To enlarge an existing hole with a hole saw, use a specialized rig or technique to center the hole saw since the pilot drill bit won't have anything to grip initially.
Here's a breakdown of the process, based on the provided (though limited) reference:
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Secure the Rig: The key is a rig that fits inside the existing hole. This rig will hold the hole saw centered while you begin cutting.
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Insert the Rig: Insert the centering rig into the hole you want to enlarge.
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Drill Slowly and Steadily: Begin drilling with the hole saw. The rig will keep the saw centered until it starts to bite into the material. The twist of the pilot bit might help secure the rig further as you drill.
Why is this necessary?
Without a pilot hole to guide it, the hole saw will "walk" or wander, making it impossible to create a clean, centered, and accurately sized larger hole. The rig provides that initial centering and stability.
Alternative Techniques (not explicitly mentioned in the limited references, but common practice):
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Using a Plug: You could tightly fit a wooden plug into the existing hole and then drill your pilot hole into the center of that plug. Then use the hole saw as normal.
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Creating a Template: Create a template from wood or metal with the desired larger hole size cut into it. Clamp or secure the template perfectly centered over the existing hole. Use the template to guide the hole saw.
