Another Slot Mortiser

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Project by measureagain posted 04-28-2015 05:17 AM 3267 views 11 times favorited 7 comments Add to Favorites Watch

I like cutting mortises by hand, but when faced with a need to cut a dozen, I looked at a machine to make it less tedious. There are a lot of choices in the low end for $250-$350, but no one seemed completely happy with them. So I investigated a build-it-yourself solution.

I looked at so many different designs that I can’t remember who to credit for any particular feature. If you see one of your ideas in use here, thank you.

Design considerations:
1) Keep the cost down. It isn’t going to get used that often. So, no laughing, OK? It looks cheap because it was cheap. All the wood came from the scrap pile. The router has been hanging around rarely used for about a decade (I built a router table for the Triton router). The only stuff I purchased new was the t-tracks, bearings, DIN rail, and aluminum plate. The router plate was $10 off craigslist.

2) Keep it as compact as possible. I have storage issues, and won’t be keeping it out and available for everyday use.

3) The Z-axis would need to be adjustable, but not complicated. I need mortises almost exclusively for 3/4” stock, so once it is set up, it shouldn’t need to be changed often, if at all. I did want to have the ability to cut double mortises in a 4” x 4” bench leg someday, or to work with 1/2” stock, so it needed a wide range of movement.

4) The y-axis will only need a limit stop in one direction, for less than full depth mortises.

5) The x-axis will need limit stops in both directions, and have to accommodate mortises on either end, or in the middle of a piece of stock.

6) Clamping for stock will have to hold it firmly in place, and be repeatable for multiple pieces.

7) Despite the best laid plans, the movable work area isn’t necessarily going to be at 90 degrees to the cutting bit. So there has to be a mechanism to tweak that alignment if required.

8) Dust collection would be nice, but will most likely be a work in progress. Sawdust doesn’t usually end up falling down in the most convenient, or planned location.

9) A single handle to move the workspace in both the x and y-axis would be desirable. I like having extra hands to assist where needed.

There are three general approaches on construction of a homebuilt slot mortiser.
A) Fix the worktable. Move the router to cut the mortise.
B) Fix the router. Move the worktable.
C) Move both the router and the worktable.

I decided that the mechanisms required to both move the router, and yet hold it in a rigid alignment would be too costly, or take too much vertical space. I decided to have a rigid mounting for the router and to move the worktable.

I wanted a slide or bearing configuration that wouldn’t have to be taken apart and cleaned after every mortise. Most of the designs I looked at appeared to have problems with sawdust buildup, cost or vertical height. Drawer slides clog with sawdust too easy, and having grease on the ball bearings just adds to the problem. Sealed linear bearings would be nice, but to be stiff enough they would need to be a large diameter and that thickness would start to stack up. Linear rails would be thinner, but would be susceptible to sawdust, and are expensive. And then I ran into the idea on you-tube where someone used DIN rail and sealed v-groove bearings. Problem solved. Simple. Cheap. Bearings are sealed, and the v-groove bearings and rail should be above any chips or sawdust.

This is the z-axis mechanism. The bottom half is two horizontally movable wedges, joined with a 3/4” piece of plywood. You can also see the attempt at dust collection down in front. Although liberally coated with Slip-it, the wedge needs that handle knob to be pulled in and out.

The next two photos show the upper half of the z-axis mechanism. It is made of 3/4” plywood, and a piece of 3/4” stock. The wedges are mirrors of the wedges on the bottom half, hopefully making for a horizontal worktable. Fixed sliders on the sides of the frame, and the wedges, allow only 90 degree movement up and down, relative to the base. Knobs, t-bolts, and short pieces of t-track lock it in place, once the final working height has been tweaked. You can also see the first pieces of DIN rail mounted.

The next photos show the y-axis mechanism. It is another piece of 3/4” plywood. The first photo shows the platform flipped upside-down, so that you can see the plates that mount the v-groove bearings. The second photo shows it right-side up and mounted onto the DIN rail. On the top you can see the DIN rail for the x-axis. On the right-hand side you may be able to see the depth stop. Better picture of that coming later.

And the next three photos show the top layer, that does the x-axis movements. Two layers of 3/4” plywood. Again, the first photo shows it upside-down, so you can see the mounting plates for the v-groove bearings. The second photo shows it right-side up and in place. Mortise height stop blocks are not installed. The third photo shows all the clamps and stop blocks installed.

The mortise height and depth stops are wooden blocks with stepped bottoms to keep them aligned in the t-tracks. They bump up against 5/16” x 1.5” lag bolts. The front of the top platform has a 1/8” aluminum fence with a window cut out to clear the spiral up-cut bit.

The next set of photos show the router mounting. The first photo shows it pulled back from the worktables so you can see the air intake for the vacuum. The second photo shows it in place, from the rear, and locked down with the orange knobs on bolts that go through the base. Those bolts go through slots on the bottom of the router assembly, in the event the assembly needs to be pulled back from the work tables. There are a couple of lock nuts with nylon inserts embedded in the bottom rear of the assembly, with bolts in them for fine tuning the tilt of the whole assembly. I used a digital angle gauge and found the worktables were slightly out of parallel with the base. I put a piece of drill rod in the chuck, and used that same angle gauge on it to tilt the router assembly to match. It took about a quarter turn of the tall bolts. It was only off by two-tenths of a degree, so I probably could have left it alone and been fine. A standard shop vacuum hose plugs in under the router.

This is the final, ready for battle, view. I never could come up with a handle mechanism to move the work platforms that didn’t involve complicated pivot and/or u-joints, so I settled for a thick wire drawer pull mounted to the x platform. It is very sturdy, and works surprisingly well one-handed.

7 comments so far

View ChrisK's profile


1794 posts in 2499 days

#1 posted 04-28-2015 12:30 PM

Great work. Projects like this are what large scrap piles are for.

-- Chris K

View kaerlighedsbamsen's profile


1168 posts in 1131 days

#2 posted 04-28-2015 05:46 PM

Looks like a keeper to me. Bet you are going to include mortices into more projects now that you got the machine!

-- "Do or Do not. There is no try." - Yoda

View brian3's profile


4 posts in 552 days

#3 posted 04-28-2015 07:23 PM

Real nice concept. The DIN rail and sealed v-groove bearings would come in handy for a few projects that I have in mind. Where did you source them?

View measureagain's profile


17 posts in 543 days

#4 posted 04-28-2015 10:54 PM

ebay for the bearings and amazon for the DIN rail, I think (primary laptop has just gone south, so mail archive isn’t available to provide links). I never could find washers to space the bearings away from the mounting plates and not bind the bearings, so I just used another nut as a spacer, and got longer bolts.

View tyvekboy's profile


1307 posts in 2431 days

#5 posted 04-29-2015 03:15 AM

Been wanting to make a slot mortiser too but just haven’t got around to designing one.

You have some great ideas here.

Thanks for sharing.

-- Tyvekboy -- Marietta, GA ………….. one can never be too organized

View MarkW's profile


2 posts in 2614 days

#6 posted 09-26-2015 03:22 AM

I was impressed with your slot mortiser and would like to make one in a similar vein. The DIN rail and v-groove bearings look to be a good solution for me
I would like a little additional information regarding this if possible.
- What is the size and/or specification of the v-groove bearing you used to fit the DIN rail?
- Did you make your own carrier plate or purchase one for the bearings, and if so, how did you set and adjust your bearing location tolerances to be in line, not too tight and still have no backlash or slop in bearing to DIN rail movement? There is a company ( ) that uses v-groove bearings with a carrier plate that works with 8020 materials, but they use eccentric installation nuts to adjust to proper tolerance. Is this along the lines of what you did?
- What material and thickness did you use for your carrier plate?
- You did not mention bronze bushings and a steel rod. Do you think that they will collect saw dust and not slide easily enough, or is it a stack up height issue?
Thanks in advance.

View measureagain's profile


17 posts in 543 days

#7 posted 01-10-2016 01:23 PM

MarkW, sorry I missed your post many months ago. I was building a deck all summer and haven’t been back to lumberjocks until now. I looked at a lot of linear bushing types for this, but decided that cost, stack height, and collection of sawdust would all be prohibitive. I made my own carrier plates from 1/8” aluminum plate, and ran a file through the holes on one side to slightly elongate them. then I just used a piece of DIN rail to snug the bearings up against. This isn’t a CNC setup, so “finger tight” is good enough. Eccentric mounts would be a lot more precise, but I felt it would be unnecessary. The hardest part was finding washers to space the inner bearing races away from the carrier plates without binding the outer races.

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