Yup, been there, did that. I had a broken tap that I did that with. If you are going to use a good tap, be sure keep it cool as you have to take off quite a bit off the end and they are tough(the good ones are anyway).
I try to design to avoid the need for bottoming taps but, in the past when I've needed one, I've made them this way...
Rather than grind the tip off, which extends the risk of overheating, I cut the tip off with a cutoff disk mounted in a Dremel tool. I have a tiny shop vac that I use for small clean up jobs. I mount the hose to the exhaust port and arrange it to blow over the tap while I'm cutting. By pecking at the tap with the disk, the risk of overheating is reduced considerably.
Still, there are very few situations that absolutely require bottoming taps; designing the need away is still the best solution.
Home Shop Freeware
1. When grinding away the tapered part of the tap you MUST KEEP THE TAP COOL so as not to ruin the temper of the tap
2. When tap has been ground down into a bottom tap you will need to provide a taper at the end of the tap over at least one thread (no more than 2 threads) otherwise you will really risk tap breakage, especially in hard material .
However, if it's not worth that effort or you don't have the setup to do that, making the bottoming tap out of taper isn't a horrible second choice.
First, thread a nut onto the tap so the start of the taper is just peaking out of the outboard side of the nut (towards the tip/away from the handle.) This nut will help dress any of the tap's threads that are damaged during the cutting. It'll likely be softer than the tap, but it can still straighten tiny cutting threads a time or two before the threads in the nut are ruined.
If you can, make your cut under water, or at least with flood cooling. A lapidary saw for cutting gem rocks is ideal for the job, but you can also direct a garden hose when using other cutoff tools.
It's important that the cut is exactly at right angles to the centerline of the tap - it won't have the taper to align the tap in the hole, so it has to be absolutely at a right angle to the hole you're tapping, else it'll start every tap job on a skew. If the cut IS at an angle, you can try to grind it perpendicular. It's a lot easier to cut it properly in the first place, especially if you don't have an accurate tool grinding setup.
The main thing is to keep the tap cool at all times. It can be warm - if you can hold a finger on it without burning yourself, that's cool enough to avoid softening. Any hotter than that though, and you risk ruining the tap.
Once the cut is made, remove the nut. If you have annealed the tap, you can dress the threads with the appropriate die. If not, run the nut on and off a few times.
Hope that helps. Let us know how it goes.
Dave Davies, Canada
yes I have done it many times slowly repeat slowly grind the bottom down till you get to the last thread or 2 that has a slight taper if you grind the end to fast and it turns blue than it is junk and it could break in the hole. when threading drill the hole to the correct size compare it to a chart then start it with a standard tap finish it with a bottom you made also use a good tapping oil for the metal you are working on thank you jimbo
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