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Straight threads with a tapered tap

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Straight threads with a tapered tap

Unread postAuthor: inonickname » Thu May 21, 2009 4:44 am

Well in a serviceable (turned/lathed) pengun I'm making the back port threaded to allow for serviceability (epoxy loses.).

The problem is, to keep the gun streamlined I only have a short space to cut threads in (see attached picture) and I'm using a 1/8 NPT tap (to mount a streamlined schrader with 1/8 threads, go figure). Now by nature, NPT taps are tapered. The schrader isn't tapered, but it will screw into 1/8 threads.

Assuming I'm understood, how do I do this properly? Is there a secret way of doing it or will I have to make the sections and go to town with epoxy..
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Unread postAuthor: Carlman » Thu May 21, 2009 5:12 am

Buy yourself a brass 1/8" hose nipple chuck it in the drill press and slowly cut in, bit at a time.
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Unread postAuthor: inonickname » Thu May 21, 2009 5:20 am

Carlman wrote:Buy yourself a brass 1/8" hose nipple chuck it in the drill press and slowly cut in, bit at a time.


Cool, I'll give that a shot! For the record, the material is aluminum so it should be plenty easy enough. Will turn the school lathe down to around 50 (rpm) then do shallow cuts with the tap so the nipple can grip, then thread it in. If I can't get a 1/8" nipple by then I'll just buy one from Nev's and do it at home.

(yes, I make concealed weapons in metalwork- except this one actually functions as a pen, as well as a 400 psi co-axial. will be indiscernible from a pen when complete.. :twisted: )
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Unread postAuthor: psycix » Fri May 22, 2009 1:40 pm

Cut the threads on a lathe, or get a "bottoming" or "plug" tap (not for tapered threads though).
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Unread postAuthor: inonickname » Fri May 22, 2009 6:27 pm

You can cut inner threads on a lathe? It would seem to me that 1/8" may be a little small.

So If I were to do this again (in a different application) with larger threads, probably around 3/8" (give or take some, theres no need for it to be exactly that) to make a flat seat for a burst disk coupler (think hybrid) in hardened steel, cutting the threads with lathe tooling is acceptable? Seen as the mixtures will be say..20X epoxy is not acceptable.

I'll look into it.

And off-topic, does anyone have experience with bluing steel?

Thanks guys
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Unread postAuthor: ramses » Tue May 26, 2009 3:19 pm

yes, you chuck a tap in the lathe (the tailstock) chuck in the work piece, and tap away. if you bottom out the tap in the hole and neglect to turn the lathe off, you will break the tap in the hole, and you may as well just throw the whole thing away.
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Unread postAuthor: Gippeto » Tue May 26, 2009 4:14 pm

I use the lathe tail stock to start the tap in straight, turning the chuck by hand, then remove the piece and finish with a tap handle.

I only rarely break a tap doing it this way. :D

Broken taps can be annealed and drilled without a problem if the piece you're making will tolerate being heated.
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Unread postAuthor: jmccalip » Tue May 26, 2009 4:24 pm

The kind of tap you want to get is a Cold Forming Bottoming Tap.

Like this:

Image

We were just using 1/8" cold forming taps today. :o

That are a real PITA because they break easily...




You MIGHT be able to get away with just using that 1/8" threaded nipple...the tap is just what you're supposed to use.
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Unread postAuthor: Technician1002 » Tue May 26, 2009 7:45 pm

jmccalip wrote:The kind of tap you want to get is a Cold Forming Bottoming Tap.

Like this:

Image

We were just using 1/8" cold forming taps today. :o

That are a real PITA because they break easily...




You MIGHT be able to get away with just using that 1/8" threaded nipple...the tap is just what you're supposed to use.


If you use a pipe tap first and then use a bottoming tap, they don't break as often.
http://en.wikipedia.org/wiki/Tap_and_die

Bottoming tap or plug tap:[1] The tap illustrated in the top of the image has a continuous cutting edge with no taper. This feature enables a bottoming tap to cut threads to the bottom of a blind hole. A bottoming tap is never used to cut threads in an unthreaded hole, as the cutting edges lack the taper required to successfully start into such a hole. In the US they are commonly known as bottoming taps, but in Australia and Britain they are also known as plug taps.
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Unread postAuthor: USGF » Tue May 26, 2009 8:05 pm

As Gippeto says, use a carbon steel tap if salvaging a part will be inportant. A HSS (High Speed Steel) tap will be near inpossible to cut out if broken. Cold forming taps are great but kinda beyond the scope of a DIY layperson.

If you have 1/8-27 straight pipe threads, then find that size tap. 1/8-27 taper pipe taps will not work the best in this situation. You can find exotic taps, generally, tapered pipe taps only come as a bottoming style. Regular bolt and screw size taps come as tapered, plug and bottoming here in the USA. The number of chamfered threads is what sets them apart. A bottom has 1 or 2 threads chamfered, a plug about 4 -5, a tapered up to around 10.

It is easy to start a plain hole with a tapered tap. To thread close to the bottom you must use a bottoming tap. If going by hand, steady half turn forward, 1/4 turn backwards to break the chip. Pull out all the way once in a while to clear chips. Use an appropriate cutting oil. 30 wt motor oil will work in steel, wd-40 in aluminum, real cutting oil is best.

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Unread postAuthor: jmccalip » Tue May 26, 2009 9:11 pm

Technician1002 wrote:If you use a pipe tap first and then use a bottoming tap, they don't break as often.
http://en.wikipedia.org/wiki/Tap_and_die






You can also use a large chamfer on the hole.

Without the small chamfer the tap broke within 5 parts. It ended up splitting the aluminum part!

With the large chamfer, the new tap is going 100 parts strong.
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Unread postAuthor: inonickname » Wed May 27, 2009 3:46 am

I think I'll use gippeto's method. I found a tapered tap to start the hole, then I'll finish up with a nipple or bottoming tap if I can get one. Will just chuck it in the lathe and manually turn the chuck to line it up, then cut them manually.

Will be a lot of work, as it's extremely hard steel. Thanks everyone

(Taps aint cheap- I'm not going to be breaking any)
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