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Drill tap chart with different % thread engagements

P

PeterT

Ultra Member
Premium Member
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.
 
There are special taps that are to be used in pairs - one is touch under size and the other is finishing. Without them in harder alloys it seems 50% thread engagement is good. Tapping anything harder then around 32 or so is next to impossible for standard thread engagement with single pass. Even if successful the tap will not last more then few holes.
 
Drill Chart is made I think 1914
Imagine type of Machines and tools they had.
Very wobble Drill Presses and HSS Drills.
Now Machines are way more rigid, and Carbide Drill is drilling holes bang on size.

We can not follow Chart 100%

It takes so many Lawyers to change the Chart, it will never happen.
 
Machinery's Handbook. Everyone should have one. Will tell you everything you ever wanted to know about thread fits plus almost everything you would ever need to know about anything to do with engineering in metal.
 
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