- Joined
- Feb 13, 2017
- Messages
- 2,138
Wire sized drills fill many different functions. A Nr 30 drill at 0.128" leaves .003" for a .125 rivet. Basicaly 1/8 inch plus .003". A common clearance hole in the aircraft industry. Another is .1875 + .003 (3/16"). The end result is 0.1905, a really off the wall number. I do have a drill(several) this size. Rather tight spiral for deep cuts in aluminium.
Machine screws line up for many of the small sizes. A size "0-80" screw is 0.060". Add (or subtract) 0.013" for each screw number. ie. #2-56 screws are calculated as (2x.013")+060"=0.086". Smaller sizes work out negative. ie. A #00-90 is 0.060 minus 0.013" =.047" . A 000-120 (3/0) is 0.034" and a 0000-160 (4/0) is .021". There are smaller sizes but 4/0 is the smallest I use for my models. The thread pitches noted are the ones I use. There are others, just as there are for larger sizes. Of course, each tap size requires a different size drill.....
I don't know the reason for .060 plus or minus .013 for machine screws but it dates way, way back. At least into my grandfather's time. (He died in 1912....) A curiosity here; the screw that holds a wall plate on your room lights is a #6, 0.138" dia. (Tap size for #6-32 TPI is a #36) It also is 3.5mm, although I don't remember what the tap size is in metric.
Metric measurement goes back to 1820 or so. Thomas Jefferson was a big proponent of metrics. But England still was using the Imperial system and Congress wanted to maintain a good trade relationship with them. Progress was set on the "back burner" that time, for sure. (Pun intended......)
I've babbled on far too much here but you must consider the fractional inch measurements as somewhat primative as a system that was held onto because of politics.
Bill Hudson
Machine screws line up for many of the small sizes. A size "0-80" screw is 0.060". Add (or subtract) 0.013" for each screw number. ie. #2-56 screws are calculated as (2x.013")+060"=0.086". Smaller sizes work out negative. ie. A #00-90 is 0.060 minus 0.013" =.047" . A 000-120 (3/0) is 0.034" and a 0000-160 (4/0) is .021". There are smaller sizes but 4/0 is the smallest I use for my models. The thread pitches noted are the ones I use. There are others, just as there are for larger sizes. Of course, each tap size requires a different size drill.....
I don't know the reason for .060 plus or minus .013 for machine screws but it dates way, way back. At least into my grandfather's time. (He died in 1912....) A curiosity here; the screw that holds a wall plate on your room lights is a #6, 0.138" dia. (Tap size for #6-32 TPI is a #36) It also is 3.5mm, although I don't remember what the tap size is in metric.
Metric measurement goes back to 1820 or so. Thomas Jefferson was a big proponent of metrics. But England still was using the Imperial system and Congress wanted to maintain a good trade relationship with them. Progress was set on the "back burner" that time, for sure. (Pun intended......)
I've babbled on far too much here but you must consider the fractional inch measurements as somewhat primative as a system that was held onto because of politics.
Bill Hudson
