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| MT Joints, Cumpiano 'n such http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=31203 |
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| Author: | dberkowitz [ Mon Feb 21, 2011 9:12 am ] |
| Post subject: | MT Joints, Cumpiano 'n such |
I have a complicated project for which I'm going to need to employ a MT joint. For those of you using one, which systems do you use, i.e., Cumpiano, or threaded inserts, how deep are your tenons, general mortise, tenon, and headblock depth dimensions. Some reading at Bill's site appears to say he's using the same 1" tenon and 2" block, but just using the KD hardware. I'm wondering whether one could reduce the size of the tenon and neckblock if you're using the cross laminated tenon Bill says Michael Millard is using at Froggy Bottom. Thoughts? All the best, David |
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| Author: | Laurent Brondel [ Mon Feb 21, 2011 9:41 am ] |
| Post subject: | Re: MT Joints, Cumpiano 'n such |
I use end grain threaded inserts and drill for a 1/2" walnut dowel in the tenon. Tenon is .800" x 3/4" deep, Neck block is 2 3/4" x 1 1/2". |
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| Author: | Arnt Rian [ Mon Feb 21, 2011 9:49 am ] |
| Post subject: | Re: MT Joints, Cumpiano 'n such |
David, I use barrel bolts in a reinforced tenon (I put in the reinforcements before routing the tenon, and make them deep enough to provide some additional strength to the heel as well). My tenon is 20 x 20 mm. I’ve made the neck blocks as small as 35 mm, but anywhere from 35-40mm deep seems to be OK. No need for a 2” (50 mm) neck block, as far as I can tell. Here’s what it looks like before the reinforcements are glued in…  …and here’s the finished tenon 
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| Author: | woody b [ Mon Feb 21, 2011 6:05 pm ] |
| Post subject: | Re: MT Joints, Cumpiano 'n such |
I use 1/4" hanger bolts with a 1/2" long tenon, and a 5/8" deep mortise. (3/4" wide......or 7/8"??? I'll have to measure one) I counter sink the bolt holes in my 1 5/16" neck block, then cover the bolts with a decal. My attachment method is similar to Collings. I use a maple dowel through the heel for the bolts to thread into. I use dovetail on ~1/2 of my models though. |
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| Author: | Terence Kennedy [ Mon Feb 21, 2011 8:20 pm ] |
| Post subject: | Re: MT Joints, Cumpiano 'n such |
I use Maple strips like Arnt. I have a little jig that keys in the truss rod slot to rout the slots after the tenon is cut. I tried carbon fiber bars but they really beat up your drill when you drill the barrel nut holes. I think Maple is plenty strong. 
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| Author: | dberkowitz [ Tue Feb 22, 2011 7:17 pm ] |
| Post subject: | Re: MT Joints, Cumpiano 'n such |
Thanks, folks. Looks like I'll use Cumpiano's joint. Arnt, I like the idea of gluing in the laminations and then routing the tenon, that makes a lot of sense. Thanks, everyone! |
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| Author: | dberkowitz [ Wed Feb 23, 2011 9:23 am ] |
| Post subject: | Re: MT Joints, Cumpiano 'n such |
Thanks, Todd. That's really useful. I'm in the midst of a doubleneck project and I'm trying to reduce the headblock(s) as much as possible. -- db |
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| Author: | Arnt Rian [ Wed Feb 23, 2011 9:32 am ] |
| Post subject: | Re: MT Joints, Cumpiano 'n such |
Todd Todd Stock wrote: The tenon reinforcement does not have to extend further than the middle of the barrel bolt - it's there to add bearing strength on the shear faces. True, if all you wanted to reinforce was the part of the tenon behind the barrel. By extending the reinforcements into the heel, it too should get some insurance against cracking. Not that it’s such a common problem, but since its so easy to do at this stage, why not?Todd Stock wrote: If you want to reduce the depth of the tenon, 1/16" x 1/2" CF rod works well as a reinforcement, and reducing tenon length to 3/4" and neck block depth to 1-9/16" is doable. Like I mentioned in my previous post, my tenon is 20 mm (very close to ¾”), and my neck blocks are between 35-40 mm (1- 9/16" is about 40 mm), so…
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| Author: | Robert Renick [ Wed Feb 23, 2011 10:49 am ] |
| Post subject: | Re: MT Joints, Cumpiano 'n such |
Question for Arnt: What is your thinking behind the grain orientation of the laminated strips, seems the vertical grain there would be prone to crack in the direction that is trying to be supported. I used the barrel bolt method in my 2 guitars, but I drilled and installed the barrels and then put the laminations on the tennon, the barrel bolt is then permanent, but it could be drilled out if needed. My thinking is that the hole in the lamination occurs right where I am trying to add strength. Thanks Arnt, your tutorials were very helpful in my completing the instruments that I have. Rob |
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| Author: | dberkowitz [ Wed Feb 23, 2011 11:36 am ] |
| Post subject: | Re: MT Joints, Cumpiano 'n such |
The cross lamination is on Bill Cumpiano's site and is mentioned as a modification that Michael Millard of Froggy Bottom Guitars has been doing. - db |
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| Author: | Arnt Rian [ Wed Feb 23, 2011 2:50 pm ] |
| Post subject: | Re: MT Joints, Cumpiano 'n such |
comfyfoot wrote: Question for Arnt: What is your thinking behind the grain orientation of the laminated strips, seems the vertical grain there would be prone to crack in the direction that is trying to be supported. Rob, the way I "read" this arrangement, the reinforcements add strength because they are glued to the tenon, which has its grain going in the opposite direction. Alone, either grain direction is weaker, than combined. As for the method you mention, if I understand it correctly, the barrels are buried behind cross grain reinforcements glued to the side of the tenon; yes, that's probably even stronger. Perhaps more trouble than it is worth, though, but if it works for you, why not? |
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| Author: | Robert Renick [ Wed Feb 23, 2011 3:22 pm ] |
| Post subject: | Re: MT Joints, Cumpiano 'n such |
Thanks Arnt, Kinda what I thought. I like to overbuild, especially on my first guitars, I was scared they would fall apart, but the main push to add the extra support for me was that the necks were Port orford cedar, the tennon felt pretty mealy and weak in the direction requiring strength. I was tempted to spline in a maple tennon, and will consider that in the future. Thanks again, Rob |
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| Author: | Terence Kennedy [ Fri Feb 25, 2011 2:22 pm ] |
| Post subject: | Re: MT Joints, Cumpiano 'n such |
I think Todd's right. This is somewhere in the archives but a couple of years ago I did some testing with a 3/4" tenon glued and bolted to a backer board.  It didn't take much force to break through the holes with plain Mahogany and they basically failed in tension.  Putting carbon fiber or maple strips in routed slots that went past the midpoint of the barrel nut holes functioned as tension bands and created a construct that I couldn't break.  The tenon cracked near one of the bolts  I use Maple now as the carbon fiber kills drill bits. I suppose with enough force you could crack the tenon neck interface and Arnt's technique would prevent that. Do you make those slots with your table saw Arnt? It looks quick and simple. |
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