OT - need machinist to explain how to use this gauge
 

I have several Starrett precision machinist tools and know how to use most of them, but I'm a little shaky about how to interpret and use this drill gauge. Any guidance would be appreciated.

Re: OT - need machinist to explain how to use this gauge
 

I think it’s two different things stuck together. Right side is a sizing gauge to tell you what gauge a wire is and the decimal equivalent in inches. Left side tells you what size body drill bit and tap drill bit to use to tap a given thread pitch and size. The body drill bit is the one used to remove most of the stock, then you finalize the hole with the tap drill bit, then you tap the hole.

It’s confusing because your instinct is that the two sides should connect but I don’t think they do, it’s just two useful things on the same sheet of metal.

Re: OT - need machinist to explain how to use this gauge
 

Thanks for the feedback. Yes, I think you're right. There's definitely a lot going on there. I figured an accomplished machinist (which I'm not) could make better sense of it and know how to read and interpret the various scales and readings.

Re: OT - need machinist to explain how to use this gauge
 

The left side is telling you which drill size to use for a specific tap. Also the clearance (body drill) drill size for that particular screw thread to pass through. The holes on the right side are for finding the sizes of drills or wire gage sizes. A pretty basic machinists or toolmakers gage.

Here’s the modern version of your drill and wire gage. Still available from Starrett.
Link. https://www.starrett.com/details?cat-no=185