The lapper is smooth and has no fibers so the lug holes shouldn't be getting dragged by material. Problem is that if your angle is off, the lapper will drag on a edge and the bowling out occurs. It happens so fast that trained personnel have to resurface the case to basically start over again. Scary stuff when you're dealing with prized watches or vintage. If you've had some time behind the wheel, you can also correct bowled out lug holes by using the right amount of pressure and most importantly proper angle of attack. A lot of thought goes into cutting cases. Pros make it look easy though:)
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