, great quote!

  3. fred says:

    For wood – many manufacturers offer bullnose router bits that cut both top and bottom in a single pass. For built up solid surface countertops – where you may not form a true half round – but leave a small strip in the center flat – this approach (round under and round over) is better. It also means that you put less stress on the router – and do not have to have a bit the conforms to the precise thickness of the countertop.
    We have Zenesis diamond bits that work in similar fashion with stone countertops.

  4. Eddie says:

    I just got my first router. I don’t think I will be adding this one to my bit collection any time soon….

  5. Jay Swift says:

    Okay, here is a question that I ask every time I use my round over bits (I flip the material because I can in no way afford a $100 router bit):

    Why can’t they make a round over bit that you can flip?

  6. fred says:

    @ Jay Swift

    Amana, Whiteside and others make – double roundover bits tthat you might presumably do just what you ask – if they sold them with only a single cutter head as an option:


  7. fred says:

    @ Jay Swift

    If you did a lot of countertops in-situ this bit might be the way to go – but controlling a big router bit handheld – especially for an undercut – is best done with a bit of jigging – e.g. supporting the router either with an extended base.

    You also need not go to the expense of buying an Amana brand bit – just Google – Amazon lists Grizzly and Magnate brand under-roundover bits for substantially lower cost.

  8. dreamcatcher says:

    I used to use these bits when I worked in a Corian countertop fabrication shop. The picture doesn’t do it justice; they are very big, stick out far and are scary as hell to run. Though this was one of the SMALL bits!

    The larger edge profile bits, like a stepped triple radius bit (AKA “waterfall”), sticks out 1-5/8″ and spins a 2-3/4″ diameter at 13,000 rpm. To run any of these bits it is absolutely necessary to make a larger base for the router….not too difficult to do when you are surrounded by Corian scraps.

    The size of the bits are too large to fit through most OEM router bases. Our base was made out of a Corian scrap about 10″ x 16″ with the router offset to one side and a large knob-handle on the other. The offset helps to keep pressure on the countertop so the router doesn’t tip. Let me tell you from experience, if that router tips you can be sure that something – or someone – is going to get seriously damaged.

    I should also mention that we used the MONSTER 3-1/4 HP Porter Cable #7539 five speed plunge router that weighs a hefty 17-1/4 lbs even before you chuck in a big 4 pound router bit.

    Typically profiling the edge takes a minimum of 3 or 4 passes, cutting incrementally deeper each pass. Since Corian is acrylic, the shavings come off as tissue paper thin, pure white ribbons that are static electrically charged and cling to your body. By the time you are finished profiling a full kitchen counter top you look like Frosty the Snowman.

    Not the most fun job I ever had, but working with a versatile material like Corian can have it’s perks; I made my self a Corian coffee mug, a few boxes, some cabinetmaking jigs, several cutting boards, a couple of Corian handled knives and a few Corian handled chisels among other fun items.


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