http://www.tapmatic.com/tapping_questions_torque_vs_thread_strength.ydev#:~:text=It stands to reason that,the stronger the tapped hole.
one of my engineering instructors used to say
that if a decent quality steel bolt
(HE WAS REFERRING TO GRADE 8)
is threaded into a cast iron or steel surface,
TO A DEPTH OF THREAD at least 1.75 times its thread diameter,
its not going to pull out before the bolt snaps under tension loads,
in aluminum, that thread engagement must be increased to about 2.5 times,
its thread diameter its not going to pull out before the bolt snaps under tension loads
Tapping Torque Vs Thread Strength
Suggested Percentage Of Full Threads In Tapped Holes
It stands to reason that it takes more power to tap to a full depth of thread than it does to tap to a partial depth of thread. The higher the metal removal rate, the more torque required to produce the cut.
It would also stand to reason that the greater the depth of thread, the stronger the tapped hole. This is true, but only to a point. Beyond that point (usually about 75% of full thread) the strength of the hole does not increase, yet the torque required to tap the hole rises exponentially. Also, it becomes more difficult to hold size, and the likelihood of tap breakage increases. With this in mind, it does not make good tapping sense to generate threads deeper than the required strength of the thread dictates.
As a general rule, the tougher the material, the less the percentage of thread is needed to create a hole strong enough to do the job for which it was intended. In some harder materials such as stainless steel, Monel, and some heat-treated alloys, it is possible to tap to as little as 50% of full thread without sacrificing the usefulness of the tapped hole.
Workpiece
Material Deep Hole
Tapping Average
Commercial
Work Thin Sheet
Stock Or
Stampings
Hard or Tough
Cast Steel
Drop Forgings
Monel Metal
Nickel Steel
Stainless Steel 55% - 65% 60% - 70% -
Free-Cutting
Aluminum
Brass
Bronze
Cast Iron
Copper
Mild Steel
Tools Steel 60% - 70% 65% - 75% 75% - 85%
% Of Thread Calculation for Cutting Taps
On page 1924 of my
Machinery's Handbook (27th ed.), under the heading "
Diameter of Tap Drill", they make the statement:
"Tests have shown that any increase in the percentage of full thread over 60 per cent does not significantly increase the strength of the thread. Often, a 55 to 60 per cent thread is satisfactory, although 75 per cent threads are commonly used to provide an extra margin of safety."
However, the table that immediately follows,
Recommended Hole Size Limits Before Tapping Unified Threads, lists min and max hole sizes for various lengths of thread engagements. For 1/4-20 threads less than 1/3 diameter long (< .083"), the table calls for 87 to 100% thread depth. For thread lengths from 1½ to 3 diameters long (.375 to .750"), they call for 75 to 85%. I haven't checked any other sizes, but they don't seem to be following their own 60% rule.
For 1/4-20, my handy-dandy Starrett reference card calls for a #7 (0.201") tap drill, which is 91% thread depth. A #2 (0.221”) tap drill gives a 54% thread depth, with a lot less effort required to tap and less chance of tap breakage.
At present, I'm dealing with 1/4-20 by 0.50" thread depth hole in 6061-T6. How big can I make the tap drill without sacrificing strength, assuming 0.50" thread engagement? Anyone know of any good references or on-line calculators to answer this sort of question?
https://www.fastenal.com/content/feds/pdf/Article - Screw Threads Design.pdf
https://fieldfastener.com/2018/03/13/rules-of-thumb-for-thread-engagement/
RULES OF THUMB FOR THREAD ENGAGEMENT
WHAT IS THREAD ENGAGEMENT?
Thread engagement has two factors, length of thread engagement and percentage of thread engagement.
Length of thread engagement is measured by the length of interaction between the fastener and nut member (i.e. nut or mating material for screw.) For example a standard thread forming screw applied in
10mm of material will have more length of thread engagement than the same thread forming screw in 8mm of material.
Percent of thread engagement is based size of the mating hole compared to the fastener and how much the height of threads of the screw are engaged. For example a #10-24 thread forming screw will have around 80% thread engagement in a .1683” hole versus around 70% thread engagement in a .1710” hole.
WHY IS THREAD ENGAGEMENT IMPORTANT?
Thread engagement is important because it directly relates to the integrity of the joint. With too little thread engagement (combination of length and percentage) the nut member may have a tendency to strip out at lower than desirable torques, in effect lowering the strip/drive torque ratio. On the other hand with too much thread engagement the torque needed to drive the screw will tend to be high, which can lead to inadequate clamp load or even breaking fasteners during assembly
Percent and length engagement can also be traded for one another; i.e. if the nut member material isn’t optimally thick, a smaller hole can be used to get a more optimal amount of thread engagement and in effect a better joint.
HOW DOES THREAD ENGAGEMENT EFFECT THE TENSILE STRENGTH OF A BOLT/JOINT COMBINATION?
Simply put, more thread engagement can result in higher tensile strength a joint. Tensile strength is the force required to pulls something until it breaks or the capacity of that material to withstand that load. If a bolt is longer than needed to develop full tensile strength in a nut member, that excess material is wasted. On the flip side, if there is not enough bolt length engaged in a nut member, the bolt has a higher probability of stripping out before full tensile strength capability is achieved. Generally speaking, you need at least 1-1.5 bolt diameter engaged in the nut member to achieve optimum joint strength with a thread forming fastener, depending on the strength of the nut material. There are different factors that affect this such as the nut material; steel is a close to a 1-1.5 relationship, while softer material, like plastic, will need more thread length engagement to achieve optimum joint strength.
