Somewhere in our past discussions we were talking about drill sizes for different percentages of thread engagement. There are formulas to calculate & on-line tools, but I found mention of this chart on another forum.
http://www.guhring.com/documents/Tech/Charts/CutTapDrill.pdf
Why would you want to know this? Typical tap/drill charts assume 70-75% thread engagement. People assume they are the gospel, but they are just the norm for many common alloys & screw sizes. But when you are dealing with tougher alloys like stainless or tool steel, you might have difficulty tapping so its generally advisable to enlarge the hole. This doesn't replace good taps, correct fluid & correct tapping procedures. Its just about hole size & resultant geometric thread engagement. On the flip side, other materials (possibly plastics? no practical experience here), you can reduce the hole size for slightly higher % engagement.
http://www.guhring.com/documents/Tech/Charts/CutTapDrill.pdf
Why would you want to know this? Typical tap/drill charts assume 70-75% thread engagement. People assume they are the gospel, but they are just the norm for many common alloys & screw sizes. But when you are dealing with tougher alloys like stainless or tool steel, you might have difficulty tapping so its generally advisable to enlarge the hole. This doesn't replace good taps, correct fluid & correct tapping procedures. Its just about hole size & resultant geometric thread engagement. On the flip side, other materials (possibly plastics? no practical experience here), you can reduce the hole size for slightly higher % engagement.
