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I just got a tongue and groove router bit, and on my first attempt on some scrap I had about 1/32"-1/16" of an offset between the two boards I was joining.

I think I went wrong by thinking I could eyeball lining up the groove bit with the already cut tongue. Obviously that didn't work though. How can I easily install the bits in the router at the correct height? (Don't have a table yet, just working with a handheld router.)

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  • I don't know how to align exactly (I have some ideas ... trial and error??), but I like it! Looks like the groove portion sits a little high in the board. – Pᴀᴜʟsᴛᴇʀ2 May 16 '15 at 20:24
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    Don't these bits come in sets which means they should be aligned already by having shafts of the same length? I would have guessed that sticking them equally far into the router should ensure a perfect joint. – null May 17 '15 at 10:33
  • @null This had occurred to me as well. Perhaps sticking them equally far into the router is the issue? Although I'm sure the put in equally as far. I wonder if his test pieces were not perfectly true – Matt May 17 '15 at 12:04
  • @Matt - The boards will be true to themselves, so that is not an issue. How the bit is adjusted or fit into the router chuck is what'll make the difference. I'm not sure of an easy way to make this happen, so have only commented. And also, aren't most routers these days depth adjustable? You should be able to compensate that way as well, but every time you change the bit out, you'd be readjusting and tearing up more wood to get it right. I am just not sure of an easy and sure way to make this happen on a consistent basis. – Pᴀᴜʟsᴛᴇʀ2 May 17 '15 at 12:29
  • @Paulster2 The boards will be true to themselves is not always correct. Yes if the board is perfectly flat to begin with yes. But there are plenty of reasons for a board to not be. Also, if you look at the picture the groove portion is reversed (see the grain) so where it was assembled the two pieces no longer would compliment each-other. I don't own a router so that is just speculation for me. – Matt May 17 '15 at 14:19
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How can I easily install the bits in the router at the correct height?

The first time you use the set:

  1. Install one cutter to a height that looks right for your lumber.
  2. Make a on a test piece. Label this piece so that you'll remember that it's not scrap. Make sure that you mark the side of the piece that was against the router's base plate.
  3. Switch to the other cutter, using the first test piece to eyeball the height.
  4. Make a cut on another test piece.
  5. Measure the error using a dial indicator or a set of digital/dial/vernier calipers, i.e. anything that'll let you measure small distances accurately.
  6. Adjust the bit height according to the error.
  7. Repeat steps 2-4 as necessary.
  8. Once you're happy with the result, label the last test from the second cutter as you did for the first.

Now you've got two test pieces that you can use as gauges in the future. For either cutter, you can use the corresponding test piece to set the height. Install the cutter and, with the test piece on the base plate, adjust the height until you can turn the cutter by hand through the test piece with as little resistance as possible. Lock the router height and proceed.

It might help to point out that most routers have some means of measuring changes in height. A Porter Cable 690, for example, has a collar with a series of tick marks that correspond to 1/128". If your router doesn't have something like that, you can also use a tool like a dial indicator to measure changes. If you can measure the amount of error in your first cut, and if you can measure changes to the bit height, then your second test cut should be spot on.

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    "make a cut on a test piece" is a big one ... a lesson my father has tried to instill in me but never really took until I made some mistakes on a koa go board I was making. Those were expensive mistakes. – Daniel B. May 17 '15 at 1:28
  • Testing on scrap pieces and keeping the final ones as gauges to use in the future is a good suggestion. To set those up, the trial-and-error method will work, but it is often overly complicated and time-consuming. Try to avoid reading and interpreting measurements as much as possible. Transfer measurements and settings directly whenever possible. – rob May 18 '15 at 5:45
  • @rob That's why I said using the first test piece to eyeball the height. You can generally get pretty close if you have a sample cut by the complementary cutter. But when you're working in the realm of measurements taken with a dial indicator, direct transfer of measurements is often difficult to achieve. – Caleb May 18 '15 at 5:54
  • @Caleb By directly transferring, I mean physically transferring the measurement, not eyeballing a setting relative to another measurement. Sometimes the hardest part is figuring out which tool is the right one for the job. In this case, it's not a dial indicator, but that will get the job done eventually. – rob May 18 '15 at 6:02
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    If you cut the final 'test' piece on the backside of the other 'test' piece, you now have a set-up jig that works for both bits. – TX Turner May 18 '15 at 14:15
3

Although you're using a tongue and groove set, the process would be very similar with other bits. However, the reference surfaces might differ.

Depending on which bits you're using, you should have at least 2 routers, one set up to cut the groove and another one (or two) set up to cut the tongue. For the examples below, I'll assume you only have 1 router.

Keep in mind that anytime you're trying to make mating parts, you should avoid measuring on each separate part. You can measure on the first part, but after that you should ideally use some type of gauge to transfer the measurement directly from one part to the other. In many cases, the first part itself is the gauge.

Method 1: no special tools required

  1. For simplicity of explanation, let's cut the tongue first.
  2. Install the groove cutter bit and line the tongue board up alongside the router bit. Don't plug in the router yet (you did unplug it to change the router bit, right?)
  3. Adjust the height of the tongue cutter until it is perfectly lined up with the tongue. With the router unplugged, use your finger to tell when the groove bit's cutter is perfectly lined up with the tongue. Whether it's true or not, the common wisdom in the woodworking field is that the human finger has a resolution of about 1/1000". Alternatively, you can lay a small flat scrap across the top of the tongue and micro-adjust the router bit until it just kisses the scrap (in that case, a backlight will help you see when the gap is closed).
  4. Cut the groove on the second workpiece.

Method 2: caliper

Caliper

(Source)

For the purposes of this example, I'll assume you have a caliper, but you can apply the same gauging concept to other measurement devices.

  1. For simplicity of explanation, cut the tongue first.
  2. Lay a shim (e.g., a flat, rigid metal ruler) alongside the tongue piece so the tongue overhangs the shim.
  3. Using the depth gauge of your caliper, measure the distance from the the top of the tongue to the shim. Lock the reading into your caliper using the lock screw. This measurement is the distance from the top of the tongue to the baseplate of your router, minus the thickness of the shim.
  4. Install the groove cutter bit and line the groove board up roughly alongside the router bit, then raise the bit upward slightly (assuming your router has the bit facing upward at this point).
  5. Register the depth gauge end of your caliper along the groove bit's cutter.
  6. Using the router's micro-adjustment knob, adjust the bit downward (again, assuming the bit is facing upward) until the caliper's depth gauge bottoms out on the shim (which is now lying across the hole in your router table or baseplate).
  7. Cut the tongue on the second workpiece.

Method 3: caliper

For the purposes of this example, I'll assume you have a caliper. Note that this example is slightly convoluted in an effort to demonstrate how to take a reading using one part of a caliper, then transfer it using a different part.

  1. For simplicity of explanation, let's cut the groove first.
  2. Using the lower jaws of the caliper, take the outside measurement of the thickness of the lip that was touching the router base (or router table), and lock this measurement into your caliper with the lock screw.
  3. Install the tongue cutter bit.
  4. Transfer your previous measurement directly when setting up to cut the tongue. Use the depth rod to gauge the height of the tongue cutter, while adjusting the height using your router's coarse and fine adjustment knobs.
  5. Cut the tongue.
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I consider that to be a very good match for not using a table.

it doesn't take much to get them off just a little bit. Incra has gauges that can be used to measure the height of bits and blades down to 64th of an inch. If you are really serious about being that close, I'd get a router table (or make a jig) and get a gauge, and you have to make sure both the wood and the router are exactly 90 degrees to one another (whether by hand or table). If your board has even a little warp across it and you don't keep the front flat, you will be off.

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