How to thread to the bottom of a dead end hole?

[jpowersny2]

Here's a newbee question: I'm need to put an air coupler into my backplate on the oscillator engine. The plans say to tap the hole 6-32 0.250 deep. I start tapping in and the point on the tap dead ends before I've started cutting full threads at the entrance (there is a smaller hole beyond, but the tap dead-ends there). Then the whole thing strips out. Do I grind of the tip on a 2nd tap and continue?

Thanks.

 

[b.lindsey]

The best option is to follow what you have already done with a bottom tap. If not available you can try grinding the 2nd tap you have but it won't be good for much else. The success of the second option can depend on how deep the hole is to begin with and if the threads at the entrance of the hole are well enough formed to guide the ground off tap.

 

[mklotz]

They make something called a "bottoming" tap which, as you might guess from the name, is meant to tap to the bottom of a hole. Rather than buy them, I simply grind a conventional plug tap to do the job. That should work in your case since you don't need much in the way of thread to attach an air line.

<rant>

IMO, there should be a special circle in hell for designers who call out a hole fully tapped to a flat bottom. It's almost always possible to design around the need for such.

</rant>

 

[BobWarfield]

They make something called a "bottoming" tap which, as you might guess from the name, is meant to tap to the bottom of a hole. Rather than buy them, I simply grind a conventional plug tap to do the job. That should work in your case since you don't need much in the way of thread to attach an air line.

<rant>

IMO, there should be a special circle in hell for designers who call out a hole fully tapped to a flat bottom. It's almost always possible to design around the need for such.

</rant>

Lads, we must never forget, those who are "they" are "us". In short, you own the machine shop, you are the engineer, if you don't like it, redesign it to your satisfaction.

Or, use a bottoming tap as the other guys have said.

Cheers,

BW

 

[zeusrekning]

I have to do this quite often for one of our customers. Even a bottom tap won't get the required depth sometimes. Usually I tap as deep as I can with the plug tap then follow with a tap that I've ground the end on. I try to grind the chamfer back to one thread before the first full thread on the tap. This usually gets me the depth I need. Just keep in mind that each tooth on the tap is a cutting "tool" If the first thread to contact the material is accidental ground poorly it will not cut well. Keep the bottom of the tap flat when grinding. If you grind to far back the tap is still usable just grind until you get a good first thread or you could try sharpening the first cutting thread but i guess on a #6 tap that would be very difficult. I use the taps that I have left some tapper on to tap holes if I have it in a machine. I would not use this tap for hand tapping unless you allready have a few revolutions of good threads so that the tap will be started straight. Hope this helps

 

[Cedge]

Pre-tap the hole with a normal tap and then finish chase it with a bottom tap. Less stress on you and the tool, when all is said and done...

Steve

 

[rake60]

That's a tough issue in the scale we work at.

My personal answer to achieving a fully tapped blind hole is to use a
bottom spiral flute tap with the point ground away, such as the taps
sold by ENCO
The spiral flute eliminates the chip binding problem in a blind hole.

Rick

 

[gilessim]

If you have a plug tap that you can grind, try grinding a "flute" on the bottom, that is, grinding a shallow "U" shape from one cutting edge , across the centre to the vertical flute on the other side, about 1-2 threads deep, this flute will have to be angled so as not to touch the other 2 cutting edges, this trick helps to clear the chips that accumulate at the bottom of the hole, also you can feel when you hit bottom!, I hope you can visualise what I mean!...Giles

 

[wareagle]

I will echo the comments from Rake. I ended up with a couple of sprial flute taps by accident, and they work great! I wish I had more of them.

<rant>

IMO, there should be a special circle in hell for designers who call out a hole fully tapped to a flat bottom. It's almost always possible to design around the need for such.

</rant>

Agreed! Very well put.

 

[jpowersny2]

I think I will try combining two suggestions. From mklotz and Bob Warfield I'm going to change the design and through tap it. Then I'll make my own air connector that is flush to the backplate with the smaller sized hole through it. I'll probably red locktite this in place. From rake60 and wareagle I'll order a few bottom spiral flute taps next time I place an order, but I don't want to do that now. My wife is becoming suspicious about the various boxes showing up on our front step

Thanks for all the help. It really great.