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Chain drilling

There are often times when a bandsaw would be useful but one is not available. This is when a piece of material needs to be sawn off a larger piece. The solution to this is chain drilling. This not only does the job but saves space in the workshop as well as the cost of replacing saw blades.

This method works on pieces of material that have a flat top surface. This means any sheet type material that can be drilled even when it is quite thick can be “sawn” up.

The trick is that where the “saw” cut would have been, as series of holes is drilled next to each other. When all of the holes have been drilled then a hacksaw is used to saw through the thin pieces of metal that are left between the holes.

If done properly the metal between the holes is so thin that the sawing becomes a trivial task.

Thickness of material

If the metal is very thin it is quite easy to saw it and there is no advantage in chain drilling it. This would depend on how hard the metal was but in general anything less than a few millimeters in thickness is probably better done with a hacksaw even if that means using just the blade because the cut is too deep for the whole hacksaw to be used. In this case wrap some insulating tape round one end of the blade to form a handle.

If the material is thicker than this then the answer is chain drilling.

The size of the drill to be used

If the size of the drill bit is, say, 3mm dia then a very large number of holes needs be drilled. If a large drill is used, say, 12mm, fewer holes are needed to be drilled but a significant amount of material might be lost.

To minimise the metal that needs to be sawn requires the metal between the holes to be minimised. It seems easier to do this with a larger drill than a small one. Also with a larger drill there are less bits between the holes to saw.

As the material gets thicker a larger drill will drill a deep hole more easily with less pecking needed to remove the swarf.

The plan is to drill the holes just short of touching each other. In practice, with a large drill, it is possible to drill holes that do touch. The problem is that if they touch too much the drill will not drill in a straight line but will veer completely off course and it might not even be possible to finish the hole. In this case it leaves a large amount of metal to be sawn by hand.

If a row of holes is drilled from one side of a piece of metal to near the other side there is bound to be a bit of metal left that for which the current drill bit is too big. It is not worth trying to drill a hole unless there is going to be some metal on both sides of the drill bit. The solution that is well worth doing is to measure the width of the metal left and pick a drill bit specially for just this last hole.

fig edge showing use of smaller drill to finish

The other snag with using a large size drill bit it that after the cut has been made to metal is left with a very ragged edge. If this can be milled or machined in some other way this is not a problem. If it has to be filed then a smaller drill will mean less filing.

Chain drilling a round edge

If the holes are not in a straight line then the saw will have to turn a little bit between each hole. As the holes gets smaller this becomes a problem. Ideally the holes will have to be least half the width of a hacksaw blade in diameter.