Workholding and consistency/repeatability, in addition to the superb accuracy and precision of the Leigh FMT jig! One question from a reader prompts me to write this.

After reading one earlier post on this blog, Charles in Houston, TX, asked if I had much trouble aligning the twin mortises with the tenons properly, to ensure a)_square fit, and b)_easy and tight assembly.

I thought it would be best to show how I do it, and what I recommend, using photos. And if you are reading this and spot a problem with my definitions, concepts, or procedure, please let me know in the Comments section of this post (linked just below the title for this post), or via e-mail to sandal_woods@bbwi.net.

First, some definitions and one image:

From Class Vocabulary at Tooling University, LLC:

accuracy: The difference between a measurement reading and the true value of that measurement.

repeatability: The ability to obtain consistent results when measuring the same part with the same measuring instrument.

precision: The degree to which an instrument will repeat the same measurement over a period of time.

And from Understanding Precision and Accuracy,CHE 353M, Fall 2002 at the Wilson Research Group, the University of Texas at Austin:

The FMT is inherently a very accurate and precise instrument – it was made to be so (bottom-right on the bull’s eye image above). This means it can make a mortise and tenon joint that fits very well; and  better yet, to do so even if multiple mortises and multiple tenons are present on stretchers like the ones below:

And the FMT can do all of this accurately, repeatably, and precisely; using calipers I can verify the thickness of the mortise and the tenons, and their length and width. Of course, I must be able to set up the machine properly to do so. But I must do the layout accurately, to get the results I want.

I want these stretchers to be centered on the legs, such that the eye cannot tell misalignment or misplacement. This is where I must also be accurate with the layout and measurements, and with positioning of the legs and the stretchers on the jig. Once I do this, I put my work on the bottom-right of the image above, close to the bull’s eye. I am helping the FMT work for me, to give me the results I need.

Now, to answer the question Charles posed: Do I have much trouble aligning the mortises and/or the tenons properly, to ensure square fit and assembly, and position on the legs? My answer is no, I have no trouble at all doing so. I take some precautions to lay out the joints; and more precautions and care to properly position not only the work pieces on the jig, but also to move and to position the jig’s sliding table to the dimensions I need. To accomplish all of this repeatably, I use the workholding capability of the jig, as I explain below.

Working with scrap pieces to set up the joinery on the FMT, I found the legs to be very heavy. In fact, I felt I needed to ensure no movement during the machining of the mortises on the legs, as the bulk of the leg would be cantilevered on one side. Enter workholding, using clamps:

Using this arrangement, I was satisfied the heavy legs would not move as I plunged the 3/8-inch router bit into the wood.

Now, some details.

First, positioning and holding the work piece is critical. This is accomplished on the FMT with clamps on the body of the jig, and on the outrigger assemblies. The jig body accepts cam-action speed clamps — the four clamps holding the leg in the photo below; on the outriggers I use any other type of clamp that won’t slip under pressure or light bumping:

The FMT ships with two Cam-Action clamps. I bought two additional ones:

In the photo above, the vertical fence in the center is used to register one side of the stretchers when the tenons are machined. I leave it in place for the duration of all machining operations. Once set, it is extremely difficult to move; it will move if bumped hard with a heavy work piece. But I recommend double-checking its position once in a while.

It is possible to use just about any type of clamp on the outriggers. However, I recommend staying away from spring clamps, as their holding ability with large pieces, such as the heavy legs I machined in this example, is marginal at best. In the next photo, taken from behind the outrigger, I show a Bessey bar clamp holding the leg, as extra insurance from (primarily) vertical movement due to the mass of the leg:

And on the right outrigger board, a FatMax Xtreme speed clamp:

This clamping arrangement on the right outrigger is the key to accurate positioning, and re-positioning of the legs, for reproducible machining of the mortises. The end of the leg is registered against the edge of the board held tightly in place by the clamp. Barring any unforeseen problem, the board stays in place until all leg mortises are machined.

In summary:

• The FMT is a wonderful jig that allows machining of mortises and tenons very quickly and accurately.
• As users, we can influence the results we get with it; care in positioning a work piece reduces the chances of moving a fence or reference point.
• As good as the FMT is, I recommend taking precautions and measures to ensure reproducible positioning and re-positioning of work pieces machined on this jig.
• I suggest care in layout and measurements.
• Always use scrap pieces to set up the machine properly, not your precious project boards.
• Check to ensure the router bit has not shifted position – it is possible that, on some routers, the bit will go deeper in the collet, resulting in shallow mortises and short tenons.
• Make sure the plunge depth on the router has not changed.
• And finally, conduct periodic checks of the jig setup, whether stops on the outriggers, or the position and squareness of the vertical fence used to machine the tenons, or the table movement X- and Y-axis stops.

My thanks to Charles in Houston, TX, for your question. I hope your question and my answers and suggestions will also help other readers!

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