Leigh super FMT confusion

[Tom DiBiasio]

Just recently purchased a super FMT and had a few hours to try it out today - left my shop after 3 hours with not a single joint created. The issue is pretty simple to describe - the tenon is to big for the mortise - no amount of adjustment up or down on the brass pin fixes the issue. Using the supplied 5/16 spiral bit and a 5/16 x 1 1/2 guide, with 3/4 cherry test boards. I also made sure that when I cut the mortise that I followed the inside of the left guide, although that did not make any difference then just going left to right in the slot.

im sure I am missing something obvious as most reviews I read say they get a "piston" fit out of the box.

Any suggestions would be greatly appreciated.

TomD

[Robert LaPlaca]

Tom, I own a FMT, haven't used it in 5 years or more... But it can create piston fit. IIRC the mortise is cut with the brass pin in the slot in the guide, the tendon is cut with the brass pin following the outside of the guide. The knob on the brass pin controls the size of the tendon slightly.. is the tendon being cut too large or small?

[Tom DiBiasio]

Mortise = .308
Tennon = .35

after another hour scratching my head I am no closer to figuring out what I am doing wrong !!!

[Dave Richards]

Mortise = .308

A 5/16 in. diameter bit should leave a mortise at least 0.3125 in. wide, right?


If the bit is smaller than the template is set up for, you'd wind up with a narrower mortise and a wider (thicker) tenon.

[Gary Muto]

If I remember right the bras pin has a taper to adjust the fit. If you turn it clockwise it should advance and make the tenon Larger and the mortise smaller. turn the opposite way to make a looser joint. I may have it backward but the taper on the brass pin is the key to adjusting the fit.

[John Piwaron]

A 5/16 in. diameter bit should leave a mortise at least 0.3125 in. wide, right?


If the bit is smaller than the template is set up for, you'd wind up with a narrower mortise and a wider (thicker) tenon.

I get bit by problems like that once in a while. A problem so obvious it's TOO obvious.

[glenn bradley]

So the supplied bit is too small?

[Tom DiBiasio]

Well Im no machinist, just an amateur saw dust producer at best.... but I have taken the following pic that shows what caliper reading I get when I measure the outside of the bit that came with the jig.

[Randy Heinemann]

Your measurement shows the bit is about .01" shy of 5/16. I don't know if that would cause the problem. I'm at a loss to help you. I owned (don't anymore) an FMT jig for a number of years. Out of the box I never had that problem. After making the suggested adjustments, I always got a perfect fit of tenon to mortise.

The first thing I'd do is call Leigh technical support (hopefully they have an easily accessible number) and ask them if the fact the bit is .01" under 5/16" would make the difference. Also, they should be able to tell you what the problem might be. I'm guessing it is something in the setup, but I'm very sorry I can't really suggest anything. I also just noticed that Leigh has a huge number of "technical bulletins" on their website, but I 'd still call first. The answer should be more direct and faster. I believe they have a good support service (at least they used to).

One thing I can suggest is, if you have not already taken the base off the router and started over remounting it, you might try that. Once I couldn't figure out a setup problem and just started over. The problem didn't exist the second time, but I never figured out what I did the second time that I didn't do the first time. Sometimes our minds skip over what seems like a minor detail and it isn't so minor.

[Paul Grant]

Call Leigh....they have some of the best technical support around. They'll have you up and running in no time....