HALVING JOINTS & SIMPLE MITRES

A bench hook

A bench hook (Photo credit: Wikipedia)

Getting joints to fit snugly is one of the major objectives in carpentry, and nothing introduces the techniques so well as the halving joint. As for the perfect finish, that’s the role of the mitre. There are many situations in woodwork when you need a joint that’s fast and simple, but also neat and strong. And this is where halving joints come into their own. Despite their simplicity, they’re very effective joints because the two pieces of wood are cut so they interlock together, either face to face or edge to edge, making the joint as strong as — if not stronger than — the timber itself. They are used almost exclusively for building frameworks, joining the rails (side pieces) either at a corner or in a cross absolutely flush. You end up with a frame that’s neat enough to be on show and sturdy enough to need no reinforcement.

Mitre joints, though not strictly speaking considered halving joints as there’s no interlocking, are halved to make up a perfect 90° angle. In this section, only the simple mitre is dealt with — the more complicated forms (eg, mitred secret dovetails) are covered in another section

Strength of joints

There are three things that affect the strength of a halving joint — the size of the timber, the quality of the timber, and any reinforcement you add.

The size of timber is important because it governs the amount of adhesive in the joint; the greater the areas glued together, the stronger the joint will be. Usually problems only arise when you are trying to join thin pieces of timber together it’s almost impossible to get the joint to stay rigid. Regarding timber quality, hardwoods rarely present a problem, but with softwoods, splitting can occur which will seriously weaken the joint. You should, therefore, reject timber containing knots, cracks and other potential weak spots.

In many cases, the correct adhesive is all the reinforcement you need — use a good quality PVA woodworking adhesive, or, if the joint will be subjected to heat or moisture, a urea formaldehyde woodworking adhesive. If still greater strength is required — this is more likely on corner halving joints than on cross halvings — you should drive screws through the overlaps, or, for a more natural look, drill a hole right through and glue in a length of dowel. Both the dowels and screws are set like the spots on a dice to stop the joint twisting.

Butt joints must be reinforced in some way to have strength, but with mitred butt joints this would defeat the decorative aim. Because of this, they are normally reserved for situations where strength is not required — picture frames and decorative edgings, such as door architraves for example.

Marking corner halving joints

Having sawn the ends of the two pieces of wood to be joined perfectly square place one on top of the other, and mark the width of the top piece on the one below. Carry this mark right round the timber using a knife and a try-square, then repeat the process, this time with the bottom piece of wood on top.

Next divide the thickness of the timber in two. You need a single-tooth marking gauge for this: it consists of a wooden shaft with a sharp metal pin called a spur near end, and a block of wood (the stock) which can be moved along the shaft and be fixed at any point with the aid of a thumbscrew.

Position the stock so that the distance between it and the spur is roughly half the timber’s thickness, and place it against one edge of the wood. Use the spur to dent the surface of the timber, then repeat with the stock against the other edge. If the dents co-incide, the gauge is set correctly. If they don’t, reset the gauge. Don’t try to make small adjustments by undoing the thumbscrew and moving the stock — you’ll go on for ever trying to make it accurate. Instead, with the screw reasonably tight, tap one end of the shaft sharply on a hard surface. Depending which end you tap and how hard you tap it, the setting will increase or decrease by the merest fraction.

With the setting right, wedge one end of the timber into the angle of a bench hook, place the stock of the gauge firmly against the timber’s edge and holding it there, score the wood from the width line to the end. You’ll find this easier if, rather than digging the spur right into the wood, you merely drag it across the surface. Score identical lines on the other side and the end.

Use a pencil to shade the areas on each piece of wood that will form the waste (the top of one, the bottom of the other), then grip the first piece upright in a vice. The lower down you can get it the better. If you can’t get it low, back it with a piece of scrap wood to reduce vibration. Using a tenon saw, carefully saw down until you reach the width line — the first one you marked. The golden rule of sawing any kind of joint is to saw on the waste side of the marked line (it’s better to saw or chisel off too little rather than too much since you can always take off a little more but you can never put it back). And remember that the closer the fit, the stronger the joint will end up. Basically, it should fit like a hand in a glove.

Remove the wood from the vice, put it on a bench hook and cut down along the width line to release the waste wood. Again make sure you cut on the waste side of the line and be prepared to make final adjusments with a chisel. Treat the second piece of wood in exactly the same way, then bring the two together and check the fit.

You can use either a chisel or a piece of glasspaper to take off any unevenness in the timber, although it’ll be quicker to use a chisel to clear out the edges so that the corners are absolutely square. When the pieces finally fit neatly, spread adhesive on both faces of the joint and hold them in place with a G-cramp (protecting the wood’s surface with scrap timber) until the glue has set. Remove the cramp, and add any re- inforcing screws or dowels that may be needed, drilling pilol holes first.

Making cross halving joints

I he difference between cross halving joints and corner halving joints is that you cannot remove the waste using only a saw. You have to make a housing’ and for this you need a chisel.

Saw down the width lines to the halfway mark and make additional saw cuts in between to break up ttie waste – these canbe the same width as the chisel blade to make chipping out easier. Grip the work in a vice, or on a bench hook, and now use the chisel to remove the waste This is done in four stages. Guiding the chisel blade bevel uppermost with one hand and striking the handle with the palm of your other hand -for this job your hand is better than a mallet — reduce the edge of the timber nearest to you to a shallow slope ending a fraction above the halfway line Don’t tiy to remove all the wood in one go or if will split Remove just a sliver at a time

The next step is to turn the wood round and slope the other edge to leave a sort of pyramid of waste. With that done, pushing the chisel through the wood rather than hitting it, gradually flatten off the pyramid until you have brought it level with the halfway lines You II get a neater finish here if, in the final stages, you work with the chisel’s blade flat but at an angle to the grain of the wood Finally, again pushing the chisel, remove any ragged fibres lodged in the angles of the housing.

Once you’ve sawn and chiselled out the housing in the second piece of wood, the next step is to try fitting the two together Don’t try forcing them if they don’t quite fit — you’re in danger of splitting the wood Instead, carefully chisel off a fraction more wood, bit by bit, until you can fit the pieces together without undue force If, on the other hand, you’ve cut the housing too wide so the ht is veiy loose, you’ll have to add some reinforcement like screws or dowels, and fill in the gaps witli a wood filler, stopping or a mixture of fine sawdust and PVA adhesive It’s not worth trying to add a wedge unless the gap is very wide (over 6mm/’/iin) because the result can be very messy

Making a mitre joint

With wood that’s square or rectangular in section, the first job is to make suie that both pieces are absolutely squarely cut. Use the Ity-square to check this if they’re not, it’s better to cut another piece of wood than attempt to make adjustments. Next, place one piece on top of the other to form a right angle. Maik an internal ai d external corner on both, then take them apart and carry the marks across tfie edge with a knife and try square. Join up the marks on each piece of wood – this will give sawing lines at 45°. Mark the waste side of each with a pencil.

Wood that is raised on one side (eg, mouldings for picture frames) cannot be marked in the same way as the pieces won’t sit flat on each other. The easiest way is to mark the point of the mitre (the corner point) and then to use a simple to cut the angle A mitre block not only helps you support the piece of wood (like a bench hook) but also has saw cuts at 45’ in the back face to guide the saw. Then you only have to line up the mitre point on the wood with the saw now set at the correct angle. You can make a mitie block yourself — see

Mitre aids

There are other devices available lo help you cut mitres accurately A proprietary for example, guides the saw either at right angles or at 45°, a is like a mitre block but has an extra side so that the whole length of the saw is kept in line

Without these devices, getting the angles right isn’t easy – – but if necessary you can use a bench hook, driving in two nails so the wood is held against the block and the line of cutting is free of the bench hook This is not as easy as using one of the other methods Mark the wood so you know the sawing line, then place it in the mitre block, box or jig, to line up with the appropriate groove to guide the saw. If the wood you are cutting is very thin, put some blocks of scrap wood under the device to bring it up to a reasonable height Insert a tenon saw into the guide slot and. Holding it level, saw away.

There are only two things that can go wrong If the block is old. The ‘guide’ cut may have widened, resulting in an inaccurate rut A larger tenon saw may help, but really the only answer is to hold the saw as steady as possible The other common error when cutting mouldings and the like is to cut two mitres the same — that is two right-handed or left handed angles, instead of one of each This can be avoided by always marking the waste on the wood, and checking that the saw is in the correct guide slot before you begin

Clean up the cut ends with glasspaper, taking care not to alter the angle, and glue and cramp the joint together. For frames, special mitre cramps are available, but you again make up your own From scrap wood, cut lour I shaped blocks, and drill a hole at an angle through the point of each L. Feed a single piece of string through the holes of all four blocks, position the blocks at the corners of the frame and tie the string into a continuous loop To tighten up, twist the siring around a stick, and keep twisting the stick to draw the blocks together You can then wedge the stick against the frame to stop it untwisting until the adhesive has set There ate three ways to strengthen mitres with timbei connectors, plywood triangles or metal angle repair irons For frames they should be fitted from behind, either by glueing or glueing and pinning.

MAKING A MITRE BLOCK

Mitre blocks and boxes already have 45 angle cuts made as a saw guide Rather than buying one, you can make one that’s used in the same way as a bench hook You’ll need:.

– a piece of 19mm (:V.nn) plywood measuring about 250 x 150mm (10 x 6in)

– a 250mm (10in) length of 50 x 50mm (2 x 2in) softwood – or hardwood such as beech if available

– a 250mm (10in) length of 50 x 25mm (2 x 1 in) softwood.Glue and screw together as shown in the diagrams, then

– use a combination square, or make a template by folding a square piece of paper in half diagonally, as a guide for the 45’ angle saw cuts

– square the lines down both faces of the block, and cut the slots with a tenon saw MAKING A MITRE CRAMP

You can make a simple cramp for small mitred frames from 4 L-shaped softwood blocks. Drill holes through one leg of each block, thread string through holes and tie tightly to hold blocks against frame corners.

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