The Ford Barn

mcgarrett · 07-25-2025, 08:31 PM

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.

alexiskai · 07-25-2025, 09:42 PM

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.

mcgarrett · 07-25-2025, 10:01 PM

Quote:

Originally Posted by alexiskai

I think it’s two different things stuck together. Right side is a sizing gauge to telu 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.

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.

AzBob · 07-25-2025, 10:51 PM

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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

07-25-2025, 09:42 PM	#2
alexiskai Senior Member Join Date: Apr 2018 Location: Mebane NC Posts: 2,848	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.

07-25-2025, 10:51 PM	#4
AzBob Senior Member Join Date: Oct 2018 Location: central AZ Posts: 200	Re: OT - need machinist to explain how to use this gauge Sponsored Links (Register now to hide all advertisements) 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 Last edited by AzBob; 07-25-2025 at 11:46 PM.