Clamping mitre joints
August 16th, 2008
A recent project (Sixty by six) used simple mitre joints on the four corners of the ‘cabinet’. I’ve used a technique for clamping this joint that I learnt from George Ingham.
Fun with clamping mitre joints
I’ve really got no idea whether this is a commonly used technique but I can’t recall ever seeing it documented and suspect it may not be widely used. I’m very interested to discover how other woodworkers deal with this problem.
Regardless of the type of mitre joint - whether it contains hidden dovetails, is the mitred corners of through dovetails or is a simple mitre relying solely on the glue - good clamping pressure is required for a pleasing visual result and a strong joint.
I often use simple butted mitres with no fancy jointing. Obviously this limits the strength of the joint as the gluing surfaces are half way between long grain (strong bond) and end grain (weak bond). In many situations this is not strong enough. But under the right conditions it is. A wood that has good general gluing properties (eg not oily) and has a reasonably open end-grain can provide a suitably strong joint at 45° to the grain. Clearly the stress that will be exerted on the joint is important which generally means this joint is suitable for smaller works. This joint relies entirely on the effectiveness of the glue so the gluing process is all important. The wood surfaces must be clean and perfectly co-planar. The glue must be fresh and strong. And the clamping pressure must be adequate - and this is the punchline to this post!
The vector of clamping force should be at right angles to the surface of the joint
It is very difficult to get a good clamping result unless you are able to exert enough force at right-angles to the surfaces being glued. Picture frame clamps in their various forms are very poor at this. Crossed sash cramps are very hard to set up effectively and still don’t provide the pressure in the correct vector. Angled clamping blocks that rely on friction (such as sandpaper-covered blocks) may work but are fraught with potential glue-up disasters and ultimately compromise the amount of pressure you can apply.
My method is to glue clamping blocks firmly to the outside faces of the elements being joined allowing clamping directly across the perpendicular of the joint.
First the choice of blocks. I use MDF because it is softer than the wood I’m gluing so unlikely to compress it. It also breaks up in layers which makes removal at the end very easy. The shape is a triangular prism, that is a right triangle in cross-section extruded to the exact thickness of the elements being joined. You must make the blocks large enough to accomodate the pads of the clamp when they are positioned so that the pads are centred on the desired lines of force.
Make the blocks large enough to accommodate the clamps
I use my normal yellow glue (Titebond II at the moment) to attach the blocks. Some makers use only a dab of glue and little pressure. I prefer a stronger bond and cover most of both surfaces and just rely on the suction pressure created by rubbing the surfaces over each other until they grab.
Make sure you leave the blocks long enough for a good bond before proceeding to the next step. I try to organise my work so that the blocks can dry overnight.
As always, do a complete dry run of the clamping setup before applying any glue to the work. Then glue up as you normally do - and use the mitre blocks to achieve solid clamping pressure across the mitres. You should be able to get strong joints and a minimal glue-line.
This glue-up required sash cramps for the tenons along the side as well as the clamps on the mitred corners
Cleaning up the glued mitre is quick and easy. I do it in three stages. First I remove the bulk of the clamping block. I use a bandsaw for this.
Remove the bulk of the sacrifical block on the bandsaw
I then remove most of the remaining material with a wide, thin chisel. This usually leaves a small amount of block material glued to the work.
Remove the rest with a chisel
Some material is left.
The remaing MDF and glue is simply planed off leaving a clean joint.
Plane away the remaining material and glue
The cleaned joint
clamping a narrow mitre joint
Gluing narrow mitres was illustrated in an earlier post.
I’m very keen to hear how other woodworkers solve this problem and hope that this technique might be of some help.
Entry Filed under: Wood
8 Comments Add your own
1. Bill Stankus | August 17th, 2008 at 9:47 am
I’ve done similar gluing on miters. The type of wood matters, some open grain species aren’t friending during clean-up. So does size of the project. Mostly tho, when I do miters I include a spline.
For the glue blocks I’ve also used hot glue because it has reasonably good holding strength in one direction - in line with the clamp direction. When removing the glued on block a hammer blow from the side will usually knock the block off. Residual glue is easily removed w/ chisel.
BTW, I’m impressed with all those Record bar clamps - haven’t seen those in some time. I have a few and always wished I had more.
2. Keith Cruickshank | August 17th, 2008 at 9:54 am
Fantastic explanation and photos! Artfully done. - Keith (www.woodtreks.com)
3. Mark | August 18th, 2008 at 4:23 pm
Bill - I forgot to mention hot-glue. I have used it but seem to recall at least one failure. All depends on the level of risk you are willing to accept. Hot glue is probably a good compromise for many situations.
I have about 17 of those sash cramps of various lengths. Picked them up in the early 90’s. You don’t see them made to the same level of finish these days. I just happened to be showing someone my mate’s report of his Churchill Fellowship trip to Europe and I came across this pic of Robert Ingham in his workshop. Check out the sash cramps! Damn close to showing off. Seems short a few G-cramps tho. He may have still been setting up. (Robert is the brother of George Ingham who I trained with here in Canberra.)
4. Mark | August 18th, 2008 at 4:35 pm
Keith - Thanks very much for the encouragement. Quite an honour when I see how professional your web site is! Happy to have found your site. Enjoying the videos. A great service - keep it up!
5. Fergus | April 21st, 2009 at 8:30 am
That is really usefull.Not a method I have seen before. I always have plenty of MDf offcuts as well-so now they have a use.
Many thanks !
6. Gayl Leake | October 13th, 2009 at 11:24 am
Hello Mark, I have just discovered this site, which is an excellent setup. I am computer challenged myself,but anyway.
Yes, I remember using large pine blocks to pull up a mitre joint which were sprinkled with water to create temporary adhesion -this created a ’suction’ that was remarkable but lost its effect once the blocks were repositioned a few times. This technique isn’t foolproof in the stressfully short time one has when gluing up with yellow glue.
The next time we rubbed on the glueblocks with yellow glue until only a thin smear of glue remained and then inserted newspaper between the block and the work. After the glueup, a sharp hammer blow to the side removed the block easily which sheared off at the paperline. I remember shavings of newsprint from cleaning up of the surface looked interesting.
7. Mark | October 15th, 2009 at 6:55 pm
Hi Gayl! Welcome to my blog.
I’ve never heard of the water technique - sounds scary. I have heard of the paper technique but never used it. Will try it and report back. Sounds like it could speed the cleanup.
8. toby muir wilson | January 17th, 2010 at 4:49 pm
I use a similar cramping technique for mitres (taught by Robert) but on batch production the cramping blocks are attached to sheets of ply which fit outside the carcass less 5 mm to see the joint, also use brown packaging tape , several layers exert a lot of pressure and are almost self squareing. Good mitre right angle keys are available from Hafele and applied into the end of the mitre using the Festo domino cutter, or make your own finger jointed keys, they give excellent side grain to side grain glueing and the finger joint can be adjusted to diferent angles. The domino places the strength of the key very close to the surface unlike a bisquit.
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