screw striping

[steelman2]

hi everybody
in our assemble line we have a problem. we use a M6 screw with a grade of T7. the screw must pass 3 steel sheets.unfortunately the holes in these 3 sheets are not concentric and that causes the screw to get striped or fractured while fastening with screwdriver. for the time being it is impossible for us to modify the sheet molds and the place of holes. is there any way that with changing the design or heat treatment of screw i can prevent stripping?
best regards

 

[MikeHalloran]

The question really boils down to , "How can we change the ONE thing that ISN'T, er, screwed up, so we don't have to admit that we haven't yet grasped that modern circa 1840 concept of interchangeable parts?"

Good thing you didn't post in the GD&T forum; those boys would have a really good laugh at your expense.


You can end up with a screw-like object in the finished assembly by filling the holes with epoxy, and finishing off with a stubby screw or just a head where you need a head showing. It won't have any strength, and you can't disassemble it, but it doesn't sound like serviceability matters much to your outfit anyway.

If you did need strength, you could match-drill the holes and use a drive rivet.

Or you could buy some special screws with reduced diameter shanks and a very short threaded point. Forget resisting shear loads with them.


I'm assuming you were forced to ask the question by an ogre boss. Been there.



Mike Halloran
Pembroke Pines, FL, USA

 

[steelman2]

dear Mike Halloran
you hit it right on the head about my boss.in fact the issue is not that simple. the screw is fastened with a welded nut but the problem is that at that point the screw has no threads anymore.i need the screw to act as a dill and can penetrate the sheet metals without being striped.i know that kind of question is laughing and stupid but that 's my problem.
anyway thanks for your reply

 

[CoryPad]

More details would be nice. What are the materials you are joining? How strong/hard/thick are they?

What is grade T7?

You could try a Taptite fastener with an induction hardened tip and an unthreaded weld nut. The induction hardened tip would create its own threads through the assembly and in the nut.

 

[Wrenchbender]

Not too familiar with the metric designations, but it sounds like harder screws, an extended pilot point, or lubricant would help. Drill point screws might work.

http://www.mcmaster.com/#tapping-screws/=9or7j7

BTW what is this for? Something we would ever buy for personal use or come in contact with?

 

[CoryPad]

Bestwrench,

This type of condition was commonplace for automotive structures (non-critical attachments) up through the 1990's.

 

[Wrenchbender]

Cory, Sorry my last sentence above was lame and facetious - like what is this thing so we don't buy it. I worked as a process engineer in an automotive assembly plant. Usually any bad lot of parts ,e.g., crooked holes, would be returned to the vendor. If something was our own fault, extra manpower was temporarily put on the job with authorized repair procedures, e.g. redrilling, welding on fishplates, etc. I recall some trim attachments where drill point screws were always used to go thru an exiting hole into body sheetmetal that had no hole (e.g. scuff plate into door jamb) - worked very well, OP should have no trouble going thru 3 sheets.