Using a tap as a thread boring tool - video

[Jon]

Using a tap as a thread boring tool. By This Old Tony. 10:02 video:

[hemmjo]

Interesting how he describes the tap as 24 TPI. Then also gives the metric thread pitch 1.058. Why is that important at all.

I do see a couple of problems with this. If the tap axis is not perfectly aligned with the bore axis both horizontal it become a huge form tool, with each tooth making a chips. This would increase the loading on the tap. Taps are designed to take load equally spread around the axis. The main cause of tap breakage is forces that are not aligned with the axis and unequal on each side of the tap.

I have in the past ground all but one of the teeth off of a broken tap and used it as boring bar to single point an internal thread. In doing this, you can cut any thread pitch you want, as long as you do not mix thread profiles. It was easier than trying to grind a proper tiny thread profile from HSS.

[Frank S]

While I do have to plead guilty of having done this myself in the past, in some soft material but I didn't silver braze the tap in a holder though. So yes his procedure will work by being careful. I also can't recommend it as a method for threading on a regular basis, especially for a novice machinist. There are very few overlapping thread pitches between large and small diameter bores. 18 TPI is one 12TPI another as well as 14TPI would be come of the more common overlapping pitches the 18 pitch is quite common for axle nuts 12 pitch for the SAE boss o ring just to name a couple.
Since I don't have an internal thread chase I do often use a tap to clean out the threads of a larger bore size in this manner and anyone who has ever had to re chuck any threaded part internal or external knows you can't just toss it in the chuck and begin cutting on the threads it does require some adjusting to match the cutter to the existing thread.

[carl blum]

Hi Gang:

I did this back in 1974. I was making an indexing holder for sharpening end mills with 12 stops, every 30°. I think the thread was 2"-24tpi threaded with a 3/8"-24 tap. Someone pointed out the geometry isn't perfect, the angle of the thread around the tap is 2° while the nut I was threading was 0.38°. The nut was steel and I was chasing the thread a few thou a pass so it worked fine. Remember when tapping a 3/8-24 hole the tap cuts the full thread in one pass. This would be a nice way to thread ring for camera lenses.
Take care, Carl.

[hemmjo]

Hi Gang: ... SNIP... Remember when tapping a 3/8-24 hole the tap cuts the full thread in one pass. ...snip
Take care, Carl.

Perhaps I have miss-understood what you said but we need to be a bit careful here. I can see being able to chase threads already cut. But, if one attempts to cut a full thread, using a tap as a boring device where only one flute of the tap is touching the work, I doubt very seriously you can do that in one pass.

While it is true the tap cuts in one pass. It is not true that a single tooth cuts the full thread in one pass. A taper tap, that can cut a full thread through a hole is tapered, so as you get started, each tooth cuts makes a tiny chip, getting cutting the thread deeper as you advance the tap. It also cuts on all sides at the same time so the load is centered equally all around the body of the tap.

[Frank S]

Perhaps I have miss-understood what you said but we need to be a bit careful here. I can see being able to chase threads already cut. But, if one attempts to cut a full thread, using a tap as a boring device where only one flute of the tap is touching the work, I doubt very seriously you can do that in one pass.

While it is true the tap cuts in one pass. It is not true that a single tooth cuts the full thread in one pass. A taper tap, that can cut a full thread through a hole is tapered, so as you get started, each tooth cuts makes a tiny chip, getting cutting the thread deeper as you advance the tap. It also cuts on all sides at the same time so the load is centered equally all around the body of the tap.

A good way to guard against breakage is to make a fitted sleeve over the tap encapsulating the shank and at least the bottom and back thread runs
What I would do would be to bore a piece of round stock to fit the shank then bore out the thread depth slice away about 120° of the now threaded end the length of the threads. Mow screw the tap into the holder. shank first once it is threaded all the way in drill and tap for a couple of grub screws to contact in the flutes clamp this in your boring bar holder or tool holder, you now have made a tool you can thread with without breaking the tap.