Jig for accurately spacing mortises for slats

  • Advertise with us

« back to Jigs & Fixtures forum

Forum topic by botanist posted 01-16-2013 03:42 PM 3431 views 0 times favorited 24 replies Add to Favorites Watch
View botanist's profile


167 posts in 3507 days

01-16-2013 03:42 PM

I’m planning to build a new bed for my wife and I that involves slats fit into mortises on the head board and footboard. I was planning on using a forstner bit in my drill press to rough out the mortises and I wanted to build a jig to streamline the process and less prone for error. I don’t trust my accuracy to do this free hand

The jig would have to be able to support a long piece (over 60” as we have a queen-sized mattress) and I need the jig to do two things: accurately space adjacent mortises, and create mortises of the same size. I can then use a kerfmaker to create the tenons on the slats. My inspiration is the spacing guide on the Festool domino (which if I could afford one would make the process even easier). Anyone have any ideas?

24 replies so far

View Lee Barker's profile

Lee Barker

2170 posts in 2819 days

#1 posted 01-16-2013 04:43 PM

You’ve got a simple math problem here. You’ll spend as much time building a device to space your mortises as you will laying them out.

If you know the span, and the width of the slats, you’re just about there.

Add up the slats, subtract that from the span, and divide that answer by the number of slats plus one and you have the gap.

If you can, clamp the top and bottom pieces together for the layout process.

Check your work carefully.

Build a carriage to hold the material vertical on the DP table; you’ll need outboard support of course.

Commence to bore! Since you’ll be cleaning out the mortises anyway, (I presume) you will be able to true up the ends where round meets square.

Ring my chime if this isn’t clear, botanist. I’d like to see this be a delightful part of the project for you.



-- " his brain, which is as dry as the remainder biscuit after a voyage, he hath strange places cramm'd with observation, the which he vents in mangled forms." --Shakespeare, "As You Like It"

View botanist's profile


167 posts in 3507 days

#2 posted 01-16-2013 04:50 PM

The other option I was thinking of (and it sounds easier because I can do it on my bench) was to use a router jig. I would screw the router to a base with a couple of pins (one to help determine the correct spacing between slats, the other to determine the correct width of the mortise). I just have trouble visualizing what it would look like. Or I would just need one pin. The spacing between the pin and the bit would represent the spacing between slats and the pin would slip inside a preexisting mortise and that would limit the travel of the bit to the width of the mortise.

View CharlieM1958's profile


16274 posts in 4187 days

#3 posted 01-16-2013 04:54 PM

Here is one way to lay out the slats evenly. Now let me quickly add that I’ve never done this…. it just popped into my head as a result of your question. So maybe someone will jump in and say why it’s a bad idea, but…

Lay out the two end slats where you want them to be, then use your measuring tape to find dead center between the two end slats, and lay out a slat there. Now just keep repeating that center-finding process between the previously-laid-out slats until you have as many slats as you want.

-- Charlie M. "Woodworking - patience = firewood"

View botanist's profile


167 posts in 3507 days

#4 posted 01-16-2013 04:58 PM

I’m not too concerned about the layout or spacing of the slats-that part’s easy. I’m concerned about creating a jig I can use to make sure that my spacing and mortise width are consistent when I’m cutting them.

View Loren's profile


10278 posts in 3616 days

#5 posted 01-16-2013 05:21 PM

Do you have a plunge router?

It is simple to make a jig that you run a template
guide in and plunge your mortises to the same

Assuming you start at one end and work towards
the other, the problem with using an indexing pin to space
mortises is compounding errors. I do not recommend
it. Your rails will not come out with the same
spacing of the mortises… not dead-on. You could
start from the center slat, but then you have to
do more setups of the jig to cut the mortises
referencing from different faces of the work
being mortised.

Indexing pins work with dowels because
the variation in hole size and centering from
hole to hole will tend to be small enough to
be insignificant.

You want your slats lined up evenly, right?

My advice is to use the plunge router, mark
the mortise locations out by hand and get
the result you want the first time.

Further, if you put shoulders on your tenons
instead of using “slats” variations in the mortises
won’t be visible.

You can go ahead and make a jig with a pin to
space router mortises, but you’ll spend a lot of
time clamping, moving and unclamping it.
The work will go quicker if you mark the joints
and cut them with a minimal router jig like a hardboard
template or a router fence i

View CharlieM1958's profile


16274 posts in 4187 days

#6 posted 01-16-2013 05:39 PM

It seems to me that once you have the mortises laid out and pencil marked, your only problem is supporting the work piece properly. A simple fence clamped to the table would ensure the cuts were properly centered front-to-back, and roller stands or some sort of purpose-built sawhorses on either side would support the ends. If you are 1/32” off in eyeballing bringing your bit into your pencil markings, it will never be visible to the naked eye. If you really want to be anal about it ( :-) ) you can make a pencil mark exactly 1/2 the distance of the bit’s diameter from the edge of the mortise layout mark, knowing this is where your bit point must make first contact.

-- Charlie M. "Woodworking - patience = firewood"

View botanist's profile


167 posts in 3507 days

#7 posted 01-16-2013 05:40 PM

Loren, my idea was to lay out everything together (top and bottom) to make sure I’ve got the spacing right and then use the jig for cutting the mortises (keeping in mind that there will be some error involved and correcting for it). If I can make small corrections for each mortise maybe I can reduce the error. I was planning on putting shoulders on the tenon to hide any variations in the mortises.

Charlie, I’m pretty picky about stuff like that, so that’s probably something I would do.

View runswithscissors's profile


2725 posts in 1993 days

#8 posted 01-16-2013 10:46 PM

I agree you should have shoulders on your tenons. If not, the slats would have to be a perfect fit, or gaps would show. One suggestion: However you make your mortises (aside from a tenoning machine, which makes a square hole), there’s no need to square the ends. Round off the tenons instead. Another time when I suggested this, one response was that rounded tenons wouldn’t be as strong. But if this were so, it would mean the Domino tenons must not be as strong, and I’ve never seen anyone suggest that. It is a lot easier to round the edges of the tenon than to square the ends of the mortise. They don’t have to be perfect.

By making the faces of the tenons a little narrower than the mortises, you gain some flexibility in correcting any spacing errors.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

View a1Jim's profile


117063 posts in 3545 days

#9 posted 01-16-2013 11:10 PM

It’s simple you are the jig lay it out and go for it,this is how we get better at each woodworking technique. I know when you haven’t done a certain operation many times before it’s a little scary but once you have done it next time you will feel more confident. You can lay it out double check the measurements and the do you drilling and chiseling,if you have sharp chisels you will have it done in no time. If your concerned with your tenon fitting your morticies do a couple practice pieces on some scrap.

-- wood crafting & woodworking classes

View pintodeluxe's profile


5623 posts in 2782 days

#10 posted 01-16-2013 11:19 PM

I would recommend a benchtop hollow chisel mortiser. They are more affordable now than ever.
The problem I have with hand chopped mortises is the workpiece will be full of dents on the back side.
Using a drill press and chisels is a time honored way to do this, however it’s not something you will do again after you get a mortiser.
Layout becomes much easier once you are using a square bit to cut a square mortise.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View oldnovice's profile


6773 posts in 3336 days

#11 posted 01-17-2013 08:30 AM

Here is a totally off the wall idea!

Make a jig as shown below:

  1. Locate the center mortise position and by using the cross hairs on the back side of the template (barely visible in the image), cut the center mortise location using a router and a guide bushing (the first mortise has been highlighted in gray).
  2. The template has a “dummy” slat that fits into the previous opening and locates the next mortise.
  3. Fit the “dummy” into the opening, using the dowel to maintain lateral alignment, and cut the next mortise.
  4. Move along the rail until the last mortise is cut.

Flip the jig 180° and repeat the mortises down the other side of the rail.
Then do the next rail in a same manner!

The jig consist of:
  1. 3/16” Plexiglas with the “dummy” slat and the router opening at the desired spacing. If the spacing is off it will be consistent.
  2. Two dowels to keep the jig square to the rail. It is important that these are properly located so when the jig is rotated that the lateral alignment is maintained!

By changing the bushing to a larger size another mortise can be cut inside of the previous one or a dowel can be located by using the hole in the Plexiglass and the “dummy” slat.

I used this method to create opening for false tenons as shown below.

-- "I never met a board I didn't like!"

View botanist's profile


167 posts in 3507 days

#12 posted 01-17-2013 03:25 PM

Oldnovice, I like the idea of using plexiglass but I’m concerned about it’s tendency to crack. How would I cut the hole for the mortise in the plexiglass? Can I use a router bit?

View oldnovice's profile


6773 posts in 3336 days

#13 posted 01-17-2013 05:52 PM

Yes you can cut Plexiglass with a router but it works better cutting a slow speed. If you are concerned about cracking you can always step up to 1/4” thick or more. I used 1/8” and raised the template so my bushings would work. If a crack starts, use some Plexiglass cement to glue it back together … that’s what I did.

Remember the template will give you the exact cut out so make it as precise as possible … trial and error is the best as it took me three attempts to bet mine “perfect”. Make it smaller and then you can always enlarge it. I cut mine with a combination of router and files to get it the size I wanted.

The dowels must be precisely located if you are going to flip the jig.

Lastly, make the template large enough to support you router base and clamp the template securely for the cuts. There is nothing more unnerving as to have the template move during a cut.

-- "I never met a board I didn't like!"

View joeyinsouthaustin's profile


1294 posts in 2041 days

#14 posted 01-17-2013 06:43 PM

+ 10 on laying it out by hand. My experience in spacing jigs for projects like this is that it doesn’t take much to get off. 1/64’ will quickly build up to 1/2” You will need to be able to micro adjust the jig to 1ooo/th of an inch. Combine the two methods. Build one jig to get consistent mortises, Your main goal, then reference that jig off of center lines. Use the negative space layout Lee Barker clearly gives (op +1) This way you just center the mortise jig on the layout you did. It will go muuuuch faster than trying to fine tune a spacing jig.

If you are not concerned with the spacing on the far left and right side, use the peg style jig that old novice, and loren describe I would add one thing to that jig. Mount the post in a channel with a compression fit from the top, so that it can be adjusted left and right, both for micro adjusting, and to make it more useful. Then cut everything long and trim the left and right equally to get to final size.

My ten cents

-- Who is John Galt?

View botanist's profile


167 posts in 3507 days

#15 posted 01-17-2013 07:07 PM

joeyinsouthaustin-I like the idea of putting the post in a channel to make it more adjustable. I don’t want to just make a spacing jig and then hope that it gives the correct spacing the entire length of the piece. I was hoping to do the layout on the top and bottom pieces together and then do the cutting with the jig, adjusting it to cut exactly where I laid out the mortises. Or I could skip the pin altogether and just have a hole for the mortises that I need, that way I’m not compounding my errors or making things too complicated.

showing 1 through 15 of 24 replies

Have your say...

You must be signed in to reply.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics