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Lock Miter Router Bit Setup

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Blog entry by RichTaylor posted 08-09-2017 06:36 PM 468 reads 19 times favorited 10 comments Add to Favorites Watch

There was some discussion about lock miter bits on Rick M’s Deals thread. I didn’t want to muck up his thread with off-topic conversation, so I am tossing this out there for anyone who is not certain about setting up lock miter bits.

First, a diagram (the numbers shown are for the Infinity Lock Miter Jr. Router Bit item #55-502). Note that the markings are for reference only and, particularly in the case of the fence location, don’t precisely represent the actual set up. That is, the fence would never be set that far back.

The key to getting a correctly aligned joint is to have the two orange lines shown on the diagram an equal distance from the outer face of the boards (outer meaning the faces that are on the outside of the joint when the boards are put together). Those orange lines mark the center of the bit both vertically and horizontally. However, it is most desirable for the joint to be centered on the board, and it’s also the best way to get an easy and consistent setup. By adjusting bit height and fence setback so that those lines match the center of the board, you will get a centered lock miter joint.

My preferred way to handle router bit settings is to pick a point on the bit and measure its height when the bit is properly adjusted. For example, on round over bits, once they are adjusted for a good cut, I measure and write down the height at the top of the bearing screw because that spot is most accessible to get a height gauge on. The next time I use that bit, I set the height to that value, make a test cut and it’s ready to go.

The lock miter bit has an extra element though, because the fence has to be set correctly too, in order for the joint to line up. For getting it set, I use the top of the bit for bit height, and the tip of the lock groove cutter for getting the fence right.

As I said, the numbers in the diagram above are specific to that Infinity bit. But, you can apply the same technique for any brand by using test cuts to get the bit set properly, and then measuring the bit height and distance from the fence to the tip and making up your own formula based on the thickness of the board.

Remember that it’s the thickness/2 that matters, so take the measurements you get for height and distance to fence and subtract half the board thickness. Those are the amounts you will add to thickness/2 for other board thicknesses.

Separate the fence position from the bit height

If you are having trouble getting a good joint on your test boards, the easiest thing to do is separate the bit height adjustment from the fence adjustment. That way you’re not fighting two variables at once. To do this, take two boards and first run each edge with the board flat against the router table top. Flip one board and slide them together. If the faces are flush, the bit height is set properly.

First off, I’ve built a couple of fancy jigs for managing the boards that wound up in the trash. Ultimately, I found that guide sticks attached flush to the edge with double-stick tape does best.

Here is the routed board.

After routing the second board, when placed end-to-end, you can see the error.

The great thing about this is that the offset is double the error of the bit height. You can measure it and adjust the bit based on that.

Offset was 0.038, so adjust the router 0.019.

...and the alignment is correct.

Adjust the fence. Make two cuts with the boards flush to the fence. You can do the same measurement of the board offset, halve it, and get what will probably be your correct setting on the next pass. The fence is tougher to get set right since you have to move it by hand.

Be sure to measure and make note of the distance from the fence to a repeatable spot on the bit. Also note the height of the bit.

Good enough for government work.

You can now take the fence and height measurements, subtract half the board thickness and have the correct setting for other board thicknesses. Write those down and keep them with your bit.

-- No matter how much you push the envelope, it'll still be stationery.



10 comments so far

View CL810's profile (online now)

CL810

3650 posts in 2705 days


#1 posted 08-09-2017 07:14 PM

Thanks Rich for taking the time to make this blog. Favorited, saving for reference.

-- "The only limits to our realization of tomorrow will be our doubts of today." - FDR

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Redoak49

2625 posts in 1705 days


#2 posted 08-09-2017 08:30 PM

Very good blog…what digital height gauge are you using.

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BenDupre

453 posts in 205 days


#3 posted 08-09-2017 10:59 PM

Thank you Rich!

-- The problem with communication is the illusion that it has occurred. – George Bernard Shaw

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RichTaylor

1305 posts in 306 days


#4 posted 08-09-2017 11:20 PM


Very good blog…what digital height gauge are you using.

- Redoak49

It’s the iGaging Digital Electronic Height Gauge. Its design makes it easy to get into places for measurements.

-- No matter how much you push the envelope, it'll still be stationery.

View ki7hy's profile

ki7hy

2113 posts in 456 days


#5 posted 08-10-2017 12:56 AM

This is interesting Rich. Very handy. So are you saying the expensive little red aluminum bits were a waste or are they useful at all?

Thank you for putting your time into this.

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RichTaylor

1305 posts in 306 days


#6 posted 08-10-2017 01:49 AM



This is interesting Rich. Very handy. So are you saying the expensive little red aluminum bits were a waste or are they useful at all?

Thank you for putting your time into this.

- ki7hy

It was useful for me in that it gave me a visual understanding of what’s involved in using the bit. Those bits aren’t intuitive until you’ve played around with one for a while, and without the little red gauge, it would have taken me longer to get a feel for it. I don’t use it for setup anymore though. It’s just not precise enough, and I can get it done in a fraction of the time with my measurement system, and have never needed to make more than one test cut.

The price is a ripoff though. $30 is at least double what it’s worth.

-- No matter how much you push the envelope, it'll still be stationery.

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ki7hy

2113 posts in 456 days


#7 posted 08-10-2017 04:49 AM

It’s a total rip off. Should be $10 a pop and that’s still spendy but probably worth it.

Either way, I’ll play with this soon. I have a couple projects on the bench to finish and then tons of small log milling to do. Then I’ll play. I need more time!

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ScottKaye

516 posts in 1670 days


#8 posted 08-10-2017 01:08 PM

added to favorites. thanks! Ive always wondered about this style bit but have stayed away from it do to the horror stories Ive read with the set up.

-- "Nothing happens until you build it"

View John_H's profile

John_H

28 posts in 1423 days


#9 posted 08-13-2017 04:53 AM

I hope I am not out of line by posting a slightly different variation, if I am, please delete this post

As mentioned, the center of the lock miter bit needs to be aligned with the center of the workpiece, both horizontally and vertically. But where actually is the center of a lock miter bit?

If you look at a lock miter bit closely, you will see two horizontal flats that help form the ‘tongue and groove’ portion of the bit. The horizontal center is exactly in-between those two surfaces and it is pretty easy to determine by making a couple of measurements

(I use a Bosch Lock Miter bit and I drew this up for my own reference – but you should get the idea if you have a different bit)

You only need to do this one time for YOUR bit.

You want to measure to the top edge of the top cutter and the distance of the top edge of the bottom cutter. (red lines on the right in my diagram) The exact horizontal center of the bit will be in-between those two edges.

I use this Wixey WR200 Digital Height Gauge

Securely mount the bit in your router at a comfortable working height and measure to the top cutter

Then measure to the lower cutter:

A little math – subtract the bottom number from the top number and divide the result by two. In the case of my Bosch bit the center is .177” below the top edge of the top cutter.

Now I just need to find the center of the board which is easy enough. The center of a 3/4” board, for example, is .375”

Just add the two numbers together – half of the bit measurement and half of the board measurement and you will get the height that the top edge of the top cutter needs to be above the router table. In this example using a true 3/4” (.750”) board, the top edge would be .552” above the router table surface.

Once you have set the bit height you just need to set the fence position:

Place one of your boards that you are going to be using up against the fence (vertically) and place a straight edge along it. Position the router bit so that it is perpendicular to the fence (the tip is towards the straight edge) and then move the fence IN so that the tip of the bit just barely touches the straight edge, and then back it off just a hair. That’s it:

Once I know the number to use with my lock miter bit (.177”) it’s easy to compensate for boards that are slightly thicker or thinner. For example, a 3/4” piece of MDO that I had actually measured .718”. To properly set the height of the router bit I take my .177” and add to it half of the board thickness (.718” divided by 2 = .359”) which get’s me to .536”. So with this piece of MDO, I set the top edge of the top cutter to .536” and set the fence up the same way as before – using the piece of MDO up against the fence as the spacer

Lastly – Don’t forget to lower the speed of your router! Here is a simple chart from Rockler I use. I run my router at the lowest speed with this bit – 9,000 rpm:

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RichTaylor

1305 posts in 306 days


#10 posted 08-13-2017 05:08 AM



I hoping I am not out of line by posting a slightly different variation, if I am, please delete this post

- John_H

Not out of line at all, John. You’re a man after my own heart. Every setup I do is based on the numbers. I’m happy to see another woodworker going about it the same way, and thanks for contributing to my post. Your input is welcome and very helpful. It’s always valuable to be able to see different ways to get the job done.

The trick with both of our methods is that, if followed, you’ll get a perfect joint first time, every time. I still do a test cut if it’s expensive wood, but since I started doing it my way, I’ve never had to make an adjustment.

-- No matter how much you push the envelope, it'll still be stationery.

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