Oliver Waters and Jim Cooper share a few techniques for establishing reliable layout lines for crisp joints
Students on a one-year course at Waters & Acland spend the first few weeks mainly focusing on tool preparation and their respective use in combination with a variety of measuring and marking equipment. To develop their skills they are required to build a Chinese puzzle as a precursor to tackling a host of decorative dovetail joints. In this article we’ll be looking at why stock preparation is key to establishing lines you can trust and why measurements alone aren’t always the best indication of size.
Stage 1
The students are provided with a length of stock machined to a 20mm square cross-section. Their remit is to use the supplied stock to produce all the components necessary to make the Chinese puzzle. Accurate marking-out, cutting and chiselling are essential in order to produce a tightly fitting puzzle. Too loose and the parts won’t stay together, too tight and it won’t go together at all. The first stage requires the students to produce six blanks exactly 80mm long. It is essential that all the ends of the blanks are square and that they are exactly 80mm long, as the ends will be used as reference for marking out. This is best achieved using a shooting board, making checks along the way with a pair of digital dial callipers. Although only six blanks are required to make the puzzle, students are encouraged to produce a couple of extra ones in case of mistakes later on in the project; itself an excellent habit to get into. The next stage is to mark out all the components, the dimensions of which are shown in the drawing, creating one each of type A and D and two each of type B and C. At this stage the components should be marked out using a very sharp H2 pencil and an adjustable square. These lines will not be used for cutting to, but rather to act as a guide for where and how long the marking gauge and scalpel lines will be. This minimises the risk of unsightly gauge or scalpel lines extending beyond the ends of cut lines and to avoid cutting the wrong side of a line. It is also good practice to introduce the habit of identifying the waste areas with some cross hatching or light shading.
The next stage is to mark all the cut lines using a marking gauge or scalpel. Different techniques are used to do this depending on whether a side or an end of the blank, or a line on another face of the blank, is used as the reference for marking. For cut lines across the length of the component (a,b,c,d,e,f,g,h,i and j), where the end of the component is used as the reference for marking out, a marking gauge is used to lightly mark the centre of the cut line and a scalpel is then used to mark the rest of the cut line, using the marking gauge mark as a reference.
For cut lines along the length of the component (p,q,r,s,t,u,v,w,x,y and z), where the side of the component is used as the reference for marking out, the marking gauge is used to lightly mark the cut line over its whole length. For cut lines which are extensions of already existing lines on other faces of the component (aa,ab,ba,bb,ca,cb,da,db,ea,eb,fa,fb,ga,gb,ha,hb,ia,ib,ja and jb), a scalpel is used to extend the line from the existing line to the new face. Note the two type B components are initially worked to be the same as type D components, and the lines ca, da and q are then marked prior to modifying them to make type B components. The key to successful marking out is accuracy. Good marking gauge and scalpel technique are essential in order to obtain the required level of accuracy.
Making gauge technique
Three different gauge settings (10mm, 20mm and 30mm) are required for marking all the cut lines. If you are using a single marking gauge, the gauge should be set to the first of these lengths (10mm), using digital callipers to ensure accuracy, and all lines requiring this setting then marked (lines p,q,r,s,t,u,v,w,x,y and z). The marking gauge should then be reset to the second length (20mm), once again using digital callipers and all lines requiring this setting then marked (lines a,b,e,h,i and j). Finally, the gauge should be set to the third and final length (30mm) and all lines requiring this setting then marked (lines c,d,f and g). If you have more than one marking gauge it is useful to retain the gauge settings, as this makes it easier should you need to mark additional lines at a later stage, for example if you need to remake one of the components.
Using digital callipers to measure the 80mm component
Setting up a gauge for success
If you use a knife-style marking gauge it needs to be set up with the bevel on the waste side of the line being marked. This ensures that there is a clear vertical line defining the edge of the waste area. Wheel-style gauges can be similarly orientated. A pin-style gauge needs to be filed to a very sharp point.
Scalpel technique
A scalpel blade has two symmetrical bevels. Consequently, when marking a line with a scalpel, the scalpel needs to be held at an angle so that the bevel on the non-waste side of the cut is vertical. This gives a clear vertical line defining the edge of the waste area. The scalpel is used in two distinctly different situations; to extend the small mark made by the marking gauge in the centre of some of the cut lines, and to extend already existing scalpel lines to other faces of the component. In the first case, where the scalpel is used to extend the small mark made by the marking gauge in the centre of a cut line, the scalpel blade should be placed in the centre of the mark made by the marking gauge, with the scalpel blade angled so that the bevel is vertical on the non-waste side of the line. An engineering square should then be placed firmly against the scalpel blade (on the non-waste side of the line) and the scalpel used to extend the line to the required length.
In the second case, where the scalpel is used to extend already existing lines to other faces of the component, the scalpel should once again be angled so that the bevel is vertical on the non-waste side of the line and the blade then placed into the end of the line already existing on the other face of the component. An engineering square should then be placed firmly against the scalpel blade (on the non-waste side of the line) and the scalpel then used to extend the line onto the new face.
Before using the scalpel to mark out the components, it is strongly recommended to practise your technique. Using a scrap piece of the original 20mm square stock, start off by scribing a scalpel line across one of the faces using an engineering square to position the scalpel and angling the blade correctly so that the bevel against the square is vertical relative to the face being marked. Now, extend this line to the other three faces of the component using the same technique. If you have done everything correctly, then you should find that the mark on the fourth face lines up perfectly with the line on the first face. If this is not the case, then you should repeat the exercise until the lines on the first and fourth faces do indeed line up.
Troubleshooting
If, after successive attempts, you cannot achieve perfect alignment you may need to check the accuracy of your square or the squareness of your blank.
Cutting out the waste
The next stage is to remove the bulk of the waste material, prior to chiselling to the lines. A dovetail or similar saw should be used for vertical cuts and a fine-bladed coping saw for horizontal cuts. This is your last opportunity to check and identify the waste area. Once again, it is a good idea to practise your technique before starting on the actual components, and if possible in the same material as the finished object. Mark some vertical and horizontal lines and practise cutting 1mm away from the vertical lines (using a dovetail or similar saw) and 1mm above the horizontal lines (using a fine-bladed coping saw). You should also practise moving from vertical to horizontal, ensuring that you remain within the waste area. Once you are happy with your technique, it is time to move on to wasting out the actual components. If you find it easier, you can make additional pencil lines 1–1.5 mm inside the waste areas and use these to saw to.
Chiselling to lines
The next stage is to chisel to the scalpel and marking gauge lines. For each face, start by making relief cuts with the chisel in the waste area some 0.5–0.7 mm away from each of the scalpel lines defining the face. For example, for the face defined by cut lines j, ja and jb, relief cuts should be made in the waste area some 0.5-0.7 mm away from the scalpel lines defining cut lines j, ja and jb. To do this place the chisel on the component, with the bevel side of the chisel towards the waste area, angling the chisel slightly so that the cut is angled away from the non-waste side of the line and then tap the end of the chisel with a mallet. Next, using a chisel slightly narrower than the width of the face, remove the ‘tent’ (raised area created due to the angling of the chisel). The next stage is to make the final cuts to the scalpel lines defining the face. The same process is followed as for the relief cuts. However, this time the blade of the chisel should be accurately positioned in the scalpel lines defining cut lines j, ja and jb.
Finally, once again using a chisel slightly narrower than the width of the face, remove the ‘tent’. Once the final cuts have been made to the scalpel lines, it is essential to only use chisels narrower than the face being pared, and to ensure that the edges of the chisel blade do not contact the crisp straight edges that you have just created. You should regularly check for both squareness and flatness of the faces that you are chiselling using the back of a chisel or a square. At this point, if all has gone well and you have produced components to the required accuracy, then they should now come together to produce a tightly fitting puzzle.
