This jig by Alan Holtham can be adjusted to suit differing grooves
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Multiple grooving is a common operation in woodworking and the router is the perfect tool for the job. The grooves may be widely spaced, as in for instance shelf housings, or they may be quite close, like drawer runner grooves or for bending. No matter what the purpose, it is always essential that the grooves are evenly spaced on matching components or the job will not line up properly.
The answer is usually to make a simple jig consisting of a plate of MDF fixed to the base of the router, with a suitably sized guide strip screwed to it to locate in the previous slot. In the past I have always custom-made these jigs, discarding them at the end of a job as they are usually of no further use - it is rare to get two jobs with the same groove and spacings; however, the prospect of quite a big variable housing job on some pigeon hole units convinced me of the need to make a proper version of the jig, one that was adjustable for an infinite range of groove widths and spacings.
The finished jig in use
MDF is ideal for jig making, so always save your offcuts, even small ones. They will all be of a consistent thickness no matter what the original source, so you can use all sorts of different pieces without worrying about them lining up. The thinner pieces in particular are very useful
This simple jig requires the minimum of material, so start by cutting out a piece of 6mm material about 4cm wider than the base of your router and as long as you want . I made mine 320mm long which allows me to accommodate the range of groove spacings I mostly use
Cut a couple of narrower strips of 9mm material to act as locating sides for the router base. As the router has to slide between them, machine dead straight and to a smooth finish. These then have to be fixed onto the base board by pinning, gluing or screwing. Use spray accelerator with super glue
Start off by gluing on one edge strip, and then use the router as a spacer to get the other one parallel to it. In order to achieve a reliable hold, make sure to apply plenty of pressure for a few seconds as the glue sets. Make sure the router can still slide smoothly between these two strips for the length of the base board
In order to hold the router firmly in position, we're going to use the ever-helpful side fence rods which pass through a locating block at one end of the jig. To cut these accurately to length, machine up a length of hardwood 38 x 35mm in section, then cut a 5mm strip off one face
I decided that it would be better if this locating block was the full width of the base plate, and had to cut away a short section on the end of each of the guide strips to locate it. Fit this section, then bring the guide strips up to it for convenience
Hold the locating block in position and use a pencil through the router guide rod holes to mark their position. Once you're sure you've got everything marked out properly, these holes now have to be drilled accurately...
...using a drill press; making sure you're working with a drill vice for safety
Countersink the entry to these holes to make it easier to locate the bars into them
To lock the bars in position you will now need a couple of threaded knobs and nuts of some sort - always worth keeping around the workshop, or failing that, ordinary bolts will do
Next, drill some cross holes into the locating block to take these bolts, countersink the holes slightly undersize to take the nuts and hammer them in flush with the surface
A touch of epoxy glue would normally be enough, but to be on the safe side I now stick the 5mm cover strip back over the top, to stop the nuts lifting out if you get a bit heavy handed on the tightening up!
This block can now be fixed into position on the end of the baseplate. I used the super glue again to get the initial fixing, but do it with the router in place as well to make sure it all lined up. Once the glue is set add a few screws for additional strength, but position them well clear of the bar holes
Your router should now slide smoothly along the bars and lock in place wherever you want it, but, more importantly, you can quickly remove it for normal work
Fit a suitable bottom-cut, two-flute cutter into the router, plunge it down through the base into a piece of scrap then slide the router along the bars to cut a slot in the base that is long enough to give you plenty of adjustment for position when you come to use the jig
The jig is now essentially complete, but you will have to add a suitable slide bar when you come to use it. This varies in size depending on the job in hand. It is just a strip of hardwood machined to be a tight, but sliding, fit in the groove you are making. To get this perfect make a trial groove first in some scrap, then keep planing the strip until it fits as you want it. Make it too tight and you will have trouble sliding the router smoothly and the groove will end up full of jumps and burn marks
Screw the strip into the base at a suitable distance from the cutter to give the desired spacing of the grooves
It only needs to be approximate as you can fine tune the spacing by sliding the router along the bars. Obviously you can change the size of this strip and its position for each different job, adding greatly to the versatility of this jig
Form the first groove with either the router side fence or using a straightedge as a guide, then position the slide in this groove and adjust the router position for subsequent grooves
Lock everything up really tight, set the plunge stop to the required depth and push the router across to form the second groove. The trick is getting enough clearance so that the guide bar slides smoothly and without snagging. I always push it across the board and then pull it backwards as well. This seems to widen the groove fractionally and accounts for any play in the guide bar. Deep grooves may need two or three passes to avoid stalling the router or burning the cutter...
...then just keep repeating the procedure until you have as many grooves as you want