Using a piercing saw

For one-off parts, there is a lot to be said for cutting by hand which, unlike most other methods, requires almost no setup time. The tools for the job are the piercing saw and saw table. A piercing saw (occasionally called a jewellers saw) is a frame saw which takes fine blades that are clamped at both ends, and are held in tension by the spring of the frame. With a piercing saw it is possible to cut intricate shapes, and holes and shapes within the metal.

Blades are available from tool suppliers, but there is a range of qualities. Avoid cheap blades, they are a false economy. Often they do not cut straight, and break easily. Good quality blades are visibly better formed, are separate, retain their sharpness, and last much longer. Buy from companies supplying jewellers, watchmakers and clockmakers rather than general-purpose modelling suppliers. Buy reputable brands and avoid “no-name” blades. Blades come in a range of sizes from no. 8 (the largest) through 0 to no. 6/0 (the smallest). Important dimensions are listed in the table.

Some suppliers have started to list blades by number of teeth per cm rather than the traditional number system.

In selecting blades, the teeth/cm is the most important dimension. One should aim to use a blade that has at least three teeth in contact with the material at any time. A moment's thought will tell you that rule will not work in a lot of cases. The smallest blade has 30 teeth/cm, or 3 teeth/mm, which means that the blade is theoretically too coarse for any material thinner than 1 mm. Etched kits use material thinner than this, and scratch builders regularly use material as thin as 0.25 mm (and even thinner on occasion).

Fortunately, the rule is not hard-and-fast, and with a little care, thinner materials can be cut. Very thin material can be cut if it is placed on a backing sheet of soft material such as brass, nickel silver, plywood or plastic sheet. It is not necessary to maintain stocks of all blade sizes. A thin blade (4/0 to 6/0) is adequate for most work in O gauge. It is worth keeping perhaps two thicker sizes, about sizes 0 and 4, for the occasions when it is necessary to cut thicker material.

Make sure you have the blade the right way round – the teeth point downwards and outwards from the frame. That may sound obvious but the teeth of the very finest blades are not always easy to see clearly. Loosen the thumb screw at the top of the frame, insert the blade and tighten the thumbscrew. Loosen the bottom thumbscrew and insert the bottom end of the blade. Push the frame up against a table as you tighten the screw. When tight, release the frame and the blade will go into tension. Pluck the blade and if you hear a “ping”, it is ready for work.

The frame can be adjusted to accept blades of different lengths. The adjusting screw should be set before fitting the blade.

To cut reliably, the material must be held firmly without wobble or vibration. At the same time, it should be possible to manoeuvre it to cut curved shapes and corners. The saw table should ideally be steel or aluminium of about 8-10 mm thick, and it should be firmly bolted or clamped to the bench. A cutout means that the material can be supported on both sides of the cut during the cutting process. For soft materials such as brass and nickel silver up to about 1 mm thick, finger pressure is sufficient to hold it on the saw table and allows it to be moved around easily as the cut progresses. For thicker and harder materials it may be necessary to clamp the material to the saw table.

To cut, hold the blade vertical, or nearly so (a slight forward lean is acceptable), and start with an upstroke to make a little notch in the material. Locate the blade against one edge of the saw table or against your thumbnail as you do so. You should be able to follow that immediately with a downstroke and continue cutting. If the blade snags and will not come down, repeat the upstroke until it does. Apply just enough pressure to keep cutting, excessive pressure simply leads to breakages. Let the saw do the work. Try to develop a smooth rhythm and do not rush.

As with any cutting operation, lubrication helps. Almost any lubricant will do. Wax from the stub of a candle or a wax crayon, wiped on the blade when necessary, is a good mess-free option. You will be amazed how much easier cutting becomes the first time you try it. You may find yourself holding the frame at a small angle to the line you are cutting. That is quite usual. Even the best blades do not cut completely symmetrically, and each blade is different in that respect. When you start a cut watch to see if the blade tries to deviate left or right from the line and adjust the frame to suit. If the angle is more than a few degrees, that is a sign of a defective blade, usually from a cheap source, that is not worth persisting with.

It is possible to cut corners of very small radius with care. As you approach the point where the corner is to be cut, remove all hand pressure to stop the blade moving forward but maintain the up and down motion. Slowly and carefully, begin to rotate the workpiece about the blade. The blade should then cut its own corner. If it starts to twist or jamb you are most likely rotating the workpiece too quickly. Stop, apply some lubricant, and start again, going slowly.

If you are uncertain about this, or have to cut a corner in a thick piece of material, drill a small hole in the corner that allows the blade to turn.

Nick Baines, March 2021