I'm not sure why you would intentionally drill a hole smaller than what you would need, but you would then use the bit you used as the pilot, and rather than having just a few mm as a lead you have as much as you want. Works on the larger and smaller arbors.
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Thanks for the link , gonna pick one up, seen so many tools and gadgets since I registered her, I need to speak to the wife
Reason for drilling the smaller holes as I said was there's so many sizes and I didnt always know so drilled for the smallest standard GU10
Not involved in that type work these days