Carb-i-tool Mitre Lock Bit

The miter lock bit is one of those that is seen as an easy solution to making quick strong joints, but so many sit unused in the router bit collection after the purchaser has given up trying to work out how to use the bit.

It isn’t too hard, but there are some definite steps to achieving the required accuracy.  Once you know them, using the bit really is straightforward, and can produce a joint that is mechanically strong as well as increasing the glue area of the joint by about 75%.

The bit is large, and can only be used with a table-mounted router.  Also, the router needs to be variable speed so you can set a suitable rpm.

Mitre Lock Bit

Mitre Lock Bit

This is the bit to the right – the item in frame to the left is the end of the Wixey Digital Height Gauge I was using to set the bit up accurately.

There are two primary orientations of joint that you use this bit for.

Producing a strong panel:

Panel Orientation

Panel Orientation

I’ve coloured one side so you can easily see the joint itself.  This is produced by running both boards through the router table, one with the good side down, and the other with the good side up, then flip one board over.  The joint is much stronger than a straight butt joint, with significantly increased glue area.

The other orientation is to produce a 90 degree joint:

Corner Orientation

Corner Orientation

This joint is created by running one board through the bit horizontally, and the other board vertically.  (The joint shown here is rotated 90 degrees, so that might be a little confusing if you are trying to work out how it was done!)

As you can see, this joint has an extremely strong direction – moving the pale-coloured board is resisted by both the glue joint, but also the mitre-lock itself.  This is an excellent joint for items such as drawer fronts, where a lot of load is placed on the joint as the drawer is opened.

Drawer Orientation

Drawer Orientation

In the example here, the pale board would be the drawer front.  Now, you could be even cleverer, and have the front of the drawer in that orientation, but the back of the drawer in the other – so the front resists the forces of pulling the drawer open, and the back helps hold the two sides together.

You can also combine 4 together to produce a wooden pillar etc.

Lock Mitre Bit Pillar

Lock Mitre Bit Pillar

Note that each side is identical – ie it has one horizontal cut and one vertical one.  This doesn’t do a lot, but does mean the whole pillar kind of clips together.  There isn’t a huge amount of mechanical strength in that, but it all helps.

In any respect, the joint is quite an easy one to measure up for, because you cut the boards to the final required width before machining the joint.  There is no need to factor in the width of the timber (such as for a butt joint, where 2 sides are much shorter than the actual side of the drawer, or for a half blind dovetail where you have to factor in the material used in the joint itself).

Now the secret of using this bit is in the setup.  There are not one, but two critical dimensions.

One is the height of the bit, and the other is the fence position.  Both need to be very accurate for the joint to work (and meet neatly at the corners)

First setting is the bit height.  The middle of the router bit must be set to 1/2 the thickness of the timber.  If this is out, then everything else will end up wrong as well, and you have no chance of working out what to move where to correct the joint.

The example here is timber that is 19.5mm thick, so the mid point of the bit needed to be at 9.75mm.  This was easily achieved with the Wixey Digital Height gauge.

Wixey Setting Bit Height

Wixey Setting Bit Height

The difficulty is determining what is the centre of the profile.  Now other writeup’s I have seen on the web have it pointing to a corner of the cutter, but unfortunately, this is close, but wrong.  The actual middle of the profile is halfway down the underside of the ‘wing’.

Mitre Lock Bit Centre

Mitre Lock Bit Centre

Set this point to 1/2 the thickness of the timber.

Next, you need to set the fence position.  This is also set so that the 1/2 way point of the vertical orientation of the profile is exactly 1/2 the timber thickness from the fence.  This point also shown in the above-image.

Mite Lock Fence

Mite Lock Fence

Detail of vertical centre

Detail of vertical centre

Fence Positioning

Fence Positioning

The Wixey certainly is an advantage for setting the fence position accurately as well.

Once both these settings have been made, you are ready to start cutting!

Small Pillar

Small Pillar

The Mitre Lock Bit is an interesting one to have in the router bit library, and this example from Carb-i-tool is typical of the quality bits they produce.  Accurate machining is a must for this bit, otherwise you never get the profiles matching, And typically for Carb-i-tool is the quality of the workmanship, and materials chosen.

8 Responses

  1. Hi Stu,
    I bought one of these about 6 months ago and couldn’t get it to work. A visit to Carb-i-tool resulted in them producing, after a couple of days, two pieces of wood joined by use of my mitre lock bit. No explanation as to how it was done. Put it aside again, but will definitely have another go after your words, (and photos0, of wisdom. Many thanks.

  2. Pce of crap is all i can say
    stuffed around with it for weeks on spindle molder and router table
    rips the shit out of the timber and burns


  3. Interesting perspective, but it looks to be one of those cases where the tool isn’t the problem.

    If you are getting massive tearout / burning, then I’d say you have the router bit running way too fast and/or your feed rate is excessive.

    Depending on what timber you are using, if it is excessively prone to tearout, then you may need to take multiple passes, using a secondary board as a supplementary fence so you don’t have to change the final pass setup. Also create a zero clearance fence which should help with tearout.

    Bottom line however, remember it is a large router bit, and needs to be slowed right down.

    If the mitre lock bit isn’t a Carb-i-tool, then I can’t speak for how sharp it may be from new, and it wouldn’t be fair casting negative comments against a different brand in a review of the Carb-i-tool bit. If it is Carb-i-tool, and was new, then I’d be very surprised if it wasn’t razor sharp and well finished.

  4. Good blog Stue

    I’ve been trying out my LM bit and I have found that knowing the center point of the bit is the crux of the problem. Thanks for showing me where it is. I am hoping it’s the same point on my bit. I did get nice clead straight cuts in end grain by making a high 7″ fence and a slider with a quick clamp to hold the work piece in position for the vertical cutting and a jig that clamps the workpiece in the horizontal position which slides along the fence with a long guide. These jigs are primarily for end grain cuts used to make boxes. They are not needed for long grain cuts as you showed, but feather boards and hold downs do promote safety and yield a cleaner cut. The sliding jig can be found on the Fine Woodworking website. It is really good. Please tell Larry Makin his American friend from Norway says hello if you see him.

  5. I bought one of these from the states.

    It came with a set up block.

    I find it works fine but you have to creep up on the final position unless you have a 3HP router.

  6. I recently bought the Carbitool bit and love the setup instructions – thanks Stu.

    I was wondering if a Mitre Lock join would be ok for a large door I have to build (23300×1100). Yes I should use tennon/mortice but how does the mitre lock join compare?

    • At first glance I don’t see why not, so long as material thickness marries up with the cutter. The extra glue area will provide a reasonable joint, especially with a stronger glue than PVA.

      Dowel, tenon or domino would certainly add to door strength, but a decent glue area is much of the battle, and the interlocking nature of the lick mitre joint is pretty significant too.

      Let us know how you get on!

      • Thanks for your comments Stu. I went ahead and built the door use tennons and as my first really big item using T&M joints, i am chuffed with the results. Now I have built all my T&M jigs and having re-read the above setup notes I will probably stay away from mitre lock joints.

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