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I'm using a rabbeting bit on a 1" x 4" red oak board.

When I cut at 1/4" depth and 1/2" width, it works fine. When I increase the depth of cut to 3/8", the wood splinters.

Is this common? Is it typical to do two runs and if so are there any rules of thumb with how deep a cut you can make with a rabbeting bit?

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up vote 17 down vote accepted

It is generally advisable to take multiple passes with a router when removing large amounts of material. Adjusting your feed rate might also help

It's also a good practice to make some test cuts on some scrap material in order to ensure your depth of cut and feed rate won't cause any problems.

Doing this also gives you an opportunity to "sneak up" on the exact dimensions of the cut you are trying to make. Similarly, often times, round-over and other shaping bits might burn the wood and making a final small pass allows you to easily remove this leaving a nicely finished edge.

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When I cut at 1/4" depth and 1/2" width, it works fine. When I increase it to 3/8" depth the wood splinters.

In addition to the Answer given by @Steven already, it may be a good idea to score the fibers that outline the sides of the cut prior to performing routing operations. Oak is notorious for splintering due to its open grain.

This can easily be done with a knife and a straight-edge:

knife
(source)

or with a table saw, running the piece backwards over the blade:

table saw
(source)

Note that the table saw method should only be attempted if you have the piece securely fastened on something like a miter sled. See the referenced web page for further details, or this video.

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1  
Good methods but use MUCH CAUTION if running a workpiece backwards over a saw. The saw will want to grab onto the workpiece and fire it across your workshop. You should only ever do this with a very shallow cut to eliminate the possibility of the wood pinching onto the blade as much as possible. – WhatEvil Jan 14 at 20:05

Yes it's normal. How much grain break-out or splitting you get is a function of several factors:

  • Sharpness of the router bit
  • Rotation speed of cutter
  • Feed speed
  • Depth of cut
  • Cutter geometry (straight-fluted, upcut spiral, downcut spiral, number of flutes)

In order to reduce splitting as much as possible you would:

  • Use a sharp cutter
  • Increase the RPM of the cutter up to either the max rating of the bit, or the max your router will go to, whichever is lower
  • Use a slow feed speed, though not so slow that you generate a lot of heat and burn the timber
  • Take many shallow passes of cut
  • Use a cutter with more flutes, or if suitable use an upcut or downcut spiral - upcut spiral pulls the chips upwards but will also tend to break out the top face of a board, downcut spiral pushes the chips downwards and will tend to break out the bottom face of a board, but if you're rabbeting (i.e. leaving some material on the bottom of the cut rather than trimming the full thickness of the board) then this is a good cutter to use. A downcut bit is also good to use for slotting (with timber either side of the cutter), provided that you have some good extraction or you're using compressed air to blow into the slot as you cut, so that chips are ejected properly. Spiral bits are a bit more expensive than straight-fluted cutters, so it depends on how much work you're doing / how often you'll use it as to if it's worth it to get one.

Practically, what this means is, once you've got a decent bit and whacked the RPM up on your router, play around with the depth/width of cut until you find something that works. It's very common when hand-routing to make several shallow passes until you get to finished depth/width. Another strategy is to use bigger cuts when hogging out most of the material and then leaving only a shallow cut for your last hit. Taking out the large cuts is called "roughing" and may leave you with a rough cut.

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