From The Workshop: Handheld Mortiser (aka poor man's Festool Domino) Part I

Backstory

Recently, it feels like there has been something of a boom in DIY handheld power mortiser videos on Youtube. The first documented attempt was by a russian woodworker in 2013 and he posted plans in english on his website. Then Stumpy Nubs, a youtuber, built a mortiser based on that design in 2014 and posted a video about it. A couple more builds of the same design popped up in the following years, including one made of CNC'd aluminium. Fast forward to 2018, Neil of Pask Makes released a video of his own take on the DIY mortiser. Taking a different approach and slightly different form factor, it then influenced the design of Keith Brown's own design for a battery powered version.

The Domino

 
The commercially available Festool domino is the template for all of these attempts and rightfully so. First to market (and patented to protect that market share), the domino system creates mortises that fit a set of standard sized loose tenons called "dominos" (for hopefully obvious reasons).

The mortiser itself is shaped, and handles, like a biscuit joiner for familiarity and ease of use. While it doesn't do anything a dowel jig, slot-mortiser, pantorouter, or beadlock system can't do, it allows for loose tenon joinery in medium production environments where hand cutting mortises would be too time consuming and a stationary slot-mortiser is not flexible enough or otherwise not justifiable for the setup time or cost. Loose tenon joinery is thought to be stronger than dowel joints due to the increased surface area of the tenon and can be slightly more forgiving of minor misalignment, but really, "loose tenon joinery" just sounds better than "dowel joinery" which brings to mind flatpack particleboard furniture. For these reasons, (and the fact it is a Festool tool), the domino system has a cult-like following among professional youtubers woodoworkers. Unfortunately, anyone who knows anything about Festool can tell you two things: they make well designed tools marketed towards professionals and they are priced accordingly. For most home-gamers, the $1000 pricetag for a tool that only sticks things together is a little too dear. And of course, that doesn't include the consumables and alternate cutter sizes.

While the patent covers any attempt to make a similar device without a drastically different cutting system, there have been attempts to build a similar looking tool, most notably by Triton with their TDJ600 Duo and by Mafell with the DD40P. While both tools are simply plunge cutting dowelers, the Triton has the unique and dubious honor of being of the most maligned and hated tools in woodworking. Suffice it to say, neither is really comparable to the domino system in terms of polish and utility.
In the end, it's not particularly surprising that clever woodworkers would attempt to DIY their own tool. Ultimately though, the form factor of the Domino mortiser will remain unique and difficult to replicate through DIY means, especially for those designs based off a compact router.

Reinventing the Wheel

One of the best parts about Keith Brown's build video is that he ultimately concludes that reinventing the wheel is a loosing proposition and he should have just built Pask's design in the first place. Tool design and ergonomics are not accidents and the engineering involved is real and critical: Keith demonstrates this in a humble and plainspoken manner that I respect. A look at any of AvE's videos on youtube will show the consequences of design decisions when it comes to things you are supposed to hold in your hand and use. All that said, of course I am arrogant confident enough to think that I can come up with a different design that meets my needs better than all the other designs out there (this post wouldn't be much fun otherwise). In the end, unlike Keith, I am generally fully satisfied with my design, although it did end up costing a little more in tooling and accessories than I would've liked. I only ended up needing two versions, though I have a rev3 in the works now that I have used it a bit.

My Complaints

Anyone who knows me can tell you I like complaining. What can I say, it's fun, for me at least. But with regards to the existing designs, my biggest complaint was with how bulky they all were, especially in the vertical direction. The vertical height below the cutting plane is a consequence of the carriage design they all share: the Russian design uses drawer slides while Keith and Neil use rods in a diy linear glide. Both approaches require at least three "layers" of material, the base layer, the carriage layer, and the pivot layer. Depending on the design, they also require a fair bit of width, although that is less of an issue (and one my version does not really address). My other complain with the existing designs was the overall level of complexity of both the pivot and the carriage mechanism. Mini-drawer slides cost money and still have too much play to use stock, the rod design is comparatively simple and robust, but requires a bit of precision to build that I didn't really want to deal with and I had a feeling I would end up repeating Keith's experience if I deviated from Neil's approach.
The pivot mechanism for all the designs are relatively similar, a triangular platform pivots on top of a platform and the router is strapped to the triangle. The design ends up requiring a clamp to keep the triangle from lifting and still requires extra vertical height. Ideally, the pivot point would also be as far away from the tip of the router bit as possible to reduce the degrees-of-arc the bit would need to travel to complete the cut: a larger "slice of pie" would result in a more pronounced curve at the back of the mortise.

If you're so smart...

With those complaints and considerations, I thought: "wouldn't it be nice to be able to combine both axes of movement into a single mechanism." My solution was to use a piston & cylinder like system built around the body of the router for linear movement, and a gimbal for the oscillating motion. The "piston" would consist of a plywood box that nests and slides inside of a larger plywood box that would constitute the "cylinder." The cylinder would serve as both the linear movement guide and the casing for the entire tool. Compared to the drawer slides or rods, I figured nesting boxes should be easier to build to an acceptable level of precision since the outer box (cylinder) could quite literally be built around the inner box (piston). The self-cleaning nature of the piston should also reduce sawdust jamming that could occur with drawer slides or guide rods.
The pivot gimbal is based around a pair of machined pins threaded into the body of the router itself. The pins would be held in place by the "piston" part of the linear motion mechanism and allow the router body to pivot inside the "piston" box. Basic stops would be added to the "piston" box, limiting the range of motion as needed. The trickiest part about the gimbal is that the router body would need to be drilled and tapped to accept the pin, voiding any warranty and possibly damaging the stator or cooling fan. I specifically chose to build the mortiser around the Makita XTR01Z trim router because it has an aluminium body and from the parts diagram, there should be enough space at the front of the router. I specify machined pins instead of a simple bolt because we need the pin to be flush with the surface of the "piston," it would need to be extractable so the head must be larger than the threads, and it will be riding in a plywood bore without a bushing so the contact surface must be as smooth as possible to prevent wear.

By combining the "piston" and gimbal into a single part and integrating the tool housing with the "cylinder," the result is a fairly compact design that should be lighter and less bulky than the existing versions. I honestly don't think there is any way to make the entire mechanism more compact without switching to metal or plastic, and even then, the linear motion mechanism would likely end up more complicated.

Stay tuned for the actual build and results.

Continued in part II...