Cramps and Cramping
Wednesday 29 October 2008
Robert Ingham says planning is the secret to good cramping
Many arts and crafts pieces, particularly cabinets, had a logic of assembly that employed the simple strategy of assembling all components in one direction, with all cramp pressure being applied in one direction, too. When you consider the very short open assembly time of hot melt animal glue this simple logic makes a lot of sense. I sometimes shudder at the prospect of such an experience and remember asking a senior cabinetmaker how he tackled the problem. His answer was: teamwork... get as many pairs of hands to help as you can and then make sure that everyone knows what to do and when, as with gluing up you do not get a second chance.
Design considerationsWith today's new and unusual designs, assembly and gluing up must be given ongoing consideration during the detailed design stages. It is possible to predict a lot of potential problems as early as at the drawing stage.
If you employ a radical construction, the value of a mock-up or a test joint gives you the opportunity to glue up and test the construction dynamically. It might even back up your product liability insurance if the construction is to be included in the making of a chair, for example. It will certainly enable you to work out the sequence of assembly, bearing in mind such important factors as directions of pressure and clamping blocks to distribute load efficiently. In short, the motto of gluing should be: plan and leave nothing to chance.
Cramp costsWhile it is true that you can never have too many cramps, economics often dictate otherwise as cramps can be expensive. In reality, it
is often necessary to compromise, and applying some thought and discussing your requirements with established cramp makers can save you money.
Base your choice of cramps on the type of work you are planning to do, not forgetting that you can expand your collection as you go along. It is also worth shopping around to get the best deals. Look out for special deals at trade shows, where discounts are often offered to encourage people to attend.
Remember the maxim: 'nothing ventured nothing gained'. Ask for a discount if you are buying more than just a couple of items. I once went to a tool supplier intending to buy four 24in (600mm) sash cramps and came away with a dozen after negotiating a substantial discount. There are cramps for all kinds of situations. It takes time to build up a comprehensive collection and buying some of the specialist cramps can be put off until later.
Sash crampsProbably the most indispensable cramp is the sash cramp. These are available in sizes from 24in (600mm), increasing in 6in (150mm) increments to 6ft (1,830mm). The ultimate goal is to have a selection of all the sizes, but a realistic start would be to think about the scale of work that you envisage carrying out and buy accordingly.
Also, bear in mind that sash cramps are used in pairs in many situations and that some sizes are used more often than others. I would recommend investing in six 24in (600mm) and six 36in (900mm) to start with.
When used in cabinetmaking, it is not necessary for sash cramps to have heavy-duty bars or T-section bars as joints should not be so tight as to require the extra force. However, if you require increased capacity it is possible to bolt two cramps together to achieve this. Use two bolts and screw the two cramps together so that they behave as one.
New sash cramps need to be cleaned thoroughly as they are liberally covered in oil. Be sure that you clean the screws, too, or you can be certain that oil will get onto that beautiful sycamore surface! Alternatives to the sash cramp are available, particularly the well-engineered deep- jawed versions that are popular in Germany and America. I personally don't favour these for assembling carcass furniture because of their excessive bulk.
In addition, I like the inner edge of the bar to contact the surface of the component being cramped and this is more difficult to achieve with deep jaws. Because it is impossible to tighten up all the cramps involved at the same time, in almost any situation that requires the internal corners of an assembly to be square it is likely that some adjustment will have to be carried out. This is done by moving the cramps towards the longer of the two diagonals. It is easier to do this progressively if one point of contact with the outside face of the assembly can be maintained. This is much more difficult to achieve with deep-jawed cramps.
However, with the inner edge of the bar in contact with the component it is quite likely that it will also come into contact with the glue, resulting in a rust mark on the surface. This can be prevented by covering the bar with masking tape. Better still, I use pieces of extruded plastic channel, which I cut from file binders. This U-shaped strip of plastic can be bought from any office supplies or stationery shop. The plastic strip can be slid along the bar to any desired position to prevent contact with glue, thus avoiding any rust on the cramp or stain on the wood.
Making your own cramps standsIt is also worth making cramp stands. The jaws of sash cramps have a small foot that is intended to prevent the cramp from falling over sideways when placed on a work surface. Sadly, the foot is often not wide enough and whole sets of cramps fall over at the most inopportune times. I have made my own cramp stands from MDF squares with a 1/4in groove (cramp bars are still made from imperial stock) routed into the face. It is easier to rout a groove into a strip of MDF 2in x 1/2in (50mm x 12mm) and cut it into squares after routing the groove.
While talking about sash cramps I should mention cramp heads, which are bought in sets for which you make your own bars from wood. While this might seem an inexpensive way of solving the problem you will find that they are bulky and cumbersome to use.
G-crampsG-cramps are extremely convenient for clamping small components together and for generally holding down components while working on them. Drop-forged from steel, with a powerful screw, they are available in a variety of sizes from 1in (25mm) upwards. I have some monster G-cramps with a 12in (300mm) capacity, which I use for laminating, where the force needed to hold thick laminations in tight curves is difficult to achieve with a bag press.
Very often the appropriate size of clamp depends on the scale of work being held, which suggests that owning a selection of G-cramp sizes should be the long-term aim of most furniture-makers. Deep-throat G-cramps are also available but my preference is for the more recently popular F-clamps.
F-clamps have also been popular in Germany and America, and have been manufactured by leading German makers for quite some time. They are now readily available in this country, too. The main advantage of F-clamps is the speed and ease with which they can be adjusted to suit the capacity being clamped. Unfortunately, over a period of time, the bar serrations wear and the grip is liable to slip. It's frustrating - to say the least - when you come to take the clamps off after the glue has set, only to find that the joint was not fully closed. This problem is now being addressed by some makers with a redesign of the bar and its serrations.
F-clamps are also available with deeper throat capacities than G-cramps, which make them more suitable where pressure needs to be applied well in from the edge of a component. Be prepared to pay accordingly for the deeper-throated sizes. Another practical advantage with F-clamps is the provision of plastic shoe covers, which very often obviate the need for protective clamping blocks. One word of warning: when buying F-clamps, avoid the economy versions sold from market stalls. They are flimsy and badly made!
Wooden-jawed cam clampsFor lightweight holding, Klemmsia wooden-jawed cam clamps are a good substitute for metal G-cramps and F-clamps. They are quick and easy to set and are ideal for holding down components, but are not really strong enough for gluing up where sustained force is required. Because these wooden Klemmsia clamps are very light in weight they are ideal when the combined weight of components and clamps would cause distortion to the assembly.
Quick-gripIn recent years an assortment of quick-grip and single-handed action clamps have become available. They come with accessories offering a versatile range of options. They can be useful for occasional use but, in my opinion, are not a suitable alternative to G-cramps and
F-clamps for sustained application of pressure when gluing up a construction.
Edging crampsI use veneers for most of my carcass pieces. The edges of man-made substrates require lipping, sometimes before veneering and sometimes after veneering. For straight-edged situations the lipping can be glued on and pressure applied with sash clamps. If the lippings are curved, however, gluing up becomes more complicated. To avoid curved cramping blocks I use edging cramps.
I am not keen on the type of edging clamp that has a G-clamp with a secondary clamping screw set at 90 degrees. The hold of the G-cramping action tends to slip and the edge clamping direction, because of its fixed position, is not always centred on the edge of the component.
A few years ago I came across the definitive edging clamp from Wurth. This is a well-engineered clamp, consisting of a sturdy diecast frame and a pair of double-acting cams, faced with rubber. The cams open like jaws to hold the thickness of the component, while a screw-driven foot applies pressure to the edge. The foot is virtually self-centred on the edge, despite the alignment of the cam jaws. It's ideal for applying pressure to a curved edge. With a little bit of imagination, you can use these clamps to apply pressure from one side to more situations than just edgings. Although fairly costly items, their versatility offsets the price.
Edge press clampsFor edge jointing solid wood to produce wide panels from narrower boards, most craftsmen use sash cramps. With clamps alternately placed at intervals applying pressure to the edges from above and below the boards, the work can be held flat at the same time. I have a set of six Plano press clamps for this purpose.
These clamps hold the boards flat and align the faces while applying pressure to the edges. The clamp consists of two extruded aluminium bars that press on both faces of the boards. When edge pressure is applied from the end-screw, the bars in turn apply pressure to the faces, ensuring surface alignment, producing perfect butt joints, without the need for splines or biscuits. These clamps are definitely only justifiable if you are doing a substantial amount of butt-jointing in solid wood. Axminster Power Tool Centre's panel clamps - though not as sophisticated - perform the same function at a more modest cost.
Other clampsThe clamps i have described form the mainstay of my arsenal of clamping hardware. I also have six jet clamps, which are ideal in situations when pressure cannot be applied due to the screw being restricted by the position of other components. A typical example would be a partition in a box. Jet clamps apply pressure from the outside, rather than the end of the clamp.
For small, lightweight, additions I use spring-cramps. These operate like bulldog clips used for organizing paperwork. They are versatile and simple to use, with an amazing amount of strength in their springs.