Full disclosure ... there's absolutely nothing boaty below! There's also a lot of images
Anyway, one of the challenges of making new gratings is the considerable number of halving joints that have to be cut in the rails. If each one was cut by hand the traditional way (saw and chisel), it would take a LOT of time 'cos there's no less than 696 of them on the gratings for Pagan!
Confucious he say that if the Gods had intended man to faff about cutting 696 halving joints the hard way, he wouldn't have invented power tools! Even so, it's not as simple as it seems. Each joint needs to be 22mm wide by 9mm deep and spaced on 44mm centres from the adjacent joints. The margin for error is small by woodworking standard - the longest rail is 508mm long and a 1mm error between joints would add up to being 15mm out at the middle (assuming, as I intend to do) starting from the ends to the middle. Needs to be a lot better than that.
I considered various methods...
Our American friends would tool up their table saw with a dado cutter set but they're not available in the UK and Europe because they're considered dangerous (and with good reason as to use them you have to remove the riving knife that prevents the workpiece closing up and grabbing the blade with potentially lethal consquences). That's a none starter then.
I contemplated and sletched designs for clamping the workpiece in a jig with a guide bar setup to use a hand held router with a 22mm cutter to cut the joints. That would have worked well enough I'm sure but then I came up with the notion of turning things on their head and fixing the router whilst moving the workpiece. In theory, that ought to be the simpler solution (in practice, I'm not sure that the first idea wouldn't, in the end, have been easier but hey ho).
So I sketched up a design for a very simple jig ...
And built it ...
The base is two layers of 9mm MDF, the carriage is made of the same stuff. Underneath is mounted a fairly hefty router donated by my brother who no longer had a use for it.
(There's numerous videos on YouTube of similar jigs and there's off the shelf parts for building them at various levels of sophistication right up to the next best thing to a full spindle moulder setup. however they invariably use trim routers. Iroko, from which the new gratings will be made, is tough stuff and hard on cutting edges so I needed a powerful heavy duty router as the basis for the jig).
It sort of worked. Conceptually, it prooved I was on the right lines. However, it proved impossible to repeatedly keep the carriage tracking at 90 degrees to the router cutter head. I also found that trying to rout a halving joint at the end of a workpiece inevitably led to tear out. There were some other niggles too all of which added up to a case of back to the drawing board to come up with Mk.II
There's quite a lot to unpack here!
First significant change is the use of several 3d printed parts. My brother adapted a design he already had on file for ball bearing track guides as a solution to keeping the carriage square when using the jig.
I also designed, and Glen 3d printed, a stop bar large enough to accomodate the full width of the Iroko boards. Based on test cuts with the Mk.I jig, I had concluded that it would be faster and more accurate to cut the halving joints into the boards before ripping them down into the individual rails.
I also reworked my cutting plan so that each rail is initially made 44mm too long with the excess to be cut off after the gratings are assembled. That avoids the problem of tearout when trying to run a cut at the end of the workpiece (at the cost of wasted timber but hey ho).
So having a design, and after a trip to Bodgit & Quit for more MDF, I set to making the thing ...
Step 1: Cut the two base plate pieces from a sheet of 9mm MDF
Steps 2 & 3: Cut the track slots and router bit clearance holes in the top plate
Step 4: Mark out and cut the recess for the router show in the bottom plate
Step 5: Glue the top and bottom plates together
Step 6: Mark the mounting holes for the router (bizarrely, only 3 of the 4 pre-drilled holes in the router shoe can be used as the 4th hole is fouled by the depth stop anvil)
Step 7. The holes were then countersunk on the top side of the base plate
Step 8: Mount the router and clamp the base plate in the old Workmate that will be used as as stand for the jig when in use.
(When not in use, the jig will be stored out of the way on a shelf).
Step 9: Cut the pieces for the carriage and step 10: glue them together ensuring they are accurately clamped at 90 degrees (the Mk.1 prototype was a bit of a failure at this stage. The Mk.II is as close to spot on as I'm likely to get!)
Step 11: Cut some reinfocing pieces and step 11; glue them in place
Step 12: Carefully measure, mark and fit the upper ball race rails 3d printed by my brother Glen.
It was vital at this stage that the rails were as close to parallel to each other as humanly possible. Measure many many times, screw down once!
Step 13: Fit the lower ball race rails with the same level of care and attention.
The rails were designed with slotted mountings to allow the positioning to be tweaked as necessary. As it happens, it was barely neccessary and the carriage glides very nicely backwards and forwards without any racking or twisting
At this point, I realised I'd dropped a bit of a clanger and made the base too small to be able to easily clamp it or the carriage to the Workmate
Easily solved ... I made up two pieces of MDF per side glued together and then ...
... glued them to the base.
That gives me the option to clamp the base to the Workmate (although it seems to clamp well enough on the router shoe) and also to clamp the carriage in place in order to use the jig for other purposes.
At this point I did some test cuts into scrap wood and it was fortunate I did so as I swapped which side of the cutter the guide bar is set (the rotation of the cutter tends to throw the workpiece to the left so having the stop bar on that side prevents that)
The tricky bit now was routing out the mounting slot for the aforementioned stop bar. It needed to be as close to a 44mm offset as possible. Although a certain amount of adjustment is built in to the design, the closer the better to start with.
The trick here was to stick some masking tape onto the base and then superglue down some spare lengths of track carefully at the right spacing ...
... then the carriage could be run on the temporary tracks with the router cutter set to the correct height to cut the slot for the stop bar.
Cutter clearance slot on the left, stop bar slot on the right
Along the way, I'd routed a slot for an adjustable 3d printed hold down clamp (I forgot to take an photos, you'll be relieved to read!).
I also decided I needed to put in a couple of cutouts in the front face of the carriage to make it easier to hold the workpiece in place.
A template was cut out of MDF and temporarily fixed in place using the tape and superglue trick. My own router fitted with a guide wheel cutter then made short work of cutting out the hand holds.
The hand hold routed out and the template after being peeled off
Test cut time!
You can better see the hold down clamp in action and I've just cut the second slot using the stop bar in the first slot I cut
Measuring up, the spacing is spot on.
However, I'd failed to consider that the carriage is held slightly clear of the base plate by the ball races and that necessitated a reprint of the stop bar with the projection thinned down to 6mm instead of 9mm.
I also found that the wood screws securing the stop bar weren't gripping ...
The solution to that problem was to use M4 T nuts. In softwood, they'll bit in and end up flush with the surface of the wood but MDF isn't very compressible to I cut a shallow recess using a Forstner bit to make sure they'd be flush
And the finished router table in all it's glory!
The next step is to run some test cuts and, if necessary, adjust the stop bar to reduce the cumulative spacing errors to an acceptable level (assuming it's not already OK).
I'll experiment with taking some video of it in action but no promises on that front!
The end result has taken a fair bit longer, and cost somewhat more (about £40 not including the router and cutter) then I expected but having made it larger and more sophisticated than stricty necessary it will have a life beyond this one project.
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