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Posted by D Sherman [72.47.9.228] on Friday, June 17, 2011 at 19:04:59 :
In Reply to: tapping steel? posted by perry [108.35.185.31] on Friday, June 17, 2011 at 18:50:45 :
What size screw are you tapping for? Every machinist's handbook and many larger tool catalogs have a "tap drill chart" that shows you what size to use. It's not necessary to use exactly the size they specify. For example, if you don't have a set of number or letter drills, the nearest fractional size is usually close enough. The charts are usually set up to give you roughly a 75% thread depth. You can go deeper than that, with a smaller drill, but the increase in holding strength is minimal and the change of breaking the tap goes way up. Or you can go to a bit less thread depth for easier tapping in a hard material that is probably strong enough anyway, especially if you don't have the exact right size drill. In general fine threads (SAE/UNF) are more critical than coarse (UNC)threads as far as the hole size.
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