There is at least one situation in luthiery where you need to drill and maybe re-drill or enlarge existing holes. If you have ever attempted to replace worn out vintage tuners with new better quality tuners you soon discover that although each tuner post fits its assigned hole by itself, all three posts won’t slide all the way down into the gear head. This is frequently true for slot head guitars.
Part of the problem is that the newer tuners have thicker posts. But just free hand enlarging the holes will not solve the problem. So a drill bit guide is what we need.
Below is an image of two Stanley No. 59 drill bit guides. The top version is cast aluminum. The bottom version is cast iron. You want the one on the bottom, not the one on top. Cast iron is the thing. These are not available in stores. The cast iron version frequently shows up on eBay.
Notice in the image below that every mating surface is beautifully machined. Also, the two little thumb screws are entirely sufficient for tightening the guide clamp (thick bent wire) and the sliding bit guide. The aluminum version requires pliers to snug down the sliding pieces.
To use, assemble the guide with the 1/4 inch bit guide. Use a smaller bit to align the guide with the slot head hole. Then use the appropriate sized bit to enlarge the hole as needed. You will have to experiment with various drill bit sizes and alignment schemes for best results. Practise on scrap wood first.
The Stanley No.59 facilitates hole realignment and simplifies re-orienting the holes as needed. New tuners just about always need some sort of hole enlarging and re-aligning. The Stanley No.59 is great for this.