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I'm trying to decide how long the cutting edge on a pattern router bit should be. I'm trimming a 2-by (1-1/2" actual) with a pattern / template on top, and I'm having a hard time finding a 1-1/2" (cutting) length bit. I found a 2" length. Is there a reason I shouldn't just use the 2" length? I can't find any technical information about selecting the appropriate length, or whether I should avoid a longer length.

P.S. I'm aware of the stability considerations with shank diameter; that's not what this question is about. The 2" length bit has a 1/2" dia shank.

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You could (and probably should) do this with a shorter bit. Unless you're just skimming off little bits to finish a cut previously made with a jig or band saw, cutting 1.5" of wood with a router will require a rather beefy router motor.

Make a first pass with the bearing riding on your template. Once this pass is complete, lower the router to cut deeper and allow the bearing to ride on the freshly cut wood. It should be exactly the same as your template after having made that first pass. Depending on what bit you have and how much wood you're actually taking off, you may even want to make 3 passes to work your way down.

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  • Thanks, but why? Why do I want to use a shorter bit and multiple passes?
    – rory.ap
    Apr 21 '21 at 12:54
  • By cutting less wood each time, it's A) easier on the router, and B) easier for you to control, thus more safe and more likely to get a good quality finish cut requiring less sanding.
    – FreeMan
    Apr 21 '21 at 12:56
  • Okay thanks. In my case, I'm making about 35 rafter tails for a pergola, and I'd much prefer the one-and-done than having to remove the template, etc. for each one. They're up high so they'll be fine after sanding.
    – rory.ap
    Apr 21 '21 at 13:13
  • If you're working in the air, you'll definitely want to make smaller passes as the control will be easier. Also, you're probably using (or would like to for ease) a smaller router, maybe even a little battery powered one, that may not have the oomph necessary to drive a 2" long bit through that much wood. I do hope you're cutting the tails to shape on the ground before you install them!
    – FreeMan
    Apr 21 '21 at 13:57
  • I'm cutting about 60% of them on the ground. Unfortunately due to the construction method I used, the exterior rafters are already in place.
    – rory.ap
    Apr 21 '21 at 18:42
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You wouldn't want a 1-1/2" bit in the first place, as that would require absolute precision with both router table depth and technique. You almost always want some buffer with woodworking tools, so a 2" seems ideal anyway.

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