I have a joint that’s a round tenon that gets glued into a hole. It happens to be in a banjo, but I think the principles would apply to virtually any joint of this type. The hole that the tenon goes into was drilled at a very slightly incorrect angle. In order to make the other end of the piece align properly, the joint has been made just a bit sloppy so that the tenon has a little room to move. For purposes of clarity, I’ll add that what I mean by sloppy is that the joint has maybe 1/32 all the way around.
Here’s where I’m going with all of this – It has been suggested to me to thicken the glue (Titebond) to make it better able to fill the gap. I’m interested in getting any thoughts on whether thickened Titebond really has any gap filling properties. I would probably use colloidal silica to thicken it since that adds strength when used to thicken eopxy. The only reason I’m not using epoxy is because I’m told that the joint needs to be reversible. Any thoughts are greatly appreciated.
Replies
Glue-up
Why not do it properly, especially if the joint may need to be dis-assembled at a later date? Re-drill and plug the hole with an appropriate sized dowel made of the 'proper' wood and grain orientation. Use epoxy to glue it in. When the epoxy is fully set, re-bore the hole to the proper size and angle. This way, the epoxy will hold the 'sleeve' p[erminantly in place while the peg can be taken apart if needed..
Another option is to increase the size of the round tennon, enough to re-cut it on an angle to match the 'wrong angle' hole. This may mean that you've got to glue veneir onto the tennon until it is built up so it can be trimmed to the proper diameter and angle. The only problem with this solution is that if it is not done properly (type of wood and proper contact, you will eventually get some buzzing as the repair shrinks / expands with the change in weather. It will also be a disaster if you, or some other repair person has to take it apart. (Yea, I hate that sort of surprise, especially AFTER I've quoted a price to repair the item)
SawdustSteve Long Island, NY (E of NYC)
Thanks for the feedback folks. Your thoughts are in line with mine that PVA really doesn't have much strength in the gap as it were. Epoxy would be my first choice not only to fill the gap, but for sheer strength - 'twere it not for the requirement of reversability.
As for the why not do it right thought....... If I could I would. This is an application where if you're off by even a fraction of a degree, the thing doesn't line up. My first thought was to plug the hole and start over, but getting my drill press table set up to align with the necessary level of accuracy doesn't appear likely. I've also tride repeatedly to turn a piece with a deliberately crooked tenon and I just can't seem to get it spot on.
I've attached a picture depicting the scene of the crime. The tiniest misalignment in the tenon throws the opposite end off enough so it won't hit the hole in the rim it needs to align with.
Is anyone aware of another gap filling glue that's reversible?
What is it and why does it need to be reversable? Epoxy can generally be reversed with the application of heat. I've used a heat gun or a hair drier.
Michael Fortune has recommended the addition of baking powder to PVA glue to reduce the potential of 'creep' in laminations. I don't know of the virtues of this additive in your case.
Frosty
Glue reversibility..
Quick,
I'm not sure you'll find PVA glue significantly more reversible than epoxy in this case. I guess if you use an older formulation like Titebond original, there are some solvents that will attack it - in fact there is one made to do just that. But Titebond III, to my knowledge, is a PVA that does not "reverse."
So it might be just as well to go with epoxy. How often does this joint need to come apart, anyway? If you're looking for true reversibility, you'd go with hide glue, which is what most instrument makers use - to insure just that quality. The bad news is that it doesn't have much gap-filling qualities either..
Zolton
Why not take a long plane shaving of the appropriate wood and wrap it around the tenon? Glue the shaving on, wrapping several times if needed to get the right minimum diameter, then sand or turn it to fit after it dries?
sawduststeve has the only right answer. do it properly
ron
Another thought
Have you tried posting at mimf.com (Musical Instrument Makers Forum) or any other similar group. From your drawing it looks like a 'standard procedure' and there must be someone out there who can help you solve this problem, because others have run into it too.
SawdustSteve
Thickened epoxy
Epoxy will soften once above 200º.
However, If you need to repair, couldn't you just cut off the part, then drill out the old dowel in the thickened / filled hole?
I recall seeing a neat trick, for injecting steam into joints like this by using a basketball inflation needle hooked up to a steam kettle, inserted into little holes around the glue joint, to heat it up.
Sometimes I wonder about trying to build something reversible. If it isn't broke why would you want to fix it? Build it well enough that it never needs to be taken apart.
Parts can be damaged in plenty of ways that don't involve a joint failure. Kids and cleaning ladies can certainly find opportunities to demonstrate this. Then if you need to remanufacture a part--like a chair rung, or a leg it's easy if the glue is reversible, a horror if not.
Although PVA does not have gap filling strength, if one makes a thick paste of sawdust and glue then would this not be a gap filler? All the sawdust particles will be glued to each other.
I can also confirm that epoxy softens at above 80 deg C. Which explained why whenever I repaired a cooking pop handle with Araldite it soon came apart again :-(
I haven't checked mimf, but I
I haven't checked mimf, but I do go to a banjo building site. The normal practice is to mix titebond (original) with sawdust. Like you guys, my background is in woodworking where this concept wouldn't fly. Ordinarily, these things (called dowel sticks) are sometimes ornate or part of a vintage instrument and people would want to save it. Also if the stick should get acidentally mis-aligned during installation, you'd want to be able to remove it and re-adjust. I guess I could try it this way and if it didn't work out at least I'd be able to get it apart for another try.
Here's an actual picture of the scene of the crime. The underside of the tenon (the part you can't see because it's hidden by the stick) is tight against the hole. Since the tenon is techincally in there crooked, there's a corresponding gap on the opposite side of the end of the tenon inside the hole.
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