I am attempting to repair a chair leg that has a caster with a wooden stem inserted about 1 1/4″ into the bottom. The original leg has a break out of the rear bottom from the Caster’s stem being forced backwards at an angle which caused the material to fail resulting in a large chip out of the chair leg. An attempt was made to hot glue the chip back into place by the owner, but this did not work.
My solution is to cutoff the damage, make a new bottom piece to match the old one, and dowel and gorilla glue the repair back together with a 1″ x 1 3/8″ piece of oak. The original type of material is not known as the damage has left the grain chipped and unrecognizable. I will then do my best to match the original finish with a Minwax polyshade.
My question is, in order to achieve maximum stability of the doweled mortise and tendoned joint, how deep would I want to bore the hole for the 3/8″ dowel? I do need to mention that the bottom of the leg is mitered at a 10 degree angle from back to front causing the vertical axis of the caster stem to be out of alignment with the upper portion of the chair leg.
I would appreciate any suggestions and or advice that you could provide.
Work Safely!
Senor Dorado
Replies
Senor Dorado,
I can't be certain without seeing the damage, and the design of the leg, but if you are planning to glue a new block shaped piece onto the end grain of the shortened leg, reinforcing it with a dowel, I doubt that the joint will have anywhere near the strength needed to resist breaking off in such a highly stressed location.
A better fix would be to clean up the damaged end of the leg, removing only a minimum amount of wood, and preserving as much of the full length grain as possible, to get a good surface to glue a patch onto. Then glue on the patch piece with a slow set epoxy. If you can, leave off the casters after the repair is done, they add to the stress on the leg.
John White
Hi John Thanks for your timely reply. Is your solution based upon the end grain to end grain glue joint reinforced by the dowel, being weaker than your patch method,that would involve some end grain to end grain along with long grain to long grain glue up and clamping? Do you think that a dowel would help, if inserted across the joint? As far as the casters go, I can suggest that the owners don't use them, however this chair is part of a set of chairs for a court room and yes the rest of the set are on casters. Another thought I had would be to cut an L shaped patch and cutoff above the damage, thereby creating the long grain joinery you are suggesting. I could then insert a dowel or two and epoxy together.
This would also create a solid bottom to bore into for the caster stem. What is the difference between an epoxy and a polyurethane glue as far as strength is concerned? I do know that they both can offer a longer open time. Would one be better than the other as far as applying a finish goes? Thanks again for your time and knowledge! Work safely! Senor Dorado
Senor Dorado,
Your analysis for my suggestions is correct, I'm trying to keep the grain continuous right down to the bottom of the leg and to avoid an end grain to end grain glue joint which are quite weak.
Adding a dowel will certainly help in spanning the joint with some continuous grain, but a 3/8" diameter dowel has only 10% of the cross sectional area of a 1" square leg, and that leg failed under the strain already.
Your idea of creating an "L" shaped patch is certainly a much stronger design, but I'd suggest a variation on the fix using the same concept, make the patch in the form of a "U" with two legs that extend up either side of the leg. This is about the strongest fix possible and the one I would use if I had to guarantee the work.
I'm recommending epoxy since it has superior strength in end grain to end grain joints and good gap filling properties which helps when making patches that can't be fitted and/or clamped as tightly as would be ideal.
No glue is especially better or worse for finishing, as long as you don't get the epoxy in the grain of the areas you will finish then you won't have any trouble finishing over the joint areas. The best approach with epoxy or poly glues is to allow any squeeze out to bead up without smearing it around and then use a chisel to lift the glue off the surface when it has cured to the point that it has become rubbery.
Try to convince the client to remove the other casters and place glides on all of the legs, though I suppose that might make the chair seats a bit low. If one of the casters broke off, the others will probably also fail over time. You might check the other chairs, if some are showing signs of approaching failure, you can probably make your case.
John White
John Thanks John for your advice, and now I think we are on the same page as far as the techniques that are involved go. Can you clear me up on the theories of doweling. I'm thinking that a dowel that is inserted across a joint and leaves the surrounding walls less than 1/4" are not as strong as one,that the wall thickness is a 1/4" or greater. Is the surrounding wall thickness of the joint as critical as the cross sectional area of the dowel? Are we still looking at the amount of long grain of the dowel being adhered to the end grain of the bored out joint? Is this a major concern or am I starting to over think and splitting hairs ? Work Safely! Senor Dorado
Senor Dorado,
On the bottom of a chair leg with a joint just an inch or two above the end, the most likely failure will probably be in shear. Picture someone dragging the chair across the floor and suddenly snagging the leg, with no wood fibers crossing the joint, the glue line will almost certainly snap off cleanly. My sense is that a 3/8 inch dowel won't add that much more strength and the dowel would also snap off cleanly, so a larger dowel would add considerable strength.
The dowel through the repair block will be glued long grain to long grain, a joint which is stronger than the wood itself, so the walls, even if thin, will be just as strong as if the block were all solid wood.
You aren't over thinking this, but analyzing a problem such as this is difficult. It was only because I studied these types of problems in an engineering school that I have some skill at sorting out what is going on, now I wish I had paid more attention in class.
John W.
Thanks again John. Now its off to my "Saw Dust Factory". I'll let you how it all comes together. You've been very helpful! Work Safely! Senor Dorado
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