Leigh FMT tenon length question

[Bill Szydlo]

I own one of the original Leigh FMTs and an aspect of it has always confused me. Assuming a 5/16" thick tenon is appropriate for 3/4" stock the max depth of the mortise is around 1". This accounts for bit length and accuracy/deflection of the bit at extended distances. Regarding what mortise depth is appropriate, I have read anywhere from 40% to 60% of the width of the mortise stock. Using 50%, a 1" deep mortise would be appropriate for 2" wide stock.

When making a door frame, rails could easily be wider than 2" so is a 1" deep mortise inadequate? This seems to be a limitation of all router created mortises. To get a deeper mortise you would have to use a larger bit. For instance a 1/2" bit could do 2" deep mortises but the resulting tenon would be inappropriate for the tenon width (in 3/4" stock).

Using 3/4" stock, would a 1" deep mortise be satisfactory or is this a slight of hand with router mortising jigs that you never think about until after purchasing?

Any help with this confusing router mortise topic would be greatly appreciated.

Thanks,
Bill

[John Erickson]

25 years with my FMT making cabinet face frames and the 1" length did bother me but in all that time I never had a failure. Frames are from 1 1/2" frame stock with only the top rail being 2", all mortise & tenons are 3/8" x 1" and (3/8" x 1 1/2" top rail).
In some builds where I wanted/needed a long mortise & tenon, I'd use the square tenon template and used my General mortiser to do the mortise. This worked very well after I attached a DRO to the cross slide of the table, this made for a quick and accurate setup.

[Charles Lent]

I've made many mortises and tenons with my FMT Pro, now about 18 years old. Other than what has been suggested, the only thing that I can add is that when making tenons longer than about 1", the vacuum port will plug up with chips quickly. If you need longer tenons, cut them in two increasing depth passes, so the wood chips are shorter, and the vacuum port will not clog.

I also added a piece of 1/8" Lexan to the front edge of the top plate of the FMT jig by attaching it with Velcro. This addition keeps the chips from hitting you as you cut the facing side of the tenon. You should never get your eyes down in line with the cutting function, so don't use this for eye protection, but it does leave you considerably cleaner when using the FMT and cutting tenons.

Charley