Big check and how to use a Dutchman

[rarefish383]

I have 3, 7' Poplar slabs. They're about 3 inches thick by 21"s wide, and my plan is to make a mortise and tennon bench. The slab I plan on using for the back has a widening check down the center. I have some dried Cherry slabs that might be 3" thick. If I cut an hour glass shaped insert, clamp and glue the crack, then install the insert, how deep should it be? Should the dutchman go all the way through the plank. I'm thinking I'll plane the Cherry insert down to about 2" and Install over the check. I think I only want it showing on the front side. That way there will be more surface area to glue the insert into. Is this the proper way to do it? Joe.

 

[BlueRider]

Since the purpose of a cross grain butterfly is to stabalize the crack to prevent any further movement the buterfly should be the same depth as the crack. Since the crack is the full depth of the board the butterfly should also be the full thickness. Also I would not recomend using cherry for your butterflies. The stress in the butterfly is along the long grain so you should use a species with interlocking grain, walnut or elm would be a great choice.

I make the hole with slightly tapered sides and I cut the butterfly out of thicker stock. this lets me carefully fit the butterfly and as I am fitting it it drops further into position. Once it is glued I trim the thickness of the butterfly. as you can see having the butterfly go the full thickness in some ways makes things eaiser.

 

[Daninvan]

I have not done it myself, so take this tip at what you paid for it(!), but what I read about Nakashima is that he did basically what you are proposing to do, but in addition he screwed the keys (AKA Dutchman, insert, etc.) in from underneath.

As for the clamping and gluing idea, I guess it depends on how wide the crack is. I thought the keys were more for preventing it from getting worse / aesthetics rather than closing up an existing gap?

Dan

 

[jimdad07]

Iron wood makes good butterflies as well, even thorn apple if you can't get the walnut or elm. Like was said above, just about any wood with a twisting grain works well, the worse the better, just make sure it is good and dry.

 

[HUCKLEBERRY]

Joe

Another option you may want to consider, especially if your goal is to "pull the crack back together and glue it", is to thru-bolt the slab with allthread. If it is 3" thick you have plenty of meat to work with. You can countersink each edge and then drill 1/2 way through from each side and meet the holes in the middle. It can take a few tries but as long as you don't poke through the face there's no problem. The countersinks should be large enough in diameter for a washer and deep enough to allow for a plug when done. The plugs can either match material and grain or add contrast. As long as you keep the thru-bolts away fron joinery you can do as many as needed to "stabilize" the workpiece. Using the all thread you can really squeeze things back together so that the only sign left of a modification is the small plugs on the edges. Depending on the size of the crack you may need to do the other end due to the stress applied to keep the same from happening there. The whole process is strong, fairly easy and cheap.

 

[BobL]

I have 3, 7' Poplar slabs. They're about 3 inches thick by 21"s wide, and my plan is to make a mortise and tennon bench. The slab I plan on using for the back has a widening check down the center. I have some dried Cherry slabs that might be 3" thick. If I cut an hour glass shaped insert, clamp and glue the crack, then install the insert, how deep should it be? Should the dutchman go all the way through the plank. I'm thinking I'll plane the Cherry insert down to about 2" and Install over the check. I think I only want it showing on the front side. That way there will be more surface area to glue the insert into. Is this the proper way to do it? Joe.

If it is just the back of the bench do you really need to keep that slab whole? Even if it going to be seen it would be easier and safer to rip it down the middle and then let it settle for a few weeks (or a month if you have time) and then square edge and glue the the two pieces back together.

 

[rarefish383]

Thanks guys, I'm in no hurry. I like the idea of the through bolts. I can get pretty much any kind of wood to use on the butterfies that grows on the East Coast. I know I can get some Iron Wood and I'll have to ask my cousin if he has any Elm in his wood pile. I just gave a couple Walnut logs away, I might be able to get a butt cut from where I took them down. I'll get some pics up today or tonight, Joe.