Tagged: Gluing mortise joints
Hello folks. I finally finished this coffee table this week, which was meant to be a Christmas gift. The reason for this post is that all the m/t joints were nip tight – nothing stupid. But the moment I introduced PVA to glue up, the sides of both the mortise and the tenon swelled, making clamping tricky and very hairy! I’ve two more tables and four cupboard doors to make in the coming weeks. The doors have centre stiles and rails. Any tips on gluing and clamping would be much appreciated.
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Terry, the only thing for this is to gain experience to get a sense of too tight vs. too loose, including the swelling, which always happens. Things that might help:
1. Do a clamp rehearsal every time. Make sure there is little or no thinking or fetching of clamps during the actual glue up because you have everything ready, all the clamps close to the right openings and at hand, and know the order of things.
2. Don’t let the glue sit after application. As much as possible, you want joints to go together as soon as gluye is on them vs. putting glue on everything and then starting to assemble. Keep this in mind during rehearsal and search for a way to make sub assemblies.
3. Don’t be too fussy. Get the glue on, get the joint together. You don’t want to slop glue all over the place, but you also don’t want to slow yourself down by being too tidy
4. Make sure there is some extra room for the glue. If the volume of your tenon is exactly equal to the volume of the mortise and you pour in a glob of glue, that glue will not have anywhere to go. The tenon will slide in, the glue will pool at the bottom, and you will not be able to drive the joint home. The chamfers Paul puts on the tenon don’t merely make it easy to start the tenon. They give a place for excess glue to gather, so don’t skip them. For hand chopped mortises, I think this is rarely a problem because the bottom of the mortise is rough, but keep it in mind.
One nice thing about OBG that Peter mentions is that it is reversible. If you get in trouble and cannot assemble the joint, you can pull it apart, let it dry, adjust the joint, and try again. The glue will reactivate with moisture and heat and reapplication of more glue. PVA glue doesn’t act like that and, once on a surface, I worry about having it dry and then applying glue on top of it. But, I think Larry posted info about that being less of a worry than it might seem. I can’t remember where / when he did, though.
I’ve had a couple M&T joints that I thought were too loose before gluing, but were fine once glued up.
Thank you Ed and Peter. I can take that advice on board. By the time I’ve got to the finish line, I always want done and that’s not a good thing. I will spend half an hour next glue up simply chamfering all the tenons to make sure there’s no compression going on. That’s helped me a lot!
Kind regards, Terry
Here are some thoughts that might help.
I was going to suggest you try hide glue like OBG also It has a 30 minute open time and the wood will slide better. Claims have been made tha it gives better full coverage in the joint than PVA in very tight joints. Use it warmed up per directions and the joint will go together more easily.
I just warm it with in a bath of hot water out of the tap ( 125° F or so – 52°C). Lastly, it is reversible with heat and moisture. For a longer open time, warm the wood.
The downside is you have to keep your project in clamps longer.
If you can’t find OBG, Titebond’s hide glue works ok too, or make your own by salting hot hide glue. Plenty of info on the web.
But that doesn’t solve the issue of the wood swelling before you complete the glue up. Work quickly and it won’t let the water in the glue time to enter the wood with either type of glue.
If you want to stay with PVA, try Titebond 3. It has a longer open time than original or type 2 ( 8-10 min vs 3-5 min) Titebond says you can thin it 10% with little loss of strength.
And while a joint fit that won’t come apart even before glueing is impressive, it isn’t always a practical way to size your joints. With larger work. Consider a thou or two allowance for glue in the joint. It will still be plenty strong enough. Try wetting the tenon and then putting the joint together without glue to judge when the joint is too tight. Shaving the tenon is a great job for a shoulder plane. A router works also. Sandpaperglued to a board works in a pinch.
Thank you Larry. That is also a very big help. I’ve spent years working towards tighter joints as my early attempts were rather woeful. I’m encouraged to hear that close enough is good enough. I now use a Lie N hand router to shave the faces of the tenon as per Paul and that is a total Godsend!
Many thanks, Terry
OBG is available at my local Woodcraft store. So, that is one source. I think it comes in two sizes. Consider getting the smaller one even though it is cheaper per oz in the bigger bottle. This is because OBG (and any hide glue) has a shelf life, so you may not get through the bottle before it expires. Second, the smaller bottle warms more quickly in a bath of water, which is important if you don’t have a glue pot. Before I had a glue pot, I did as Larry describes, but I had to heat the water in the microwave a little since my water heater runs on the low side to save energy. I suggest you use a thermometer to see that you are getting to 140F. (Going much higher isn’t better and can degrade the glue.) You want the glue uniformly warm, so jostle the bottle about, let it sit some more, etc.
Make sure to check the date on the OBG bottle before buying or you may get an old one.
I use both OBG and freshly made hide glue. The OBG has a longer working time. Something weird has happened in my glue making that I haven’t figured out, but when I make fresh glue (from granules), I have trouble with it gelling up too fast when I apply it. haven’t figured out if it is age, moisture level, temp, or something else. You shouldn’t run into this with OBG.
Ultimately, I think you can use PVA glue. Almost everyone does. OBG is just an idea to give you something reversible. I’ll bet 99% of the people here on WWMC have gone through exactly what you are going through, i.e., learning to get the tightness just right, and have all done it with PVA and nail biting. I’ve been switching more and more to hide glue, but it is “one more thing to learn.” OBG does make it simple.
Oh! Store your OBG in the fridge so that it lasts longer. Before using it, after heating, put a drop on your thumb, press it against your pointer finger, open and close them a few times. Make sure it feels tacky and develops little strings between your fingers. That’s a quick test for whether the glue is still good.
- This reply was modified 1 year, 1 month ago by Ed.
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