Rework

Deep hole drilling is a high-risk process. When something goes wrong – a drill breaks in the bore, the bore wanders off position, or the surface finish is out of spec – the part is not necessarily scrap. I have salvaged thousands of dollars worth of components by knowing how to repair and rework bores. The key is knowing when a part can be saved and when it cannot. I have a simple rule: if the repair costs less than 60% of the replacement cost, it is worth doing. Anything over that and the customer is better off starting fresh. ...

Problem Description A bore drilled off position is a costly problem in deep hole drilling. I’ve had parts where the bore exit was 0.5 mm off location on a 20 mm diameter hole through 300 mm of 4140 steel. The part was otherwise perfectly machined, but the positional error made it scrap unless I could rework it. In deep hole drilling, bore position errors are especially painful because so much machining time has already been invested when the error is discovered. ...

Every shop drills a crooked hole eventually. The question is not whether it will happen, but what you do when it does. I have a simple framework for deciding whether to salvage an off-center hole. It comes down to three things: how far off the hole is, how much material is left around it, and what the part is worth. Step 1: Measure How Bad It Is Before I decide what to do, I need accurate numbers. I measure the hole position at both ends and, if possible, at the midpoint. ...

Assessing the Breakage Before Extraction Every deep hole driller breaks tools eventually. When a gun drill breaks in the bore, the first step is not to panic. I assess the situation before deciding how to extract the broken tool. I ask myself three questions: where in the bore did it break, how deep is the broken piece, and is the tool jammed or loose. The location of the break matters more than most people realize. If the tool broke near the entry, I have more options. A break at the far end of a deep bore changes my strategy entirely. I use a borescope to examine the break point before committing to any extraction method. In my experience, thirty minutes of inspection saves hours of trial-and-error extraction work. ...

When Repair Makes Sense Not every bad hole is scrap. I try to repair a bad hole before scrapping a high-value part, especially when the part is large, complex, or made from expensive material. Repairing a deep hole is almost always cheaper than scrapping a part that may have hundreds of dollars of material and hours of machining already invested. The decision to repair depends on three factors: how much material can be removed, how deep the defect extends, and what tools are available to reach the defect. I have successfully repaired holes up to 2 meters deep, but only when the right tooling was available. ...