Mitre Biscuit Jig

Wednesday 11 July 2018

One of the best ways of securing a mitre joint on a box or carcase is by fitting biscuits, but even with the proper mitre fences in place, cutting the necessary slots can be a bit hit and miss if you try and use a biscuit jointer. This simple jig overcomes the need for the biscuit jointer altogether and instead uses your router and an inexpensive cutter. It is just a means of holding the work at precisely 45 degrees to a horizontal slotting cutter mounted in the router. It is one of the easiest jigs to make, but does require you to have the router mounted in a table of some sort. There is very little precision involved either, a few accurate 45 degree cuts and make sure the material stop on the jig is at 90 degrees to the table surface and the joint becomes foolproof.

Another advantage of these jigs is that they are cheap to make and allow you to use up all sorts of scraps of material, the finished size to some extent being dictated by what you have available. I always use MDF as it is flat, stable and cheap, but use hardwood if you prefer. MDF is quite soft so I cover the wearing surfaces of jigs that I am going to use regularly with offcuts of laminate, but this is not essential for occasional use.

Build the jig

Start the construction by cutting out all the necessary pieces. In this case the base is 10mm stock, the support board and stops are 16mm and the ends 25mm.

The support board has to be fixed at a true 45 degrees to the base, so start by planing this angle along one edge.

The supports for this also need to have a 45 degree angle so ideally cut these on a chop saw, but do check the angle produced, it may not necessarily be spot-on straight off the saw.

The laminate is stuck on using contact adhesive, the new solvent free ones seem to work well and are much more pleasant to use. Cut the laminate roughly to size leaving plenty of spare all round and stick it on firmly.

A tight sueeze in the vice often helps the bonding process. Then use a straight trimmer in the router to cut back the excess to a perfectly flush finish.

The support board needs to slots to take the adjustable stop, so cut these with a straight cutter suitable for the bolts. The coach bolt head needs to be a sliding fit in the slot, but not so loose that it can turn. I used an 8.5mm cutter for 8 mm bolts. Use the side fence to guide the cutter, and mark the start and stop positions with a pencil line.

A deep cut like this with a narrow cutter will have to be made in several passes, or there is so much vibration that the cutter chatters and may even snap. A cut of 5 mm at a time is plenty deep enough, and with a little care you should end up with two perfect slots.

Use these slots as a template to mark and then drill the bolt holes in the sliding stop. This stop and the fixed one also need a 45 degree chamfer cutting on the end.

With the bolts and wing nuts in place the stop should be a nice sliding fit without being too sloppy.

The angled supports need to be screwed to the base from the underneath, a little PVA glue will help to hold it all rigid.

Then turn the jig back up and fix the support board, screwing through the end stop as well at the one end, and just through the support board at the other. This fixed end stop must be at 90 degrees to the edge of the support board or the biscuit slots will be angled, this is the only crucial part of the making.

That is all there is to the jig, it should only take you an hour or so to knock up, but will save hours of frustration while trying to glue up mitre joints.

The bolts should slide freely, but if there is any problem you can always access them from the back of the jig.

Using the jig

To use the jig you will need a dedicated biscuit cutter for the router. These are supplied with a range of bearings to suit the size of biscuit you are going to use.

The router has to be mounted in a table and the height of the cutter adjusted until it is in the centre of the mitre joint. A fine height adjuster on the router makes this much easier.

I initially just hand held the work in the jig, extending it down onto the table and used the bearing on the cutter to control the depth of cut.

A few trial cuts soon showed up a tendency for the work to 'creep' as it was pushed into the cutter. This means the slot ends up slightly over- width so the effectiveness of the biscuit is lost, and the joint also goes slightly out of square.

Instead set the work back in the jig so it lines up with the edge of the jig and use this edge as a guide against the table fence to control the cut. Put some pencil lines on the table fence to give you the start and stop positions.

You will also need to clamp the work firmly onto the jig to eliminate any movement; it must stay fixed in place during the cut.

With everything firm, just gently push the whole assembly onto the revolving cutter. With the weight of the jig and the massive area to hold, there is little danger of kick back.

The result should be a perfect, clean slot in the mitre.

Repeat the procedure for the other part, the pencil lines will ensure you get the matching slots the same length.

The end result is a really strong and 'frustration free' mitre joint that locates perfectly and is dead easy to glue up because it cannot slide about.