I was tapping a blind hole this weekend and concluded that one could not really do this with a standard tap because the "point" protrudes too much. My answer was to grind a bit off the tip; however, even so, I did not feel I was getting into the hole as I should.

My proposed solution it to get me one or two more taps and grind the first with less tip and the third with none at all, such that it can go to the bottom of the hole.

How do you pros do it?? Kudude

You're describing a bottoming tap. You can either buy them or grind them yourself as you suggest.

even a bottoming tap can have three to five threads of lead.

I use a bottoming tap to get close to the bottom and another one that I have ground off most of the lead to go the balance of the way.

quote:

Originally posted by vapodog:
even a bottoming tap can have three to five threads of lead.

I use a bottoming tap to get close to the bottom and another one that I have ground off most of the lead to go the balance of the way.

Actually you are describing a "plug" tap.

a Bottoming tap has only one to two.

A taper tap OTOH has 6-7

quote:

Originally posted by kudude:
How do you pros do it?? Kudude

Well, at the risk of stepping on a pro's toes I would suggest the following. First use a carbon tap such as the ones Brownells sells so when it snaps, you can break it into little pieces without severely screwing up the hole.

You will need a couple of taps. The frist one should be ground to get you within 3 - 4 threads of the bottom and the second one to finish the hole to within 1 thread of the bottom.

To help reduce the risk of breaking these small taps by "over flexing" you will want to shorten the length of the threaded section to just the length needed for the depth of the hole you will be working with.

Using a pair of pliers, break 2 carbon taps so that the threaded section is roughly the same length as the depth of the hole you will drill. Next, "LIGHTLY", and I mean "LIGHTLY" grind the face of each tap square frequently quenching it in cold water. If the tap gets hot, it WILL break apart in the hole.

The first tap should have about a 3 thread taper, and the second tap, or, "plug" tap should have a very slight relief cut, ground on the trailing edge of each flute. Using a precision, ground center in the machines chuck as a guide, carefully run the tapered tap into the hole until it lightly bottoms. Next run the plug tap until it bottoms. If everything was done correctly, you should be within 1 thread of the bottom. That is close enough.

Keep the "tool" cool when grinding and use a good sulpher based tap oil. Go slow. Practice on some scrap pieces of similar material to develop a feel for hitting the bottom. The practice will also come in handy for developing the vocabulary used by pro's while picking broken pieces of the tap from a blind hole because they lost their concentration and got a little cocky.

And don't forget the part about the flat bottom hole.

As an alternative you might just shorten the screw a couple of threads and go with the standard plug or bottom tap. Most of the "blind" holes for small screws have plenty of depth to have 3-5 threads engaged. It only takes .125 inches for 5 turns of an 40 tpi screw. Turning them in further adds little to the holding power of these small screws.

Roger

quote:

Originally posted by Westpac:
The practice will also come in handy for developing the vocabulary used by pro's while picking broken pieces of the tap from a blind hole because they lost their concentration and got a little cocky.

Truer words have never been spoken. That getting cocky part will get a guy every blasted time.

quote:

Originally posted by RogerR:
As an alternative you might just shorten the screw a couple of threads and go with the standard plug or bottom tap. Most of the "blind" holes for small screws have plenty of depth to have 3-5 threads engaged. It only takes .125 inches for 5 turns of an 40 tpi screw. Turning them in further adds little to the holding power of these small screws.

Roger

Yes, but how deep does the hole have to be to get that .125" of enagement? In most cases that will take you well into the chamber or bore. That's what makes blind holes so tricky. You forget that at the end of the hole is an angle left over from the drill bit. That adds depth but no threads.

quote:

Originally posted by Allan DeGroot:

quote:

Originally posted by vapodog:
even a bottoming tap can have three to five threads of lead.

I use a bottoming tap to get close to the bottom and another one that I have ground off most of the lead to go the balance of the way.

Actually you are describing a "plug" tap.

If you look up the manufacturers specs on bottoming taps you'll find from three to five threads of lead.

quote:

Originally posted by rem721:

quote:

Originally posted by RogerR:
As an alternative you might just shorten the screw a couple of threads and go with the standard plug or bottom tap. Most of the "blind" holes for small screws have plenty of depth to have 3-5 threads engaged. It only takes .125 inches for 5 turns of an 40 tpi screw. Turning them in further adds little to the holding power of these small screws.

Roger

Yes, but how deep does the hole have to be to get that .125" of enagement? In most cases that will take you well into the chamber or bore. That's what makes blind holes so tricky. You forget that at the end of the hole is an angle left over from the drill bit. That adds depth but no threads.

Yeah...

I keep hearing people say "3 threads is all you need". I don't know where they come up with that. Probably some higher education, book learning thing.

3 threads might well be enough IF the screw and the material that it is threaded into are of a sufficient hardness so as to resist tearing.

I don't know how many stripped receiver holes I get in here from someone using too short of a screw for the material. If I recall, it seems that it is usually the first 3 or 4 threads in the hole that let go. What does that say? When it comes to gun screws, put me in "the more threads the better" camp.

For me you get more threads by going deeper, and when you are limited in your depth, you get more threads by being able to tap the hole clear to the bottom.

quote:

Originally posted by vapodog:

quote:

Originally posted by Allan DeGroot:

quote:

Originally posted by vapodog:
even a bottoming tap can have three to five threads of lead.

I use a bottoming tap to get close to the bottom and another one that I have ground off most of the lead to go the balance of the way.

Actually you are describing a "plug" tap.

If you look up the manufacturers specs on bottoming taps you'll find from three to five threads of lead.

I think Allen was commenting that the tap that you described having ground "to go the balance of the way" is referred to as a "plug" tap. That is how I read it. But I may be wrong.

quote:

Originally posted by rem721:
Yes, but how deep does the hole have to be to get that .125" of enagement? In most cases that will take you well into the chamber or bore. That's what makes blind holes so tricky. You forget that at the end of the hole is an angle left over from the drill bit. That adds depth but no threads.

Mr. Kobe already addressed this but it needs repeating... that's why you make a flat bottom hole so you can get threads to with in 1 turn of the bottom. Look at virtualy any thread in a barrel, the holes are flat bottomed.

Thanks, guys. I thought I was on to something and you confirmed it. Isn't this a wonderful place. Thank's Saeed, again, for sponsoring and maintaining this wonderful resource. Kudude

3 threads gives you about 93% of the load carrying capacity of something like 10 threads. Going on a very old memory here.

If 93% of the strength will not hold without failing, then you have the wrong screw for that application. 3 threads will work fine if that is all you can get in there.

Everyone mark this date. I think this is the first time I knew something about guns that Malm didn't know!

BTW-taps are made as taper, plug, and bottoming taps.

quote:

Originally posted by rem721:

Yes, but how deep does the hole have to be to get that .125" of enagement? In most cases that will take you well into the chamber or bore. That's what makes blind holes so tricky. You forget that at the end of the hole is an angle left over from the drill bit. That adds depth but no threads.

Sorry, I guess it never occured to me that anyone might drill holes in the barrel that were anywhere near the chamber.

For sight base holes in the barrel, I try to use the screw to locate the correct position (more like a pin); then soder or locktight them on. The holding power of the screw is then not important.

For other applications where it is desireable to make a blind hole, MSC and Brownell's sell letter sized and fractional end mills that make nice flat bottom holes. If you make blind holes on a mill you can get the depth correct, so why not use an end mill to make the hole, then use standard plug or bottom taps. Remember to leave the screw 1/2 to 1 turn (thread)short so the screw doesn't "bottom out."

Most commonly I have seen stripped scope base holes in the rear bridge if it has been ground thin, and then usually if the bases have been "adjusted" or changed. These are small holes, and with small soft screws they can easily strip. "Over sized" #6 screws and moving up to number 8 screws are solutions. I usually use a "too long" screw and trim from the underside. For these, I like to use the red goo.

Roger