1: Essential tools when marking out mitre joints are a protractor (a) and either a sliding bevel (b) or combination square (c).

2: Set the required mitre angle on the sliding bevel with the protractor and use it to mark out the cut lines on the work piece.

To mark out a mitre you will need a pencil, marking knife, rule, try square and a means for marking angles— either a combination square or a sliding bevel. The combination, or sliding, square (fig. 1) is capable of marking angles of 90° and 45° and has a sliding rule which may be adjusted to give pro­jections from 1 mm to approximately 260 mm. It is extremely useful when the blade of an ordinary try square is too long, because it can double up as an adjustable try square. The sliding bevel with the aid of a protractor (fig. 2) can be set to any angle, the blade being locked in posi­tion until released.

Start marking by deciding the over­all dimensions of the frame to be made and then add 5-6 mm to both ends of each separate component to allow for waste. Measure and rough-cut the individual pieces of wood, marking each face side and edge with the proper identification marks. Then, using a rule, marking knife and try square, carefully mark out the overall dimens­ions on the face edge of each work piece, allowing enough for waste. If the frame is longer one way than the other, mark opposite pairs together to avoid confusion.

If the mitre is to be cut across the wide surface, or 'side' of the piece, use one of the angle marking aids and a marking knife to set out the cut on the face side. Slide the combination square or sliding bevel up to the knife and score across the timber. Mark across the next edge at 90° to the side then finally mark the angle across the surface opposite the face side. If the mitre is to be cut across the narrow surface, or 'edge', of the piece start by marking the overall dimension line with a try square allowing for waste across the face edge; and then mark the face edge to the mitre angle with the sliding bevel or combination square. Next mark the sides at right angles with a try square, and use the bevel to mark the surface opposite the face edge as a mitre.

[edit] Cutting The Mitre

3: Use a mitre box to guide the saw when cutting 45° mitres. Hold the work piece tight. Packing underneath it will protect the base of the box.

4: Adjustable mitre jigs are not expensive and they can be used to guide the saw on cuts at a number of different angles.

5: Use a mitre shooting board to trim off the waste from a mitre which has been cut across the face side of the work piece.

6: Use a donkey's ear and a plane to trim off the waste from a mitre which has been cut across the face edge of the work piece.

It is quite possible to cut a mitre by holding the work pieces at an angle in a vice and sawing them direct. But for greater accuracy and ease of handling use a mitre block or, better still, a mitre box (fig. C above). Both of these devices are available from any hardware store. Cramp the block or box in a vice, or fix it to a worktop with large G-cramps.

A fine toothed saw should be used to cut into any moulding on the work-piece and not out of it. This ensures that the rag of the saw cut is at the back of the joint and not on the face. Always remember to saw slightly to the waste side of the cut and trim off the excess afterwards with a plane.

Trimming off the waste. Large sections can be angled in a vice and trimmed freehand, taking great care and using a fine set No. 4 smooth plane or a block plane.

Small sections may be more accurately trimmed with the aid of a mitre shooting board (fig. 5) for mitres in the width or side, and a donkey's ' ear (fig. 6) for mitres in the thickness, or edge, of the work piece. When using the mitre shooting board, plane away from your body to trim right hand mitres and towards you for left hand mitres.

The donkey's ear will give the best results if you first of all plane away from you, to just beyond the centre of the work, then place the work on the other side of the stop, reverse the plane and trim towards you. Finally, reverse the procedure for a last, straight through trim to finish the mitre neatly and cleanly.

[edit] Joining Mitred Frames

7: Use a picture framer's cramp to hold the components of the mitre in position while you pin them together and let the adhesive set.

8: Alternatively use a proprietary clamp system which sets strong springs into the wood and leaves small indents which are later filled.

The big problem when joining simple mitres is that the adhesive acts as a lubricant—causing the pieces of the frame to slide about and making the use of sash cramps impossible.

Light frames can be joined together using adhesive and pins. Use a picture framer's cramp to hold the mitres in position while the pins are driven home, then tap them below the surface with a fine pin punch and fill.

Another method of clamping mitres in light to medium section timbers uses special spring clamps (fig. 8 above). This clamping system, which is relatively cheap and easy to use, employs three sets of spring clamps in different sizes to suit different sections.

The springs which are set into the frame using a special tool, hold the mitre together by closing spiked ends onto the frame, one at each corner, rather like a hawk's talons. The indentation they produce is relatively small and can be either ignored or filled.

Larger frames can be joined together with a web cramp or a simple straining twine. Both of these tools should be used with L shaped wooden blocks, the outer corners of which must be well rounded to allow the web or cord to move around them.

Place clean paper between the block and the work piece to stop the block sticking to the work. Once the web or cord is tensioned, check the frame for square by measuring the diagonals, which should be equal.

E: Strengthening a mitre joint with a tongue insert. The extra gluing area provided makes the mitre less susceptible to warping or splitting.

F: Edge mitres are best strengthened by using veneer inserts like these. Always make sure that the grain of the veneer crosses that of the mitre.

G: Dowel inserts provide the best way of strengthening a mitre joint and are completely invisible when the work is finished.

[edit] Stronger types of mitre joints

9: To make a cross tongued mitre, mark out the with of the tongue with a mortise gauge and use a dovetail saw and chisel to remove the wood.

10: When the tongue groove is cut, glue the frame together with the strengthening tongue inserted as shown above.

11: Using a coping saw to trim out the waste tongue when the glue has set lessens the likelihood of disturbing the mitre joint.

12: For a really neat finish, pare away the last few slivers of the tongue insert with a sharp bevel edged chisel.

The cross tongued or feathered-mitre joint gives a greater gluing area in the mitre and is used where sideways movement is likely to occur (fig. E above). Grooves are cut in the faces of the mitres into which a tongue, or feather, is then inserted.

The tongue should be cut either from matching timber, in which case the grain must run at right angles to the mitre faces or from multi-layer plywood. The thickness of the tongue can vary between one quarter and one third of the thickness of the timber and will considerably strengthen the joint. The width of the tongue depends upon the size of the frame section, but a measurement based on one fifth of the face width is about right.

To mark out the grooves, set a mortise gauge to the width of the chisel you will use which should also match, exactly, the thickness of the tongue. Then set the gauge so that the two points are evenly spaced across the thickness of the mitre and mark parallel lines along the mitre face (fig. 9).

To mark the groove depth, use a marking knife and combination square or sliding bevel and make a faint line on the face across which the mitre has been cut parallel to the edge of the mitre. The line should be half the tongue's width plus half a milli­metre from the mitre edge. Use a try square to carry these lines across both edges of the piece.

Set the piece in a vice so that the mitre face is horizontal and use a dove­tail saw or fine tenon saw (backsaw) to make two saw cuts to the waste side of the groove. Chisel out waste.

When all of the grooves are cut, piece the frame together to ensure everything fits. Make any adjustment necessary for a good fit and, when all is well, glue the frame together.

The best cramping method to use for this type of joint is the spring system used in conjunction with G-cramps across the thickness of the section. Trim off the waste after the adhesive has set completely (fig. 12).

[edit] Veneer Keyed Mitre Joints

13: For a veneer keyed mitre joint, cramp the mitre in a vice and use a panel saw to cut angled slots for the veneer.

14: Cut slivers of veneer so that the grain runs across that of the work piece, glue them in place and trim off the waste.

Keyed mitres are used to strengthen edge mitred joints, especially those which are to be veneered. Cut the mitres and check that each side is of the correct length, then assemble the frame without glue to ensure it goes together well. Mark each pair of mitres on the face side, A-A, B-B and so on until all are marked.

Next, take a pair of mitres and place them in a vice, ensuring that the correct angle is maintained, use a try square or sliding bevel to check this. Using a fine saw with a veneer thickness kerf of approximately 2 mm, make opposing angled cuts across the corner of the frame (fig. 13). Check that the cut will take the veneer you propose to use, if the veneer is too tight a fit, open the cut slightly using either a thin file or a piece of glass paper fixed to a steel rule with double sided tape.

When all of the joints are cut and ready, apply adhesive to the mitres and assemble the frame using either spring or sash cramps. If sash cramps are used they must be prepared with protective waste blocks before you apply the adhesive. Two cramps should be placed underneath the frame close to the mitres, the other two go on top of the frame at right angles to the first pair and also close to the mitres.

When the frame is in place tighten the cramps so that they are just squeezing the protecting waste blocks against the frame. Check that the mitres are correctly placed and insert the pre-cut veneer slips (that should be coated with adhesive) with the grain running at right angles to the mitre. Carefully tighten the cramps and ensure that the mitres slip evenly, if at all, so that the frame remains entirely square and does not twist. When the adhesive has set, remove the frame from the cramps and cut off any excess veneer with a dovetail saw.