Made scrap from parts due to a typo on LMS tap & clearance chart!

I have been using the LMS chart for years and like having the two columns. I will deviate from either column if the job requires it.
 
I had this question as well. In 1" steel (same 1/2 x 13) I went a little > than 50% engagement and the threads looked weird but seem to work just fine. Very informative conversation this. Can definitely see the utility in deep holes.
 
The LMS 50% thread engagement spec is good for steel IF the tap is good quality, the drill drills on size, the part is thick enough, and the screw is the correct size. If any of these conditions are not met, the screw may not hold. It has happened to me, so don't let anyone tell you it cannot happen. There are two things you can do to prevent this unfortunate occurrence. First, good taps are good economy. Cheap taps are poor economy, but they make great boring bars. A drill point gauge is a very useful tool to make sure that your sharpened drill makes on size holes. Most high quality drill bits are good out of the box, but watch out for the cheap ones. It may be useful to test their hole size before scrapping parts, especially if there is a lot of work in the part.
 
A 27/64" drill with a R/O of .015 and a 29/64 with R/O of .001 won't be giving you a hole much different in diameter , and 29/64 will be easier to thread especially in hard steel. In a 1/2" hole with 5 threads engaged- in most situations this is a strong assembly.
If you need class 3 or 4 thread then this is a different issue and more difficult to atttain.
 
I’ve found that depending on the material you’re working with and the size of thread your trying to tap that 50% thread depth can be troublesome to tap at best. I’ve worked with some materials that getting a 1/2-13 tap through 1/2” tick material with 50% thread depth would break a good sharp tap every time. Keep in mind most of what I’m talking about are automotive castings. I’m those cases the solution is to keep enlarging the hole size until the tap will go in without breaking. Often times this means less than 50% thread depth. In the instances I’ve had to do this the tapped hole has always served it’s purpose. My point is that the LMS drill chart isn’t necessary wrong or right but also that we need to be aware of what we are doing when making things and when things don’t turn out the way we want them to we should learn from that so we’re less likely to do it again. For me in the OPs situation I’d leave that chart on the wall as a reminder of what happened when I used it that one time. But I’m forgetful so I need those reminders hanging around.


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According to a study done by U of Tulsa engineering department:

Internal threads, 5/8-18 in .26 % carbon mild steel, precision bored holes. Cut threads maintained their strength from .578 to .597. 35,000-lb strip out strength was used with a 5/8-18 ,.5" long grade 8 steel setscrew . Setscrew was pressed out with a steel rod while recording the required force(35k lbs).
In fact tap holes must be increased to .602( about 33% thread engagement) before the strip out force needed dipped below 35,000. both spiral and plug type taps were used.

test done in Jan of 2011.
 
finsruskw said:
Tapping some 1/2" 13TPI holes in 3/8" stock for Cub Cadet lift arm parts when I noticed the threads didn't look quite right, peaks were kind of flat and the bolt felt loose when turned in.
Click to expand...
Would 50% thread engagement not work for your application or were you just not happy with how it felt?

Tom
 
One other thing to concider is what size hole did the drill actually make? A 1/2-20 thread drilled at a true 29/64 diameter will give a nice thread fit, if the hole is oversize then it can be a problem. I use 29/64 all the time in SS, saves taps, important when doing 100's of holes. It only makes the thread crests a bit flat, the major diameter is controlled by the tap.
 
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