Woodworking often seems to be about building or making something to do a job you need done, but don't have the tool for. I would love to have a 20 or 24 inch planer, or 24 inch belt sander, unfortunately I would use either so seldom it would not be a cost effective purchase.
So, when I need to plane a board that is too thick, if it is larger than my 15 inch planer, I have to devise another way of planing the board down. There are a few options, you can find a local woodworking shop to plane or sand the board down (for a price), or you can use a hand plane (which is very tedious if it is a large board), you can take the fence of many jointers and make multiple passes, but then you have re-install the fence and adjust it, you could use a hand power planer (but they can gouge the wood if you are not careful) and the last option ... which is the one I opted for is to make a planing jig or planing sled that I can use my router on.
These jigs are not new and have been around in one form or another for years. I you purchase rough sawn lumber from local mills or suppliers, this is something that will be a Must Have in your workshop. Or if you don't have a planer of any size, this allow you to plane and re-surface boards in fairly short order, and do a great job of it.
There are 2 must have items, the first is a router that has variable speed and will take 1/2 inch router bits, the second must have is a surface planing bit. There is not a large selection of these bits on the market and the one that we found one of the most cost effective and readily available was the surface planing bits, available from Amazon. They have a few to choose from, the Magnate bits rage from 7/8 inch diameter to 2 3/4 inch diameter for brazed carbide, and are quite affordable. This bit does not have interchangeable cutting heads which means when it gets dull or if the carbide chips, it means you will likely need to purchase a whole new bit. But, if you don't need this function very often, this might be your best bet.
If you want a much better bit with interchangeable carbide bits, the Amana RC-2250 is great choice at Amazon.com
The Amana bit, with the angle the heads are at, combined with the number of heads, will not only make it easier for your router to plough through the wood, it will also give a much cleaner surface because the blades are cutting more that scraping the wood. You will pay a lot more for the Amana bit, but if you can change the cutting heads and it will help your router to last longer because it is easier on the motor, it could still be the cheapest investment, especially if you are purchasing rough lumber and find yourself needing to plane down larger boards on a regular basis.
In order to build the planing or surfacing jig, you really need to have the router and bit in place before you start. The reason for this is the type of router you have will determine how far apart you rails need to be on the top of the jig and the size of the bit you select will help determine the width of the hole that the bit will need to fit through on the base of the jig.
If you have a smaller router, you will probably want to opt for a bit that is somewhat smaller in diameter, if you router is a 1 1/2 hp or more, it should be able to handle even the largest bit.
The size of your jig, in general terms is going to want to be at least 3 feet, even up to 4 feet long. The base can be any stable material, a good quality plywood is probably best, and should be 1/2 to 5/8 inch thick, but no more.
The rails that will be fastened to the top need to be sufficient that they will help prevent the top of the jig from sagging, even under light pressure, so that the board you are planing does not finish up concaved slightly.
The thickness of the feet is going to vary, depending on how thick the board is that you want to plane down, the thickness of the base of your jig, what planing router bit you finally decide to use, and what sort of height variations your router is capable of. Make sure that you provide the ability to easily replace the feet of the jig, even if they are just attached with screws to the base, there is every probability that you will need to replace them with thicker or thinner feet depending on your work. If you can devise and adjustable foot, even better.
All in all, this is a great jig to make for anyone who is using rough lumber or who has the need to plane down thicker boards.
Copyright - Colin Knecht