Saturday, 27 December 2014

Glass Grinder Maintenance

Glass grinders are often the main machine used in stained glass.  They are important machines to many of us and so need proper maintenance and use to give us long service.  This note is about the things you need to keep in mind about maintaining the grinder.

Glass grinders are very handy tools. They mainly consists of a spinning diamond coated cylinder (the bit) extending above a grid surface that can rapidly and accurately grind glass to the desired shape.

Don't purposely cut glass large with the intention of grinding. Ideally no grinding would have to be done. It should be used only for minor adjustments or for grinding tough-to-cut inside curves.


The water reservoir should be emptied each day after use.  It is not advisable to have standing water in your grinder for some health reasons, avoiding unpleasant smells, and as moving a grinder with water risks damaging the grinder’s motor and other equipment and materials in the studio.  Even if you do not move your grinder about, the water needs changing just as frequently as the glass grit needs removing.  Some people line their reservoir with aluminium foil to catch the grinding residue.  This may not be possible on all grinders, but does reduce the effort of cleaning the grit from the bottom of the reservoir.

Ensure you clean your sponge (if the grinder uses one) when cleaning out the reservoir.  If you do not clean it regularly, it will become hard and stop bringing water to your grinding head.

The accumulated glass grit should be cleaned out with scraper or paper towel and disposed of in the waste, not down a sink.  The glass grit is relatively heavy and will become solid and block your drains.  If you have a garden, the glass grit is a good soil modifier, by helping the drainage.  The glass grit will accumulate and restrict the flow of water to your grinding head.

Clean any plastic eye shield with running water, as rubbing it will scratch the surface.

Ensure there is water in the reservoir before starting any grinding, but do not over fill as the excess water could get into the motor. Empty the reservoir daily. This keeps the water from producing a smell, and allows you to clear the glass residue from around the grinding bit.

Grinder Head Maintenance

Wear - There seem to be two main indications of wear – chipping of the glass, and slow grinding.  There are a number of reasons that may cause the grinder to chip the glass surface and not all are indications of wear. Some of the things to check are:

Too much pressure will cause the glass to chip.
It may be that you are pressing the glass into the grinder head too hard. The grinder head should do the work. Firm rather than hard pressure should be applied. If the grinder motor slows, it is an indication that far too much pressure is being applied. It will also lead to cuts on your fingers. Run the glass lightly against the grinder all the way around to remove any overlooked sharp shells on the glass.  Then light pressure will not cut your fingers while grinding.

This shows some finger protection being used, possibly to protect fingers from too much pressure

Grit size has an influence on the way the edge appears.
It is possible that you may be using too coarse a grit on the grinder bit/head. The coarser the grit is, the larger the chips will be taken off the edge surfaces. Smaller grits take smaller chips off the edges, and so are less obvious.

New bits can be a source of chipping.
Oddly, new bits can cause chipping. If it is a new bit that is causing the chipping, consider dressing it. New bits often need to be dressed – removing protruding diamonds, or cleaning and exposing new ones on a worn bit. To dress the bit you can grind some scrap glass, brick, or use a dressing stone to lightly grind some of the abrasive material away. This most often settles the bit and avoids chipping.

A worn or damaged grinder bit/head can cause chipping.
Inspect your bit carefully for smooth areas showing that the diamonds have been worn away. Also look for dents, and other irregularities on the surface, indicating that the bit is damaged. Any dents or smooth places on the bit cause a vibration that is similar to a tiny hammer tapping the edge of the glass.

This shows that chipping of the glass is most often comes from the way the machine is used rather than the grinder head needing replacement.


Water for the grinding head
Insufficient water supply can reduce the life of the grinder head.
If there is too little water reaching the head to lubricate the diamonds and keep the glass cool it can reduce the life of the grinder head, by overheating it too. If you are getting a white paste or a powder on or near the glass, you need to increase the water supply. This may indicate a low water level, too much grit in the reservoir or sponge clogged preventing sufficient water reaching the grinding head.

The white reside on the grinding head shows there is insufficient water reaching the grinding surface

Vaseline for the shaft and grub screw.  Smooth any corrosion from the shaft with fine wet and dry sandpaper and lubricate the shaft. Periodic removal of the bit and lubrication of the shaft should be part of the regular maintenance of the grinder.  Also ensure the grub screw is kept clear of glass grit by putting some Vaseline in the recess.  This keeps the threads and the slot for the alan key from clogging up with glass grit.

Removing the Head

The first step in removing the grinding head is to loosen the grub screw that fixes the head to the shaft. This is often full of glass grit, so clean the recess for the alan key first. I do this by using a needle or other thin sharp object to clear out all the glass powder from the recess in the screw and the threads. I am sure there are other things that can be used to clean out the hole too. When clean, insert the alan key as far as possible and while holding it in turn the key in an anticlockwise direction.  If it will not move, you need more leverage.  If the alan key simply spins in the recess, you need a stud bolt remover tool. 

This is a tool that many mechanics and tool shops have. It is normally square or triangular. It is used by drilling into the broken off bolt. The tool is tapped into the hole and then with a wrench/spanner the grub screw is loosened. In the case of the grub screw the recess is already there so you only need to hammer the removal tool into the alan key recess and proceed as normal.

The more modern and safer method is to use a tool with a reverse thread on it. This means that as you turn anti-clockwise, the tool thread bites into the recess and brings the grub screw out.

If the grinding bit is already seized, the method of removal is based on how fast it is stuck. If there is a bit of movement around the shaft when the grub screw is removed, you can probably remove it with simple tools. First use very fine wet and dry sandpaper to remove all corrosion and roughness from the upper, exposed part of the shaft. Put a thin film of lubrication or penetrating oil on the shaft and then you can hold the top of the shaft tight with smooth-jawed pliers while you twist the bit. Be careful not to mark the shaft or you will create another obstacle to removal of the bit. Alternatively, while pulling up on the bit, you can tap the top end of the shaft gently with a plastic hammer to shake the bit loose.

If this does not work, remove the grid and turn the dried grinder upside down and spray WD-40 or other penetrating oil to the bottom of the grinder bit.  This should be left for a few days with renewal of the penetrating oil every half day. Then try the methods above to free the bit from the shaft.

If the bit is still firmly stuck, you will need a small wheel puller to get the bit off the shaft.  Small wheel pullers are available from most DIY or mechanics suppliers.

Once you have the bit off, smooth any corrosion from the shaft with fine wet and dry sandpaper and lubricate the shaft. Periodic removal of the bit and lubrication of the shaft should be part of the regular maintenance of the grinder.

Adjusting height

If your grinder bit is too low or too high the diamond surface will not grind the whole of the glass edge. This can lead to chipping of the surface of the glass at the edges.

A good practice is to start with the bit as high as possible to allow for differing thicknesses of glass. As high as possible is with the bottom of the diamonds just below the platform of the grinder. This will ensure that you can deal with varying thicknesses of glass without immediate adjustment. You can then lower the bit as it wears.
This bit is only just high enough for this style of grinding

An example of  the right height, but you can see that too much pressure is being applied

Here the head is only just low enough to catch all the glass. It should be a little lower.

Be certain that you secure the grinding head with the grub screw facing the flat part of the shaft.  Failure to do that will both mark the shaft, making removal difficult, and potentially allow the shaft to spin while the head remains stationary.
This shows the minimum level for 3mm glass

Of course, you need to ensure there is adequate water reaching the grinding bit to avoid overheating the glass, and to keep the dust from grinding from getting into the air.

Care in Use

Water is the primary lubricant.  It washes the grit from between the diamonds and keeps the head cool.  It also cools the glass, of course.

You can buy an additive for the water – often called a diamond coolant – which is intended to provide a kind of lubrication for the diamonds. This may also extend the life of the bit.

Grinding head/Bits

The diamond bit must be kept wet in order to reduce wear on the diamond and prevent glass dust from developing and being inhaled. There are several grits available. "Fine" grinds slower but leaves fewer chips out of the glass. "Coarse" grinds very fast but leaves larger chips. "Standard" is a central compromise.


Smooth any corrosion from the shaft with fine wet and dry sandpaper and lubricate the shaft. Periodic removal of the bit and lubrication of the shaft should be part of the regular maintenance of the grinder.

The maintenance is not only on the shaft but also on the fixings. Putting a dab of Vaseline or thick grease into the grub screw socket will help keep it clear of the glass residue. 


It may be that you are pressing the glass into the grinder head too hard. The grinder head should do the work. Firm rather than hard pressure should be applied. If the grinder slows, it is an indication that too much pressure is being applied.

Extending the life of your grinder bits is a matter of recognising that you should not force the glass into the grinding head. Excessive pressure against the head heats the bit and allows the diamonds to become free of the binding material, so reducing its life. If the motor slows as you press the glass to the bit, you are applying too much pressure. That kind of pressure also puts a lot of wear on the bearings of the motor.

If the grinder is not taking glass off fast enough for your purposes, you should put a coarser bit on the grinder, rather than press harder. The bits do come in a variety of grits. Try out some different grits to find the one that works best for the speed at which you want to remove the glass.

Maintenance routine

A maintenance routine for your grinder is a practice that will reward you with longer lives for your grinder heads and the machine itself.  A sample routine might be like the following:
·         Empty water reservoir
·         Clean out glass grit
·         Clean eye shield

  • Inspect bit for wear. Adjust up or down or replace
  • Inspect grub screw is free of glass grit. Clean and re-fill with Vaseline
  • Check head moves freely up and down on shaft

Saturday, 27 July 2013

Stepping Stones

Making stepping stones is a relatively simple process if a few processes are followed. 

A range of examples

Work Area
Pick a work area where the mold can be left undisturbed for 48 hrs.
Prepare the area by putting a protective layer of newspaper or plastic around the work area. Place several pairs of gloves, bags of concrete, chicken wire, mould, stick back plastic, cut glass and water ready to hand. A mixing container is essential, and you should have the reinforcing chicken wire ready cut to size for the mould. A nearby dust bin is a good idea to be able to put waste into without moving wet materials far. Make sure the work area is level and the mould sitting on it is also level.

Design and positioning of glass
Opalescent glass is the best to use for stepping stones, as it reflects the light. Transparent ones appear dark or black.
The cut line cartoon should have lines at least 3mm wide – up to 6mm is also suitable. The cement needs to get between the glass pieces. Cut the glass to the edge of the cartoon lines.
Ensure the glass pieces do not come closer than 12mm to the edge of the mould to allow the concrete to form an edge that will protect the glass pieces.
Cartoon, cut glass, and examples of the process

Placing and adhering glass
Use clear sticky backed plastic and fix it – sticky face up – over your cartoon. Place the glass pieces firmly onto the sticky plastic. Cut the plastic to the size and shape of the mould to ensure no concrete gets under the glass.

Glass placed and ready for pouring

Preparing the mould
Put a separator on the mould – spray on oils can work, but Vaseline is a more certain separator. All of the mould surface must be thoroughly coated. Disposable plastic or latex gloves will help with greasing the mould.
Carefully lift the sticky back plastic with the glass on it and place it in mold making sure placed in center.
Examples of ready made moulds

Concrete mix
If you are making small quantities of cement, ready mixed concrete for general purposes will be suitable. 

If you are making a number of stepping stones ,mix the concrete yourself as it will be cheaper.

A ready mixed quick setting concrete

The ratios for general purpose concrete are 1: 2: 4 concrete: sand: stones. The water needed will be about 1.7 times the volume of the dry mix.

Calculating the amounts: Square and rectangular moulds are easiest – multiply the length by the width to give the area and multiply that by the depth in centimeters. This will give you the volume in cubic centimeters. Divide the result by 1000 and you have the total number of litres required.
For circular or hexagonal moulds multiply one half of the diameter (the radius) times itself and then times 3.14 to give the area. Finally multiply by the depth of the mould to get the volume. All of this is measured in centimeters.
You have seven parts of dry material, so divide by seven and the one seventh will be the concrete needed, two sevenths the sand and four sevenths the volume of stones needed. The stones should not be larger than drive-way type gravel.

You can of course, just fill the moulds with water and measure the water required to give you the volumes you need.

Mix up the dry materials, whether ready mixed or measured by yourself, to ensure everything is coated with the concrete.

Dry mixing and then progressively adding water

To get a fine, smooth surface to the top of the stepping stone, sieve out the stones from some of the mix to give you about 1 cm depth to pour between the glass pieces without any stones sitting there. (Multiply area by 1 cm to give you the volume of dry material you need to put down the top layer.) When you have sieved that amount of material out, put the coarser material back into the dry materials and mix again. The stones are important to the strength of the whole stepping stone.

Pouring the concrete
Mix the cement, sand and small stones with about 1.5 times water until a thick pancake batter. Pour it slowly over the glass to ensure you do not disturb the placement of the glass pieces. When poured, tap the sides of the mould with a piece of timber to encourage air bubbles to rise.

Mix the remaining dry materials with water and gently pour half of it into the mould. Then add the chicken wire for re-enforcement and fill with the remainder of the cement. The metal is better than the plastic version. 

Tap the sides of the mould again to drive air bubbles out of the mixture. There should be no pools of water on top of the cement, as too much water will weaken the concrete, as will too little. If there is excess it should be absorbed with some paper towels.

This is the time to sign the stepping stone. There is a relatively short time after about an hour to do this while the concrete is stiff enough to hold the marks and not too stiff to resist any marks.

Let the stone cure at least 48 hours before removing from mold. Do not move mold once the concrete is in it – you will get concrete between the contact paper and glass which causes the top to be uneven and more importantly can cause cracks in the concrete.

When you are ready to de-mould, turn the mould face down onto a board. Ease the sides away from the concrete gently. It may drop out with just that encouragement. If not, press gently at the centre of the mould until it beings to release. Finally, if necessary tap the sides of the mould with a rubber mallet. Difficulty in removing the stepping stone usually relates to too little separator.

Pealing off the plastic

Turn the stepping stone over with the assistance of the a board on the back. Look at the top of the stone and decide whether it is fully cured. If not, leave the sticky plastic on for a few more days before gently pealing it off. If you can see that there are gaps or depressions in the surface, you will need to take the plastic off immediately and make a small mix of concrete and sand (1:2) with water to fill those blemishes.

Clean the concrete from the glass before fully cured as it is more difficult to get the dry concrete from the glass without scratching it.

Let the stepping stone sit for 30 days before sealing. You can leave them flat or stand against a wall. In either case you should put some paper between so they don't get scratched. Clear concrete sealer, with two to four coats brushed on, will be sufficient for protection from moisture.
Weather, temperature and time of year play a big part in the curing and de-molding. Do not put your mold in the sun to cure, as that dries out the water before the chemical reactions can occur. Concrete needs to cure slowly to achieve its maximum strength. For the same reason, you do not want to leave the curing concrete in freezing conditions as that will also stop the chemical reactions. If it is wet, you need to protect the curing cement from the rain as too much water can weaken the concrete too.

Thursday, 18 October 2012

Light Boxes

Description and Use
Light boxes are in many ways development from the glass easel. The glass easel was used in studios to wax up the painted glass and display it as it would be seen in a window. Sometimes the glass painters painted across all the glass at once, so this method enabled them to see the results immediately.
Nowadays people tend to use back lighting for these and other purposes, so the light box has become more popular. Some of the uses are outlined here:

The light box is very useful when tracing or altering designs. The back lighting enables you to use other paper than tracing papers to transfer the design elements. You can fold the paper along the lines of symmetry to check on how the lines match, or to copy the lines from one side onto the other side at the designing and cartoon stages.

The light box can help select glass colours either initially or when the main pieces have already been established. The combination of the glass over light shows how they interact with each other. At later stages when the main glass is cut, it can help avoid unwanted bright or dull areas.

Possibly the most common use is in cutting dark or opalescent glass. The additional light allows you to see the cartoon through the glass and so cut directly from the cartoon. This can be enhanced by blanking out the excess light from around the glass or cartoon.

The light box enables the arrangement of the cut glass pieces to be assembled to view the colour balance and have a virtual view of how the panel or window will look as a finished piece. An additional step toward the result is gained if each piece is outlined in white-board markers – use black pens - to represent the lead or copper foil and their widths. This stops the light between the pieces from causing you pupils to contract, and gives a more accurate representation of the appearance of the final pane.  White-board markers can just be wiped off the glass without using spirits.

The light box is important in painting. The back lighting shows the effect of the painted line or level of shading immediately. This allows adjustments to be made quickly and accurately before firing,

Depending on how the light box is built, it also can be used as display lighting. This can be as up-lighting or backlighting.


The top surface needs to be firm and scratch resistant. Toughened or laminated glass is good for this. The larger area covered, the thicker the glass needs to be, or there needs to be support under the glass to avoid breakage from pressure. The toughened or laminated glass resists breaking from dropping material onto the light box.

You need to have daylight corrected light sources for you light box, especially if you are doing any glass selection on it. Fluorescent tubes are easily available, but other light sources can be used. You need to have ventilation to allow the heat generated by the lights to disperse. Fluorescent tubes do not generate much heat and so are the common choice for lighting.

You need diffused, even light across the whole surface. This requires a diffuser and there are a number of solutions. You can sandblast the back of the top sheet, but I find this does not provide enough dispersion. You can sandblast both sides, which gives better dispersion of the light, but is difficult to clean and so needs another sheet on top. The best dispersion of light comes from using a sheet of opaque acrylic with about 80% light reduction. The difficulty with this is that it is flexible and needs support if any glass cutting is going to be done on the surface. I place the acrylic sheet underneath a sheet of 6.4mm laminated glass. This gives both solidity and dispersion.

Light is a central consideration in building the light box. The intensity is controlled by two things mainly – The number of lumens and the intervals of the light sources. The best way toward even light distribution even with good dispersion sheets, is to have multiple light sources. It would be possible to pack the box with light fixtures, but this is expensive and generates a lot of heat. It also may make the light too intense to be comfortable to work with. In general, fluorescent tubes placed at about 150mm centers apart will provide all the light you will need.

To make sure you get all the benefit of the light you need to build an enclosed box with ventilation holes or slots that is painted matt white on the inside. This allows the light to be reflected upwards through the surface without bright spots that can be caused with gloss paint.

You need to consider the size of the box in terms of surface area. This will relate to the space you have available and the scale that you work at. In addition to a separate surface the box can be an area of the work bench, or covered by a separate work board – whether permanent or temporary.

The height of the box will need to be considered. Will you be sitting or standing while working at the light box? It needs to be high enough in either case for you to maintain a straight back.

You need to consider the ability to screen parts of the light so the light is directed only at the work area. Large areas of light will overwhelm the glass, making it appear darker than the finished piece will actually be.

You need to think about the amount of flexibility your box requires. If you want to use it as part of your display equipment, it needs to be mobile and relatively light. This will interact with the materials to be used in construction.


Light boxes can be constructed in a variety of ways. The simplest to construct is the free standing, horizontal, single purpose light box.

You need to consider the size of the box in terms of surface area. This will relate to the space you have available and the scale that you work at. Having determined the surface area required or possible, you need to think about the height. The top should be of a height so you can stand or sit with a straight back while drawing, cutting or painting. This will vary according to your height and whether you will be standing or sitting. Typically these heights will be the heights of the benches and desks you already use, but you need to check again that you are actually working with a straight back, as this will reduce the fatigue you might otherwise experience.

Note that if the box is going to be sat at, it will need to be narrower to be able to reach to the opposite side. If you will be standing at the box, it can be at least half again as wide as the sitting version. A sitting version will also affect the depth of the box containing the lights. It may not be possible to have anything deeper than 100mm. This will produce some problems with the evenness of the light, but nothing that will make it unusable.

Then you need to consider the depth of the box. In principle, the deeper the box the better diffusion of the light. But there are limits. If the box is really deep, more lights will be required, and potential storage space is lost. I recommend about 150mm for the depth of the box. I then place the fluorescent tubes at 150mm centres across the box. It does not matter which direction they are oriented. That will be more determined by the available fittings and the dimensions chosen.

The flexibility you have in building your own box includes a number of things which could be constructed separately or in combination.

You can cover the light box with a sturdy work board to do all kinds of work on top. So this makes a combination light box and work bench. This top can be hinged so you don't have to lift it off each time you want to use the bench. It should have some support mechanism so it does not fall on your or your work. I have used a chain that allows the board to go back just beyond the vertical. These chains can sometimes get in the way of your work.

In addition to a separate surface the box can be an area of the work bench. The important element is that the rest of the surface of the bench should be at the same level as the light box top. Any variation runs the risk of breaking the glass you may be working on. The cover for this can be hinged to protect the surface when the light is not needed.

Often you will be working on pieces smaller than the illuminated area. It is possible to arrange things so that each light fitting can be turned on and off independently to allow light reduction or intensification as you need. It is simpler to have sheets of opaque card to place around your work area to reduce the extraneous light that will overwhelm the glass that you are selecting or painting, for example. In the case of too much light the glass or the painted lines and shading look darker than they really are as a result of your pupils contracting against the light.

You could add a variation to allow the light box to be used as a near vertical illuminated glass easel. This requires a set of hinges, a ledge on the hinged side and a support of some kind at the back, similar to a piano lid support. This is most useful in painting and in waxing up the pieces to view the whole panel before leading or foiling.

You need to think about the amount of flexibility you require the box to have when considering the materials to be used. If you want to use it as part of your display equipment, it needs to be mobile and so relatively light. This will interact with the materials to be used in construction. In this case you may want to make greater use of metals for their strength in relation to weight. 

 You probably will use opaque acrylic sheet as the surface. If you do, you will need to give it internal support to keep it from bowing. The best for this is another piece of acrylic – clear this time – glued to the top sheet and to the bottom of the box between the light fittings.

Working surface

I recommend your top should be 6.4 laminated or 4mm toughened glass for anything up to 610 by 1000mm. If it is larger, you should go to 6mm toughened, as 8.6mm laminated glass is pretty expensive. I suggest glass because it is strong, rigid, scratch resistant and easy to clean.

You can use a router to form a ledge for the glass to sit on. You can use a less machine intensive method, by nailing thin battens or quarter rounds around the glass. But the structure which confines the glass should be no higher than the glass surface. If it is higher than the glass, you can simply plane or sand it down. Insure there is no part of the fixings of the glass higher than the glass surface. This is especially important when cutting glass on the light box. If the surround is higher, you run the risk of breaking glass that is for one reason or another overhanging the edge. It also makes it easier to get the glass on and off the light box.

To get the appropriate diffusion you need to do more than sandblast the underside of the glass. While this will provide some diffusion, it is not enough. You can put another sheet of glass, sandblasted on both sides, underneath the top sandblasted sheet to provide good dispersion of the light. However, I have found a 3 or 4 mm sheet of white acrylic that is 70% -80% opaque provides the best diffusion of the light elements, even though it is more expensive than glass.

You also need to have a method to be able to get at the lights. This can be by having a removable section of the boundary. You can also make use of the ventilation holes, if appropriately placed, to lift the glass. A portion of the box sides can hinge to allow access to the lights through the side, although this is more awkward than fitting from above.


Light is a central consideration in building the light box. 

The best way toward even light distribution even with good diffusion sheets, is to have multiple light sources. I recommend placing them at the same distance apart as the depth of the box. It would be possible to pack the box with light fixtures, but this is expensive and generates a lot of heat. It also may make the light too intense to be comfortable to work with. If you can control the general lighting of your studio and you can turn it off or down, you will not need such intense lighting in your box.

An alternative, but more complicated method is to build the light box with baffles so the light is never directly under your work. Commonly, this would require the box to be built wider than the glass upon which you will be working. The light reflects from the sides and bottom of the box to give an even light. In this case, the single sandblasted surface would be sufficient to disperse the light and keep your eye focused near the surface of the glass or cartoon on which you are working.

You need to have daylight corrected light sources for you light box, especially if you are doing any glass selection on it. Fluorescent tubes are easily available, but other light sources can be used if they can be found in daylight colours. Fluorescent tubes do not generate much heat and are available in daylight corrected colours. So these are the common choice.

You still need to have ventilation to allow the heat to disperse, though. Ventilation can be provided in a number of ways ranging from drilling holes in the sides, to providing a slot in the side or bottom.

You need to have access to the light fittings to replace bulbs. It is easiest if this is by removing the glass top. You can provide tabs on or under the glass to lift it with, but these often interfere with other uses. You can use the ventilation holes if they are high on the box to stick a lifter under the glass to be able to grasp the edge. You can have a removable section to the beading that holds the glass top in place. You can provide a couple of finger holes at the top edge of the box to enable more direct lifting of the glass without disturbing any of the box fittings.

Another important element in getting the maximum amount of light out of your box is to paint the inside white. This should be a matte or at most silk finish. Any glossier finish will produce bright reflective areas. Shiny surfaces such as aluminium foil also produce these unwanted bright areas. In fact, a matte white surface gives more apparent light than aluminium foil in the light box.

The lights should be wired in series so they all come on at the same time. It is of course possible to have a switch for each fitting, to vary the intensity of the light for the work you are doing. This does add a bit to the expense, but may be valuable for your way of working.

Tuesday, 18 September 2012

Measuring openings

There are a number of measurements that are critical for a good design and a sound installation of window panels.

1. Tight Size: This is the full size of the glass opening with no allowances for expansion and contraction. In a wood or metal rebate frame one would measure from steel to opposing steel or wood to wood; in a stone groove installation, from the bottom of one groove to the bottom of the opposing groove. Depending on the size of the opening, this measurement should be checked in multiple areas; at a minimum at the top, bottom and middle horizontally and at the left and right jamb.

2. Sight Size is the daylight opening or the largest opening that allows light to pass through.

3. Rebate or groove details. With a rebate frame, the depth and the width of the rebate must be measured, as well as the interior return if round bars will be used (this dictates what size bar will fit and how long the bar should be). These dimensions are also necessary to determine the dimensions of the retaining moulding if one is to be used. If it is a groove, the depth of the groove and the width of the groove (measured from interior to exterior) are important.

4. Panel Size. This is the ideal size of a panel that will be installed into the opening in question. Typically, this will be a function of the tight size less 3mm in both width and height for a leaded glass panel, to allow for expansion and contraction. One must also recognize if the size varies throughout the frame and make allowance for this as well. With dalle de verre, you need a deep rebate or groove and allow at least 5mm in both directions for expansion.

5. The depth of the rebate or the width of the groove are also critical measurements. To allow for a proper installation, allow a minimum of 13mm to be added to the thickness of the panel to provide room for a proper putty fillet.

Based on comments from Art Femenella

When measuring older openings and especially doors, measure the diagonals in addition to all the other measurements. This provides a check of all your other measurements and also tells you whether the opening is a true rectangle or parallelogram.

Measuring a Rectangular Opening

Measure sight and tight sizes at top and bottom, and left and right. You should also measure the middle of the horizontals and verticals in addition to the details of the rebate.

Measure the diagonals to determine if the opening is “square” - all angles square. If these measurements are equal or +/- 5mm you can consider the opening to be a rectangle.

With bigger variations you may set out the cartoon using the measurements for the opening. Still, you need to know where the right angles are, if there are any, to be able to set out the cartoon to properly fit the opening. You can check for ”squareness” with a try square, although that is not completely accurate. If it is difficult to determine where the right angle(s) are, you need to take a template of the opening.


If it is not possible to tell where the right angles of the opening are, a template is called for. The material to be used for taking templates should be stiff, easy to cut, unaffected by moisture, and relatively inexpensive. This eliminates paper and some cardboards. If you can find stiff corrugated cardboard this works well. Mounting board works well too, but is expensive. Foam board is excellent, but also expensive. Hard board or other thin pressed board is inexpensive but difficult to cut with a knife. Thin plywood is also a good material for templates, especially if the opening is relatively regular. The more complicated the opening, the more cardboard, mounting board, or foam board becomes useful for its ease of shaping to the opening.

What ever material you use, you must mark which is the interior and exterior and for further checks, which is left and right. Fit this template into the opening to make sure it fits into the opening smoothly. This template will form the external extent of the built window when it is installed into an opening with a rebate.

Where the window is to be fitted into a channel, as in stone, you need to make the template of stiff material so you can determine the panel can be installed and that there will be enough of the panel within the stone channels to ensure the stability of the window in the future and still be able to manipulate the leaded panel into the opening.

Irregular rectangles

If you have found or can see that the opening is not a true rectangle and cannot determine where any right angles are, you need to take a template.

The objective is to make a piece that will fit into the opening without bending or being too small for the space. It will be the same size as the finished panel and so you will be able to put the finished panel into the opening without needing to trim or expand the panel.

  • First, trim the sheet of material you have chosen to use to a size a little larger than the measured size. Place the uncut side along one of the long sides of the opening. If the opening is a portrait format, place it on the right or left side as convenient to you.
  • Next, adjust the bottom by marking a line on the sheet. This is where a second person is very useful. One person can hold the sheet in place on outside of the opening and the other do the marking from the inside –in the case of the rebate being on the outside and vice versa if the rebate is on the inside. The marked line should be as close to the edge of the rebate as possible. The special case of an opening in stone will be dealt with separately.
  • Then take the sheet to a place where it can be safely cut. A long metal straight edge and craft or “Stanley” knife are often the best aids to cutting straight lines. Replace the sheet into the opening after cutting, and make any adjustments to the size and angles of the sheet at the bottom by marking and cutting as necessary.
  • When the side and bottom are adjusted, start on the other side. Proceed as for the bottom.
  • When the side is finished, start on the top.
  • Finally, present the whole sheet to the opening to make sure it slips into place with no snags, or bending of the sheet.

It may be that the opening is too large for a single sheet. In that case you will need to work with two or more sheets and try them together for the final fitting into the opening. You can put them together in the window. You can fasten them together with tape or other fasteners to make one sheet. You can also make two parallel lines both at angles and at intervals across the sheet so that when you get back to the studio you can exactly reproduce the full sheet by matching the marks and then firmly fastening them together. This makes transport of large templates much easier.

You will know that a panel made to a template made in this way will fit into the opening, no matter how irregular the opening may be.


Occasionally the window is circular and sometimes an oval. In both cases a template is important. The circle rarely is exact. Take the template in the normal way and then ensure you mark the verticals and horizontals for the opening. You often can use the jointing in the woodwork to help with these. Also mark any other reference points from the opening. Finally, mark which is the outside and which the inside.

This procedure will ensure that you will be able to fit the panel into the opening.

Round headed openings can be considered as a special case of a circle.

The horizontal you must find is the shoulder of the window. This is the place from which the curve springs on each side. The opening is generally vertical up to this point and then the curve begins. In stone, there is most often a joint at this point. This is often the case in wood too, if you can find the joint under the paint.

You need to make sure you have marked where this shoulder is on the template. You should indicate any reference points from the frame onto the template.

The join to the lower part of the window must be made obvious. Normally there will be an overlap between the lower rectangular template and this approximate half circle. You need to mark where this overlap occurs, if you do not fasten the two sheets together. This can be done by marking across the two sheets in a few places. This will enable you to join them exactly back at the studio.

Stone openings

When the opening is in stone, slight variations occur in the process of taking a template. The main difference is that the rebates are concealed. The rebates are slots into the stone. Thus, the template must slip into the slotted rebate. In these cases, the stiffer the material being used to take template, the better. Usually, thin plywood is the best material, as it has to be manipulated many times and in ways similar to the final panel.

Things are further complicated, as tracery is more common in stone than in timber framed openings. A complex opening shape may require two or more parts to enable the panel to be inserted. The taking of a template will help greatly in figuring out how the panel will be inserted into the opening.

Additionally, when the template is in position, you should mark the visible portion of the opening onto the template. Mark which is the inside and which the outside. Finally, mark on each template which side has the deeper slot as this will help in installation.