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Looking for chart of strength of tapped holes by percentage of engagement?????

C

charosenz

Plastic
Joined
Apr 14, 2018
I am looking for a chart that shows the actual strength (PSI) of the threads of holes as it compares against various percentages of engagement of 50%, 55%, 60%, 65%, 70% and 75% of bolts in the various diameter holes.

I would like to get beyond the typical statements of XX% is "Acceptable" or "normal" or "Good, best or better".

Just for clarification, I am not talking about the depth of the bolt in a hole, but rather the size of the drilled hole prior to tapping.

Here is a very good explanation of percentage of threads....

best description of percentage of thread for tapped holes-resized.JPG

Charlie
 
Have you checked the machinery handbook? I recently was looking at hole min. and max hole size for percentage of thread engagement, but don't recall if there was anything on strength, etc. or not. The one i was looking through is one of the newest, the anniversary edition, really fat in comparison to the older one I have and has what appears to be quite a bit more info in regards to engineering. Otherwise you could check out Penn Engineering webiste, if I recall correctly. They may have what you need if machinery handbook doesn't, otherwise I don't know. I would also assume material will play a big part in it, so you may have to just experiment with it, I am not a engineer.
 
Does that attachment say 6/8ths? Or am I seeing things?

I would think that 16/64ths would be better than 6/8ths, or maybe 750/1000's. But really we would need Gordon to clarify the Thread percentage thing for us.

Machinery's Handbook has all the info. you need.
 
I did that on a spreadsheet years ago. No longer have it, I looked. IIRC, you entered OD, TPI, material, length of engagement, and ID. Output was calculated pull-out.
JR
 
I have not found it in my copy of the machinist handbook but it is almost 40 years old.

I am sort of shocked that as much as this topic is discussed that there does not seemed to be documentation on it.....
 
charosenz said:
I have not found it in my copy of the machinist handbook but it is almost 40 years old.

I am sort of shocked that as much as this topic is discussed that there does not seemed to be documentation on it.....
Click to expand...

It's because on a basic level, as long as the materials aren't too different in tensile strength, and you've got at least 1.5X diameter engagement, it's (almost) always the bolt/screw that will fail first, not the tapped hole. If you go beyond reasonable limits (say, 50% engagement), perhaps it's something to consider, but there's very few applications where more than 75% makes a difference (except in broken taps).

If you're grasping for that last 5% of theoretical joint strength, it's time to reevaluate your fastener size, quantity, materials, etc. Don't look for meaningful strength improvements from that last bit of minor diameter female thread.
 
charosenz said:
I am looking for a chart that shows the actual strength (PSI) of the threads of holes as it compares against various percentages of engagement of 50%, 55%, 60%, 65%, 70% and 75% of bolts in the various diameter holes.

I would like to get beyond the typical statements of XX% is "Acceptable" or "normal" or "Good, best or better".

Just for clarification, I am not talking about the depth of the bolt in a hole, but rather the size of the drilled hole prior to tapping.

Here is a very good explanation of percentage of threads....

View attachment 225817

Charlie
Click to expand...
The strength in psi is a function of the material discussed, not the size or any other dimension or shape of the part.

I do not think any explanation that equates 3/4 or 6/8 with 100% can be called very good.

Larry
 
The "percentage of full thread depth" concept is native to the US, and its first use was in the context of the US Standard (aka Sellers, Franklin Institute, and American National)threadform.

That threadform -- I prefer to call it the Sellers threadform, as it ceased being the official standard threadform of the US government shortly after the end of WWII -- nominally features 60 degree flank-to-flank angles (measured in a plane containing the axis of the screw) and 1/8 x Pitch flats at both the Major and Minor Diameters.

The radial-to-screw-axis separation between the Major and Minor Diameters defines the full Single Depth of the screwthread, as shown in the diagram.

The US government did not modify the Sellers-form basis of the depth of thread calculation after replacing the Sellers theadform with the Unified threadform, which has the same 60 degree flank-to-flank angle and 1/8 X Pitch fat at the Major Diameter, but a 1/4 x Pitch flat at the Minor Diameter. Instead, the US government defined the maximum permissilble Depth of Thread for government service to be 83 percent.

John
 
There is no chart. you need to calculate for pull-out. There might be something in Engineers toolbox. If this is something for a home shop or just something that you think you need, your wasting your time. Most tap drills are chosen to give between 65-75% depth. Anything smaller makes it difficult to tap. Anything larger and you drop strength drastically.

The reason I looked was for high nickal alloys that we thread milled. Don't drill as deep and raise the contact area vs conventional. It was about a wash.
JR
 
Milland said:
It's because on a basic level, as long as the materials aren't too different in tensile strength, and you've got at least 1.5X diameter engagement, it's (almost) always the bolt/screw that will fail first, not the tapped hole. If you go beyond reasonable limits (say, 50% engagement), perhaps it's something to consider, but there's very few applications where more than 75% makes a difference (except in broken taps).

If you're grasping for that last 5% of theoretical joint strength, it's time to reevaluate your fastener size, quantity, materials, etc. Don't look for meaningful strength improvements from that last bit of minor diameter female thread.
Click to expand...

I appreciate the reply. I do. But it is impossible to make an informed decision without data to back that decision. To put this another way, it is not possible to know if 50% or even 60% is "OK" without knowing the results of how they compare in strength.

No a doubt the data exist, in an chart or in a calculator, but oddly enough, no one seems to be able to come up with one.

I do appreciate the help....
 
charosenz said:
I appreciate the reply. I do. But it is impossible to make an informed decision without data to back that decision. To put this another way, it is not possible to know if 50% or even 60% is "OK" without knowing the results of how they compare in strength.

No a doubt the data exist, in an chart or in a calculator, but oddly enough, no one seems to be able to come up with one.

I do appreciate the help....
Click to expand...
You just need to challenge these guys, watch this
Yeah, there probably isn't anyone here with that type of knowledge. :D
 
charosenz said:
I appreciate the reply. I do. But it is impossible to make an informed decision without data to back that decision. To put this another way, it is not possible to know if 50% or even 60% is "OK" without knowing the results of how they compare in strength.

No a doubt the data exist, in an chart or in a calculator, but oddly enough, no one seems to be able to come up with one.

I do appreciate the help....
Click to expand...

I agree pull-out is what you need to be looking at. Exactly what is it you are working on?
 
Why? do you really have enough drill sizes to make the hole that tiny bit bigger or are you looking at a custom tap? I would think at these close percentages drill wobble etc could become significant as just a few percent wobble may increase hole size enough to make the percent engagement less by say 5% or more of the total expected.
Also at those higher loading any thread faults such as micro chipping smearing etc will quickly reduce a bigger percentage of the safety factor.
Unless your threads are perfect in a perfect material and inspected you do not want to reduce your margin of safety too much. But how much is too much and how much can you trust the installer s down the line not to overtorque the bolts and damage the threads in doing do.
You might want to look up the failed windshields in a BAC 111 plane due to over tightening screws.
Bill D.
Bil lD.
 
....and, as much as you massage and tweek your hole, you have no control over
the fastener.

Unless you want to start making those too....:skep:
 
charosenz said:
I am looking for a chart that shows the actual strength (PSI) of the threads of holes as it compares against various percentages of engagement of 50%, 55%, 60%, 65%, 70% and 75% of bolts in the various diameter holes.

I would like to get beyond the typical statements of XX% is "Acceptable" or "normal" or "Good, best or better".

Just for clarification, I am not talking about the depth of the bolt in a hole, but rather the size of the drilled hole prior to tapping.

Here is a very good explanation of percentage of threads....

View attachment 225817

Charlie
Click to expand...

There is a chart for this presented on page 69 in the guide "The use and care of taps" by the Cleveland Twist Drill Co. The chart is based on test results for a 1/4 -28 thread. The chart shows that a 75% thread is slightly stronger than a 100% thread. The strength drops by 1/2 when using a 50% thread depth for steel alloys. The strength drops by 20% with a 50% thread depth in aluminum and brass alloys.

Robert
 
charosenz said:
No a doubt the data exist, in an chart or in a calculator, but oddly enough, no one seems to be able to come up with one.
Click to expand...

The data is out there. How good is your calculus? That's what you'll need to get through it. Your strength of materials will need to be very good also because you'll need to know what happens first, plastic deformation or shear. There wouldn't be A chart, there would be thousands for each instance of occurance. They just don't exist. Now, I know that I didn't get my degrees from the highest ranked engineering school in the US, but after taking statics, dynamics, strength of materials, and a whole lot of calculus, I figured it out.
JR
 
Seems to me that someone requiring this amount of data for technical decisions should be capable of generating or deriving it themself.

Sent from my SM-G930R4 using Tapatalk
 
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