How to Cut 45 degrees

Hi Nicolas,
Essentially, yes. However:
1. That bit, though nice and solid, may be too small - I cannot be sure from the picture. The carbide edge should be long enough to do the final pass(es) in one go, without adjusting bit height. It will be enough to adjust the fence progressively back between passes, without fiddling with bit height. You can work out whether the carbide edge is long enough, by using Pythagoras' theorem.
2. I know you said the wood is the wrong way around, but it bears remembering that you will be moving the wood from right to left.
3. That setup is fine for long edges, such as you show. But if you want to do short edges, you will need some kind of support, to stop the wood rocking during the cut. The simplest would be a plywood right-angled "triangle", with a piece of wood glued/screwed under one short edge. This rail would run along the front edge of your table, keeping the workpiece at 90° to the line of cut. Or you could use a piece of square timber, making sure you keep it pressed along the fence.
4. Be aware that you may well get tear-out at the end of each cut, so you would want a scrapwood backer, that the bit could cut into (reusable for the final cut). If you are cutting short edges, the "triangle" or square guide can serve as the backer, provided it reaches close to the fence.
5 . Keep your fingers well away from flippy-spinny things, use push sticks and feather boards. You do not have grooves for feather boards, but they can be clamped to the table and the fence, especially if you make them yourself, and make them long enough. The goal is 10 in, 10 out (fingers, that is).
6. For efficiency, rout all the edges at a particular fence setting, before moving the fence back.
7. It pays to rout an extra board in each size, in case of snafu's.

Good luck. This is really a job for a table saw, and I have never gotten that clean a cut on Chinese ply. This is something where you don't want any chip out. You want as many layers as possible, which only happens with some premium big box ply, or Baltic Birch, or ApplePly, a premium hardwood ply made in America.

You are also likely to have some issues with holding the box in place when you glue up the corners. That can be alleviated by using wide tape, face up, and laying the outside pieces end to end and just touching. You can fold this up so the tape holds the joint in place.

It is a very weak joint. Pre glue the ends, then add more when you put them together. That will help some, but half the ply shows the endgrain, so the adhesion is a little spotty.

If you have a table saw with a really good blade, you would first make a groove for the bottom on the inside of all four pieces. Be sure all four pieces are grooved the same. The bottom cut will be hidden at the corners after glue up. Cut the miter on one end, then use a stop block clamped to your miter gauge's fence and cut the other end. You can do this on your table with a miter gauge, or by clamping the stop block to whatever you're using to push the piece through. The piece will want to fly away when it hits the bit, so make sure it's secure.

On a router table, you can likely do this by butting the end of a piece precut for length against the fence, and aligning the fence to the center of the cutter. Instead of making so many passes and getting it just right. Set the fence for the final cut, and move the workpiece up toward the bit a little closer with each pass. Don't move the fence!

Unstead, put some sticks at the end of whatever you're using to hold the workpiece. Place them between the fence and carrier. That way you can control depth of cut and have it be the same for every piece. If you move the fence, you'll never get your pieces to match.

For real strength, you can use splines, see pix, either vertically inside the miter cut, or cut into the corners after assembly. For the pictured spline on a router, you will need a carrier you can get to 45 degrees. That means a jig of some sort slanted to 45 degrees and a small straight bit to cut the vertical groove.


For edge splines, you need a carrier that holds the box at 45 degrees. I generally use 1/8th inch blades and the the same thickness material for the splines, but you can make them any size you want.

I find it easy to make mitered boxes using my table saw, but set the blade angle with a Wixey blade angle gauge for exactly 45 deg. I frequently then spline these mitered joints full length with cross grained splines that I make using my table saw and a tenon jig, but setting the jig so it cuts the spline from what would normally be the waste between the jig and the blade. The resulting tenon becomes scrap when chopped off with the miter saw. I can then flip the board over to cut another identical spline from the opposite side, then flip the board end for end to cut two more splines from the other end of the board. I then use my miter saw set to 90 deg with a stop in place so as to cut each spline to the correct width (2x the spline cut kerf depth) off the end of this board. This yields 4 cross grained splines, but then I can go back to the table saw and tenon jig to make 4 more cross grained splines and repeat the whole process to get as many, plus spares, as I need (being cross grained, they break in the grain direction easily, but you can stack pieces in the joint to produce the full kerf spline. It doesn't need to be just one long spline). I make these splines about the thickness of my table saw blade kerf, so I can make the slots for the splines using just the table saw and blade.

Careful positioning of the table saw fence and flipping the mitered box sides over allows cutting the kerfs for these splines at 90 deg to the mitered box ends.

The photo shows the final result of one corner of a box that I built this way. The spline keeps the mitered box corner from slipping off position as the box is being glued up. Keeping the grain in the spline running crosswise, and in the same direction as the box sides, makes for a very strong glued joint.

On the first try, cutting these mitered box corners and then cutting the kerf in each mitered end is a bit of a mind bender. Then making the cross grained (not long grained) splines poses a bit of a challenge too, but once you have made your first joint this way and you get your head wrapped around the process, it all becomes quite easy and relatively quick. This spline shown in the photo runs the full height of the joint. Looking at both the top and bottom, you will see that the spline shows from one end of the joint to the other, so the image will look the same.

Charley


Edit - Tom posted the same joint (just before me) with a great photo (his first photo). So now you have good info from both of us on this type of joint. His second photo is another way to achieve strong mitered corner joints, but different jigs and process are required for this joint. With both methods, the grain of the spline needs to run in the horizontal direction and in the same direction as the grain in the box sides. Running the grain top to bottom in the spline will produce a very weak joint.

jw2170 said:

Some good advice given here, but if you still want to use the router, I would go for a cutter such as this...

Torquata Chamfer Moulding Router Bit

Chamfer bits are great for finishing the edge of your work with an edge at a variety of angles from 15 – 45 degrees.

www.timbecon.com.au

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That bit reminded me that I do have one which looks similar and perhaps I do have other ones better? As mentioned the set I have is at least 20+ years and never been used.

I never understood how to use the bearing on these bits. I know the bearing will follow a surface but how do you control the depth of cut using the router free hand?

CharleyL said:

You should also look into Lock Miter bits, if you are set on cutting the box corners at 45 degrees with a router. The Lock Miter Master Gauge from Infinity tools makes setting the height of the bit and the fence position based on a center line on the thickness of the work pieces. A locked together joint as well as a perfect 45 degree cut are possible when using these bits, but you need a follower scrap piece to keep the bit from splintering the trailing end of the work where the bit exits the cut. Don't try to cut full depth in one pass either. Use shim boards between the fence or table to make a light pass before cutting full depth.

This is why I prefer not using these bits and just cutting slots in 45 deg ends for the cross grained splines. It just goes faster and easier, with more accuracy for me, but lock miter bits do work and produce good joints, if you can work through their splintering problems.

Charley

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I have the Infinity gauges and they make the bit much easier to use. Without them you have to do trial, trial, trial, error, error, error and toss the bad pieces. I use it with the Rockler center marker. Pix of both below. One shows the joint finished, the other shows how to align the jig with the wood's centerline. The jigs are held in place by small but strong magnets. The bits come in two basic sizes, so do the jigs, although they are really easy to misplace.

This is one method that I have used successfully. I've also used the lock-mitre which is the ultimate corner joint