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I would like attach a 2"x2" wood rail so that it's flush inside of the 4"x4" wood post, like in the drawing I've included.

My question is: how would I be able to cut out the 2" cube in the 4" wood post so that the 2" rail fits nice and flush in the wood post? What tools would I need and is it something that a novice wood DIY guy could do?

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  • Would it be acceptable that the 2x2 extend to the outside of the post? What sort of precision are you after? (Is this furniture or what?) – isherwood Jul 17 '18 at 20:02
  • it would take 30 seconds with an oscillating saw; best $20 i ever spent at harbor freight... – dandavis Jul 17 '18 at 20:59
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You could rough it out with a drill, maybe a router. Then tidy up the corners and edges with a chisel and mallet.

You could use a saw to establish the edges, then finish off with chisel and mallet.

Given that people such as Paul Sellers routinely cut blind mortices remarkably quickly using only a chisel and hammer, this could certainly be done that way too.

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Drill one corner hole on the top and one corner hole on the face. Use a saber saw to cut the vertical side cuts. Use the holes to turn the saw for the two cross cuts between the side cuts (one across the top, one across the side face).

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Cut a 2"x2" notch in the 4"x4" post.

Put glue in the notch and on the end of the 2x2.

Put end of 2x2 in the notch.

  • For such a small cut, would you use a hand saw? Could you make the notches “square” with a chisel? Is there a router or grinder that could work? – Lee Sam Jul 17 '18 at 18:24
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    Sorry, that wasn't the question asked. :D – The Evil Greebo Jul 17 '18 at 18:56
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    (But I'll tell you how I would do it - I would build a jig and use a 2" long, very thin router bit and cut out the cube. Then I'd use a narrow wood chisel to square the corners the router bit missed.) – The Evil Greebo Jul 17 '18 at 19:18
  • I think that was helpful...+1 for the answer. – Lee Sam Jul 17 '18 at 20:18
  • Thanks for the answer. What router bit would you suggest? – rbhat Jul 17 '18 at 20:31
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You can not do it freehand with a router. You could do it with a utility knife and chisel, but adding a saw and drill will make things a bit more foolproof. Remember that everything will be out of square, so make the rough opening undersized and trim to fit. (If looking good from above is the primary concern consider tapering the ends of the rails a couple of degrees so that the bottom is slightly narrower than the top. That hides any imperfections in fit.)

One you have the cube marked out on the post, drill two holes on the long grain side tangential to the lower corners. That will allow you to cut the sides with a dovetail or back saw, the toe of the saw moves back and forth in the hole. That will give you the cleanest edges, cutting the fibers with a utility knife and straightedge is quicker but rougher.)

With the edges cut place a chisel (1" is good size) on the face about a quarter inch below the top of the post and strike it sharply to sever the wood fibers. Once you have a piece cute on three sides (2 saw cuts, 1 chiseled line across) place the chisel on the top of the post at the same depth (Or slightly shallower) as your first chisel cut. Strike the chisel again to split the wood along the grain down to the cut and voila, you've removed a little piece of the cube.

Repeat the process, cutting the end of the split then splitting along the grain to enlarge the hole. As you go you'll get a better idea how big of a bite you can take without making a mess. As you get close to finished you'll want to stop with the mallet and push the chisel along the bottom and inside edge by hand, paring them down flat.

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I would use a router. Within reason, the bigger the bit, the better. I'd say 5/16-inch, plus/minus 1/16-inch.

Within reason, the diameter of the router bit won't matter much, except that a thinner bit:

  1. Removes less material per revolution.
  2. Tends to bend more and could easily break (especially at 2-inch length/depth) so the cut has to be made slower.

Because of that, the thinner the bit, the longer it will take to make all the cuts, and the more the cutting speed will effect the accuracy of the cut.

Depending on the hardness of the wood, and the various cutting factors (bit hardness, sharpness, and diameter, router spindle speed, cutting feed-speed, cutting depth), you may need to make several cuts, starting with a shallow cut then increasing the cut-depth until you reach the final depth.

The larger radius created by the larger diameter bit is not relevant, because the radius material will be (mostly) cut away when the router is turned 90-degrees to cut from the other face.

For example, when you make the cut (basically a square-U) with the base of the router against the end of the 4x4, it will leave a radius (inside) for the full 2-inch depth at each of the 2 corners (turning points) of the "U". If you use a larger diameter router bit, this radius will be larger. But, the bottom (the deepest part) of the cut will be square, with no radius along the full cut.

Then, when you turn the router 90-degrees, to make the same (square-U) cut with the base of the router against the the side of the 4x4, the cut at the bottom will remove the radius that was left along the depth of the first cut.

The only thing that will remain is, at the bottom, at the 2 spots (inside corners) where the 3 cut faces meet, there will be some material that needs to be removed. You could handle that in (at least) 4 ways:

  1. Use a chisel to clean that out (should be relatively easy).
  2. Switch to a smaller (thinner) router bit to re-cut only the 2 inside corners, then follow up with a chisel to remove the rest.
  3. Use a drill bit, a little larger in diameter than the router bit, and drill into the 2 corners at about 45-degrees, to a depth of about, or a little more than the diameter of the router bit. This will create a relief "pocket" at the 2 corners to allow room for the corners of the 2x2.
  4. Ignore it. Just knock off the 2 corners of the 2x2 that will fit inside the cut. This part of the 2x2 is not visible once it is assembled, so the cutting away of the corners does not have to be very precise at all. Just cut enough so the 2x2 fits all the way into the cutout in the 4x4. If you cut a little too much, or even a good bit too much, it won't matter because those corners of the 2x2 are not visible in the final assembly.

I'd say the best choices are #3 or #4. Some may say they're a "hack", but they're probably the easier and quicker ones and perhaps less error prone. I'd probably go with #3.

This could be done with a router, by marking or scribing the cut lines, then cutting "freehand", but if the accuracy and finish of the final fit are very important to you, the use of a table mounted router with fences, or well constructed custom jigs/fixtures are recommended. Also, it would probably be helpful to do a trial run with scrap lumber first, to familiarize yourself with the process. And as always, measure twice, cut once.

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