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I'm working on some landscaping projects with interlocking halved joints. The material is 100x50mm treated pine so my cuts are 50x50mm.

I'm currently achieving my cuts by clamping several lengths together and using multiple passes with a Skilsaw as if it was a dado. This is working but the cleanup with chisel and rasp is quite time consuming. I'm considering using a router to speed things up but I have some questions before I spend money on chunky bits:

  1. Is a 50mm-deep cut practical and safe with a hand-held router?
  2. Is there such thing as a deep (50mm+) straight bit with a pattern-following bearing? This will impact the type of jig I'll need to build.
  3. Will this actually save me any time?

I've had a look around at wobble blades and stacked dado sets but I can't find any which will cut a 50mm wide swath. That and I don't have access to a saw bench.

  • Have you considered installing your router under a table ? – Nelstaar Jun 3 '15 at 5:52
  • @Nelstaar Yes I have thought about using a router table but with up to 2.7m lengths (~100kg with 6 pieces clamped together) it seems more practical to take the router to the material rather than the material to the router. I could be wrong though, I'm very much an amateur! – tristanm Jun 3 '15 at 6:39
  • +1 to @rob's laissez-faire approach (don't clean up much; use screws), but if you must clean the bottoms quickly, what about a router with a more common bit (1/2", 1-1/2" length, unguided, flat bottom), a guide collar, and a template? You'd do the circular saw work; knock most of it out with a hammer; clamp your template on, and then rattle around inside the groove for a shallow pass, then set full depth and rattle around again. (This plan might need a template guide that'll let the collet stick through the router base to get a full 2" cutting depth, as longer router bits are uncommon.) – Aloysius Defenestrate Jun 3 '15 at 13:53
  • What about a handheld jigsaw? You could probably freehand it, or make a jig for the jigsaw to follow. – Jeremy Jun 4 '15 at 1:18
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    ...but I can't find any which will cut a 50mm wide swath You don't need a dado cutter that cuts the entire width at once. If you need a dado that's wider than your cutter, you simply make several passes. – Caleb Jun 4 '15 at 4:39
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First, to answer your 3 questions:

  1. Yes, you can safely make a 50mm deep cut with a handheld router, but only if you take several passes, only cutting part of the depth at a time. If you try to make the entire cut in one pass, you'll very likely either damage the bit, stall your router, injure yourself, or some combination of these.
  2. There are certainly bottom-bearing bits this long, but it seems to be harder to find top-bearing (pattern) bits that will make that deep a cut. However, you can buy a collet extension which will increase your maximum depth of cut. Keep in mind that a pattern bit's cutters don't need to be long enough to cut the entire depth all at once. For your first pass, the bearing rides along your pattern, then for deeper passes the cut from one of your previous passes becomes the pattern (assuming your bearing is the same diameter as the cutter--which isn't always the case).
  3. If you are asking whether this will be faster than making several passes with a 3/4" dado stack, then the answer is probably no.

The main reason you cannot find a dado set that cuts a 50mm wide dado is that no consumer-grade table saw has an arbor long enough to accommodate that width, at least in the US (and my understanding is that dado blades are prohibited as unsafe on European table saws). Such a wide dado stack would probably also strain or stall the motors of many consumer-grade table saws. In the US, radial arm saws were also commonly used with dado blades, but have declined in popularity. If you want to use a dado stack or wobble dado blade, you'll have to make multiple passes.

There are several options available for cutting this with a router, from setting up guides that register against the router's baseplate and using a large straight bit, to a spiral bit with a bushing and pattern, to a straight or spiral pattern bit. You can buy large spiral pattern bits, but they're expensive. You can buy a collet extension if your bit isn't long enough to cut 50mm deep. Regardless of which bit you use, you will still need to make several passes and depending on your setup you may be just as well off hogging out the bulk of material with some other tool (even a bandsaw, drill press, hand saw, jigsaw, table saw, or circular saw) before cleaning it up with the router.

Of these two overall options, the table saw will still probably be the fastest. I'd suggest ganging up several boards, clamping them together (in a crosscut sled, if using a table saw), and cutting the dadoes in all of them at once.

Personally, if I was building landscaping projects from pressure-treated wood, I wouldn't worry about cleaning up the joints too much and would just use construction screws that are designed not to corrode when used with modern pressure-treated lumber. Depending on the application, I might not even bother to use glue.

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    Great answer, @rob. Thanks for the detail. I may look at investing in a table saw and cut multiple passes, possibly with a single blade and a jig with several shims. – tristanm Jun 3 '15 at 7:32
  • @tristanm Glad to help. An easy way to do it with a table saw sled is with 2 stops, one for the left side and one for the right side of the cut. Then you finish cutting out the middle, but you don't need to worry about precise positioning for the middle cut(s). – rob Jun 3 '15 at 8:14
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Really, do you want to turn that entire 50x50x50mm cube into sawdust?

Since you don't have access to a bench saw and the dado alternative , this is what I would advise:

Cut each end of the slot with your Skil saw. Knock out the 50x50x50mm piece with your hammer and clean up the bottom of the cut with your a wide chisel.

Most of your cuts will require little more than a touch up and you will get the thrill of doing something that requires a little hand craftsmanship.

If you still want to go with the router, you could use this approach and use the long router bit only to smooth the bottom of the cut,

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    I have found that two saw cuts and one mallet tap to the chisel pops out the block of wood and it is often clean enough as is to put the joint together! – ewm Jun 5 '15 at 15:52
  • If I want a cleaner cut than I get with the chisel smack, can I cut notches most of the way down both ends of the planned cut, break out the wood in between, then cut the rest of the way with repeated shallow passes with a blade with flat-top teeth? – Steve Nov 18 '17 at 0:26
  • Sounds like a plan. Go for it. – Ast Pace Nov 18 '17 at 4:11
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50mm is a trifle under 2 inches. There are certainly 2-inch-long spiral upcut bits, and you could use a guide collar, so this sounds entirely doable -- in multiple passes, preferably, and with a secondary base to bridge the wide opening for stability.

Unsure about relative speed.

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