Tapping hole through two welded plates
Tapping hole through two welded plates
(OP)
I have two pieces of carbon steel A36 plate that were welded together to make a thicker plate. Is it acceptable to tap a hole through both of them at the same time to get the bolt thread engagement I need? Or should I replace with a single thicker piece of plate? Any concerns about corrosion at the gap between the plates after we tap through it?





RE: Tapping hole through two welded plates
RE: Tapping hole through two welded plates
I am mounting a centrifugal compressor to a baseplate which we are designing. On the baseplate design, I have to weld a mounting pad (1" thick plate) to the top of an I-beam. This plate will be machined flat.
To minimize the thickness of the pad, we would prefer to drill and tap (for the compressor hold down bolts) through the pad and the I-beam underneath. This would give enough thickness for engagement of the hold-down bolts.
Otherwise, I can beef up the pad by going to a 2" or 3" plate. However, that would mean reducing the I-beam height and selecting a different beam.
RE: Tapping hole through two welded plates
I wouldn't worry too much about corrosion, as you'll use plenty of cutting oil when threading a 1" thick plate...
RE: Tapping hole through two welded plates
I am assuming the use of nuts on the back side of the plate is not allowable because the nuts would be right where the beam is?
Is the process: weld the plate to the beam, grind the plate flat, then tap and drill through both the plate and beam? This sounds perfectly acceptable at first glance assuming your hardware selection is appropriate and the thread engagement length is adequate. Remember the A36 will have a lower strength than most hardware steels. This is a good resource for threading problems: http://www.fastenal.com/content/feds/pdf/Article%2...
RE: Tapping hole through two welded plates
RE: Tapping hole through two welded plates
Dik
RE: Tapping hole through two welded plates
I have mounted pumps and motors on frames with welded pads to a beam and tapped holes straight through as you describe and never had a problem.
i wouldn't weld a nut to the underside of a beam although I have seen that done also, the reason I wouldn't do it is the heat from the welding can affect the nut material properties.
RE: Tapping hole through two welded plates
Could you please let us know what the expected loading is on the bolted connection and what size the hardware is. That would help us help you with this dilemma. I am picturing a 3/4" bolt or something around that size, in which case tapping through 1" plate + the flange would be fine, but if this is a 2 1/2-4 thread or something huge than that would change things a bit.
RE: Tapping hole through two welded plates
I was wondering more about the method of fastening, though. I had opened another thread to discuss this topic: Link. So if you have any comments on whether a threaded fastener connection is better than a through bolt connection, please answer them there.
RE: Tapping hole through two welded plates
Using the formula:
Engagement Length = 2*Tensile Area / (0.5*pi*(Major Diameter of Screw - 0.64952/#threads))
Tensile Area = 0.40816 in^2
Major Diameter = 0.816 in
# Threads = 10.16
I get 0.691 inches of thread engagement. Now this is assuming that the thread and the steel plate have the same strength.
If the yield stress of the bolt is 92,000 psi and the plate is standard A36 steel with 36,000psi yield than the minimum strength to ensure that the bolt will fail before it rips the threads out is: 92/36*0.691 = 1.765 inches
I think these numbers are very conservative. Kamax fasteners recommend 1.0*d for grade 8.8 in A36 blind holes, which means your threads are fine.
So, because your load is only about 4,000 lbs AND because most of the load is carried by the first few threads I don't see a problem with this arrangement even if you didn't engage any threads into the I-beam. Of course some other items come into play here such as pre-load and fatigue and whether or not people are standing under this compressor (ie is this bolt a single point of failure?)
Nut-Bolt vs threaded plate each have their pros and cons. Nuts are easier since you don't need to tap parts.
Best of luck on your project!
RE: Tapping hole through two welded plates
We do not find this practice to be a cause for tap breakage. The causes of tap breakage we see are the use of taps past the point where they're dull (this is the most common), drilling the wrong sized root hole, or an over-aggressive or careless operator.
RE: Tapping hole through two welded plates
rmw
RE: Tapping hole through two welded plates