Housing and Slotting Jig
Thursday 14 August 2008
Ron Fox introduces you to one of the most useful jigs and aids for router work
Cutting housings is one of the bread and butter applications of the router that appears quite straightforward. You mark the positions of your housings, put the appropriate-diameter straight cutter in your router, clamp a straightedge across the workpiece, set the depth of cut, and run the router base against the straightedge to make the cut.
Well, that's the theory. In practice this apparently simple job presents several problems which tend to be glossed over.1 It is usually impossible to find a cutter whose diameter exactly matches the thickness of your shelf board to give a snug-fitting joint. If you do find one by sheer luck, your very next shelf board might be of slightly different thickness, and in any case your cutter will wear with use and with honing. If you are preparing your own timber you could, of course, thickness it to suit a particular cutter but you can't do that with veneered chipboard, plywood, etc. 2 Positioning the cut can be a tedious business. With a router the cut takes place at a distance from the edge of the router base, and this distance varies with the diameter of the cutter. I have seen advice ranging from 'clamp the straightedge to the board' to 'clamp the straightedge at a distance from the marked line equal to the distance from the cutter blade to the edge of the router base'. Something far less annoying and error prone is required. 3 With a simple straightedge it is easy to let the router wander off the cut. With a D-shaped router base there is also the question of whether to run the straight or rounded part of the base against the straightedge. There is also a curious school of thought that seems to believe that you cannot use a circular-based router against a straightedge. You most certainly can, but with this jig you don't have to. 4 With stopped housings it is easy to end up with slightly misaligned housings in the left-hand and right-hand shelf uprights, a subt le problem that is not always recognised. For about 10 years now I have been using a simple guidebush-based jig to cut housings and also to slot and counter-slot boards. The original version first appeared on these pages in issue 13, May 1999, but over the years I have refined it to make it more user friendly, photo 1.
SOLUTIONThe only critical dimension is the width of the slot, which is made to be a snug fit for the intended guidebush. Mine is 30mm wide to take a 30mm guidebush, a common size with large routers.
If you have a number of different-diameter guidebushes you will get much more out of the jig if you cut the slot to match the largest one. If your router does not offer a good set of bushes, it would pay you to invest in a sub-base such as the Trend Unibase plus a set of Trend bushes.
CONSTRUCTIONThis jig is made from a piece of 12mm MDF plus a length of 32 x 50mm batten.
The first step is to decide what width slot to cut in the MDF. Those with only one guidebush with their router have no choice; the slot will be cut to be a snug fit for that guidebush. Those with a range of bushes of different diameters would do well to make the slot to take the largest bush, for reasons that will become apparent.
Begin by cutting an accurate rectangle of MDF to the dimensions shown. Next, draw the outline of the jig on the board but do not cut to shape yet. The rectangle will simplify the cutting of the top-bar slots and also make clamping the board easier while machining it.
Now mark the slot positions, photo 2.
Rough cut the interior of the guidebush slot leaving a trimming margin, which will be finished to exact size with the router and a template trim cutter. The rough cutting can be done with a bandsaw, scrollsaw, jigsaw or whatever you have to hand.
STRAIGHT EDGESNext, cut and trim a couple of straightedges in 18mm MDF. You will find these invaluable for many other applications. The ones shown are 500 x 55mm. Attach one straightedge with double-sided tape to one of the pencilled guidebush slot lines. Now tape the second straightedge to the other line, ensuring that the two straightedges are exactly parallel and exactly the correct width for the bush. Use the guidebush itself to set the second straightedge. A scrap of 18mm MDF can be used to block the inside end of the slot and give a square cut, photo 3.
With the straightedges pressed firmly down, fit a template trim cutter (the sort with the bearing on the shank, immediately above the cutter blades) to the router, and clamp the workpiece to the bench or cutting table with scrap material under it to give clearance for the cutter. Set the depth of cut so that the bearing runs on the straightedges and the blades span the thickness of the MDF, then trim the slot, cutting in a clockwise direction, photo 4.
Now cut the slots and counter-slots for the stop-bar bolts. By making the original workpiece a rectangle you can use your router side fence to position these cuts. Alternatively, use your straightedges and guidebush to locate the slot.
Make the first cut to suit the diameter of the bolt shank, then change the cutter for one large enough to give clearance for the head of the bolt. I use 6mm roofing bolts and wing nuts to fasten the stop bar. The head of these is 12mm in diameter, so a 1/2in cutter gives comfortable clearance, photo 5.
The bar itself is cut from another piece of 12mm MDF and two holes are drilled in it for the adjustment screws. If you drill these holes off the centre line of the bar you will have two different positions. These will allow you to turn the bar round to cater for a wider range of stopped housing lengths.
Now you can finish the jig by tapering the top, rounding the corners and gluing a length of batten at right angles to the slot. The joint is a simple rubbed one - no screws or clamps. I use ordinary white PVA and when it begins to grab I square the batten to the slot with an engineer's square.
Remember when you glue on the batten that the counter-slot is on the underside of the jig, photo 6.
Finally, the stop bar is installed with the roofing bolts and wing nuts. The square part under the screw head prevents the bolt turning while you tighten or loosen the wing nut. The lines drawn across the jig below the stop bar help to position it parallel with the edge of the workpiece.
SETTING UPTo initialise the jig, fit the guidebush and install a wide straight cutter (as wide as will pass through the bush). Make a cut through the batten slightly deeper than you expect your housings to go. Now fit a V-grooving cutter in the router and cut a groove across the batten. This will be dead centre and can be emphasised by drawing a line across it with a black ballpoint pen. This is your instant positioning device after marking the centre line of the housing on the workpiece, photo 7.
HOUSINGs METHOD 1To cut a housing with the jig, mark the centre line of the housing, fit the guidebush, install a cutter as close as possible to the required width of housing, clamp the jig with quick-release clamps and make the cut. Make more than one pass to get the required depth of cut if necessary.
If, as is likely, the housing is a fraction too narrow for the shelf, undo the quick-release clamps and move the jig very slightly by eye. The cutter should be chosen to be as close as possible to the required size without exceeding the shelf-board thickness. If it is a lot smaller you will not be able to reposition it by eye and you'll be back to the tedious error-prone measuring method.
By the time you have made a couple of practice cuts in a given piece of material you will have your eye in and will be making the first cut, moving the jig and making spot-on housings.
This jig and method of use overcomes the first three of the problems that I outlined at the start.
Being based on a guidebush, this jig is not specific to a particular make and model of router.
Widening the cut by moving the jig often arouses dark mutterings from some of the older workers who tend to describe it as bad practice without saying why, or giving their own solution to the problems. I've been doing it for years.
Housings method 2Method 1 works well but the situation becomes much more interesting when you use the jig with guidebushes of smaller diameter than the width of the slot. With a range of smaller guidebushes and a number of straight cutters, we can cut a series of housings - without moving the jig - with different combinations of guidebush and cutter. The jig now becomes a template and, with each different-diameter bush, a given straight cutter will cut a different-width housing.
If you take a length of veneered board and try different combinations of guidebush and cutter you will end up with a 'pattern board' which you can refer to whenever you have a project involving housings. With reasonable luck you will find a combination to fit your requirement.
Most guidebushes, either individual or in sets, are made in even metric values, e.g. 20, 22, 24, 26mm etc, but Trend also offer a range of plastic bushes in odd metric sizes, plus a range in imperial measure. These offer tremendous flexibility for cutting housings. I have a set of the Trend odd-size metric bushes and have made a second pattern board using these with the same cutters as for the 'even' board. Between them, these two pattern boards have so far met all my requirements for housings, whether through or stopped, photo 8.
This simple jig overcomes all four of the basic housing problems without having to move it to widen the cut.
Slotting and counter-slottingThis jig also makes an excellent slotting and counter-slotting device. The first cut is a through cut made with a cutter that gives clearance for the screw-shank diameter. This is followed by a shallower cut, at a depth to recess the screw head, made with a cutter that gives clearance for it, photo 9.
If you do much housing and slotting work, you might make yourself a second jig, smaller than the first, to be more convenient for smaller workpieces. The ability to slot and counter-slot enables a wide range of adjustable jigs and templates to be made, quite apart from its value as a housing jig, photo 10.