Introduction: Splined Mitre Bench
Runner Up in the
Furniture Contest 2017
A beautifully simple looking bench of solid Oak with mighty mitre joints.
Sharpen up your plane blade, dust off your power tools and follow along with this Instructable to learn how to achieve perfect big scaled mitre joints and a great bench that looks as good as it is to sit on.
Step 1: Design
The bench has a super simple aesthetic, it was conceived as an occasional bench to be sat by the front door of a clients house for them to be able to sit on while donning shoes.
The house has a contemporary staircase with 90mm solid thick treads and the bench was designed to complement the steps.
Made from featured European Oak to a finished thickness of 90mm, it is 270mm deep, 600mm long and 450mm high off of the ground.
This bench would work with any timber of choice or even with glued up layers of ply.
A basic top and two legs with a mitre joint at each corner.
The mitre joints are strengthened with splines of American walnut as a contrasting colour and for visual impact.
A slight cut out to the bottom of the legs allows the bench to sit well on slightly uneven floors.
All finished with a couple coats of hardwax oil.
Step 2: Get Your Bits Together
100mm x 150mm x 3500mm long thick slab of timber of choice - I used European Oak with a featured grain [knots and possible shakes]
70mm x 50mm x 1000mm length of American Walnut for the splines - to be sliced up
Hardwax oil - clear satin
MDF for jigs - various sizes of offcuts
General woodworking hand tools including a try plane for shooting in the timber
Planner/thicknesser if you have one
Hand held circular saw
Router with wide flat cutter bit
Sanders - palm + belt
Brush + rags for oiling
Step 3: Prepare the Timber Slabs - Cut to Length
Due to the thickness of timber I could not purchase slabs of the 270mm width, so I opted for 150mm wide slabs and would joint two slabs together to make the width that I required.
Rough cut the slabs to length.
Step 4: Prepare the Timber Slabs - Plane Square
Really important that there is no twist within the slabs when making these big mitred joints for the bench.
Hand plane one face and one edge square and flat.
Step 5: Prepare the Timber Slabs - Twisting Sticks
Check that the slabs are flat with the use of twisting sticks.
Super simple, two sticks of the same cross section, one light in colour, one painted black.
Place at each end of the timber, bring your eye down to line them up and check that the tops are parallel to each other.
If they are out, one side higher than the other, plane out some more from that corner to plane out the twist.
Check again, and move the sticks to mid points to check along the length of the timber.
Step 6: Prepare the Timber Slabs - Check for Flatness
Use a straight edge, such as the edge of the plane to check for flatness across the timber.
Step 7: Prepare the Timber Slabs - Plane Other Face + Edge
When one face and edge are planed flat and squared to each other...
...take to the planner/thicknesser and finish off the other face + edge...
...this can be down by hand as well if you wish to.
It is possible to square off all faces and edges on the planner/thicknesser though I prefer to do the first stage by hand as described for ultimate accuracy.
Step 8: Shoot It In
Select a pair of the timber slabs to be jointed together.
Shoot the edges to be glued with a try plane [long soled hand plane] until sitting nice and tight together with no gaps.
Do this for all three parts of the bench.
Step 9: Joint Up
On wide slabs such as these I find it best to use two rows of biscuit joints.
Mark out the extent of the finish sizes so that the biscuits do not fall across any cut lines.
As such these wont be seen though it is good practise to keep them out of sight.
Glue in the biscuits, add glue to the face of the joint and clamp all together...
...and leave to cure.
Step 10: Sand
Clean up the slabs ready for mitreing.
Step 11: Rough Cut the Mitre
Set the circular saw to 45 degree and rough cut the mitres with a couple millimetres extra to trim back to .
As the mitre is so big, my circular saw couldn't quite cut all the way through even cutting from both sides!
Finished off with a handsaw.
Step 12: Perfect Mighty Mitre Joints
The trick to making perfect mitre joints at such a large size is with the use of some MDF jigs and a router.
First make two 45 degree jigs as of the photo, the 45deg angle will go in opposite directions on each of the jigs.
Make them wider than the slab width and carefully clamp them to the slab to be trimmed making sure that they are square to the timber and line up at the 45 degree angle.
Fix a thin strip of MDF, I used 4mm thick, to both of the jigs, these act as lift off strips to raise the router slightly from the rough cut, giving space to trim the joint.
Fix a piece of MDF to the base of your router, this will require to be able to span the mitre trimming jigs at all points of the cut.
Step 13: Carefully Trim
Fit a flat and wide cutter bit to the router.
Remember that wide cutters require the router to be run at a slower cutter speed.
You will be router cutting on effectively end grain [at a 45 degree angle] and great care must be taken to avoid the cutter grabbing the timber.
Only take a fraction of a millimeter cut at any time.
Trim the whole face of the mitre.
Set the router to take another fraction of a millimeter cut and repeat until the whole of the surface is trimmed to the mitre line.
Step 14: Final Tickle
Check the mitre cut for flatness and plane out any inconsistencies with a block plane.
Perfectly mitred, only three more to do!
When a pair has been cut, bring the joint together to check for tightness.
You want to achieve a joint that is fully closed all around with no gaps.
Step 15: Put the Mitre Together
To help locate the mitre whilst glueing and to add some additional strength to the mitre joint...
...add a row of biscuits to the inner edge of the mitre, set in about 15mm and at 90 degree to the mitre face.
Add PVA glue to the mitre face and clamp it all together...
...check the joints are tight and square...
...and leave to cure.
I admit that my joints were out by about by about 1 degree, though without a square it is not visually noticeable.
Step 16: Splines - Jig
I wanted to add some contrasting timber splines to the mitres, partly to add strength to these large glue joints and also for the aesthetic appeal of this detail on the bench.
The splines are easy to achieve with the use of a jig to cut the slots.
The jig is made mainly from MDF boards with two 45 degree timber boards to keep all of the boards in place and at the correct angle.
You need to achieve a cutting surface for a circular saw to slide along set at 45 degree to the mitre joint.
Fix a couple pieces of thin MDF to the cutting board to keep the circular saw in place and set square to the joint.
Step 17: Splines - Cut
Work out how many splines that you want across the joint and their spacings, mark these on the timber.
Clamp the jig at the first cut, set the circular saw to 90 degree and full depth cut...
...and slowly cut up the jig and into the mitre joint, continue right through.
Move the jig to the next cut line and continue until all of the spline slots are cut.
Step 18: Splines
Cut thin strips of the contrasting timber for the splines, checking that they will fit snugly into the slots.
Glue into the slots with PVA glue...
...and when fully cured, cut off the excess timber, plane flat and give it all a good sanding.
Step 19: Feet
Add a little curved cut out on the bottom of the legs to help the bench sit nice and solidly on the ground.
I used a belt grind to sand away a couple of millimeter dish to the bottoms, hardly noticeable though effective.
Step 20: Oil Finish
Hand finish with some abrasive paper, giving the arrisses a nice little softness.
Finish off with a couple of coats of hardwax oil for durability and to punch out the beauty of the wood.
...ready to be enjoyed.
Step 21: Glamour!
Super pleased with how it worked out.
I admit that even with a lifetime of joinery and making experience I wasn't 100% convinced that I could achieve the fantastic finished result. A project like this requires complete accuracy all the way through and if you are meticulous in each step then a great bench can be achieved.
I am entering this project into the Furniture Competition and if you have enjoyed this project, I would really appreciate your vote...thank you so much.
This project is part of my YouTube channel where I try to make cool and interesting projects.
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