Fitting New Tenons and Reassembly of Sash Windows
Often, a tenon is broken or damaged and needs replacing. This is best cut off and replaced with a new tongue. Afillet provides the new tenon, half of which is inset into the horizontal member of the window sash, the rail.
To make this, cut a piece of hardwood to the same depth and thickness of the old tongue but twice the length, and add 6mm. The extra length allows the new tongue to project through the mortise so it can be sawn almost flush and planed smooth.
The new tenon slots into the rail, and may be drilled and pinned with dowels. First, cut off the old tenon flush with the end of the rail. Put the rail in a vice, mark out with a try-square the distance of the tongue back along the rail. Cut a slot for the section of hardwood to fit into, using a.
Next, drill three pilot holes in the form of a triangle through the rail so that the hardwood is just marked and remove the piece of hardwood. Make a mark with a nail punch at a distance of 1mm on the outside of these three points and then drill 10mm holes. These off-line holes pull the joint tight when it is later dowel fixed. Drill 10mm holes through the rail and assemble the rail and hardwood fillet. Coat the concealed part of the fillet with, slightly point three 10mm dowel pieces, cut slightly over-size for trimming. Coat these with an exterior grade and tap home; allow the joint to set.
The joint can then be planed smooth. Finally, the tongues can be haunched back to fit the mortise slots.
After new tongues have been fitted, re-assemble the sashes. Coat the tenons withand slide these into the mortises. Sashes must be quite square; check this by placing the inside of a try-square on the outside corners of the frame; these should be square at all points. Another way is to measure the diagonals; if these are equal in length, the frame is true.
Once the sashes are assembled, you need to drive in hardwood wedges to consolidate the mortise-and-tenon joints. These are the same thickness as the tenon and should be glued with an exterior grade of, then driven in from the outside at the edges, using a mallet or rubber-headed hammer.
It is important to hold the work steady while doing this as the frame may go out of square or the tongues of the tenons might become stressed and damaged. It is best to use sash cramps; if you do not possess these, they may be hired.
An improvised wedge can be made up by nailing blocks of wood to a surface, and supporting the frame between these. In this method, the blocks are set at a distance slightly greater than the length of the frame. The frame is tightened between the blocks by driving in four small wedges between the block and the frame at one end.
Another way is to make up a tourniquet of rope, tightened with a piece of wood, to cramp-tightly round the outside edges of the assembled sash.
Before thedries check carefully that the frame is true and wipe off any excess glue.
Another problem is tightness of sashes. This may be caused by excess paint on the outer surfaces. Strip any build-up of paint and prepare and repaint the surface.
Reglazing of sashes follows the techniques of glazing. However, beware of old or weathered glass. This may be brittle and break easily. You may wish to contemplate replacement with a decorative, patterned glass. Similarly, casement or of various types may be removed and replaced with proprietary, aluminium-framed louvres.
You may also wish to extend and reinforce frames, so that you can fit one of the forms of double glazing.
On sash windows, if attention is needed to repair the frames, it is probable that sash cords may also need replacing. You can obtain pre-stressed wax cords, or allow for stretching in use.
The lead mouse
You also need a length of string and a small, flat piece of lead called a ‘mouse’. This is used to weight the new sash cord and is later removed.
The mouse is rolled round the end of a 150mm or 180mm length of string. It is about the thickness and half the length of a cigarette. The mouse should be bent slightly in the middle and fed over the groove of the outer pulley wheel until it falls down behind the stile.
Next, tie the new sash cord to the other end of the string, and pull this over the wheel and out through the pocket opening. The mouse can now be removed.
Tie the sash cords to the top of the weights. Use either a flat-finish knot, or bind the loose end of cord so that no knot or lump can interfere with the action of the window opening.
Pull the weights up about 50mm from the bottom and partly nail through each cord into the pulley stile. This is to hold the weights temporarily in position. Next, cut each cord level with the pencil marks made earlier on the stile.
The outer sash should be positioned so that you can fit a cord into its groove. Line up the end of the cord with the pencil mark on the edge of the sash, fixing it with four or five clout, starting at this mark.
Once both cords have been fastened, the temporary nails can be taken out of the cords and stiles and the sash can be lifted into place. Test the sash operation by sliding it vertically.
The weights for the inner (lower) sash are similarly fitted-except that these should be pulled up almost to the pulleys.
Finally, replace the pockets, spring back the parting bead, and lift back the inner sash. To ensure that the sash slides smoothly, put candle wax in the two channels and on the edges of both staff and parting beads.
10. November 2011 by admin
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