Re: [ATM] Cutting thread in aluminium

["Peter John Smith" <pjifl@bigpond.com.au>]

To add to much good information from others.

If you are stuck with 'gummy' Al, under some circumstances you will definitely
have problems.  The difference between some of the really nice Al alloys and
gummy (usually pure) Al when machining and tapping is incredible.  Many are
stuck with what we have.

If possible, design so the Al thickness is substantial and the holes are through
holes.

If the holes are through holes, try to use a spiral pointed tap.  (often called
gun taps)  They can also be used in deep holes.  Usually this fixes the problem.

If using normal hand taps, a number 2 (intermediate) may be best - for starting
as well.

If the wall thickness is sufficient, it is better to choose a quite coarse
thread.  The problems were with an M5 thread.  This is quite a fine thread.
Since no commonly available small metric fasteners are available with anything
but very fine threads, you are stuck with extremely fine threads unless you move
over to non metric.  In this area, the metric standard threads leave a lot to be
desired.

Be particularly careful when starting - apply sufficient force but not too much.
It must ensure feeding.  Maintain this for a few turns of the tap.  It can be
difficult sometimes to judge the right amount of force for a taper tap so some
overlapping of threads results well down into the hole.  The thread will never
be very strong after this.

Others have mentioned hole size and cutting fluid.  Makes a big difference.
Almost anything here is much better than nothing but it should be oily.
(Alcohol wont work very well).   In an emergency, butter or cooking oil or fat
work quite well.

Peter Smith.





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