How are self drilling & self tapping scews manufactured? 1

[quiry13]

I need to know how self drilling and self tapping screws are really manufactured. The heads on a batch of screws I ordered for site are breaking off and the sales guy is giving me the run-around. Could anyone provide a breif description of the process of manufacture? Or if there is a "net site" which is particularly good or a book, I would appreciate recieving the information.

 

[Billybaru]

The screws are typically cold-headed. Heads can break from many causes. If they are breaking during installation, excessive torque could be the problem. The specs for the screws have specific torsional strength requirements they must meet. If air wrenches are being used to install the screws, torque must be monitored. If the screws are installed and later on the heads pop off, hydrogen embrittlement could be the cause. There are too many causes to address. What are the specifics of the screw and installation that is having the problem? What is the torque value being applied to the screws?

 

[diamondjim]

I wish you were more specific. How strong are
the self tapping/self drilling screws and how hard
it the material that you are tapping into with
these? I would guess you are using these in
very hard material in relationship to the screws.

 

[TBP]

The problem is not necessarily with the self tappers. I've noticed, over the last number of years what I perceive to be a lot of inconsistancy in lower grade carbon steel pipe. (Pipe being what I work with for the most part.) We've hit chunks of pipe that we just couldn't cut a good thread in. We changed dies, cutting oil, different guys doing the threading. We still had torn threads. That is until we tried a different piece of pipe. Everything was fine then. We've been drilling holes for tubing supports on a machine frame, and hit spots on that assembly that were almost impossible to drill. Yet a few feet either side - no problem. New bits, same drill, same guy.

I can't prove it, but I think there's a lot of scrap/crap in much of the material that's on the market now, and that quality control at the mills is not nearly what it used to be. However, I don't necessarily blame the manufacturers. They provide exactly what the customer will pay for. As Pogo said: "We have met the enemy, and he is us."

 

[RHK]

According to the local manufacturer's catalogue, his self-drilling screws are forged.

I know that the workers always complain that the self-drilling screws will not drill into the flanges of hot-rolled sections. They overcome the problem by pre-drilling, which takes time. Perhaps if they were a little more patient with the self-drillers they would not have as many problems. The catalogue suggests "a 500 Watt drill running at at least 2500 rpm. Use maximum revs for drilling then slow down when the thread engages in order to set the screw properly. No need to push too hard. Let the screw do the drilling."

Are you able to watch the installation. It could be poor technique in installation, with the operator applying too much force.

Our hot-rolled sections have a nominal ultimate of 410 MPa, and yield of 320, but obviously this is a lower limit. For typical Australian production, the mean yield is 118% nominal, with a coefficient of variation of 10%. There is a portable hardness tested, that you can use to compare the surface hardness of the rolled steel with a standard.

Russell Keays