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Bolt Hole Sizes

Bolt Hole Sizes

Bolt Hole Sizes

My company is both an engineering and fabrication business for pre-fab industrial steel structures.

The problem we're having is the fit-up between two sets of assemblies. Lego block 1 has two points that fasten to Lego block 2 (for example). The two fasten points are 10 ft apart. AISC's 0.2% tolerance allows for about 1/4" out-of-true dimension. That means the holes won't line up. Things get worse when we have fasten points farther apart.

So, one of the things we're looking at is the bolt hole sizes. We're using 1" dia bolts instead of 3/4" bolts as a standard, so we can have that 1/8" extra wiggle room for the hole size. We also have a plasma cutter that is said to be accurate to 1/64". So, we could take advantage of the 1/16" allowable tolerance on bolt hole size, giving us a total of 3/16" dia wiggle room.

What are some of the mechanisms you use to remedy the bolt fit up issues on your jobs?

RE: Bolt Hole Sizes


If possible I try to use slotted holes or otherwise I try to tolerance the dimensions so that the two parts fit together in the worse case scenario.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Bolt Hole Sizes

By the time the steel erector gets done slotting the hole with their oxy-acetylene torches, you will be able to drive a flatbed trailer truck through the hole. Yes, I'm being sarcastic, but what some of the erectors do in the field is just shy of criminal. Its best if the designer uses short slotted holes and uses the proper size and number of bolts to accommodate the slotted holes. Anything the designer can do to minimize field modifications is to the owner's benefit.

Best regards - Al

RE: Bolt Hole Sizes

I third the recommendation for slotted holes above. Now, you can't just do that mindlessly. Attention needs to be paid to slot direction and if a slot introduces a slip-critical connection where you now have to specify pretension on the bolts.

For anything that I dub to be massively critical, I will call up safety connections + field drilling of hole locations once fit is known. This is not the norm though and thought must be put towards how an erector will do so in practice as it can be very dangerous (hence the safety connection being specifically called out). First choice is to always mitigate the need for this via the slots mentioned above.

RE: Bolt Hole Sizes


I have posted the following article on Calculating Locational Tolerances.


Basically, there is a relationship between your bolts, your clearance hole sizes and your fabrication tolerances.


RE: Bolt Hole Sizes

yes as drawoh stated. true position and hole tolerance must be applied, when this is not possible such as in the aircraft, then pilots holes have to be predrilled, then temporary fasteners installed to line up the holes.
then match drill in place to exact hole location and precise fit. same as always quality has to be involved that the requirements are held and approved.

RE: Bolt Hole Sizes

Typically engineering controls fit via careful attention to tolerance stacks and reference features/planes. The shop controls fit by controlling the material and process. Oft-forgotten details such as accounting for gravity, thermal expansion/contraction, stress relieving, or other stressors during fabrication can cause a lot of scrap. Another common mistake is misorienting symmetrical parts, if assembly reference planes don’t align then your stackup is screwed, hence good fabbies marking reference edges. If all the above is done correctly then your 10’ job should be simple fabrication, no need for sloppy design like slots or other nonsense.

RE: Bolt Hole Sizes

Without drawing it is difficult to visualise. But is it possible to make a 10 ft template with accurate hole locations of Lego 1 and use it for punch marking and drilling holes in lego 2 ? You may get better tolerances.

Engineers, think what we have done to the environment !https://www.linkedin.com/in/goutam-das-59743b30/

RE: Bolt Hole Sizes

Quote (goutam_freelance)

...exact hole locations...

There is no such thing as "exact". Anything you design must take manufacturing tolerances into account.


RE: Bolt Hole Sizes

to clarify as well a precise fit to the attaching hardware, and the with in the true position of the engineering requirements. not sure what is being used today because of the updated technology.
on aircraft in my day it was not uncommon to use a Mylar to locate holes to drill pilot holes and temporary fasten details onto an assembly. usually done when there was 2 or more details that had to be

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