Re: [ATM] too thin tube

["Don Clement" <clement.focuser@verizon.net>]

Ellen's suggestion of a sturdy collar over a thin walled tube is very good.
If the collar is of the same aluminum alloy then the collar could be held
permanently in place using an interference fit. By using a thermal
difference (heating the collar or cooling the tube before fitting) An
alternative is to use a slip fit with Loctite anaerobic adhesive (red 271)
between the tube and collar. Loctite 271 expands slightly and tends to even
out stresses better than a thermal difference technique.



A technique that has worked well for me to get the maximum strength from
threads is to use a cutter that has the exact thread profile for a
particular pitch. For a sharp V thread the depth of thread is 0.866 x pitch
(for 60* threads). If one uses a unified or ISO thread form then depth of
thread is ~ 0.54 x pitch for internal threads.  For me, the best method for
getting the correct thread form is to use a topping insert type of cutter.
Another advantage of insert cutters is that an anvil with the correct helix
angle can be used. The correct helix angle allows for better chip clearance,
less stress when cutting in thin material. Also setting the compound (top
slide) at 29* and feeding with the compound so that the cutting is on only
one side of thread as opposed to feed with the cross-slide straight-in cut
on both sides at once.



Finer pitch will make for smaller depth of thread. I lowered the thread
depth on lead screws by one-half by halving the pitch. Then using a double
thread maintained the same lead as the leadscrew with twice the pitch only
with ½ depth of thread.



Don Clement

Running Springs, California

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