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Taping 18mm hole ( aluminum )

N

Ndx

Plastic
Joined
Feb 8, 2018
Hi Guys,

Not sure if this good forum but I will give it a shot.

I have an 18mm hole in water distributor... I need to thread so it accepts a fitting ( to be custom made ) current was glued in.

Is 20 x 1.5 tap a good choice ?
I read somewhere that there is something like starting tap its conical to help center the tap.

Any recommendations on specific tap brands, I will be doing it manually.

Thank you
 
If you got the old one out cleanly, why not clean the hole up and glue in the new piece?

20x1.5 threads expect 18.5mm hole. Don't know how much problem that .5mm will cause if you tap.
 
20x2.5 is a standard size..

Major minus pitch gives you your hole size, 17.5mm..

You're at 18mm, and that would still give you a better than 60% thread,
which should be MORE than strong enough considering the previous plug was
glued in. If it was a J thread, you would be at 70%..

20x1.5.. In aluminum, you can get that tap to go.. Its not going to be fun,
and its not going to be pretty, but it'll go... And you might get some nice
self locking threads out of it in the end. Or you could ream it out and then tap.
Though 20x2.5 seems a lot easier.

I read somewhere that there is something like starting tap its conical to help center the tap.
Click to expand...

I'm assuming you are talking about hand tapping... 2 fluters are a bitch to start straight,
3 or more flutes will generally let you feel when you are lined up nicely.. Though I would think
a 20mm tap of any flavor would have at least 3 flutes.
 
Metric hand taps are serial.
For standard coarse it's a three tap set, start, rough, finish.
For fine threads (which 20 x 1.5 is) it's a two tap set, start and finish.

In aluminium I'd use ethanol or kerosene for lube. WD40 also works.
 
Starter taps were a tap with a tapering nose that would take a smaller bite and so would tap easier than a more blunt nose tap. Because of the taper one may have needed to followed with a normal tap to make good the threads for a shallow hole. One style of a blunt nose tap was called a plug tap and was often designed to push the chip ahead of the tap so was a very good high production machine tap..with a threw hole or enough hole to fill with chips.
Irwin has come out with a starter tap that has perhaps 4 threads at the beginning of the tap that are made the diameter of the tap drill, so leading a starter design tap straight into the drill hole. They need even more depth of hole to make good thread at the beginning of the hole. Hole need be perhaps 1" deep to get perhaps 1/2"+ good thread..Or yes one could follow with a normal tat.

I used the Irwin tap yesterday for the first time (3/8-16) in aluminum..
 
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