Depth of the thread cut by a tap?

If I was to drill a deeper hole, I buy a longer drill bit of the intended diameter.  I am sure the same thing apply to the taps as well.  However, in their specifications, I do not see anything about how deep a cut they would make.  So, my question is, what should I be looking if I need to make a deeper thread than than the tap I already have?  I mean is there a number which would say that this tap goes only one inch deep and so on?


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kelseymh2 years ago

Um. Do you mean how _long_ of a hole you can tap? That you can measure just by looking at the tap. It can only cut where it has threads.

In machining, the term "thread depth" refers to the radial distance, from the outside of the screw inward to the bottom of the "V" formed by the thread.

kabira (author)  kelseymh2 years ago
>> Do you mean how _long_ of a hole you can tap

Yes, that is exactly what I mean. The problem is it is hard to gauge it by ordering online. The specifications do not mention it.

I can tell you from experience that tapping a really long and deep hole is no fun at all.
Does not really matter if you have a long tap or a standard, the problem is binding and friction.

You need oil for the tapping but this oil also binds the metal shavings, so you have to get the tap out, clean it, clean the hole and continue - many times.

A bit can be avoided if you can tap upside down as this helps to loosen the metal shavings but again a very hard task.

Be aware that especially thin taps (under 6mm or 8mm) will also create some flex, with a long tap it can be as much as a quarter turn (spread over the entire shaft).

In any case your best option is to use 3-gang taps, so one for the pre-cut, one to form the thread and the last to finnish it.
Making deep threads with a single tap will end in broken tools most of the time.

Better still, use a machine tap- either pushes the chips through a hole, or spirals them out the back, depending on the profile. Harder to start square though, for a casual user.

Good one! Always forget about the special tools for a job.
But I think someone not used to tapping on a daily base might struggle with them a bit.

kabira (author)  Downunder35m2 years ago

Thanks so much. I get the picture now.

Same thing goes for dies too. As long as you can spin it around to drive it deeper you can go all the way on a rod or pipe but you wouldn't want to.

If you have ever tapped a hole...the tap which is like a drill bit is put into a tap wrench or holder to turn it. It is "chucked" into the handle tool like a drill bit so you can get shaft extenders for the drill bit to make that drill go deeper into a hole. That said, you can tap all the way down any depth hole as long as you can extend that shaft. In practicality, it is a tricky operation to do, you might bind the expensive tap deep in the hole or break the shaft extenders and may need to destroy the piece trying to get the tap back out. I guess look at the overall length of your tap to see how far it is capable of tapping. Tapping a hole deeper than what the tap was made for probably means wasted work on getting a hole tapped for a screw or bolt longer than it needs to be.

You have to purchase a specific tap if that's what you want to do. Usually the tap is made by cutting threads partway along a tool-steel rod. The part above the thread still has the full OD, so won't fit down the hole being tapped.

That may be for high end CNC milling attachments. My Harbor Freight set taps taper back to the smaller shaft diameter to fit in a standard tap wrench which also allows for the chips and shavings to flow through.

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