Leigh Mortise & Tenon Jig

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Leigh Mortise & Tenon Jig

Now in its fourth year, the outstanding Leigh FMT Jig is appearing more and more in workshops around the world. After all, it is mortise & tenon joinery that holds most of the world’s fine furniture and cabinetry together. There are tenoning jigs on the market and mortising jigs too, but only the FMT uses a single guide and bit to cut both mortises and tenons, with speed and precision that you have to see to believe. The FMT is so easy to use and so efficient that it brings professional quality joinery within reach of any shop, from basement to custom studio to factory floor. As a result of our previously mentioned inhouse manufacturing savings, we have significantly reduced the price of the FMT. 

Here’s what woodworkers are saying about the FMT:

Ease of Use “…the Leigh Jig is really simple to use.” –Andy King

Versatility “Spot on accuracy and utmost flexibility...the Leigh Jig (FMT) makes perfect straight and angled mortise and tenon joints in a huge variety of sizes...” –Randy Johnson

Speed and Convenience “I know of no better or faster way to cut mortises and tenons...” – Christopher Schwarz

User Guide “…the user guide…is superbly written and illustrated.” –Carl Dugay

Precision Engineering “Everything looks and feels first class. The parts that move, do so smoothly, while the clamping mechanisms hold stock securely without requiring you to exert a lot of torque.” –Carl Dugay

Value and Performance “...the FMT is without doubt the best affordable Mortise and Tenon Jig available... this jig is one special animal.” –Christopher Schwarz 

Quotes above: Randy Johnson, American Woodworker Magazine; Carl Dugay, Canadian Woodworking Magazine; Andy King, Good Woodworking Magazine (UK); Christopher Schwarz, Popular Woodworking Magazine. 

Leigh Mortise & Tenon Jig

FMT Frame Mortise & Tenon Jig 

US Patent no. 5,123,463 

CDN Patent no. 2,061,568

EURO Patent no. 0 511 898


• Over 70 sizes of mortises and tenons 

• Standard joint guides available in 1/4", 5/16" (included), 3/8" and 1/2" 

• Maximum 1/2" x 5" joint size 

• Double, triple and quadruple joints 

• Easily recorded, repeatable joint tightness adjustment 

• Angled and compound angled joints 

• Works with virtually any plunge router  Standard equipment: 

• Universal sub-base that attaches to virtually any plunge router 

• Five 5/16" joint guides (5/16" by 1/2", 3/4", 1", 1-1/4" and 1-1/2"), which can make over 21 sizes of mortises and tenons 

• Joint guide stand 

• 5/16" HSS spiral upcut bit w/1/2" shank (Leigh No. 170-500) 

• Two cam-action speed clamps 

• Adjustable side stop fence for tenoning 

• Two outrigger support bars 

• Screwdrivers and mounting hardware 

• Fully illustrated Leigh FMT User Guide 

Note: A 1/2" (12mm) plunge router is essential to achieve the full potential of the FMT.  

Anatomy of a Precision Jig

It took five years to complete the design and testing of the Leigh FMT. As we worked through the process, it became clear the new jig would be mechanically demanding to produce. The final version incorporates 88 extruded, die cast or injection molded parts, each requiring a custom die or mold. Another 18 components are CNC (Computer Numerically Controlled) machined to very precise tolerances. 

Leigh Mortise & Tenon Jig

The jig that took us five years to develop takes you only five minutes to understand well enough produce a perfect mortise and tenon joint. At a cost which is less than a third of its nearest serious competitor, the FMT is clearly the best value on the market today. 

1. The clamp plate is made from a rugged 6063-T5 aluminum extrusion, CNC machined with a totally reliable non-slip, non-marring textured surface. 

2. The clamp plate can be tilted upward from 0° to 30° for angled or compound angled cuts, and it features a positive 90° return stop.

3. Two powerful 3" clamps feature Leigh's Cam-Action Speed Clamp levers and die cast ZA12 alloy rocking arms. T-slots in the clamp plate provide plenty of mounting options for holding both mortise and tenon workpieces.

4. A die-cast aluminum side stop fence has a positive 90° stop and can be set at any angle up to 45° left or right. 

5. Dust collection is handled by a die-cast aluminum dust collection vacuum box behind the clamp plate. The hose port has a 1-3/8" connection. 

6. The table is made of 1/2" Mic 6® plate, a special proprietary brand of continuously-cast aluminum plate that is rigid, stress free, and precision ground flat on both sides to thickness tolerances of less than .005". The channels, recesses and openings are all CNC milled to exacting standards. Under the table, two 6005A-T5 aluminum extrusions provide both jig structure and table movement. UHMW strips serve as slide bearings between these two plates,  and between the plates and the table. Delrin V-blocks on ground steel pins in machined V-grooves ensure true table alignment in both X and Y axes. 

Leigh Mortise & Tenon Jig

7. The table moves side to side, front to back, and locks in any position with the flip of the clamp lever. The lever operates a sophisticated system incorporating five CNC turned brass and steel parts, making it highly effective and easy to engage with a soft touch. 

8. A retractable sight made of Fortron PPS, a high-tech ultra rigid resin, is used to center the table opening over the layout marks on mortise or tenon workpieces.  

9. Table limit stops can be set for precisely controlled re-positioning of the table as needed for double, triple or quadruple joints. 

10. Joint guides snap into a recess perfectly aligned with the bit centerline and the right side pin track. 

11. Both guides and track are made from injection molded Delrin for smooth, durable operation. 

12. The router sub-base is CNC-machined 6061-T6 tempered aluminum. 

Leigh Mortise & Tenon Jig

13. The sub-base has two stainless steel tapered guide pins, threaded through brass bushings with neoprene o-rings that ride in the pin track on the right and either within or around a joint guide on the left. Adjusting the left guide pin up or down provides exceptionally fine adjustment for joint fit; i.e. .001" on the glue line per increment. 

14. Teflon bearing pads on the table and router sub-base make routing very smooth and stable regardless of the dimensions of the joint or the size of your router. 

15. The FMT's sub-base mounting system works with virtually any plunge router. In most cases, the router can be removed or refitted to the base in less than a minute, making a dedicated router unnecessary. 

Leigh Mortise & Tenon Jig

How the Leigh FMT Works

The basic concept is very simple. Mount your plunge router on Leigh’s sub-base. The sub base is positioned by two stainless steel guide pins projecting from the bottom of the base. The right side guide pin always runs in the track to the right of the bit opening. The left side guide pin steers the router, traveling within the joint guide groove for mortising or around the guide’s perimeter for tenoning. One bit, a spiral upcut matching the guide size, cuts both mortise and tenon.   

Leigh Mortise & Tenon Jig

Routing Tenons 

The first step is creating layout lines on the end of the tenon stock. Then, clamp the tenon piece in the jig and using the table movements, center the retractable site over the layout lines. Next, place the router/sub-base on jig table and set the left side guide pin to track around the outside of the joint guide. While gently easing the bit into the tenon piece, rout the tenon at full depth in a clockwise direction (climb-cut) to cut sharp, clean shoulders. Next, reverse the direction of the cut (counter clockwise) and continue to feed the bit into the tenon stock until the tenon is fully formed. The tenon is done. 

Routing Mortises 

Routing mortises is equally as simple. Mark the left-to-right center for the mortise and clamp the mortise piece horizontally in the jig. Place the router/sub base on the jig’s table with the left guide pin running in the center slot of the joint guide. Set the depth to approximately 1/16" deeper than the tenon length. Make successive full depth plunges from one side of the mortise to the other and then follow up with a clockwise cleanup cut. That’s it, the mortise is done. 

Leigh Mortise & Tenon Jig

Adjusting Joint Tightness

The FMT’s adjustable guide pins allow precise adjustment of joint fit, so precise in fact that adjustments can be made in increments of .001" on the glue line. A graduated knob (4) turns the tapered guide pin (5) up or down. A simple height washer (6) indicates the number of full turns of adjustment and the markings on the top of the graduated knob indicate 1/8 increments.  The right hand guide pin is adjusted only once to allow smooth movement along the track. The left guide pin is adjusted up or down in .001" increments until the desired fit is achieved. Adjusting the left guide pin up produces a smaller tenon and larger mortise. Conversely, a downward adjustment produces a larger tenon and smaller mortise. The reference marks allow you to record and return to the same settings for any given guide/bit combination. 

Leigh Mortise & Tenon Jig

1 Joint Fit Too Loose 

2 Joint Fit Too Tight 

3 Perfect Joint Fit 

4 Graduated Knob 

5 Guide Pin 6 Height Washer “...a test cut establishes the exact settings of the adjustable pins, after which either mortise or tenon can be cut with extreme accuracy...the positions of the adjusting knobs can be recorded so that for each guide the setting can be dialed straight away.” –Ron Fox, Routing Magazine (UK) 

Leigh Mortise & Tenon Jig

How to Rout Mortises and Tenons One guide, one bit, one setup! 

The Tenons 

1. Mark layout lines for the center of the tenon on one end of a single workpiece. 

2. Select the joint guide and matching bit for the desired mortise and tenon size. Simply snap the joint guide into the guide recess. 

3. Extend the table sight. 

4. Clamp the tenon piece flush up under the table sight and against the side stop fence. 

5. Move the table to position the table sight over the layout lines. 

6. Once the sight is centered over the layout lines... 

7. ...lock the table and retract the table sight. 

8. Place the router/sub-base on the table and plunge and lock the router bit to the tenon length. 

9. Rout the tenon. All other similar tenons may now be routed without having to mark or sight the tenons, or remove the router from the jig.  

The Mortises

1. Mark the left-to-right mortise center position on one mortise workpiece only.

2. Remove the router and extend the sight. Move the mortise piece left and right to sight for center and clamp it in place. 

3. Place the router/sub-base (not shown) on the jig and plunge rout the mortise.

See Outrigger Bars (below) for positioning the workpieces for the remaining mortises. 

Outrigger Bars 

The FMT comes with two outrigger bars which can be mounted in slots in the edges of the clamp plate. Simple shop-made outriggers of 1/4" plywood are attached to the bars with nuts & bolts provided.  When routing multiple mortises on the same workpiece, you can simply make reference marks along the top edge, clamp stop blocks on the outriggers or use a flip over stop type system (see photo, right). Now you can rout all workpieces with similar mortises without having to mark them, reposition the table or even remove the router/sub-base from the jig.  As you’ve  seen, you can rout one joint or a hundred alike by making a few layout marks and installing just a single bit. Mortise and tenon joinery has never been simpler. 

Innovative Features  Perfect Multiple Joints 

It’s remarkably easy to set up perfectly aligned multiple mortises and tenons on the FMT. Precision and repeatability are made possible by adjustable table limit stops (circled here and below) that control the table’s range of motion in both X and Y axes. With each mortise or tenon position sighted and limit stops set, you’re ready to rout double, quadruple, or even triple joints with the same ease and precision as a single mortise and tenon. 

Double and Quadruple Joints 

The sequence below shows each step in making double and quadruple joints. In steps 1& 2, the side-to-side limit stops are set to produce double inline tenons.

Steps 2 & 3 set the front-to-back limit stops which are used to produce side by side tenons. 

1. Rout the rear left tenon. 

2. Move table to the right; rout rear right tenon. 

3. Move table forward; rout front right tenon. 

4. Move table to the left; rout front left tenon.

Triple Joints 

Triple joints are set up just like quads. Make a small stop block to fit between the front-to-back limit stop and post, to position the table for routing the third mortise and tenon.  

Wider or Narrower Joints 

It’s always faster and easier to use a joint guide made to cut the exact mortise and tenon size you want. However, if you don’t have the right guide on hand, or if you want to cut a joint that’s wider or narrower than the nearest guide size, you can use the table limit stops to change what any given joint guide allows you to do. You can cut tenons as short as twice the diameter of your bit, or up to almost twice the guide length.

Bonus Feature: Floating Tenons and Doweling 

Traditional mortise and tenon joinery isn’t practical on miter joints, though often they need all the strength that tenons provide. The FMT makes it easy to mortise both sides of a miter then glue in a shop-made floating tenon for the strongest possible joint. Doweling, though not as strong, is just as easy to set up and is faster overall when you don’t have time to prepare floating tenon stock.

Mortise and Tenon Joints Combine Beauty and Strength

The traditional mortise and tenon joint is indisputably the strongest way to assemble frames for fine furniture and cabinetry. No other jig or machine method can match the speed and accuracy with which the FMT produced all of the mortise & tenon joints used to build the bedroom suite shown

Production Routing and Smaller Joints 

When you’re engaged in a high-volume production job, you can speed up the process considerably by using the largest bit possible to cut tenons smoothly and quickly in one pass. Using a tenon bit larger than the joint guide size will make tenons that are smaller than the guide, so you’ll have to use a bit smaller than the guide to cut matching mortises. The principle is simple: any two bit diameters which add up to twice a given joint guide size can be used to make joints the size of the smaller bit.  In the extreme, this technique lets you cut joints much smaller than the FMT’s smallest 1/4" joint guides. Delicate furniture parts such as small cabinet door frames, doll house architecture and furniture, and very accurate miniatures all call for mortises and tenons as narrow as 1/8" or even less.

Speed and Precision...Hallmarks of  the Leigh FMT

Incredible Miniature Joints are Easy The FMT can rout joints not just smaller than 1/4" but so tiny they’re almost beyond belief — all with the same easy setup and adjustable fit you expect in larger joints. Miniature joints show the extreme versatility of the FMT.  These matchstick joints actually creak when assembling! And if you’re wondering how these piece are held in place, here’s how: there are twelve through-holes in the clamp plate with which shop-made auxiliary plywood plates, blocks, and work-holding fixtures can be attached and used together with the Leigh cam clamps.

Note: This is just a summary from the User Guide. In depth instructions are in the easy-to-follow Leigh User Guide which comes with the Leigh Frame Mortise & Tenon Jig

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