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drill and tap bolts

drill and tap bolts

drill and tap bolts

I have a case where an angle with slotted holes needs to be fastened to the wall of a rectangular tube with a wall thickness of 5/16".  If I drill and tap a hole, is the bolt capacity the same as a typical bolted connection?..Also can I tension the bolt to be slip critical?

RE: drill and tap bolts

You did not specify what size angle or hole size (bolt size) but in general the answer to your questions is no.  5/16 inch would be ok , I guess, for 1/4 inch cap screws, but no more.


RE: drill and tap bolts

I guess my question really is what is the difference in designing a drill and tap situation as opposed to conventionally bolting thru?...I've never done a drill and tap.  The tensile capacity seems to be the only thing that would be different

RE: drill and tap bolts

The rule of thumb is to have wall thickness (depth of the tapped hole) equal to the height of the nut to maintain the same length of engagement. If this condition is met then there is no difference between bolt/tapped hole and bolt/nut application. In your case, you have 5/16" wall thickness which is more than height of regular 5/16" nut (0.273") but slightly less than height of 3/8" nut (0.337"). So, you can safely use 5/16" bolt but to use 3/8" bolt would be pushing it. All this assuming that strength of material of your rectangular tubing is equal to the strength of the nut. You can verify strength of your joint by using formulas on p.1324 in Machinery Handbook , edition 24.
Hope this helps.

RE: drill and tap bolts

Lindapter supply cavity fixings suitable for bolting onto rectangular or circular steel hollow sections. The “Hollo-bolt” and the “Lindibolt”. They act rather like an expansion anchor into concrete. There are safe load tables given in their catalogue for different section sizes and bolt diameters.

Refer to their website.

RE: drill and tap bolts

I hope this is still relevant - but I just saw this series/thread.

Quattro's rule of thumb is used quite a bit by the shop-workers I've known - and it works fine.  The 'real' rule of thumb, based on thread-deformation, is to have at least three threads engaged for full strength.

If it critical to use even fewer threads, it can be done, but I've not yet run into such a situation.

My own suggestion - tell the shop-workers to use the rule of thumb quattro suggested and to talk to you if less is available - then use the three-thread rule for yourself.
(If its less than that - re-engineer.)
(Scotty Engineering - - make yourself look like a brain, capable of quick decisions on 'hard' problems, but giving a 'good' range to the guys on the floor - - so you don't have to look at every single simple application)


RE: drill and tap bolts

I disagree with the use of the 3 threads rule, as applied to the structural use of normal steel bolts.  (It may well be valid in some other specialised field).

Normal steel hexagon nuts are designed principally to resist  1) bearing stress on the face of the nut,  2) shear stress through the threads of the nut  3) bursting stresses in the nut (transverse tension due to slope of threaded faces) and  4) to distribute thread shear stresses along a sufficient length of bolt that the bolt threads do not fail in shear.

Now a designer will not normally know which of these cases is the controlling one for a particular bolt, so the rule of thumb that should rather be followed is to make the make the tapped hole depth at least equal to the thickness of the nut, adjusted by the ratio of steel strengths nut/tapped material.

Below is a table of typical turns of thread in standard steel hex nuts, and it can be seen that this is NEVER as low as the 3 threads.

Metric standard pitch  M3 to M18   :  4.8 to  6.5   threads
Metric standard pitch M20 to M36   :  6.4 to  7.4   threads
Metric fine threads are 20% to 100% greater depending on pitch.

Unified UNC pitch 1/4'' to   5/8'' :  4.4 to  6.0   threads
Unified UNC pitch 3/4'' to 1+3/8'' :  6.4 to  7.4   threads
Unified UNF pitch 1/4'' to   5/8'' :  6.1 to  9.8   threads
Unified UNF pitch 3/4'' to 1+3/8'' : 10.2 to 14.1   threads

Richard Beneke

RE: drill and tap bolts

I do not entirely disagree with Richard - but I would point out that 'Jam Nuts' are much closer to 3-thread engagement.
(This is from a mech. engineering standpoint, so bear with me regarding my naivete' re. heavy structural practices.)

That said - I entirely agree that, if the usage is 'heavy' or critical, then it is far better to have a depth of thread at least equal to one standard nut thickness.
So much so that, several times, when shop-workers came to me about similar situations, I recommended cutting a small hole in the tubing at 90 degrees to the bolt/joint, so a nut could be installed, rather than depend upon the tubing.

Another reason for the above action - - tubing material is rarely designed to the same specific conditions as are nuts.  40K common steel ( 1018 or the like) will NOT have the same properties as a designed nut. By no means would I recommend using tubing as the 'nut' for high tension requirements.
By the same token, tubing subjected to tension and moment from one side only (as from a bolt threaded into the tubing wall) will have a much greater tendency to failure - - especially with the stress risers that the threads in the tube-wall create.  If feasible, it is much preferable to 'through-bolt' - - which again would allow you to use a regular nut.

If the only requirement is simple ('light') fastening - then 3 thread engagement should be quite adequate.  In more critical or 'heavy' cases - - use more caution - - better yet, use standard components designed for the task.


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