Spot Drilling – Get it Straight

If you’re going to drill a hole, it stands to reason that you want the hole to be straight and accurate. There are many ways to accomplish this, but all involve getting the hole started properly.

The main obstacle standing between you and drilling a straight hole is drill walk—the amount the tool deflects from a straight path. Avoiding this usually involves starting a hole with the shortest drill possible followed by successively longer drills until the desired hole depth is attained.

The hole is started by “spot drilling” the workpiece, which involves making a dimple in the face of the part with a very stubby and rigid drill known as a spot drill. This dimple minimizes drill walk by capturing the point of the subsequent drill as it enters the cut.

Quite often, spot drilling is unnecessary. A solid-carbide drill usually doesn’t need to be spotted because it’s very rigid and resists walking. The same holds true for indexable carbide drills. In fact, spot drilling the workpiece before using this type of drill will usually cause the inserts to chip or break.

Another drill that typically needs no spot drilling is the screw-machine-length drill, also known as the stub-length drill. It is so short that spot drilling is unnecessary, assuming that the hole to be drilled is shallow and part tolerance permits.

One reason shops avoid spot drilling is because the process is time consuming. Assume that you have an order for 5,000 parts made of 303 stainless steel. Two 3/8″-dia. holes 1½” deep need to be drilled in each part. A solid-carbide drill would perform admirably in this situation, but you may have a tough time convincing your boss to spend $30 for one when you could buy a dozen HSS drills for the same price. This will force you to use a jobbers-length (medium-length) drill, which requires spot drilling.

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