Re: [ATM] Cutting thread in aluminium

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

> Don,
>
>   As one of those who seems to break taps, is there a summary of
> recommended hole size/drill bit to taps? Maybe something I could print
> out and keep in the case with the taps. I'm gathering from your other
posts
> that different materials will also affect hole size.
>

Tom,

Here is one tap drill chart  for cutting taps I Googled from the web
http://www.newmantools.com/tapdrill.htm  The Machinery's Handbook is always
a good reference to have. See http://www.industrialpress.com/en/mh.asp

Roll or forming taps use larger tap drill sizes than cutting taps and are
much more sensitive to tap drill hole size and to the material to be tapped.
I see the tap drill sizes given for roll taps as a suggestion, or a starting
point as a first approximation to tap drill hole size. The best tap drill
size for roll taps is best found by experimentation. see for roll tap info
http://www.balax.com/catalog.html

>From my experience the number one cause of tap breakage in hand tapping is
that some side force is applied to the tap. Taps are hardened, not flexible,
and most taps have a thin cross section with flutes to accommodate chips. It
is best to use some sort of tap guide to keep from applying side forces when
tapping. The best gadget I found for hand tapping in the mill, drill press
or lathe is the Fisher micro tapping guide. See
http://www.cartertools.com/fmpdtg.html  The micro tapping guide is also
useful in "floating" a reamer when reaming a hole. I find it prudent to buy
the best grade ground HSS taps from reputable manufacturers. OSG
http://www2.osgtool.com/product/products.asp#taps  and  Balax are just two
good manufacturers of high quality taps. Taps come is different oversize
ratings given with the suffix H (for inch sizes) for every H number the tap
will be 0.0005" larger diameter than the nominal size. e.g. ¼-20 GH3 is a
ground tap that is 0.0015" larger than nominal. Increase the tap drill
diameter to account for higher H numbers.



There is a lot more info out there about tappping and these are just some
suggestions from my experience with tapping. Hope this helps.



Don Clement

Running Springs, California



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