How do I make an existing hole bigger?

This is a small problem that behaves badly. The point of a twist bit is what locates it, and in an existing hole the point is in fresh air. The lips catch on one side of the rim, the bit deflects, and the enlarged hole is oval, off position and often torn. All three fixes work by giving the bit something to be guided by other than its own point.

Step by step

1. Decide whether the position must be exact.If the fixing simply needs to be bigger and roughly where it was, the guide methods below work. If the hole has to stay in exactly the same place to a millimetre, plug it and start again.
2. For wood, use a guide block.Take a piece of scrap, drill it with the finish-size bit, clamp it over the workpiece with the holes aligned, and drill through it. The block holds the bit concentric and it cannot deflect.
3. For sheet material, use a step bit.A step drill self-centres in an existing hole because the leading step is smaller than the hole and the shoulder above it enters evenly. It is the ideal tool for this in metal and plastic.
4. For masonry, step up in stages.Do not jump from 6mm to 12mm. Go through the intermediate sizes, letting each new shoulder centre itself on the previous hole. Keep the tool square, because a masonry bit in an existing hole wanders very readily.
5. Plug the hole to keep the position.Glue a hardwood dowel or a plug into the old hole in timber, or fill a masonry hole with a proprietary repair mortar and let it cure. Then redrill from scratch. This is the only method that guarantees position.
6. Never use a spade or Forstner bit to open a hole.Both rely on a full circle of material to steady the centre point. In an existing hole they grab and spin the work, which is genuinely dangerous with a large bit.
7. Clamp the workpiece.A bit that catches on a rim generates a sudden lateral force. Anything not clamped will move, and in sheet material it will spin.
8. Accept that an enlarged masonry hole is weaker.The material round it has already been fractured by the first hole and again by the second. If the fixing carries anything, move over and drill fresh rather than reworking the old one.

Tips

• A tapered reamer or a round file will open a hole a small amount by hand with far more control than any powered method, and is often all that is needed to make a fixing fit.
• When enlarging in a drill press, the workpiece is much more likely to be snatched than when handheld, because the machine does not give. Clamp it to the table without exception.
• If the problem is a hole that is slightly out of position rather than too small, a slotted hole cut with a round file lets the fixing move rather than requiring a bigger hole.

Common mistakes

• Going straight to the final size with a twist bit — The point has nothing to locate on, so the lips catch one side. The hole comes out oval, out of position and torn at the rim.
• Opening a hole with a spade bit — The centre point has no material to bite into and the wings grab. The bit snatches, the workpiece spins and the hole is ruined.
• Reusing an enlarged masonry hole for a heavy fixing — Two drilling operations have fractured the surrounding material. A fresh hole 75mm away will hold far more.

Questions people ask