Tapping lots of 1/4 x 20 blind holes in 6061

[arvidj]

Here is the use case:

I have to tap 48 blind holes 1/4 x 20 in 6061 aluminum. The holes are drilled about 25% deeper than what needs to be tapped.

Tools available:

I have a drill press with a Tapmatic head, A9 tapping fluid and both an OSG two flute spiral point tap and an OSG three flute spiral bottoming tap. The point tap is not long enough to put threads as far down the hole as are needed. The bottoming tap does create threads far enough down the hole.

Conundrum:

It seem like there are two possible ways to move forward.

The first is to tap all the holes using the point tap, clean the chips out the holes as best I can and then go over them again with the bottoming tap. The advantage would be considerably less strain on the bottoming tap as it only has to cut a small percentage of the threads. The disadvantage is the extra work of cleaning the holes and going over all of them a second time along with the concern that the Tapmatic and the bottoming tap will actually pick up the existing threads and not try to create new ones. [side note: a few experiments with scrap suggests it actually will pick up the existing threads but the research was not exhaustive enough to determine if that was the norm or just luck]

The alternative is to skip the point tap and just go with the bottoming tap. The advantage is a lot less work. The disadvantage is the stress on the bottoming tap as it cuts 100% of the threads for almost 100% of its length. The thought of having to remove a broken bottoming tap and\or scrapping the workpiece is not high on my list of things to do.

Suggestions? Thoughts? Concerns? Advice?

Thanks,
Arvid

 

[tq60]

We believe normal process is multi step.

Cut with taper then finish with bottom.

Yes takes time but blind hole is going to require lots of peck motion to remove tap from hole and clear chips.

Tap will catch the thread if carefull.

Take your time as clearing a broken tap will be lots of work.

Be certian it is secured well as the tapping via machine can lift the part causing angle on tap that breaks it...been there.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk

 

[JimDawson]

[side note: a few experiments with scrap suggests it actually will pick up the existing threads but the research was not exhaustive enough to determine if that was the norm or just luck]

I have had pretty good luck doing that in hundreds (thousands maybe) of holes.

@tq60 pretty much said it all.

 

[Uglydog]

I had a job this summer where I needed to tap 280 3/32 holes 2inches deep around two 6inch 6061 cylinders as part of a vacuum assembly.
There were also multiple taped holes.
I broke two 3/32. Try getting those out! Buyer was pleased enough with the rest that he didn't care.
All I can suggest is go slow and peck.
Always clear the chips.
Use appropriate lube.

Daryl
MN

 

[T Bredehoft]

I would suggest using a "Spiral Flute" tap. they are engineered to direct the chip up the flutes and out of the hole. They work well in aluminum if kept lubricated with either a tapping fluid or a light oil. They will leave the same thread in the bottom as a bottoming tap. This eliminates the ball of chips in the bottom of the hole and the necessity of changing taps and tapping each hole twice.

 

[mcostello]

Done in the hundred of thousands in a screw machine shop. Can You grind the point off Your tap to gain the needed threads? Never 2 tapped threads, Boss would have had a breakdown if that much time was wasted.

 

[jbolt]

Another option is a thread form tap. Taps are stronger and no chips to deal will.

 

[wawoodman]

A spiral POINT tap pushes the chips forward, and is recommended for through holes. A spiral FLUTE tap pulls them back.

But I agree, a thread forming tap might be a better choice. I've never used one in a Tapmatic, though. Any people who have?

 

[willthedancer]

Go with the spiral flute tap. A9 is good stuff. Use it generously. You might back the tap just enough to let the fluid back into the cutting surface, but not so far as to break the chip.

 

[Fabrickator]

I would proceed with your Option #1. That's how I do a project with multiple 1/4"X 20 holes. Pointed tap followed up by a bottom tap.

Question: Are all 48 holes in one piece, or is that the total for multiple pieces. If it's multiple pieces, you can make a few extra and try both Option/methods. If you break a tap, just scrap the single piece and move back to Option #1.