Shortening taps

[Alberto-sp]

Hello.

Here in Spain the offer of tools is not very wide, so things like bottoming taps are hard to find. So I usually cut the point of my taps with the angular grinder to make the threads until almost the bottom of the hole (putting oil during the cut to prevent overheating). Buuuuut this time I cut to much and the 3rd tap can't start the thread. So my question is, Is there a way to shorten the taps or only the rule is not to short them too much?

I tried to smooth the point of this tap and it hasn´t work. My question is not to fix it but to prevent this in the future.

thank you

best regards!

EDIT: I forgot the picture!

 

[graham-xrf]

I have not tried this, but it occurs to me that a first and second tap are just taps with some part given some taper, to allow the tap to start while cutting a thread at a reduced diameter. It is not just about getting the end of the tap to get started nicely. The depth of the cut is definitely less.
The second tap has less taper, and cuts nearer the full diameter over the duration of the taper.

The final "bottoming tap", which you achieve by shortening a more tapered tap, does come with the assumption that a "second" taper tap has already been there, and it only has to cut the final few threads, perhaps as few as two or three. For this, I am thinking your method is OK, so long as at least some of the threads down to the bottom have seen at least a partial cut. Then, a bottoming tap with only a tiny ground off lead-in, as you make them, might still have a chance.

I guess you still need two taps available. Bottom taps cannot do it on their own.

 

[Jake M]

Even a bottom tap doesn't "fully" thread to the bottom. There is some geometry involved, with steeper and shallower chamfered (tapered) ends. If you've only got less than two threads chamfered, there's a lot shorter taper compared to a plug/second tap where there are closer to 5 threads tapered. The big end of the taper obviously is the largest diameter of the thread. The small end of the taper is always as small or smaller than the root diameter of the threads.

Take a look at the first picture in this link. (Maybe the advice too, but I havn't read it....)

Different Types of Taps and When to Use Them - ProMAXX Tool

If you’ve worked on a car repair at home or in any automotive, industrial or fabricator shop before you may know a tap is a common yet overlooked hand tool. Machining technologies have advanced and common taps have become more specific in use and in care. Learn about taps in their basic and complex

www.promaxxtool.com

Look at how the taper changes the tooth shape, and the number of teeth involved in the cut to support each other. Your home made bottom taps (and mine.... I'm guilty too), will never be as good as a proper bottom tap, but if you pay attention, shorten a little bit at a time, and study the result carefully before proceeding, you can usually make a workable tool. It's twice as much up front, but if you keep a plug tap, and a "custom" tap in each size, you'll be five times ahead, because the home made bottom taps have far too much cutting load on the first tooth or two. Tough to use, and they don't last that long. They work best when they can be screwed into an existing thread (from a plug tap), so that they are evenly supported all the way around, as the one or two leading teeth try to do all of the work, which unsupported, means it just won't dig into a hole to get started.

 

[benmychree]

There are two types of tap sets, first, what is most common in USA, the taper, plug, and bottom tap, in this set, the 3 taps all have a full body size (all the same diameter, only the tapers differ. The other set is Serial hand taps on that set the diameters start smaller than the finish tap.
Most any tap in the former tap sets can be made into bottoming taps by cutting the taper off and carefully grinding a short taper on the end by hand, grinding a bit of clearance on the bevel. I have an attachment for my cutter grinder to sharpen taps in that manner.
If making a bottom tap from a serial tap set, one would need to use the full size tap to make the bottom tap.

 

[Huub Buis]

I prefer spiral flute taps because they are bottom taps that can also be used for through holes. I have some spiral flute machine taps that only have 3 lead in threads. They require the lowest force to tap the hole.
For my cord less drill, I use hex combi taps. For blind holes, I grind off the drilling part of this ombi tap. I also use these taps on the lathe for rigid tapping

 

[RJSakowski]

I grind the tip of a tap to gain a thread or two but never past the tap drill diameter. That portion has no function in the tapping process. You can salvage the tap by carefully regrinding the chamfer.

 

[Alberto-sp]

Thank you for your answers!