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I'm looking to finish my woodworking project by rounding the edges of the surfaces with a bullnose routing bit. Like other furnishing in our home. I'm working with 22mm MDF and don't want the straight edges so looking for a radius of at least 1" or even bigger to make it a bit more 'flat'

However I can't seem to find anything on 1/4" shank that is big enough. Is there a particular reason for this or just that it's something amateurs are not supposed to do?

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There are multiple reasons you don't find 1/4" shank bits with larger profiles. For example:

  • Safety - the larger your profile, the greater the stress on the bit overall, including the shank. Nobody wants a 1" bit shearing at its shank while cutting. In addition, as the bit's mass approaches the mass of the router (e.g., a small trim router), it will become more difficult to control. Requiring a 1/2" shank incidentally also requires you to use a larger router with greater inertia. As Jacob Edmond and Ljk2000 pointed out in comments, variable speed is another important feature, since you need to slow down the motor for large bits in order to maintain a safer operating speed at the outer edge of the bit.
  • Power - routers that only handle a 1/4" shank generally have smaller motors. If you put a large cutter on a small router, you are more likely to stall the motor, especially if you take large bites out of your material.
  • Cost - once you go beyond a certain size, it does not make economic sense to manufacture bits in both 1/4" and 1/2" shank varieties because larger bits are already more expensive and most likely sell in smaller quantities, and most routers capable of spinning larger bits can accept a 1/2" shank.
  • Heat - I have a hard time believing this one is very significant once you get to the larger bits, but I'll include it for completeness anyway since it does come up in discussions of 1/4" vs. 1/2" shanks. To some degree, the shank helps pull away and dissipate heat from the cutters, and a smaller shank won't be as effective as a larger shank.
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  • I guess it's primarily the safety. Still think the router I'm using is pretty powerful and in my opinion heavy enough :-) – Sander Cox Apr 11 '17 at 12:29
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    The 1/4" shaft is not enough regardless of the router. When you start getting that big, not only do you need the bigger shaft, you also want a variable speed router. The wider the bit's diameter, the greater the speed is at the leading edge of the cutter. – Jacob Edmond Apr 11 '17 at 17:21
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    Agree with Jacob Edmond big time. I have heard of some stories how a large bit spinning fast can explode. And here is a example of how fast a bit could be traveling (at blade edge) at 20,000 RPM. a 1/4 straight bit will (at the tips) be traveling at 14.96 MPH (27.07 KPH). A one inch bit (at tip) will be traveling at 59.49 MPH (95.73 KPH). Figured that would be a 'fun fact' for some people. – Ljk2000 Apr 13 '17 at 2:31
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Checking some of my catalogs I could not find round over bits much over 1/2" radius for 1/4" shanks. I believe the reason is the because the torque generated is too great for such a small diameter shaft. A 1/2" shaft will have four times the cross sectional area in the shaft affording a great deal more strength. I found round over radii up to 1 1/4" for 1/2" shafts. If your router cannot accept a 1/2" shaft colette you may want to consider borrowing or renting one for the task.

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