Independent Review of the Leigh FMT Jig
© John Bullar 2004
previously published in Furniture & Cabinetmaking magazine
Leigh established a good reputation with their dovetail jigs many years ago, so I was particularly interested when offered the chance to test their new Frame Mortice and Tenon (FMT) jig - a tool for use with most makes of router, to produce accurate, repeatable mortice and tenon joints. The price of the FMT might be one of the first things you notice. Okay, there may be a lot goes into manufacturing a jig of Leigh’s quality, but to justify a price tag like this, a tool is going to have to really perform, as a mainstay of the workshop.
The manual is a well-produced spiral bound book with 126 pages of clear diagrams, instructions and explanations. A short learning session gets you producing simple competent joints with the FMT and there are plenty of more complex tricks up its sleeve if you want to explore them.
Router jigs can only cut round ended mortices. I cannot see any technical problem with this, but if you wanted to make traditional square through-tenons for example, the FMT would not be the tool to use.
The main parts of the FMT are the jig body - a sturdy little aluminium table for bolting onto the workbench, and the oddly shaped sub-base. This fixes on the underside of a router and slides about on Teflon pads set into the jig body tabletop. The horizontal tabletop of the jig body is a beautifully machined plate of 12.7mm thick aluminium. It is 420mm wide by 275 deep with a number of apertures cut in it. The cast aluminium stand is equally robust.
Beneath the table is a clamp plate with a knurled surface, which normally sits in the vertical position but is hinged to swing out for angled cuts. You clamp the timber on to this plate beneath an aperture in the table, using a pair of quick adjusting over-centre clamps. The clamps grip well, but are a bit awkward to fit. Leigh supply the simple tools needed for assembly and adjustment of the jig.
A crosshair alignment aid slides into position to help you centre the jig over the timber. By releasing a clamp, the whole tabletop can be slid from side to side or fore and aft, making precise adjustments easy.
Positioning of the router on the sub-base is critical. A simple plastic collar fits temporarily into the router collet and lines it up accurately. The router is clamped onto the 5mm thick sub-base by a pair of rods, passing through the fence rod holes in router base. These aluminium rods are quite flimsy - about half the diameter of the steel rods fitted to the router fence and a fraction of their strength.
Designing a mounting arrangement that is compatible with the numerous router bases must be tricky. For a few models of router, there are fixing holes for bolting directly through the sub-base. If I owned an FMT jig I would drill it for bolting my routers.
Two tapered steel pins project below the sub-base to control its movement. While cutting around a tenon, the left pin follows the outline of a template set into the table. The right pin simply slides from side to side in a slot. The router cutter follows a path of the same dimensions as the template while going sideways but half the dimensions of the template while going fore and aft. This is because the cutter is mounted midway between the pins, while the sub-base pivots about the right pin. For cutting a mortice, you need to clamp the timber horizontally under the table and this time the router simply slides from side to side with the pins trapped in two slots. Each template is designed to make both matching halves of the joint. It has the appropriate outline for the pin to run around while cutting the tenon and the corresponding slot length up the middle for the mortice.
An excellent feature is that you can fine-tune the fit of the joint. The guide pins are adjustable in depth so their tapered sides ride further in or out on the template by fractions of a millimetre.
The templates themselves are made from tough plastic, which should not wear down quickly. They are held by a sprung catch at one end while the other end has a moulded pair of lugs that slot into the aluminium top. Each lug is barely 2 mm thick by 3 mm wide. These lugs are so small they would not last long in a busy workshop. You might need to cost the templates as consumables.The length of timber you can cut a tenon on is restricted by the ground clearance of the jig and bench. This is likely to be around 1 metre altogether, so it is more than adequate for making door rails for example, but it could be limiting for large cabinet frames. The Leigh manual proposes a raised setup if you need to use longer timber.
The jig comes with a set of Leigh Spiral Upcutters, looking a bit like stubby twist drill bits. These make a lovely job of removing material from deep mortice slots without any hint of burning. They also cut clean tenon cheeks and shoulders on end grain without spelching. Being made from High-Speed Steel, regular users of Tungsten Carbide Tipped bits may find them less long lasting, but the sharpness of steel is most rewarding in the quality of cut it produces. With the aid of a dial calliper and Leigh’s clear instructions, I had a couple of practise runs on softwood to get the adjustment on the guide pins right. Then I cut some very tidy half-inch wide joints in oak and elm. The fit was superb. The joints pulled up snugly with zero visible gaps on any shoulders. There was a small misalignment between the faces of rails and stiles, but just a fraction of a millimetre - a couple of plane shavings. You could probably eliminate this by a bit more fine adjustment.
Dust extraction is provided through a vacuum nozzle cast into the back of the jig’s stand. This works fine for cutting the back of tenons but not while cutting their fronts. Also, the hole in the sliding sub-base is so small it prevents the router’s own extraction from working. This means wood fibres simply shower from the open front of the jig onto the cabinetmaker. The manual suggests you wear a mask, but that does not prevent the dust spreading over your clothes and through the air.
I enjoy making chairs from time to time, usually with angled joints between awkward shaped parts fitting these is always a time consuming business. With this in mind, I was particularly interested in the FMT’s ability to cut angled tenons. Of course, you must know the precise angles of the joint and will probably need an accurate drawing to work them out, but having decided on these and the position of the tenon shoulders, cutting the joint could not be simpler. Cutting awkward angled joints on the FMT is just as straightforward as right-angled joints and with the same superb results. The clamping plate on the FMT swings out to calibrated angles up to 30 degrees allowing precisely angled joints to be cut. As the manual explains, you would need to be wary of short-grain weakness when cutting large angles.
You can vary the angle at which you clamp the timber to the plate for producing compound angles with two directions. It is easiest to cut joints on straight timber that can be shaped afterwards, but if this is impractical, such as for jointing laminated components, a shaped cradle can be used clamp it to the FMT.
The ability to unclamp and slide the tabletop to different positions opens up other possibilities for other special joints. Double tenons end-to-end on wide boards or twin tenons side-by-side can simply be produced by sliding between the adjustable end-stops on the tabletop. In addition, this feature allows you to produce different lengths of tenons using the standard templates.
The FMT is straightforward to start with, but it has many features that make it more versatile if you want them.
Fundamentally, it is a robust and accurate tool suitable for moderate duty. At the end of the day, you judge a tool by the job it does - and the results from this one are superb! Accurate, clean-cut, joining at odd angles is the feature that really sells the FMT to me, and would make saving up for it worthwhile. Joints of such high quality, which do not take all day to cut or to set up, are something for which you would expect to have to pay.
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