Using a router as a thicknesses or surfacing machine is a VERY cost effective alternative to a large stationary machine. Not only is it probably a tool you already have, it takes up very little space compared to a planer/thicknesser. and it is not limited but the size of the machine (every tried to thickness a 14"wide board in a 12" planer??!! LOL!
Also it is far safer than a stationary machine, and give a much finer finish as well! (ever tried to put a 3" oft through a 12" planer) LOL!
Regardless, this is a much more available option for woodworkers than potentially getting a large, expensive machine that needs floor space and maintenance, and is limited in the size of work it can cope with.
Step 1: Making the Jig, and Some Considerations.
The Jig is really simple to machined consists of only 5 parts. The main section can be of virtually any material that is as flat and as consistent in thickness as possible. The jig here is made from a section of Hi-macs (corian), but Plywood, MDF, Chipboard(UK)/Particle board with a smooth face such as melamine will also work. The thicker the material is the more stable the jig will be under that weight of the router in time. I like to use 12mm/1/2" material as it lasts well.
This thickness of you jig is up to you but remember its always a trade off between the jig sagging because it's too thin, and loosing the plunge capacity of your router if it's too thick.
The sides of the jig help keep it rigid and stable for longer spans, and also act as a stop to keep the router from carving into the jig rather than the work piece. This is made from the same material as the main section of the jig.
At this point you can cut the slot in the jig. I used a simple two flute straight cutter, but you can either cut it out with a jigsaw or even use two strips of your jig material cut down the middle and spaced slightly apart. Personally I prefer to route the slot, as it stops any catching of the router base later on.
The feet that supports the jig over the work must also be flat, and consistent in thickness. Here I'm using some bits of Oak from a worth/counter top. but you can also use several pieces of you main jig body material and stack multiple bits on to of each other. If you're using MDF or Ply, this work great, just stuck enough bits up glue, pin or screw them together and fix it all to the base.
The jig needs to be high enough to clear the top of the work you are flattening/thicknessing/surfacing.
Step 2: A More Flexible Option for You Jig (optional)
I like to use single thicker parts for the feet, then have plates made from MDF that can be temporarily fixed to the feet to raise and lower the jig for other work. You can make the jigs and just throw them out after the project since they are so quick and simple to make, but I like to have one around of a medium size for general use. to save time making them, great if you can store it.A
Step 3: Choosing Your Cutter.
A simple 2 flute straight cutter is all you need. size doesn't really matter, but the bigger the slab you're working on and the smaller the cutter, the longer it'll take. Generally a 3/4" or !9mm (it) cutter is as small as I would use. if you have a large 2" cutter then all the better. Just remember to leave a big enough gap, or cut a large enough slot for your bit in the Jig.
The best kind of cutter is a 3 or even 4 flute surfacing (sometimes called a tenon cutter by some suppliers), the more flutes the finer the finish. These generally have rounded or chamfered corner to the cutting edge to stop nasty show lines being seen at the edge of every pass you make. They also tend to have a sheer cutting edge rather than a square face like a standard straight cutter. But If you only have a straight cutter use that and go slow for you passes!
Wherever possible try also to make your passes ALONG the grain rather across it (this tends to hide the shadows within the grain patterns of the timber).
Step 4: Your Work Surface and Fixing to It!
Your work surface is as important as you jig in terms of flatness. If you can use a piece of MDF/Ply/Corian and fix your work to that. It is the datum to which the jig will reference. if your surface is a half pipe, the board you're trying to flatten will be too!!
For really large slabs, you might need to get a bit creative, and use long batons that will support your jig over the work. In some cases long strips of MDF on edge, or even angle Iron can be used to create a frame or gantry to cover the slab. But 90% of the time nothing bigger than a full 8'x4' sheet of 3/4" MDF will suffice. (any thing that big that needs to be flattened is likely to be an expensive undertaking any way, so whats £15 for a sheet of MDF?
At this point it's worth mentioning fixing of the work to the base surface. For small and medium sized parts, I use exclusively HOT MELT GLUE, it's quick, cheap and hold really well. It also acts as levelling aid/wedge if needs be for really twisted up bits on the first side.
If you don't have hot glue, double sided tape and wedges and fixed blocks can be used to hold the part still when routing.
When using hot glue, run a bead (like a weld), along the edge to hold it in place rather than a few blobs under neath. It holds the work more securely, uses less, and is easier to remove afterwards!
Step 5: Setup the Router and Find the High Spots.
Now you have the jig, cutter, router, work surface sorted and the work fixed, you can make your set up.
This is the important bit. TAKE TINY CUTS!!! small passes, shallow cuts, TAKE YOUR TIME!
If you try and rush things now you run the risk of it all going wrong.
Place the jig over the work and check it'll clear the work with a small clearance gap everywhere the jig needs to travel.
Put the router on the jig and pluge down lightly until you make contact with the work and lock the plunge depth
WITH THE ROUTER OFF, move the jig around until it finds a high spot.
with the high spot located, raise the cutter and then lower it again to make contact with the new high spot.
Now repeat all the way over the work until it clear all the way, locking the plunge at each new high spot you find.
Step 6: Set Router Depth and Make First Cut.
At this point you have found the highest point of the work. I generally mark this point with a pencil incase I move the router, or get side tracked, say for lunch. Set the plunge stop/depth bar on the router until it is in contact with the pin/stop on the plunge base.
Now raise it about 1mm so it allows the router to plunge a further 1mm (or, what ever amount you have raised the bar/depth stop).
At this point you are ready to start cutting. move the router and jig to your start point and make your first pass across the whole of the work piece.
Take it slow and work carefully on the first few goes around as you'll hit parts that won't cut and some that will and you'll not always be expecting it.
The important art is patience, don't be tempted to feed the router to fast, and don't be tempted to take too deep a pass, 1mm is quite enough for this job!
Also pay attention to you direction of cut. Alway cut against the direction of the cutter, you'll know if you're going the wrong way as the cutter will act like a wheel and the machine might try to run away with itself. (again a good reason to keep you cutting depth to no more than about 1mm.
Step 7: Check Your Progress, and Resetting Your Depth of Cut.
After your first pass you'll find some bits have been surfaced and some missed.
Now you can reset your depth of cut and go around again.
Simple raise the plunge bar/depth stop a further 1mm lock it in it's new place and go again. If you have a fine height adjuster fitted to your router you can also manually lower the bit with that, but keep it to that 1mm (ish) amount.
Repeat until the entire surface has been faced, with no spots left un-routed.
Step 8: Surface the First Face and Flip It Over.
With the last pass on the first face done you can simply flip the work over and repeat until both sides have been flattened.
It will not likely be far easier to fix in place as you should have a nice flat surface to rest the work on.
These are the kind of finishes you can expect. It's really very easy, so don't be intimidated at any stage. The whole process is quite safe (But not fool-proof), if you've never done it before or are a bit nervous, take your time, take shallow passes, and be patient with it.
Step 9: Points to Remember!
Take your time, rushing simply messes you up.
Have faith in the hot glue, it holds very well.
If the part moves. STOP AND START AGAIN FROM SCRACH!
Take nice shallow passes, you'll find it actually works faster over all, and you get a much nicer finish too.
Try to cut along the grain, rather than across it. For long slabs you might have to keep the router in the same spot on the jig and move the whole assembly up and down the slab rather than the router along the jig, and reset the jig (as in the video).
If there's any confusion, let me know, really!