Strength of a 3/4th Tapped Hole

As a rule of thumb, for a tapped hole to be considered full strength you need a minimum thread engagement of one bolt diameter. The shear strength of the threads can be calculated using the guidlines found in your Machinery's Handbook.

No more need be said.

Sorry but it has something to be said:

The engagement length depends on what is the stuff you have the hole in. The above mentionned rule of thumb is valid only for steel with a yield strength accorded to the one of the bolt. For other combinations as for instance steel-aluminium or cast iron or even high strength bolts in soft steel the ration is >1. There are rules determined by as well experience as theory of thread shear under load and which consider also the non uniform distribution of load on threads.

Bob R. has given a nice and short answer, but I am of the opinion that since the question poser is a beginner, some further info. would have been more appropriate.

like which Type of threads he has in mind, whether it is Fine or Course, Size of threads, and its TPI etc. so that the question raiser gets good feel of the situation and understands better in all regards, which is relevent to his case in mind.

as a thumb rule, what Bob R. has said and the reference of Machinery Handbook is alright but to go through the exercise with full understanding still remains a question mark, hence needs further elaboration here in someway.

the material to be used is also a bit of problem for the beginner etc...

you would forgive me, if this has sounded as a critisism, but what I have seen on such forum is that there are lot of enthusiasts and they need to be put on right path of learning, and the practical aspects of engineering in general. I believe that Fastners and its associated field is vast and needs a lot more deliberation at such forums, so that the beginners can be more conversent with such topics, as it is a backbone of Mechanical Engineering as such.

Just to add a footnote the hole should also be no closer to the edge of the material than one diameter.

The bigger the bolts diameter the more the depth of thread matters simply because the turning force (Torque will be greater) and more likely to damage the thread. A courser thread is better as you go up in size. For very small holes and fasteners a and a fine pitch of thread UNF or BA Metric fine etc only a few threads may be required. But a bigger size say 1 1/2" 37mm approx size hole the a depth of 2" 50mm is about the min into blind hole. If the hole is cut through and no nut is provided what ever the plate thickness is will have to do. As for the location it is dictated by the local conditions and may get rather close to the side, but as long as you are not going to try to bolt into thin aluminium or another very soft metal where tearing through the side could be a problem a distance of 1/2 the diameter of the bolt can be tolerated but more is better.

100% = 1.5 thread diameters engagement