Lately, I’ve been doing a great deal more woodworking. I came across this very interesting bit that you can use to make 90° angles in boxes. It is called a 45° lock miter bit. You use a router to cut the joint into the boards to make a very tight connection.
Unfortunately, the router that I normally use isn’t precise (or maybe strong) enough to do the joint reliably. The bit keeps shifting, so the joint is not as clean as it should be.
I thought I’d share the instructions here anyway though, since relatively few folks know how to use this bit. The design creates a box that is just large enough to hold a double 12 set of dominoes.
Tools used:
| Table saw | 3/8” router bit | Planer |
| Router table | Drill press | Chop saw |
| ¾” lock miter bit | ½” drill bit | Band saw |
| ½” router bit | Set-up blocks | Digital height gauge |
| 1/8” round over bit | Various router featherboards |
- To start off, prepare a finished board that is 6 ½ -7” wide ¾” thick and 40 inches long at the start of class.
- Using the chop saw cut from the finished board, a 12” board (the front and back) and one 13” board (the two sides). Note: Originally, I cut the boards into their final length before using the lock miter bit but found that anything less than about 5 inches long was very difficult to add the lock miter cut and likely dangerous. This plan doesn’t cut the side boards until after the lock miter joints are in place.
- Trim the 12” board width to 5 5/8” on the table saw and the 13” board width to 6”. Note: Keep the scrap wood, since we will use these during the router setup.
- There are several ways to set up the lock miter bit on the router table. If your board is EXACTLY ¾”, you can use the setup guide (white plastic) that came with the bit. I prefer to use the digital height gauge to measure and adjust the height of the bit to the same as the board thickness. Set the height of the bit to ¾”. Note: Lock the router height when you have adjusted the height appropriately.
- Set the guide up against the bit and adjust the fence horizontally until it just touches the guide. Once you have the fence set, you can lock down all the fence knobs. Note: You can lock down the main adjustment knobs and use the micro adjustment to move the fence in or out. Once you have the fence in place, lock down all the knobs.
- If your board is not exactly ¾ of an inch, it still needs to be less than ¾ of in inch validate the setting of the depth of the bit by aligning the board horizontally and ensuring a straight edge laid along the board would almost touch the cutting edge of the bit. To orient the board vertically and do the same with a straight edge, checking that it almost touches the front cutting edge of the bit.
- Make a test cut on a board or boards that are the same thickness (hopefully the scrap boards from earlier). Note: This is a very aggressive bit, so be sure to use feather boards to hold the board up against the bit when you run it through.
- Test to make sure they fit together appropriately. If you have it set up right, make sure the bit height is locked in place. If the boards don’t line up, go back to step 5 and make sure the bit hasn’t moved during the testing process. Note: If you do the alignment check on a single board, you will need to cut it in two and then connect the boards together. When the bit is setup properly, the edges of the two boards should be flush.
- Run the longer dimension of the 13” board (these will be the two side pieces once cut) through the lock miter bit in the router while laying the board horizontally on the router. Be sure to do both sides of the board and that the cuts are aligned in the same direction. Note: Be sure to use a feather board to help keep the board up against the router. Holding the board consistently against the deck and the fence is important.
- Run the longer dimension of the 12” board (the two end pieces once cut) through the router oriented vertically. Be sure to do both sides of the board. Note: Use the tall featherboard and ensure as you press down and run the board through that your hands hang over the fence, in case the board shifts unexpectedly. Be careful not to press down too hard, since your routing to a fine edge. If your board is bowed make sure you place the bow away from the bit (on the outside).
- Using the chop saw cut the 12” board into two 5 5/8” pieces. Cut the 11” board into one of 4 7/8” and one of 5 5/8”. You can now check the box to ensure that it fits together. Mark the left, right, front, back boards, as well as their top and bottom. Note: If you suffer any tear out, you may be able to address it when you trim the boards. Cut so the lock miter joint is facing up. Use a sacrificial board if the board you’re cutting doesn’t touch both sides of the fence.
- From the remaining large board, cut a 5 5/8” board, using the chop saw. This will be the bottom of the box.
- Using the band saw cut the bottom board width to 6”.
- Now we need to cut the grooves into the sides, so the top can slide into it. Install the 3/8” router bit and adjust it vertically until it will cut 3/8” into the board. Lock the height once you have the correct setting. Note: When pulling out any bit from the router DO NOT raise the router all the way, since this can warp the deck or damage the router.
- Adjust the fence so it will cut into the side boards, 3/8” from the top of the board. Note: A more accurate and repeatable approach would be to use the 3/8” set-up blocks located in the tool room (on the right-hand side as you enter the door – see the attached photo). Place the set-up block between the bit and the fence and adjust accordingly.
- Now we shift our attention to the side boards, using a 3/8” router bit, cut the groves into the sides and the larger end piece, 3/8” down from the top and 3/8” into the board. The top board will slide into this groove when the box is assembled. Note: You should be able to lay the board flat on the router for greater control when making the cut.
- Now install the ½” router bit and adjust the router height to cut 3/8” into the board, similar to what was done in step 17. Lock the router height once you have it set. Adjust the router fence so that the bit will cut 3/8” into the edge of the board. Lock down the fence. Note: We moved to the ½” router bit, so the bit extends beyond the edge of the board. We wouldn’t want any slivers of uncut wood if our adjustment is further than 3/8”.
- Cut a 3/8” deep rabbet around the entire bottom of the box, so it can fit inside the box sides when assembled. Note: After you make your first cut, be sure to check the width of the rabbet. It is OK to be a tiny bit wider than 3/8” but you cannot be less than 3/8”, since then the box will not fit together.
- Use the same settings to cut a groove (rabbet) along the bottom of all 4 sides, on the inside of the box. Now the bottom should fit within the assembled box.
- Run the remainder of the original board through the planer until it is 5/16” thick. This will give us a sixteenth inch clearance to slide. Note: The board should be of sufficient length to meet the length limit listed on the planer storyboard.
- Measure the size of the opening at the top of the box. We’d like it to fit fairly tightly within the sliding rails at the top of the box and extend out to the edges of the box.
- Measure the top board so it has a T shape to cover the front joints of the box.
- From this thin board, cut it down to the measurements identified, using the chop saw and the band saw.
- Optionally, you can use the drill press to make a ½” diameter hole ¾” from the edge of the board at the center of the top board.
- Use the band saw to trim off the unused lock miter on the sides boards above where the top slides in.
- Fit, sand and glue the box together. Note: Once glued, this joint is not coming apart. In fact, sometimes I have trouble getting it apart even when it is not glued.
- Optionally, install a 1/8” round-over bit into the router and shape the edges of the assembled box, on the sides and the top.
Beyond the intersection of technology and life 