Constructing a Mobile Saw Base

In making the torsion box base, I made a great deal of use of the ideas and designs of many who had gone before.

I decided to make some "refinements" of my own, the first of which was to rout some shallow dados to locate the crossmembers, as I thought it might help when gluing up. I clamped the top and bottom of the base together - side by side - to help with the layout. Then, whilst still together, I cut the front to rear dados, having already dealt with the side to side ones, using a fence on the router (a Ryobi RE601 2HP).

Having ganged together the short ribs and notched them, then dealt with the notches on the long ribs, I had a dry fit to see how it was coming along. I was quite pleased. The dados located the ribs firmly, which was going to be a great help when it came to assembly, particularly when trying to turn the whole thing over on my own to complete the glue-up.
You'll notice that there appears to be a short rib missing, but that's deliberate, as I had already decided that the left-hand vertical side of the main carcass was going to go right through a slot in the top of the torsion box and after some careful morticing, would form a structural rib itself. More of that later.
Here's a close-up of the amount of dado I used - hopefully just enough to give a good glue shoulder, but nowhere near enough to risk weakening the integrity of the box construction.
Having applied a good measure of glue to the dados in the bottom of the box, I rolled glue onto the ribs and dropped the base onto them. Having made sure that they all located correctly, I then pinned the bottom to the ribs with 18 gauge brads (using my air nailer). This enabled me to then turn the whole assembly over without assistance. Then I glued the top of the ribs, then the top dados and applied the clamps to do their stuff.
Once the glue had cured, I removed the clamps, then cut some packing pieces for the castor wells. These were glued and pinned on, then drilled through. I fitted the 4" braked castors with large washers and self-locking nuts - I don't think they'll be going anywhere! The castors have good quality bearings and replacement shouldn't be an issue - by the time they wear out, I'll be on the wrong side of the grass anyway!
The next job was to cut a suitable mortice in the top of the torsion box to allow the left-hand end of the main carcass to pass through to form the "missing" rib. This was easily achieved with the router, after some careful layout. I then cut a "T" shape out of the lower edge, fine tuning to fit. As the carcass side passes through the slot, it locates in the dado directly beneath it (in the base of the torsion box), ensuring a perfect 90° side. The other refinement was to insert biscuit slots in the thickness of the "shoulders" of the side and into the torsion box base to match. Probably unnecessary, but I'm only going to make one of these and if it's over the top, so what?
The final job to firm up this design was to fashion two inserts to fill the outside spaces of the "missing" rib. I preferred this method for two reasons - the dados were already cut, so dimensions were identical to the ribs themselves. Cutting a slot between the sideways-mounted ribs (the long ribs) did not impinge on the integrity of the box unit as a whole. The inserts may not have been necessary, but not only do they "tidy things up," but the ribbing now has no gaps anywhere.
Now I set about locating the right-hand end of the carcass. I decided to use biscuits rather than screws, as MDF hasn't got a great holding capability on edge. In order to get the end in position, I needed to use number 20 biscuits in the base and number 10s in the side - you simply couldn't locate it with all number 20s as they don't bend and I was well past the stage of having a free floating torsion box top.
Time to measure and cut the top, wrap the end in oak, measure and cut the intermediate uprights, then dry fit to inspect it - not bad! Before I go any further, I want to deal with the sliding blade caddy, as that will be a small space to work in once it's assembled permanently. I intend to use a couple of Accuride™ sliding hinges that I "won" from a previous job.
Although the Accuride™ hinges are "barrister bookcase" type (lift and slide), I modified the fitting so that they worked as a straight pull, with the oak wrapping taking most of the load and all of the wear. I bolted the carry plate through from the back, reinforcing with screws at the front. I intend to fit a door with European hinges which will act both as a cover and a pull handle - I hope I can transfer my idea into reality, we'll see! There are thirty pairs of roller bearings in each hinge, so the caddy should slide easily enough.

Also in this picture are the oak-wrapped front and back carcass rails and the right-hand carcass end. I've machined adjustable shelf-pin holes, so I'll have a useful storage area, for which I'll just have to cut a shelf - at least I won't have to struggle to drill the holes afterwards (been there, didn't like it!).

Having wrapped the sides of the torsion box and made an end plate, also wrapped in oak, it was time to finish off the base. This involved some speedy glue-up (MDF soaks up glue like hungry blotting paper!), then out came the clamps again. You can see the wrapped end panel, which has been treated with Tung Oil as an experiment - I quite like it. The weight of the assembly was almost too much for my poor old hinged-flap router table.
I decided it would be easier to mount the three pairs of full extension stainless steel drawer runners before assembly. Whilst I was at it, I treated the inside surfaces of the uprights with lemon oil - it seals the wood and smells pleasant too. The underside of the torsion box was coated with sanding sealer, flatted off then a second coat applied. That's the last I'll ever see of that, hopefully!
After firstly assembling the uprights and gluing them into the base, it was time to mount the top. After the glue had cured, the front and back rails were prepared and glued on - these had to be right, as the Wide Table extension rails are going to fit directly to them. If the final measurement is under specification, shims could be used to fit the extension rails. However, if the final dimension is over specification, that would be a totally different matter!
A multitude of small jobs were dealt with next - fixing oak trim to all the front vertical upright faces and putting on the back (also ¾" MDF for strength and rigidity). The back can be seen in this picture, which mainly shows the three drawers undergoing final assembly. The drawer fronts are wrapped with mitred-corner oak, the ¼" ply bottoms sit in grooves in the front and both sides and are pinned to the back, which sits in a dado - standard "NYW" method, apart from the fronts being held to the sides with double number 20 biscuits.
Having treated the drawers (sanding sealer inside, tung oil outside), they were fitted to the full extension runners and the knobs attached. The doors for both the storage cupboard and the blade caddy were made next, treated with lemon oil on the inside, then hung with European hinges and the knobs fitted to match.
Now the most important part of the construction was needed - the drawer carcass box upon which the saw will be mounted. This HAD to be at the correct height, if the saw table was going to exactly match the mobile base top. I decided to make the top of the box as a hinged flap, in order that I could tip the saw up for maintenance and cleaning.

The fact that MDF is not very robust material for screwing into made wrapping the hinged flap vital - I chose a 35mm x 25mm oak frame, with mortice and tenon joinery as a strong and reliable frame to receive the mounting screws for the saw and the screws for the hinges. I then cut a piece of MDF of the exact inner dimensions of this frame, then biscuited it on all four sides into the frame. I felt that this would provide the greatest rigidity, especially as I was going to cut an access hole into the MDF to get at the saw innards.

The box top was made in an identical way to the flap, then two solid brass hinges were morticed into the oak. The upright panels were then cut and wrapped in oak where necessary to trim them to match the base. These were also biscuited into both the box top and the base platform.

Once the glue had cured, a test fit of the saw proved OK. Sanding and treating with Tung oil followed, then I was able to fix the Formica™ top and trim it with a laminate trimming bit in the router. Sheets of Formica™ are really hard to find here nowadays, as worktops now come pre-laminated and post-formed with rolled edges. I managed to find some in an "old fashioned" DIY store and - luckily enough - the color wasn't too bad.
Next, the saw was mounted again, this time for measurements in order to fix the extension rails to the front and back rails of the base - a vital task if everything is to work smoothly and accurately.

Having made sure that the saw and table top were identical, I slid a T-nut into the end of an existing rail and marked the center with a bradawl. This measurement was then duplicated with a mortice gauge and replicated at the location points for the extension rails.

Whilst the front extension can be fitted directly to the front rail, the rear extension requires the spacer (supplied in the Wide Table Kit) in order for the fence to clear. You can see the amount of stand-off of the rear rail, compared to the front rail in these two pictures.

Having fitted both extension rails, I added an additional measuring guide on the front rail. It doesn't go all the way to the end, but it's better than nothing, until I can source a self-adhesive tape (in millimeters) that is long enough.
I was quite pleased with the time I'd taken to get the saw box height correct. This is how the fence rides over the top of the unit.
Here is the blade caddy pulled out on its Accuride™ hinges, with European hinges to attach the door. The slanting blade storage is 6mm MDF, screwed to the oak wrapping top and bottom. The chippers for my 6" dado set are kept in the drawer below the saw - more convenient for me than keeping them with the blades. The posts for the blades are turned cedar spigots (29mm and 15mm for the 30mm and 16mm bores of the blades), glued in from the back.
The end cupboard just needed an MDF shelf, faced with oak, to complete the project.
So here's the unit. Eagle eyes will notice that I've moved the power switch to the other side. I'm predominantly left-handed, so it suits me better and will make tipping the saw up on the hinged platform easier. The vertical handles (left-over larch) make moving it about easier too - I'll stain them to match. I would have preferred to have gone with just two swivelling castors, but I've got to store the unit in such a tight space that I would have been struggling - four swivelling castors makes for some interesting manoeuvers, though!

To see the design considerations used (which may help you with some ideas for your design) click here