Using the LMI/O’Brien neck mortise jig

[Andy Bounsall]

I’ve always used the method outlined in the Cumpiano book to do acoustic guitars mortise/tenon and inevitably end up spending several hours fitting the neck, adjusting the angle, etc. Finally got tired of all that and decided the LMI/O’Brien jig looked to be much simpler.

Well, I bought one and used it for the first time today. My first first impression...it’s ALMOST a great tool. One thing that I found a bit aggravating is that the jig cuts the tenon oversized. You end up having to do some fettling to trim a bit off each side every time you cut a tenon. Seems to me that it’d be simpler to trim a little off each side of the tenon jig so that the tenon comes out the right size to begin with. Has anybody done this? Can anybody see a reason why it would be a bad idea? Not sure why they didn’t build it that way to begin with.

[Marshall Dixon]

I wasn't familiar with this jig so I looked it up:
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PRODUCT DESCRIPTION

This jig is produced from medium density fiberboard (MDF).

The SPRB12 straight bit (see below) is recommended for this jig.

The dimensions of the mortise and tenon prior to fitting them are:
• Mortise - 7/8" (.875")
• Tenon - 15/16" (.938") The tenon is slightly oversized so you can remove just a bit of material and get a perfect fit.
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I made a tenoning jig like this and designed the mortise and tenon to be the same size (3/4"). It had been a few years since I'd done one and in the past they went together snug but I didn't need to shave down the tenon. I just did one and the fit was snug enough to glue together; no play at all. It required a little tailoring even so.

Don't know why they would do it this way.

Back in the 1950's Vance Packard wrote "the Hidden Persuaders" about the psychology of advertising. I recall one part where he talks about how researchers found that the "housewife" would be more apt to buy a cake mix if the recipe called for the addition of eggs. They could make a mix without added eggs that tasted as good but apparently adding eggs gave a sense of satisfaction because of the "active role" involvement.

So maybe that's it. You get a more "active role." What? an hours worth more "active." Definitely more time than cracking a couple of eggs.

I'd email LMI and ask them what they were thinking.

Can you use another collet on your router to take in the cut a thirty secondth inch?

[Andy Bounsall]

Yeah, I know it’s in the product description. Doesn’t make sense to me that I have to do extra trimming to get a perfect fit.
I think i will email LMI and ask why it’s like that. I’m seriously considering just trimming 1/32” off each side of the tenon pattern.

[Barry Daniels]

It is fairly common to make neck jigs like that to allow some fitting. You might need this if you need to tilt the neck slightly.

[Mike Spector]

So you might get a "perfect" fit with the neck not attached to the body but where you gonna be when you stick the neck on and the centerline's off in relation to the full length of the body? You might want to leave it and "sneak up" on the geometry by whittling it a little at a time to get side to side right..

[Brian Evans]

I made a jig that is functionally identical to that one. Would have been easier if I had seen it first, but I invented mine from whole cloth, so to speak. Mine is two jigs, I have one to route the neck dovetail and one to route the body. I used a tapered bit, to get the dovetail, they kind of confuse me with recommending a straight bit, then talking about a tapered bit. What I found was that the fit is significantly altered by the depth the tapered bit was set to, the width varies as the taper bit is extended more or less into the hole. I do the neck first, since I am also setting the cheeks of the heel and the neck angle, it is the "master". I set up the body jig and put several iterations of scrap in to do test holes, looking for the correct depth for the width to perfectly match the neck dovetail. i guess I could then measure and make finding that depth repeatable, but I haven't built a depth finder/setting jig since I do so few dovetail neck joints. What I find is that when I am finessing the fit of the heel cheeks onto the body, I always have to sand them a bit, so the fit of the dovetail loosens up anyway, and when I finess the fit of the neck extension (archtop) onto the top it tightens up, so I always end up fussing with the dovetail at the end of the day anyway.

[Dick Hutchings]

You can always remove some material from it and add it back on with tape in the future if you found it doesn't work. Personally I can't imagine working that precise and would prefer to leave some wood for adjustment. I have always cut mine with a hand saw so I really don't know a thing about doing it this way.