Help! > Mistakes, muckups, and dangerous behaviour

A Novice Mistake

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Damau:

My current project, b.lindsey's ME-! mill engine uses a screw made from hex steel and threaded 4-40 to fit through the connecting rod and into the crankdisc.

All was well with the machining of the hex and turning down the shaft for threading with my 4-40 die until I reached for the die that I thought I had.  I did not have a 4-40 but I had a 4-36 and my first thought was, Ök, 4-40 vs. 4.36, what a little difference that is so I will use the 4-36 die.

Wait a minute!!!!!!!

I had already tapped the hole in the crankdisc to 4-40 and the 4-36 was not going to "fly".

Off I go to my favorite online supplier and ordered a 4-40 die and it arrived on the little brown truck today.

I still had the part chucked in the lathe and threading was a "breeze"  (simple chore).

I had allowed some extra length on the screw and knew I could bring it to the correct length after the threading was done.  That was the beginning of my big mistake.  I stepped off the length and had my DRO set to zero and proceded to face the end of the screw to the correct length.

I am sure a lot of you folks know what happened when I tried to face the end of a screw that is less than .100 dia.  I made 2 cuts of about .005 each before the work piece climbed up on the tool and bent the the work piece.  UUUGGGHHH

If you people in Europe, New Zealand and Bill at UNCC in Charlotte and the rest of the world had been listening you would have heard something like a sailor has a reputation for doing.

I had machined the screw with a hex and turned it to the proper length, waited on the little brown truck to arrive, cut the threads and was proud of my job until my bubble went "pow".

Guess what????? I straightened that little booger out and it ran concentric'.

Thanks for another blessing!!!!

b.lindsey:
Bummer Dale. It should be fine, but where it bent could be weaker now, especially if you undercut where the thread and shoulder meet.

Bill

Don1966:
Dale I know I have done this once, you seem to learn quick after a mishap like that. Bummer isn't it?

Don

Damau:
Bill and Don,

Thanks for your replies.  I think it will be ok since it was not undercut.  I cut the threads up to the shoulder that prevents it from getting tight on the connecting rod.

Don, the old saying, "experience is the best teacher" certainly applies in this case.

Jo:
Dale,

Bending the bolt has caused a weak point (stress fracture) so it would be better to make a new one, especially if you are going to use it associated with a crank.

If you want to shorten a thin bolt the easiest way for a beginner to do this is to put a nut on it, hold it in a vice cut it down with a small hacksaw and finish with a file rounding the ends. As the nut is unthreaded it will clear out any burrs on the threads  ;).

Jo

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